DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and...

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Corrosion Behavior of Titanium Based Ceramic Coatings Deposited on Steels (Korrosionsverhalten einer auf Stahl abgeschiedenen Keramikbeschichtung mit Titanbasis) Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades DOKTOR-INGENIEUR vorgelegt von Frau. Dipl.-Ing. Rania Ali Erlangen- 2016

Transcript of DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and...

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Corrosion Behavior of Titanium Based Ceramic

Coatings Deposited on Steels

(Korrosionsverhalten einer auf Stahl

abgeschiedenen Keramikbeschichtung

mit Titanbasis)

Der Technischen Fakultät der

Universität Erlangen-Nürnberg

zur Erlangung des Grades

DOKTOR-INGENIEUR

vorgelegt von

Frau. Dipl.-Ing. Rania Ali

Erlangen- 2016

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Als Dissertation genehmigt von

der Technischen Fakultät der

Universität Erlangen-Nürnberg

Tag der Einreichung: 11.12.2012

Tag der Promotion: 13.11.2015

Dekan: Prof. Dr. Peter Greil

Berichterstatter: Prof. Dr. Sannakaisa Virtanen

Prof. Dr. Andreas Roosen

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Acknowledgements:

One of the joys of completion is to look over the journey past and remember all the

friends and family who have helped and supported me along this long but fulfilling road.

First of all I would like to express my heartfelt gratitude to my supervisor, Prof. Dr.

Sannakaisa Virtanen. This thesis would not have been possible without her help, support

and patience. She unconditionally and readily shared her knowledge and offered

support, providing me with valuable insight and many ideas for the research.

I would also like to thank my examining committee, Prof. Dr. Andreas Roosen, and Prof.

Dr. Nadja Popovska-Leipertz, Prof. Dr. de Ligny, who provided encouraging and

constructive feedback.

I am very thankful to my friends and colleagues, Dr. Manuela Killian, Dr. Emad

Alkhateeb, Dr. Florian Kellner, Dr. Florian Seuss, Dr. Leonhard Klein, Dr. Hanadi

Ghanem, Dr. Giorgia Obigodi-Ndjeng, for always being there and ready to share their

experience with me over my PhD period.

I also thank Prof. Dr. Patrik Schmuki for giving me the possibility to carry out my thesis at

LKO.

I appreciate all my colleagues and friends who have helped me with surface analysis

and technical stuff. Also many thanks to the LKO staff, particularly, the corrosion group,

who enabled me to enjoy every day during this research process.

Special thanks go to Mr. Hans Rollig and his family. You have been like surrogate family

sheltered me over the years.

I would like to thank my parents, my sisters and brothers. Without their love and support

I would have not completed this road.

Finally, I would like to thank my husband, Ghadeer Diab. He was always there cheering

me up and stood by me through the good times and the bad times.

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Dedicated to:

My small family,

my beloved husband,

and my little angel, my daughter Leah

You are the sunshine of my life…

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Abstract

Titanium based ceramic films are increasingly used as coating materials because

of their high hardness, excellent wear resistance and superior corrosion

resistance. Using electrochemical and spectroscopic techniques, the

electrochemical properties of different coatings deposited on different steels

under different conditions were examined in this study.

Thin films of titanium nitride (TiN), titanium diboride (TiB2), and titanium

boronitride with different boron concentrations (TiBN-1&2) were deposited on

stainless steel and low carbon steel by chemical vapor deposition using the

hydrogen reduction of TiCl4, BCl3 and N2 at a reduced pressure of 600 mbar and

a temperature of 900°C. The factors evaluated were the substrate material, the

coating composition, the boron content, the thickness and the boron content.

Different alternating current and direct current electrochemical methods

(corrosion potential screening, potentiodynamic techniques at low scan rate and

electrochemical impedance spectroscopy) were used to study the

electrochemical behavior of the different coated steels in different electrolytes at

ambient temperature. The porosity of the deposited coatings which is essential

for the estimation of the corrosion resistance of coated components was also

measured.

Results showed that different coatings deposited on different steels have different

morphologies and crystal structures and consequently different corrosion

resistance. The resistance to corrosive attack of the coatings deposited on

stainless steel was relatively poor for TiN, better for TiBN-1&2 and best for TiB2.

On coated low carbon steel, TiB2 showed the worst corrosion resistance,

followed with TiN and TiBN-1 with relatively better resistance; TiBN-2 was the

best.

Thicker TiBN-3 with higher boron content deposited on low carbon steel was

tested in simulated soil solution, simulated seawater and 1 M HCl. The corrosion

resistance was also evaluated with immersion tests. The effect of different

temperatures (15, 35, and 45°C) was evaluated.

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Finally, applied cathodic protection and interrupted cathodic potential

measurements were also carried out.

To elucidate the corrosion protection mechanisms of the coatings, coating

morphology, chemical composition and crystal structure was studied by different

characterization techniques before and after corrosion testing.

Results showed that coating with good corrosion resistance has to meet the

following requirements: fine and dense structure with low porosity, good adhesion

to the material substrate.

Surface analyses indicate that the coatings do not only offer a physical barrier

which protects the substrate material from aggressive species, but also oxidize to

form an oxide passive layer on the coating surface, which consists mainly of

titanium oxide and titanium oxynitride. This layer enhances the corrosion

protection due to its chemical inertness; it also fills the cracks existing on the

surface and decreases the number of pathways which allow the electrolyte to

penetrate into the underlying substrate.

It is also shown that coatings with nano-crystal structure, and intermixed phases

with different crystal orientation, as TiBN-3 on low carbon steel, can provide a

superior corrosion protection in neutral test solution up to 90 days immersion

days. In acid medium, the coating is less protective due to the dissolution of the

oxide layer. Applying cathodic protection was found to decrease the protection

effect of the coating due to the reduction of the oxide film. Interrupting the applied

cathodic potential leads to coating damage and peeling off due to hydrogen

embrittlement and the reduction and reformation of the oxides filling the cracks

which leads to chipping off of the deposited coating.

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Zusammenfassung

Titan basierte keramische Beschichtungen werden mit zunehmender Häufigkeit

wegen ihrer hohen Härte, guten Abriebresistenz und überragender

Korrosionsbeständigkeit verwendet.

Mittels elektrochemischer und spektroskopischer Untersuchungen wurden in

dieser Arbeit die elektrochemischen Eigenschaften unterschiedlicher

Beschichtungen auf unterschiedlichen Stählen und mit unterschiedlichen

Beschichtungsbedingungen evaluiert.

Dünne Filme aus Titannitrid (TiN), Titanborid (TiB2) und Titanboronitrid mit

variierender Borkonzentration (TiBN-1&2) wurden sowohl auf Edelstahl als auch

auf kohlenstoffarmem Stahl mittels chemischer Gasphasenabscheidung

aufgebracht. Dabei wurden TiCl4, BCl3 und N2 unter reduziertem Druck

(600 mbar) bei einer Temperatur von 900°C reduziert. Das Substratmaterial, die

Beschichtungszusammensetzung, -dicke und der Borgehalt wurden ausgewertet.

Um das elektrochemische Verhalten der unterschiedlichen, beschichteten

Stahlproben in verschiedenen Elektrolyten bei Raumtemperatur zu prüfen,

wurden unterschiedliche elektrochemische Methoden mit Wechsel- oder

Gleichstrom angewandt (Screening des Korrosionspotentials,

potentiodynamische Techniken mit geringer Rasterfrequenz, elektrochemische

Impedanzspektroskopie). Die Porosität der abgeschiedenen Beschichtungen,

welche im Hinblick auf die Bestimmung der Korrosionsbeständigkeit der

beschichteten Komponenten essenziell ist, wurde ebenfalls bestimmt.

Die Messungen zeigten, dass unterschiedliche Beschichtungen auf

unterschiedlichen Stählen unterschiedliche Oberflächenstrukturen und

Kristallstrukturen aufweisen, was wiederum in einem unterschiedlichen

Korrosionsverhalten resultiert. Die Beschichtung von Edelstahl mit TiN wies

relativ geringe Resistenz gegenüber korrosiven Angriffen auf, TiBN 1&2 wiesen

bessere Beständigkeit auf, am besten schnitt TiB2 ab. Auf kohlenstoffarmem

Stahl wiederum zeigte TiB2 die schlechteste Korrosionsbeständigkeit, gefolgt von

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TiN und TiBN-1, welche im Vergleich stabiler waren; TiBN-2 zeigte die besten

Resultate.Dickere Beschichtungen mit erhöhtem Borgehalt (TiBN-3) auf

kohlenstoffarmem Stahl wurden in künstlicher Bodenlösung (simulated soil

solution), künstlichem Meerwasser und 1M HCl getestet.

Die Korrosionsbeständigkeit wurde auch mittels Tauchtests evaluiert.

Der Einfluss von Temperatur (15°C, 35°C, 45°C) auf die Korrosion wurde

ebenfalls untersucht. Schlussendlich wurden kathodischer Schutz und

kathodische Potentialmessungen durchgeführt.

Um den Mechanismus der Korrosionsprotektion aufzuklären, wurden die

Oberflächenbeschaffenheit, chemische Zusammensetzung und Kristallstruktur

der Beschichtungen mit verschiedenen Untersuchungsmethoden vor und nach

den Korrosionsbeständigkeitstests bestimmt.

Die Ergebnisse lassen den Rückschluss zu, dass für gute

Korrosionsbeständigkeit folgende Voraussetzungen notwendig sind: feine und

dichte Struktur mit geringer Porosität und gute Substratanhaftung.

Oberflächenanalytische Untersuchungen legen nahe, dass die Beschichtungen

nicht nur eine physische Barriere gegenüber aggressiven Medien darstellen,

welche das Substrat schützt, sondern auch durch Oxidation stabilen Passivfilme

auf ihrer Oberfläche ausbilden, welche hauptsächlich aus Titandioxid und

Titanoxynitrid bestehen. Diese Schicht erhöht den Korrosionsschutz wegen ihrer

chemischen Inertanz, füllt gleichzeitig auf der Oberfläche vorhandene Risse auf

und verringert die Anzahl der möglichen Wege, auf denen der Elektrolyt das

unterliegende Substrat erreichen kann.

Es wird auch gezeigt, dass Beschichtungen mit nanokristalliner Struktur und

vermischten Phasen unterschiedlicher kristalliner Orientierung (z.B. TIBN-3 auf

kohlenstoffarmem Stahl) für bis zu 90 Tage überragenden Korrosionsschutz in

neutraler Testlösung bieten können. In saurem Medium ist die Beschichtung

weniger effizient, da der passive Oxidfilm angegriffen werden kann.

Das Anlegen eines kathodischen Schutzpotentials zeigte eine Verringerung des

Schutzeffekts der Beschichtungen, da diese während des Prozesses reduziert

wurden. Unterbrechung des angelegten kathodischen Potentials führte zu

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Beschädigungen an den Beschichtungen und deren Ablösung durch

Wasserstoffversprödung und Reduktion und Umgestaltung der Oxidschicht, was

zu Volumenvergrößerungen des Oxids in den Rissen der Beschichtung und

daraus resultierendem Absplittern der Beschichtung führte

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Table of contents

Chapter 1: INTRODUCTION …………………………………………….1

Chapter 2: LITERATURE REVIEW……………………………………..5

2.1: Steel…………………………………………………………………………………5

2.2: Corrosion and corrosion protection…………………………………………..6

2.2.1: Corrosion forms…………………………………………………………..6

2.2.2: Corrosion protection……………………………………………………..7

2.2.3: Passive Procedures………………………………………………………8

2.2.3.1: Organic coatings (polymer coatings or paints)…………………9

2.2.3.2: Anodic protection: Passivation…………………………………11

2.2.3.3: Cathodic protection………………………………………………12

2.2.3.4: Barrier protection………………………………………………...12

2.3: Titanium based ceramic coatings……………………………………………13

2.3.1: Titanium Nitride coatings (TiN)……………………………………….13

2.3.2: Titanium diboride coatings (TiB2)……………………………………15

2.3.3: Boron nitride (BN)………………………………………………………16

2.3.4: Titanium boronitride coatings (TiBN)…………………………........18

2.4: Corrosion protection of titanium based ceramic coatings……………...20

2.5: Deposition process: Chemical vapor deposition (CVD)…………………24

2.5.1: Process principle and deposition mechanism……………………..24

2.5.2: Chemical precursors and reaction chemistry……………………...26

2.5.3: Advantages and limitations……………………………………………27

2.5.4: Applications………………………………………………………………28

2.5.5: CVD process parameters………………………………………………29

2.5.5.1: Temperature and pressure……………………………………..29

2.5.5.2: Coating-substrate adhesion…………………………………….30

2.6: Research Objectives: Protective coatings…………………………………32

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Table of contents

2.7: The electrochemical testing methods………………………………………34

2.7.1: Potentiodynamic polarization methods (potential-current

diagrams)………………………………………………………………...35

2.7.2: Electrochemical Impedance Spectroscopy (EIS)………………….38

Chapter 3: EXPERIMENTAL WORK AND METHODS…………….40

3.1: Materials and electrolytes…………………………………………………….40

3.2: Samples preparation…………………………………………………………..42

3.3: Coating of steel…………………………………………………………………42

3.3.1: Coating of stainless steel……………………………………………..42

3.3.2: Coating of low carbon steel…………………………………………..44

3.4: Coating characterization………………………………………………………46

3.4.1: X-ray Photoelectron Spectroscopy (XPS)………………………….46

3.4.2: Scanning Electron Microscopy and Energy Dispersive X-ray

(SEM & EDX)……………………………………………………………..46

3.4.3: X-Ray Diffraction (XRD)………………………………………………..47

3.4.4: Glow Discharge Optical Emission Spectroscopy (GD-OES)……48

3.4.5: Focused Ion Beam Microscopy (FIB)……………………………….48

3.4.6: Metallographic microstructural study………………………………49

3.5: The electrochemical measurements………………………………………...50

Chapter 4: RESULTS…………………………………………………...55

4.1: Characterization of different coatings on stainless steel……………….55

4.1.1: Surface morphology and optical metallographic cross-sections

of different coatings……………………………………………………55

4.1.2: Chemical composition of the deposited layer……………………..59

4.1.3: Crystal structure of the deposited coatings (XRD)……………….60

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Table of contents

4.2: Characterization of different coatings on low carbon steel……………..62

4.2.1: Surface morphology and optical metallographic cross-sections

of different coatings……………………………………………………62

4.2.2: Crystal structure of the coatings…………………………………….67

4.2.3: Chemical composition of the coatings……………………………..68

4.3: Characterization of thick TiBN-3 coating deposited on low carbon

steel……………………………………………………………………………….78

4.4: Electrochemical investigations on coated metals……………………….91

4.4.1: Electrochemical characterization of coated stainless steel……91

4.4.1.1: Open-circuit potentials and potentiodynamic

polarization measurements…………………………………...91

4.4.1.2: Electrochemical impedance spectroscopy

measurements.....................................................................94

4.4.2: Electrochemical characterization of coated low carbon steel….96

4.4.2.1: Open-circuit potentials and potentiodynamic

polarization measurements…………………………………...96

4.4.2.2: Electrochemical impedance spectroscopy

measurements.....................................................................99

4.4.2.3: Surface characterization after electrochemical

measurements……………………………………………….101

4.4.3: Summary: The electrochemical and corrosion behavior of

different coatings on different steel substrates…………………103

4.4.4: Electrochemical characterization of TiBN-3 coated low carbon

steel……………………………………………………………………..104

4.4.4.1: The results of 48 hours measurements……………………104

4.4.4.1.1: Open circuit potential measurements in simulated

soil solution and simulated seawater…………104

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Table of contents

4.4.4.1.2: Potentiodynamic polarization measurements in

simulated soil solution and simulated seawater

…………………………………………………...106

4.4.4.1.3: Electrochemical measurements of TiBN-3 coated

low carbon steel in 1M HCl……………………108

4.4.4.1.4: Electrochemical impedance spectroscopy

measurements of TiBN-3 coated low carbon

steel in different test solutions: 48 hours results

…………………………….……………………..110

4.4.4.2: The results of long time immersion (90 days)

measurements……………………………………………………...113

4.4.4.3: Potentiodynamic cyclic voltammograms of TiBN-3 coated low

carbon steel at different immersion times in simulated soil solution

and simulated seawater…………………….……………………..116

4.4.4.4: The electrochemical behavior at different temperature…………121

4.4.4.4: The electrochemical behavior under cathodic potential………..121

4.4.4.5: Pitting corrosion……………………………………………………..122

4.4.4.6: The electrochemical behavior TiBN-3 coated low carbon steel

under applied cathodic potential..............................................125

4.4.4.7: Interrupted cathodic polarization measurements………………..127

4.4.4.8: Surface analysis of TiBN-3 coating after different corrosion

tests………………………………………………………………….131

4.4.4.8.1: SEM and FIB-cut analysis……………………………..131

4.4.4.8.2: X-ray diffraction analysis………………………………132

4.4.4.8.3: XPS surface analysis…………………………………..133

4.4.4.9: XPS analysis after measurements at different temperatures….136

4.4.4.10: XPS analysis after measurements at 48h in 1M HCl………….138

4.4.4.11: XPS analysis after interrupted cathodic protection

measurements……………………………………………………...139

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Table of contents

4.4.5: Summary: The electrochemical and corrosion behavior of TiBN-3

deposited on low carbon steel ………………………………………………..…141

4.5: Equivalent circuit for CVD coated steels………………………………….142

Chapter 5: DISCUSSION……………………………………………. 146

5.1: CVD process parameters…………………………………………………….146

5.1.1: The effect of substrate microstructure and chemical composition

on different deposited coatings……………………………………146

5.1.2: The influence of boron flow rate on the morphology of TiN…..148

5.2: Corrosion and electrochemical behavior of different coatings on

different steels………………………………………………………………...150

5.2.1: The electrochemical and corrosion behavior of different coatings

on stainless steel and low carbon steel…………………………..151

5.2.2: The electrochemical behavior of TiBN-3 coating on low carbon

Steel……………………………………………………………………..154

5.2.2.1: The influence of test solution………………………………..157

5.2.2.2: The effect of test temperature……………………………….159

5.2.2.3: Passivity and localized corrosion……………………………159

5.2.2.4: The effect of interrupted cathodic polarization…………….161

6: CONCLUSIONS……………………………………………………..163

7: OUTLOOKS (FUTURE WORK)…………………………………...165

8: BIBLIOGRAPHY…………………………………………………….166

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Chapter 1: Introduction

1 Introduction Metallic Materials have always been an important part of human culture and

civilization; different types of materials are strongly needed in daily life

applications. Figure 1.1 shows the world consumption of diverse materials in the

mid-eighties. Likewise, today’s advanced technologies involve sophisticated

materials, since all of them utilize devices, products and systems that must

consist of various advanced materials. The current technical development is

strongly dependent on new materials with particular mechanical, chemical,

electrical, magnetic or optical properties.

Figure 1.1: World consumption of various materials in the middle of 1980’s [1]

Steels, in particular, are unquestionably the dominating industrial construction

materials. The combination of low cost, good mechanical properties and

manufacturing characteristics make their unique universal usefulness, although

steels, are from a corrosion viewpoint relatively poor materials since they rust in

air, corrode in aggressive environments.

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Chapter 1: Introduction

Figure 1.2: Steel production worldwide from 1950 to 2007

Figure 1.2, introduced by the World Steel Association (WSA), shows the

permanent increase in steel production worldwide.

Corrosion and/or degradation of steel structures can cause dangerous and

expensive damage to everything from automobiles, home applications, and

drinking water systems to pipelines, bridges, and public buildings, like other

natural hazards such as earth quakes or severe weather disturbances. The first

significant report about the cost of corrosion was introduced by Uhlig in 1949 [2],

where the annual cost of corrosion to the United States was estimated in the

report to be $5.5 billion or 2.1 percent of the 1949 GNP. According to another

corrosion study in the U.S. in 2002, made by NACE International Association, the

direct cost of metallic corrosion was $276 billion on an annual basis representing

3.1% of the U.S. Gross Domestic Product (GDP). Unlike weather-related

disasters, corrosion can be controlled.

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Chapter1: Introduction

However, preventing and controlling corrosion depend on the specific material to

be protected, environmental concerns such as a soil resistivity, humidity, and

exposure to saltwater or industrial environments, and many other factors [3].

The most commonly used methods in controlling corrosion include organic and

metallic protective coatings, plastics, and polymers, corrosion-resistant alloys;

corrosion inhibitors and cathodic protection- a technique mainly used on

pipelines, underground storage tanks, and offshore structures [4]. A combination

of one or more protection techniques (e.g., protective coating and cathodic

protection in pipelines) is in many applications very necessary to achieve

effective protection [5].

Conventional coatings (organic, and inorganic), considered as the most widely

applied protection method, lose their effectiveness with time and need

maintenance and/ or must be replaced, which is again very costly [6]. This made

the demand of developing new coatings with excellent corrosion resistance,

especially for structure exposed to erosive-corrosive environments it is an issue

of great importance [7].

Ceramic coatings, known as hard coatings, are promising candidates for this aim

[8]. Titanium based ceramic coatings (TiN, TiB2, and TiBN) characterized by high

hardness, wear resistance, and corrosion resistance, are obtaining widespread

use for strengthening and protection of constructional steels subject to wear and

corrosion [9, 10]. The properties of the coatings i.e., morphology, porosity and

other defects, strongly vary in function depend on deposition parameters, and

consequently, influence their corrosion behavior [11-13].

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Chapter 1: Introduction

Since many potential applications require a suitable corrosion resistance, the

understanding of surface degradation processes and the mechanisms influencing

corrosion is one of the key factors for the optimization of the materials.

Electrochemical techniques are powerful tools, which can be used very

effectively for probing corrosion processes, and life prediction, where relatively

simple techniques can yield powerful information.

The present study had multiple objectives. The first goal was to study the

corrosion behavior of four different types of titanium based ceramic films (TiN,

TiB2, TiBN with different boron contents) deposited on two different types of

steels in different test solutions. Furthermore, the influence of changing

deposition parameters on the morphology and the corrosion resistivity of TiBN

coating deposited on low carbon steel was studied.

Thicker TiBN was deposited on low carbon steel in order to produce coating free

of defects. This coating was tested in solutions with varying chloride contents, at

several temperatures, and under cathodic polarization. It was of our interest to

evaluate the corrosion protection of the coatings by electrochemical methods and

to develop a propitiate model from electrochemical impedance spectroscopy

measurements (EIS), which can be used to fit the experimental data and extract

the parameters which can characterize the corrosion process.

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Chapter 2: Literature Review

2 Literature Review

2.1 Steels

The term steel usually refers to an iron-based alloy containing carbon in amounts

less than about 2%. Carbon steels can be defined as steels that contain only

residual amounts of elements other than carbon, for deoxidation and better

corrosion resistance (such as manganese, silicon and aluminum), to improve the

mechanical properties (e.g., copper, nickel to improve strength; molybdenum to

help resisting embrittlement).

Carbon steel, although susceptible to corrosion, is one of the most widely used

materials in the industry and in daily life applications. This material is used in

water- and steam- pressure systems of power plants, in oil structures (pipes,

tanks and transporting ships), and as support for many other structures

throughout the world due to their fairly low cost (compared to other different

materials e.g. Ti, Cr, Al,…), good properties, high strength and ease of

fabrication, availability, weldability.

As the description implies, the primary alloying element of these iron-based

materials is carbon. Because carbon is such a powerful alloying element in steel,

there are significant differences in the strength, hardness, and ductility

achievable with relatively small variations in the levels of carbon in the

composition. However, other important factors- such as heat treatment and

fabrication processes can result in significant changes to the properties of the

carbon steel components. Steels are from a corrosion viewpoint poor materials

since they rust in air, corrode in acids and scale in furnace atmosphere, thus,

providing steel structure with additional corrosion protection is of high importance

to prolong the service life of steel structures.

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Chapter 2: Literature Review

2.2 Corrosion and corrosion protection

Corrosion is defined as the deterioration of a material, usually a metal, because

of a reaction with its environment which leads to a measurable alteration of the

material (properties, behavior), and may cause functional impairment of a

component or the whole system.

Thermodynamics and kinetic basic principles of the corrosion reaction allow the

prediction of whether a corrosion reaction is possible or not (thermodynamics)

and how fast it proceeds (kinetics).

2.2.1 Corrosion forms

Almost all corrosion problems and failures encountered in service can be

associated with one or more of the seven basic forms of corrosion [14]:

1. Uniform surface corrosion

General corrosion occurs on the entire surface at nearly the same rate.

2. Pitting corrosion

Corrosion with locally different abrasion rates; caused by the existence of

corrosion elements, known as shallow pit corrosion.

Local corrosion resulting in holes, that is, in cavities expanding from the surface

to the inside of the metal.

3. Crevice corrosion

Local corrosion occurs in any confined spaces caused by component design or

joints (metal or nonmetal).

4. Galvanic corrosion

Dissimilar metal corrosion occurs at contact surfaces of different metals.

5. Intergranular corrosion

Corrosion takes place in or adjacent to the grain boundaries of a metal.

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Chapter 2: Literature Review

6. Stress corrosion cracking (SCC)

SCC is the result of straining a metal (residual or applied stresses) in a corrosive

environment.

7. Erosion corrosion

Corrosion of a metal which is caused or accelerated by the relative motion of the

environment and the metal surface

Mechanisms and characteristics of different corrosion forms were thoroughly

discussed by many authors [15-24].

2.2.2 Corrosion protection

All methods, measures, and procedures aimed at the avoidance of corrosion

damages are called corrosion protection [3, 15, 25-30]. Modifications of a

corroding system in so far as corrosion damages are minimized.

Figure 2.1 gives an overview about how corrosion can be mitigated.

Figure 2.1: Methods, measures, and procedures of corrosion protection (van

Oeteren, Korrosionsschutz – Fibel [31]).

Corrosion Protection

Active Corrosion Protection

Avoidance of corrosion

Passive Corrosion Protection

Keeping corrosion substances

away from steel substrate

Metallic coatings and

organic layers

Intervention in the

corrosion process

Influencing aggressive

substances

Artificial cover and

protection layers

Intervention in the

electrochemical process

Removal of aggressive

substancesCorrosion Protection Planning

Practical design Suitable

for the material

Construction selection

Corrosion Protection

Active Corrosion Protection

Avoidance of corrosion

Passive Corrosion Protection

Keeping corrosion substances

away from steel substrate

Metallic coatings and

organic layers

Intervention in the

corrosion process

Influencing aggressive

substances

Artificial cover and

protection layers

Intervention in the

electrochemical process

Removal of aggressive

substancesCorrosion Protection Planning

Practical design Suitable

for the material

Construction selection

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Chapter 2: Literature Review

2.2.3 Passive Procedures

In passive corrosion protection, corrosion is prevented or at least decelerated

through the isolation of the metal from the corrosive environment by the applied

protective layers. These protective layers must fulfill the following technical

preconditions:

It has to be pore-free; impermeable to ionic moieties [32] and if possible,

to oxygen.

Maintain adhesion (to the metal) under wet service condition.

Corrosion resistant.

It must be resistant to external mechanical stress; and possess certain

ductility.

The main types of protective coatings are classified as follows, Figure 2.2.

Figure 2.2: Main types of protective coatings [33].

Protective coatings

Non-metallic coatingsMetallic coatings Organic coatings

1. Paints

2. Varnishes

3. Lacquers

4. Enamels

1. Surface or chemical

conversion coatings

(a) Chromate coating

(b) Phosphate coating

(c) Oxide coating

2. Anodizing

1. Hot dipping

(a) Galvanizing

(b) Tinning

2. Metal spraying

3. Cladding

4. Cementation

(a) Sherardizing

(b) Chromizing

(c) Calorizing

5. Electroplating or

electrodeposition

Protective coatings

Non-metallic coatingsMetallic coatings Organic coatings

1. Paints

2. Varnishes

3. Lacquers

4. Enamels

1. Surface or chemical

conversion coatings

(a) Chromate coating

(b) Phosphate coating

(c) Oxide coating

2. Anodizing

1. Hot dipping

(a) Galvanizing

(b) Tinning

2. Metal spraying

3. Cladding

4. Cementation

(a) Sherardizing

(b) Chromizing

(c) Calorizing

5. Electroplating or

electrodeposition

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Chapter 2: Literature Review

2.2.3.1 Organic coatings (polymer coatings or paints)

Organic coatings are inert organic barriers applied to metals substrates. They

can be effective barriers to protect steels when it is anticipated that the coating

can be applied to cover essentially all of the substrate surface and when the layer

remains intact in service. The key to maintaining corrosion protection by an intact

coating is sufficient adhesion to resist displacement forces, since; at a point of

weak adhesion between the surface and the substrate, the stress can lead to

disbandment. If the coating covers the entire surface of the steel on a

microscopic as well as a macroscopic scale, and if perfect wet adhesion could be

achieved at all areas of the interface, the coating would protect steel against

corrosion indefinitely. Practically, it is very difficult to achieve both of these

requirements in applying coatings and to assure full coverage of the entire metal

surface as required for barrier coating. Furthermore, coatings that were intact

initially may be damaged during their service lives, even those designed to

minimize the probability of mechanical failure. In such cases it is generally

desirable to design coatings to suppress electrochemical reactions rather than

primarily for their barrier properties. This can be achieved by the use of

passivating pigments, which promote the formation of a barrier layer over anodic

areas, passivating the surface. For the pigment to be effective, the binder must

permit diffusion of water to dissolve the pigment, which in turn must have

minimum solubility. However, if the solubility of the pigment is too high, the

pigment would leach out of the coating film too fast, limiting the time that it is

available to inhibit corrosion. The use of passivating pigments may lead to

blistering after exposure to humid environments if water permittivity was too high.

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Chapter 2: Literature Review

Anticorrosive pigments be divided into three types [34]:

1. Pigments with a physical protective action are chemically inactive or passive.

These lamellar pigments are packed in layers; they lengthen the pathways of

ions and inhibit their penetration.

In addition, they improve adhesion between substrate and coating and

protect the underlying binder. An example is micaceous iron ore.

2. Pigments with chemical protective action contain soluble components and can

maintain a constant pH value in the coating. Their action depends on reactions in

the interfacial areas between the pigment and substrate, pigment and binder, or

between pigment and ions that penetrate into the coating. An example is red

lead. Redox reactions can occur to form protective compounds (oxides or oxide

hydrates that may contain pigment cations). The somewhat soluble PbO raises

the pH and neutralizes any fatty acids formed over time. The toxic hazards of red

lead have resulted in prohibition of its use.

3. Pigments with an electrochemical protective action passivate the metallic

surface. Those that prevent corrosion of the iron by forming a protective coating

(e.g., phosphate pigments) are regarded as being active in the anodic region of

the metal surface (anodic protection). Pigments that prevent rust formation due to

their high oxidation potential (e.g., chromate) are said to be active in the cathodic

region (cathodic protection). Soluble chromate has also been established as

carcinogenic to humans. They must be handled with appropriate caution.

Electrochemical corrosion can also be mitigated without the use of organic

coatings; this can be achieved by suppressing the anodic reaction; suppressing

cathodic reaction; or by preventing water, oxygen, and corrosion stimulants from

contacting the surface.

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Chapter 2: Literature Review

2.2.3.2 Anodic protection: Passivation

The mechanism of anodic protection is based on the theory, that if the oxygen

concentration near the anode is high enough; ferrous ions are oxidized to ferric

ions soon after they are formed at the anodic surface.

Ferric hydroxide forms a barrier over the anodic areas, since; it is less soluble in

water than ferrous hydroxide. Suppression of corrosion by retarding the anodic

reaction is called passivation. However, a variety of oxidizing agents are used as

passivators. Chromate, nitrite, molybdate, and tungstate salts are examples. As

with oxygen, a critical concentration of these oxidizing agents is needed to

achieve passivation, since lower concentrations may promote corrosion by

cathodic depolarization. The most extensively studied reaction is the reaction

with chromate salts. Partially hydrated mixed ferric and chromic oxides are

deposited on the surface, where they presumably act as a barrier to suppress the

anodic reaction.

Other certain nonoxidizing salts, such as alkali metal salts of boric, phosphoric,

and carbonic acids, also act as passivating agents. Their basicity may result in

passivating action. By increasing pH, they may reduce the critical oxygen

concentration for passivation below the level reached in equilibrium with air.

Alternatively, it has been suggested that the anions of these salts may combine

with ferrous or ferric ions to complex salts of low solubility to form a barrier at the

anode. Possibly, the corrosion protection effect could be a mixture of both

mechanisms to some extent.

Recently, a new approach to passivation is developed, the use of a film of

electrically conductive polymer to a steel surface to protect it from corrosion. An

example is polyaniline; it is deduced to be effective by leading to the formation of

an adhesive, very thin, metal oxide passivating layer on the surface of the metal.

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Chapter 2: Literature Review

2.2.3.3 Cathodic protection

This type of corrosion protection is achieved by coating steel with zinc to make

galvanized steel. The steel is protected in two ways: Zinc has a more negative

electrode potential than does steel in most environments, so zinc is again the

anode when it is coupled to steel.

When defects such as a pinhole or a crack develop in the outmost zinc coating,

the underlying steel is protected by the sacrificial corrosion of zinc. The surface of

zinc becomes coated with a mixture of zinc hydroxide and zinc carbonate, after

exposure to the atmosphere. Both are somewhat soluble in water and strongly

basic.

2.2.3.4 Barrier protection

Barriers are films that can prevent corrosion by hindering oxygen and water from

reaching the surface. The zinc layer on galvanized steel acts as a barrier. Tin

coating on steel in tin cans acts as a barrier and it is effective as long as the can

is closed. After a can has been opened, the cut bare edges expose both steel

and tin to water and oxygen, and the steel corrodes relatively fast, because tin is

nobler than iron in the electromotive series.

Nevertheless, those coatings degrade by time elapsing [35-37], particularly, when

environments become severe i.e., in oil production and refinery and chemical

process industries, where conditions are aggressive and mechanical loads are

high, those coatings do not provide sufficient protection and become

economically not feasible, therefore the use of more corrosion resistive coatings

becomes very important. In the last few decades a new group of coatings was

developed and found their way into steel industrial applications, these coatings

exhibit a high wear and corrosion resistance, they are titanium based ceramic

films (known as hard coatings).

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Chapter 2: Literature Review

2.3 Titanium based ceramic coatings

In modern production technology, the surface treatment of tools and other parts

are important to improve the properties of work pieces, such as the corrosion

resistance, wear resistance, etc... Titanium based ceramic coatings are widely

used in practice, in particular titanium based ceramic films.

The high interest in Ti-based ceramic coatings, TiN, TiC, TiB2 and TiBN, as

coatings arises from the unusual combination of properties that characterize

these compounds and fulfill the requirements for good coatings. These

compounds exhibit, on the one hand, ultra hardness and high melting points,

typical characteristic of covalently bonded compounds. On the other hand, they

also, because of their metallic-like bonding, display metallic properties, such as

high thermal conductivity. The chemical inertness of those compounds favors

their application as corrosion protective coatings in wide variety of aggressive

environments [38]. In a relatively short time, these coatings have become major

industrial materials with numerous applications such as cutting and grinding

tools, drilling, bearing, textile machinery and many others.

These coatings can be deposited by a wide variety of methods such as chemical

and physical vapor deposition (CVD, PVD), ion plating and sputtering.

2.3.1 Titanium Nitride coatings (TiN)

Titanium nitride (TiN) belongs to the family of refractory transition metal nitrides, it

has a cubic structure identical to TiC with exceptional combination of chemical,

physical, mechanical and electrical properties, and a decorative golden

appearance [39, 40]. It is not as hard as TiB2 and TiC but is more chemically

resistant and has a lower coefficient of friction.

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Chapter 2: Literature Review

Its basic application is in improving the wear-resistance of cutting and cold

forming instruments [41, 42], it is most commonly used as a coating for drill bits,

saw blades, and other grinding and shaping tool. Drill bits coated with TiN last up

to three times longer than those without it. TiN coatings are deposited on

stainless steels as well for medical applications (artificial hips, knees or teeth).

The protective properties of TiN coatings on various constructional materials

including corrosion-resistant steel, are discussed in [43-46]. The conclusion was

that the properties and the protection efficiency of TiN coating depend on the

method of deposition, the coating thickness and the substrate type.

Synthesis of TiN can be performed by different deposition techniques, i.e.

physical vapor deposition (PVD) [47, 48] and chemical vapor deposition (CVD)

[49, 50]. The first commercial TiN coating was deposited on tools by CVD.

Several different CVD reactions have been used to deposit TiN, the most

commonly used one is the reaction of titanium tetrachloride TiCl4¯ with molecular

nitrogen. The range of temperature for this reaction is 900-1200 °C [51]. This

deposition temperature can be relatively high for some materials; therefore, the

use of ammonia, NH3, is sometimes preferred as a source of nitrogen, since is

facilitates the reduction of the deposition temperature to about 500 to 700 °C [40,

52].

The corrosion behavior of deposited TiN was thoroughly studied in various media

[48, 53-56], the evaluation of the electrochemical oxidation behavior of TiN

showed that it has a high stability against oxidation in wide-pH range. The

electrochemical oxidation resistive properties were attributed to the presence of

the nitrogen-enriched surface layer of titanium oxynitride with a large electron

density that screens the underlying titanium ions and inhibits the oxidation

reaction [54, 57].

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Chapter 2: Literature Review

Although TiN is already used in many industrial applications, its use as protective

corrosion coating (especially on steels) is still limited due to the presence of

pores, microcracks and other defects [48, 58, 59].

The porosity decreases when increasing the thickness but at the same time

strains increase, causing spalling in the deposited film [60]. Since, the

effectiveness of the corrosion protectiveness by the TiN coatings is determined

by its continuity and its good adhesion to the substrate, more work and efforts are

demanded to develop coatings with better protection quality.

2.3.2 Titanium diboride coatings (TiB2)

Titanium diboride (TiB2) is a ceramic material with a hexagonal structure in which

boron atoms form a covalently bonded network within metallic Ti matrix. TiB2 is

well known for its outstanding chemical and mechanical characteristics, such as

high hardness, high stability at high temperature, high resistivity to corrosion,

oxidation and chemical attacks [61]. In addition, because of its metallic-like

bonding, TiB2 also exhibits very good thermal and electrical properties [62]. This

combination of properties makes TiB2 very interesting as a coating material for

various applications, especially for cutting tools.

Similar to TiN, the deposition of TiB2 can be carried out by different techniques.

Chemical vapor deposition (CVD), in particular, was reported to have several

advantages over the other conventional methods, e.g., good reproducibility and

ease of controlling the growth rate, which are of great importance for good

coatings [63].

The most extensively studied reaction for CVD deposition of TiB2 is between

TiCl4 and BCl3 by Peshev and Niemyski, 1965 [64].

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Chapter 2: Literature Review

It was shown that the different morphological forms of TiB2 strongly depend on

the deposition temperature.

TiB2 films are characterized by a strong [001] texture of the columnar grains and

the grain boundaries perpendicular to the surface represents short cracks path

that can also impair the toughness [65].

The literature on the corrosion behavior of titanium diboride is rare and mainly

concerns acid environments [66, 67].

Under these conditions, the corrosion products of titanium diboride are found to

be the titanyl ion, TiO2+ and the boric acid [68]. In addition to these compounds,

also Ti3+ forms under acidic deaerated conditions. In simulated ocean water, the

complex TiO2.H2O is formed which reduces the material dissolution rate [67]. In

NaCl solution at room temperature, titanium diboride behaves like a passive

metal due to the formation of a surface oxide film, whose protectiveness

decreases with the temperature and disappears at 65° C [69].

The use of TiB2 films as hard protective coatings has been extensively studied

due to their mechanical and tribological properties [70, 71]. Results indicated that

TiB2 coatings have better wear resistance but a lower adhesion level than TiN

which limits the real applications of TiB2 coatings [72, 73].

2.3.3 Boron nitride (BN)

Boron nitride (BN) has been utilized as a significant coating material for cutting

tool applications in recent years due to its superior mechanical and chemical

properties. BN coatings possess good thermal conductivity, high electrical

resistivity, high wear resistance and chemical inertness at high temperature.

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Chapter 2: Literature Review

It is also superior to diamond due to its chemical stability against oxygen and

ferrous materials at high temperature [74, 75].

BN exists in two main crystalline polymorphs: the cubic BN (c-BN) and the

hexagonal BN (h-BN) phases. Hexagonal boron nitride has a layered structure

similar to graphite but is a transparent material, refractory and corrosion-resistant

material. It is soft, lubricating at low and high temperature, has a low friction

coefficient, and an electrically insulating and thermally conductive material. It has

wide applications as a solid lubricant in metal forming dies and metal forming

processes at high temperature in any environment [76]. In contrast, c-BN has an

extremely high hardness next only to diamond. The combination of outstanding

thermal, electrical, optical, and mechanical properties of c-BN puts it forward as a

suitable coating material for fabricating cutting tools.

Recently, sintered cubic boron nitride cutting tools have been used extensively in

the market. The problem associated with making the use of a sintered c-BN

cutting tool possible includes its high cost, poor ductility and difficulty of forming

them into various cutting tool shapes [77]. Currently, different deposition

processes have been explored to synthesize BN films. Among them PVD [78-80]

and CVD [81-83] processes.

Of all the techniques employed so far, chemical vapor deposition is the most

common and involves formation from reactive compounds by thermal means, i.e.

thermal decomposition of BCl3 using NH3 [84, 85]. These techniques generally

require a high substrate temperature. The handling of toxic and hazardous

precursors such as B2H6, BCl3 and BBr3 for the deposition of BN by thermal CVD

technique is also required. Phani et al. [86] proposed the use of aminodiboride

(ADB) as single-source precursor to solve this problem.

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Chapter 2: Literature Review

Boron nitride films were successfully deposited on different metal substrates. i.e.

Nickle and low carbon steel [87]. It was also used as interfacial coating for SiC

fibers [88]. As for the majority of hard coatings, the conducted investigations were

mainly focused on the influence of deposition parameters on the growth of BN

films, the microstructure, and the mechanical properties. Recently, attention was

drawn to their corrosion resistance properties. According to Moreno et al. [89], it

was deduced that applying a multilayer system of TiN[BCN/BN] n/c-BN on

stainless steel substrate would improve its corrosion resistance by 15 times

higher that the uncoated steel.

2.3.4 Titanium boronitride coatings (TiBN)

The development of TiBN coatings rose from the necessity of developing new,

higher performance coating systems more closely matched to particular

applications, with better mechanical, physical and chemical properties than those

of single phase coatings i.e., TiN, TiB2 and BN.

Adding borides to titanium nitrides coatings was reported to increase the

hardness, improve the corrosion resistance, and yet maintain good toughness

[90]. In a defined composition range, TiBN coatings exhibit wear resistance at

least 3 times better than for TiN or CrN [91] and hardness over 50 GPa [92, 93].

This high level of hardness was explained by the nanocomposite structure of the

film [94].

Ti-B-N coatings can be synthesized by almost all deposition techniques, having

been initially prepared by chemical vapor deposition (CVD) with deposition

temperatures above 1050 °C [95].

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Chapter 2: Literature Review

Subsequently, various film types within the system Ti-B-N-C based on TiB2 were

successfully synthesized by adding nitrogen to TiB2 films grown by non-reactive

and reactive DC magnetron

sputtering [96, 97], it was reported that the addition of nitrogen caused further

improvement of technically relevant properties such as morphology, hardness

and oxidation resistance of Ti-B films. This was attributed to the formation of

mixed-phase structure consisting of compounds based on TiB2, TiN, and BN.

The tribological [98] and corrosion [99] properties of Ti-B-N coatings prepared by

a similar sputter technique [100] were found to be promising. On the other hand,

a systematic investigation of the electrical properties of reactively sputtered films

in the Ti-B-N system in connection with the nanocomposite structure of the films

was reported in [101].

In order to synthesize coatings of different Ti-B-N compositions several other

methods have been employed such as a plasma-assisted CVD (PACVD) [102,

103].

C. Pfohl et al. reported that optimizing the PACVD process parameters permits

the deposition of dense titanium based coatings, hard and corrosion-resistant at

the same time. However, substrate corrosion and a consequent delimitation were

observed, which was related to the presence of pores in the deposited coating.

Other different deposition methods were arc physical vapor deposition [104], Ti-

implantation into (hexagonal) BN [105], interdiffusion of Ti/BN multilayer films

[106] and co-sputtering from Ti and BN targets [107].

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Chapter 2: Literature Review

CVD of TiBN coatings was reported only by few authors. After Peytavy et al. [95]

Holzschuh [108], deposited very good adherent TiBN coatings onto cemented

carbides using CVD technique at moderate temperatures (700-900 °C). The

deposition was performed by the addition of boron to the TiN coating. CVD-TiBN

was used as interlayer for the deposition of adherent diamonds films of high

quality onto steel substrates [109]. Despite the great importance and the huge

relevance between the corrosion resistance of this coating and its possible

application, the major focus of the conducted studies about TiBN coatings was

about their mechanical and tribological properties, whereas little information on

the electrochemical behavior and corrosion resistance has been published for

this material [99, 104, 110].

2.4 Corrosion protection of titanium based ceramic

coatings

The corrosion resistance of titanium based ceramic coatings is not related to their

barrier effect only but also connected to the passivation of the coatings. TiN and

TiB2 are not stable in aqueous solutions, but react spontaneously to titanium

dioxide forming a thin (few nanometer) passive layer, which prevents further

diffusion and dissolution of oxygen in the lattice [39, 111] and hence, the further

oxidation of the coating.

The electrochemical oxidation of TiN was evaluated by many research studies

[54, 112], it was reported that the electrochemical oxidation of TiN in a potential

window of (0.5 to 0.8-0.9V) leads to the formation and the growth of

oxide/oxynitride layer.

TiN + 2H2O → TiO2+1/2 N2 + 4H+ + 4e¯ (2-1)

TiN→ Ti3+ + 1/2N2 + 3e¯ (2-2)

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Chapter 2: Literature Review

Whereas at potential between 1.0-1.5 V: the oxidation results in formation of

hydroxide [112]

TiN+ 3H2O→ Ti(OH)3 +N2 + 3H+ + 3e¯ (2-3)

TiN + 3H2O→ TiO2.H2O + 1/2N2 + 4H+ + 4e¯ (2-4)

Milośev et al. [54] attributed the increase in current density in this potential region

to the formation of TiO2 on the surface of TiN rather than hydroxides (reaction (2-

3)), but accompanied by a similar liberation of N2.

The explanation was that the hydroxides are more soluble and less protective

than the oxides, and can cause an increase in current in this region. At potentials

above 2.0 V, oxygen evolution takes place with a simultaneous oxidation of

titanium nitride to TiO2.

The electrochemical oxidation of TiB2 was also studied [111]. The authors

studied the kinetics, formation mechanism, and the composition of oxide films

resulting from the oxidation of SiC-TiB2-B4C ceramics containing 10 wt% and 40

wt% TiB2 in 3 % NaCl solution. In this composite only TiB2 is the corrosive

component, SiC and B4C are fully inert to corrosive environments. It was reported

that the oxide film formed in both cases fundamentally changes its composition in

transfer from one type to another: at 10 wt% TiB2, an internal oxide layer about

100 nm thick contains of trivalent titanium oxide Ti2O3 forms in the lower layer of

the oxidized sample over the potential range -0.18 to 1.20 V.

2TiB2 + 15H2O → Ti2O3 + 4BO33- + 30H+ + 18e¯ (2-5)

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Chapter 2: Literature Review

At anodic potentials between 1.20 V and 1.90 V, an external oxide layer

containing higher titanium oxide TiO2 forms. This layer is about 50 nm thick.

TiB2 + 8H2O → TiO2 + 2BO33- + 16H+ + 10e¯ (2-6)

At 40 wt% TiB2, unlike the composite containing 10 wt% TiB2, TiB2 dissolves

insignificantly at -0.40 to -0.20 V to first TiO2+ ions and then successively Ti3+

ions pass into solution as follows:

TiB2 + 7H2O → TiO2+ + 2BO33- + 14H+ + 10e¯ (2-7)

TiB2 + 6H2O → Ti3+ + 2BO33- + 12H+ + 9e¯ (2-8)

However, those reactions are too slow and the sample surface is immediately

passivated to form stable TiO at -0.05 to +0.40 V:

TiB2 + 7H2O → TiO + 2BO33- + 14H+ + 8e¯ (2-9)

This film is stable only up to 1.00 V. With further increase of the potential the film

becomes unstable and destroys at potentials more positive than 1.00V as Ti3+

ions pass into solution. The measured thickness of the film is 250 nm with the

following chemical composition: the uppermost layer (out of four) contains

physically absorbed molecular oxygen and (deep inside the sample):

nonstoichiometric β-Ti1–xO3, stoichiometric β-Ti2O3, and β-TiO1–x as the lowest

layer.

The efficiency of the corrosion protection of titanium based ceramic films is

strongly related to the quality and uniformity of the passive film, which in turn very

strongly depends on the microstructure of the coatings (presence of pores and

defects in the deposited coatings), and their adhesion to the metal substrate.

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Chapter 2: Literature Review

Small pores in the coating may accelerate the corrosion by many mechanisms,

e.g., galvanic corrosion, crevice or pitting corrosion mechanisms. As the

corrosion reactions are initiated at the coating-substrate interface, measurements

of the porosity are essential in order to estimate the corrosion resistance of the

whole coated component. The determination of porosity is possible by means of

optical methods but difficult because of the small defect sizes.

By using electrochemical measurements, oxidation and reduction rates on the

sample surface can be measured and porosity can be estimated from these

values.

If the coating is of a good quality, no significant changes occur at the sample

surface before measuring the anodic polarization curves. Very porous films (of

porosity greater than 1%) will, however, already have failed after analysis of the

corrosion potential. On the assumption that the coating is electrochemically inert

at low anodic overpotentials, the porosity of the coating was calculated using the

following equation [113]:

𝑷 = (𝑹𝒑,𝒔 𝑹𝒑⁄ )𝟏𝟎−|𝑬𝒄𝒐𝒓𝒓|/𝒃𝒂 (𝟐 − 𝟏𝟎)

Where, P is the total coating porosity of the coating, Rp,s Ω.cm2 the polarization

resistance of the substrate, Rp Ω.cm2 is the measured polarization resistance of

the coated steel system, and is calculated according to:

𝑹𝒑 =𝟏

𝟐. 𝟑𝟎𝟑𝒊𝒄𝒐𝒓𝒓

𝒃𝒂𝒃𝒄

𝒃𝒂 + 𝒃𝒄 (𝟐 − 𝟏𝟏)

ΔEcorr is the difference of the corrosion potential between the coating and the

substrate, ba and bc the anodic and cathodic Tafel slopes, icorr the corrosion

current density in A.cm-2.

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Chapter 2: Literature Review

2.5 Deposition process: Chemical vapor deposition

(CVD)

In the last years chemical vapor deposition (CVD) has been gaining in popularity,

becoming a widely used materials-processing technique. CVD is a distinctly

different coating process than the physical vapor deposition (PVD) process or

vacuum evaporation, ion plating, or sputtering. A heat-activated process, CVD

can be defined ‘’according to Broadly’’ with the formation of solid products on a

heated substrate via chemical reactions of gaseous precursors introduced into a

reactor [114].

2.5.1 Process principle and deposition mechanism

In general, the CVD process involves the following keys steps [115, 116]

1. Generation of active gaseous reactant species.

2. Transport of the gaseous species into the reaction chamber.

3. Gaseous reactants undergo gas phase reactions forming intermediate

species.

Depending on the process conditions, homogeneous reactions may lead

to the creation of gaseous intermediates.

4. The precursors and reactive intermediates diffuse to and adsorbs on the

surface, where the heterogeneous reaction occurs at the gas-solid

interface (i.e., heated substrate) which produces the deposit and the by-

product species. The deposits will diffuse along the heated substrate

surface forming the crystallization center and growth of the film [117, 118].

5. This is accompanied by the production of chemical by-products that are

exhausted out of the chamber along with unreacted precursor gases.

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Chapter 2: Literature Review

The different CVD process steps are illustrated in Figure 2.3.

Figure 2.3: Schematic representation of the basic process steps during CVD

[119].

For the deposition of dense films and coatings, the heterogeneous reaction is

favored. In contrary, a combination of heterogeneous and homogeneous gas

phase reaction is preferred for the deposition of porous coatings.

main gas flow

diffusion and

adsorption of reactive

species

deposition and

diffusion of volatile

reaction products

gas phase

reaction

surface diffusion

and reactions

film growth

main gas flow

diffusion and

adsorption of reactive

species

deposition and

diffusion of volatile

reaction products

gas phase

reaction

surface diffusion

and reactions

film growth

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Chapter 2: Literature Review

2.5.2 Chemical precursors and reaction chemistry

The common precursors used in CVD process are metals and metal hydrides,

halides, and halo-hydrides, and metalorganic compounds. Generally, metal

halides and halo-hydrides are more stable than the corresponding hydrides. The

selection criteria of a suitable chemical precursor for coating applications are that

the precursor should:

be stable at room temperature.

have low vaporization temperature and high saturation of vapor pressure.

have suitable deposition rate, i.e., high deposition rates for thick coatings

applications.

generate vapor that is stable at low temperature.

undergo decomposition/chemical reaction at a temperature below the

melting temperature and phase transformation of the substrate. For

instance, the deposition of hard coatings (e.g., carbides, nitrides, and

borides) can use halides which tend to react at high temperatures and

offer high deposition rates.

have low toxicity, explosivity and inflammability for safety of handling

chemicals and deposition of the unreacted precursors.

be cost-effective for coating deposition.

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Chapter 2: Literature Review

2.5.3 Advantages and limitations

CVD has several features which make it the preferred process in many cases

[120, 121]:

It is not restricted to a line-of-sight deposition which is a general

characteristic of sputtering, evaporation and other PVD processes. Deep

recesses, holes, and other difficult three-dimensional configurations can

usually be coated with relative ease.

CVD films are quite conformal, i.e., the film thickness on the sidewalls of

features is comparable to the thickness on the top.

In addition to the wide variety of materials that can be deposited, they

have high purity. This results from the relative ease with which impurities

are removed from gaseous precursors using distillation techniques. The

deposition rate is high and thick coatings can be obtained.

CVD does not normally require ultrahigh vacuum as PVD processes. Its

flexibility such that it allows many changes in composition during

deposition; co-deposition of elements or compounds is readily achieved.

However, CVD does not just present advantages:

One of the major disadvantages lies in the properties of the precursors.

Ideally, the precursors need to be volatile at near-room temperature. This

is non-trivial for a number of elements, although the use of metal-organic

precursors has eased this situation.

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Chapter 2: Literature Review

Another primary disadvantage is that the films are usually deposited at

elevated temperatures of 600°C and above; many substrates are not

thermally stable at these temperatures. Moreover, it leads to stresses in

films deposited on materials with different thermal expansion coefficients,

which can cause mechanical instabilities in the deposited film.

CVD precursors can be highly toxic, explosive, or corrosive; the by-

products of reactions can also be hazardous and must be neutralized,

which may be a costly operation.

2.5.4 Applications

The major applications of CVD take advantage of the unique characteristics of

the process, such as good throwing power, the ability to deposit refractory

materials at temperatures far below the normal ceramic processing temperatures,

and the capability of producing materials of exceptionally high purity. Typical

cases for the CVD process include the fabrication or coating of tubing, tungsten

boride crucibles and dinnerware. Its applications in solid-state microelectronics

are of prime importance. Thin CVD films of insulators, dielectrics (oxides,

silicates, and nitrides), element and compound semiconductors and conductors

are extensively utilized in the fabrication of solid-state devices [122].

A substantial field of CVD exists for the hard and wear-resistant coatings such as

nitrides, borides, carbides, oxides, oxy-nitrides and carbo-nitrides of almost all

the transition metals, these coatings have found important applications in tool

technology [123], corrosion resistant coatings of cutting tools and surfaces

needing erosion and/or corrosion protection [124-127], decorative coatings, anti-

reflection and spectrally selective coatings on optical components.

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Chapter 2: Literature Review

CVD processes are also used in the manufacturing of objects with complex

shapes (e.g., refractory crucibles) out of materials such as tungsten,

molybdenum, and rhenium which resist conventional machining and fabrication

[128].

2.5.5 CVD process parameters

The deposition process and processing parameters such as temperature,

pressure, reactant gas concentration and total gas flow, and substrate

cleanliness influence the deposition rate, film growth, and the properties of the

deposited coatings. Therefore the thermodynamics and kinetics need to be

defined.

2.5.5.1 Temperature and pressure

The temperature at which the coating is deposited is critical as it controls both the

thermodynamics and kinetics of the coating process. The deposition temperature

must be achieved and maintained in order for the reaction to occur on the

substrate and not in the gas phase, and to form coatings with appropriate

microstructure (e.g., grain size and shape) [12, 129-131]. Small changes in the

temperature may change the reaction, and/or its kinetics, resulting in an inferior

coating. Increasing the substrate temperature results in an increase in the

deposition rate, mainly due to an increase in the chemical reaction rate [132].

Substrate temperature affects the growth and crystallographic structure of the

deposited films, the grain size and the preferred crystal orientation.

The total pressure of the reactor and the reactant gas partial pressure control the

transportation of the reactant gases to the substrate surface. CVD processes are

performed from atmospheric pressure to high vacuum.

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Chapter 2: Literature Review

At atmospheric pressure, the growth processes are often considered to be

‘’transport controlled’’. Parameters such as the substrates temperature, gas flow

rates, reactor geometry and gas viscosity all affect the transport phenomena in

the boundary layer. This influences the structure and composition of the

deposited films. In order to reduce the dependence of growth rate and film

composition on the hydrodynamics in the CVD reactor, many CVD processes are

carried out at total gas pressure below 1 atm where chemical reactions become

rate controlling, rather than the mass-transfer processes in determining the

characteristics of the deposited films [132]. Deposition rate varies with the

deposition temperature, pressure and gas flow, consequently, the properties and

characteristics of the deposited films will vary. The relative changes between the

partial pressure and the total pressure strongly influence the deposition rate

[133].

2.5.5.2 Coating-substrate adhesion

The adhesion of the coating to the substrate can be enhanced by avoiding [116]:

Substrate contamination (e.g., an inherent oxide layer due to oxidation),

therefore substrates must be cleaned prior coating deposition. This can be

achieved by mechanical grinding and polishing up to (desired or

requested) roughness. Additional sputter etching of the substrates can

also be performed directly before the deposition [96, 104].

The attack of corrosive unreacted precursors and/or by-products on the

substrate, hence, this will lead to the formation of weakly bonded

compounds at the interface of the coating-substrate.

Depletion of a gaseous precursor which can cause differences in gas

composition and coating thicknesses with different stress concentration.

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Chapter 2: Literature Review

Stress due to the deposition conditions or resulting from a mismatch in

thermal expansion coefficients between the substrate and the coating

when cooling down after deposition. Stress can be reduced to a certain

extent by depositing a ductile buffer layer prior to the final CVD process.

Total stress can also be reduced by decreasing the thickness of the

coating as well as by changing the grain size and morphology of the

coating.

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Chapter 2: Literature Review

2.6 Research Objectives: Protective coatings

Titanium based ceramic films were reported to be promising coatings that are

economically feasible and shows good corrosion protection for parts subject to

high stresses and corrosion environment [100]. However, a main part of the

studies carried out on this type of coatings was concerning the mechanical

properties (e.g., hardness, toughness, and adhesion) where there is still a

substantial lack of literature on the corrosion behavior of the coatings, in

particular, the deposited titanium based ceramics on steel substrates.

In this study, the aim was to evaluate the electrochemical and corrosion behavior

of three types of titanium based ceramic coatings (TiN, TiB2, and TiBN) deposited

on stainless steel and low carbon steel in different natural solutions which mainly

simulate the soil and seawater environments, since most of coated steel

structures, made for outdoor applications, are in contact with one/or both of these

environments.

In many industrial applications, the mechanical loads occurring are superimposed

by corrosive attack. Requirements for coatings therefore are chemical inertness,

a smooth surface, a dense morphology without micro pores and diffusion

pathways, a good adhesion between the coating and the substrate material.

As reported in the literature, there are many factors influence the corrosion

resistivity of these coatings, begins with the material of interest, to the coating

process and deposition conditions, the properties of the deposited coatings,

applications and the environment. For better understanding of the corrosion and

protection mechanism of the coated systems, each of the previously mentioned

factors needs to be closely investigated. Parameters, such as the morphology of

the deposited coatings and its relation to the deposition conditions (e.g., flow

rates of gaseous precursors, deposition time and coating thickness, and

substrate preparation), will be investigated in this study.

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Chapter 2: Literature Review

There are, as known in wet corrosion studies, many factors which precisely affect

the corrosion process and its velocity (e.g., pH value, chloride concentration,

chemical composition of solution, salt concentrations, and temperature).

Therefore, will also be explicit investigated in the study.

There are different corrosion tests to determine the corrosion resistant of

materials (e.g., salt spray test, electrochemical method). In this work,

electrochemical methods were chosen since they permit the separate

determination of coating and substrate corrosion. Therefore, a short introduction

to the electrochemistry and corrosion protection is provided in the next section.

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Chapter 2: Literature Review

2.7 The electrochemical testing methods

Electrochemical techniques are powerful tools for the study of corrosion [134].

These techniques provide the technologist with the ability to monitor corrosion

rates is service, giving early warning of conditions that could adversely affect

performance and integrity. They also provide the experimentalist with the ability

to determine the corrosion rate with high sensitivity, assess rate controlling

mechanisms, and in some cases make life prediction. The applicability of various

test methods depends on the exposure conditions e.g., immersion in solution and

atmospheric exposure. In this study the immersion is used for predicting the

corrosion performance of the coating systems. The used immersion test solutions

were mainly neutral solutions simulating natural environments, i.e., seawater, and

soil. In this study, the primary used electrochemical methods are:

potentiodynamic Polarization methods (PDP), Tafel Extrapolation Method,

Electrochemical Impedance Spectroscopy (EIS).

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Chapter 2: Literature Review

2.7.1 Potentiodynamic polarization methods (potential-current

diagrams)

It is very useful to know if a metal/ composite is immune to corrosion in given

circumstances using the potential-pH diagram (Pourbaix diagram), but practical

situations are most often completely different than the standard diagram and

must be independently evaluated. For achieving this, polarization methods are

very useful. They involve changing the potential of the working electrode and

monitoring the current which is produced as a function of time or potential.

Several methods may be used in polarization of specimens for corrosion testing

[135]. Potentiodynamic polarization, PDP, is a technique where the potential of

the electrode is varied at a selected rate by application of a current through the

electrolyte (Figure 2.4). It is used for determining the corrosion current and to

identify specific corrosion reactions, such as pitting and crevice corrosion.

Figure 2.4: Schematic polarization curves for Fe in an aqueous solution in the

presence of hydrogen ions.

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Chapter 2: Literature Review

A variant of potentiodynamic polarization is the cyclic potentiodynamic

polarization, CPP, test, described in [136], which provides a reasonable, rapid

method for qualitatively predicting the propensity of an alloy to suffer from

localized corrosion in the form of pitting and crevice corrosion. This technique

was developed for stainless steels and nickel-base alloys but has been

increasingly used for other alloys. This ASTM (American Society for Testing and

Materials) standard provides details on conducting CPP tests but is very limited

with respect to interpretation. Beavers et al. [137] has published a paper

concluding that from the analysis of CPP curves for the stainless steel-aqueous

chloride system and other alloy-environment systems, both a forward and

reverse scan should be performed in order to maximize the information on

localized corrosion obtainable from the test technique.

For passivating systems (e.g., containing titanium, zirconium, chromium, etc), the

cyclic potentiodynamic polarization technique is probably the most useful tool in

assessing localized corrosion [138]. In this technique, the voltage applied to an

electrode under study is ramped at a continuous rate in the anodic direction

(forward scan) up to a chosen current or voltage. At that point, the voltage scan

direction is reversed toward the cathodic or active direction (backward or reverse

scan) to a chosen voltage (usually either the corrosion potential or some active

potential) where the scan is terminated. The corrosion behavior of the system is

predicted from the structure of the polarization potential. If the reverse scan

traces, nearly, the same path of the forward scan in the region beyond the critical

current density, the material has little tendency to pit. If the current density was

different between the forward and reverse portions of the scan, a hysteresis loop

will be created [139, 140]. This difference is a result of the disruption of the

passivation chemistry of the surface as the potential increases; it reflects the

ability of the system to restore that passivation as the potential decreases back

toward the corrosion potential [141].

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Chapter 2: Literature Review

The risk of localized corrosion is greater when the current density of the return

portion is greater relative to the forward portion; this case is referred as negative

loop. Pitting and repassivation potentials are two characteristic potentials in terms

of localized corrosion can as well be defined from the hysteresis loop during the

cyclic polarization [142, 143]. The tested specimen would be expected to resist

localized corrosion if the corrosion potential if the corrosion potential lay cathodic

with respect to the repassivation potential [144]. The repassivation potential can

be chosen by several ways: as the potential at which the anodic forward and the

reverse scan cross each other, or alternatively, as that potential at which the

current density reaches its lowest readable value on the reverse portion of the

polarization scan. Choosing the latter case is done when the forward and

backward portions of the polarization scan do not cross each other [138].

Figure 2.5 represents a schematic representation of a cyclic potentiodynamic

polarization scan; it shows the case of pitting/repassivation. The first potential at

which the current density increases significantly with the applied potential is the

break down potential Ebd, the second feature is the potential at which the

hysteresis loop is completed during reverse scan after localized corrosion. This

potential is the repassivation potential or sometimes called protection potential

Epro.

Figure 2.5: Schematic representation of a

cyclic potentiodynamic polarization curve

[139].

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Chapter 2: Literature Review

2.7.2 Electrochemical Impedance Spectroscopy (EIS)

The EIS technique involves the application of a time varying voltage and

measuring the current response. The ratio of two gives the frequency-dependent

impedance [145]. The use of EIS in the evaluation of protective coatings on

metals and their interaction with corrosive environments provides new

information which cannot be obtained with traditional dc techniques, such as,

open circuit potential measurements, OCP, and/or polarization resistance, and

polarization curves [139, 146, 147]. An important advantage of EIS over other

techniques is the possibility of using tiny a.c. voltage amplitudes (10 to 50 mV)

exerting a very small perturbation on the system, and hence, EIS is considered

as a non-destructive technique relative to some dc techniques e.g., PDP.

Impedance is usually measured over a domain of discrete frequencies which is

determined according to the system under investigation [148]. For corrosion

studies, the high frequency end of the measurement domain is determined by the

frequency required to short the interfacial capacitance. Under these conditions,

only the cell solution resistance will be contained in the complex impedance. The

frequencies required to short the interfacial capacitance are related to the

measured system, i.e., uncoated/coated metal, coating types. For instance, for

bare metals, conversion coated metals; the interfacial capacitance is shorted at

frequencies ranging from 5 to 20 kHz. As the frequency is lowered, interfacial

resistances and reactances will contribute to the complex impedance.

Electrochemical and diffusional processes associated with corrosion are detected

at frequencies between about 10 and 10-6 Hz. As mentioned, the low frequency

limit of the impedance magnitude can be related to Rp which can be obtained

from Bode plot.

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Chapter 2: Literature Review

Thus, with Tafel slopes, the corrosion rate can be calculated using the Stern-

Geary equation. Figure 2.6 shows a schematic representation of a Bode plot.

Figure 2.6: Schematic diagram of a Bode [149]

Electrochemical reactions consist of electron transfer at the electrode surface;

mainly involve electrolyte resistance, adsorption of electroactive species, charge

transfer at the electrode surface, and mass transfer from the bulk solution to the

electrode surface. Each process can be considered as an electric component or

a simple electric circuit. The whole reaction process can be represented by an

electric circuit composed of resistance, capacitors, or constant phase elements

combined in parallel or in series. All this information can be extracted from EIS

measurements [150]. This is usually done by fitting the impedance data to an

equivalent electrical circuit which is representative of a physical process taking

place in the system under investigation. Through appropriate modeling, EIS can

provide some critical information on barrier and/or passive coatings, coating

degradation, development of coating defects, coating delamination, and under

film corrosion mechanisms [151, 152].

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Chapter 3: Experimental Work and Methods

3 Experimental Work and Methods

3.1 Materials and electrolytes

The chemical composition of the materials covered in this study is given in

Tables 3.1 & 3.2.

The first coated metal was stainless steel type X46Cr13. It has a martensitic

microstructure with very good mechanical properties and a moderate corrosion

resistance.

C Si Mn P S Cr

0.43-0.50 Max.1.0 Max.1.0 Max.0.04 Max.0.015 12.5-14.5

Table 3.1 The nominal composition in weight percent of the stainless steel

The corrosion resistance is limited due to the relatively low content of chromium

(ca. 13%). At this level of chromium, a thin protective passive film forms

spontaneously on steel, this acts as a barrier to protect the steel from corrosion,

but for a limited range, especially in Cl‾ containing environments [153].

While the second was a commercial low carbon steel from production pipelines, it

has API (American Petroleum Institute) grade X-52 with ferrite-pearlite structure.

C Mn Si P Al Cr Cu V Ni S

0.1544 1.262 0.313 0.0155 0.31 0.027 0.0464 0.03 0.0279 0.01

Table 3.2 The nominal composition in weight percent of the low carbon steel

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Chapter 3: Experimental Work and Methods

As previously discussed, the alloying elements that are used in low carbon steel

are limited primarily to carbon, manganese, and silicon with very small amounts

of other elements, such as chromium, aluminum and copper.

Such steels are hot rolled at elevated temperatures when they have an austenite

() crystal structure followed by relatively rapid cooling [154]. During the rapid

cooling, the austenite partially transforms to proeutectoid ferrite, (α), and the

remaining () transforms to pearlite, which leads to their microstructure a plus

pearlite. The pearlite consists of pearlitic ferrite and cementite [155, 156].

In the last part of this study and additional to low carbon steel, tantalum

specimens were coated with the same coating. Its immunity could help for a

better understanding of the protection mechanism of the coating.

The major part of the electrochemical and corrosion study was performed in

artificial types of water:

a. Simulated soil solution (SSS)

b. Simulated seawater (SSW)

The chemical composition of both solutions is presented in Table 3.3.

Solution Compound Concentration g/L

Simulated soil solution

NaCl Na2SO4

NaHCO3

0.272 0.71 0.21

Simulated seawater

NaCl MgCl2

CaCl2 KCl

Na2SO4

NaHCO3 KBr

24.53 5.20 1.16 0.70 4.09 0.20 0.10

Table 3.3: The chemical composition of test solutions (SSS) and (SSW)

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Chapter 3: Experimental Work and Methods

The used aqueous media have different chemical compositions, different salt

concentrations (in particular the chloride content), and different conductivities.

This gives them unique properties which are expected to differently affect the

tested specimens. Conventional electrolytes, as well, e.g., 0.5 M NaCl and 1 M

HCl were additionally used for the simplicity.

3.2 Samples preparation

The as received stainless steel has cylindrical shape with 24 mm diameter; it was

cut into smaller pieces of 6 mm height, whereas low carbon steel samples were

prepared into square shape with dimensions of 20mm*20mm*4 mm.

The samples were ground with silicon carbide paper up to 2400 grit, rinsed

thoroughly with ethanol in ultrasonic bath for 15 min. After that, the samples were

dried in a stream of air.

In addition to the ground low carbon steel samples, prepared for the deposition of

thicker TiBN-3 coating, part of samples were sand blasted under highly

pressurized air (4 bars) with 30 µm SiC powder for cleaning and providing a

rough surface. This was done for the purpose of studying the effect of surface

preparation on the corrosion behavior of the coating system.

3.3 Coating of steel

3.3.1 Coating of stainless steel

The deposition of the different hard coatings on different steel types was carried

out in CVD equipment from Surmetal Company (Switzerland) with a hot wall

reactor at a reduced pressure of 600 mbar and a temperature of 900°C. Figure

3.1 shows a schema of the CVD equipment used.

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Chapter 3: Experimental Work and Methods

Figure 3.1: Schema of the chemical vapor deposition (CVD) equipment.

The deposition parameters of the different coatings were as following:

a. Titanium nitride (TiN): TiN was deposited from a system TiCl4/ N2/H2. The

partial pressure of TiCl4 was 11.3 mbar and the N2/H2 ratio was 1:1.

Deposition of TiN occurs according to the following reaction.

2TiCl4 (g) + N2 (g) + 4H2 (g) → 2TiN (s) + 8HCl (g) (3-1)

b. Titanium diboride (TiB2): Precursor mixture of TiCl4, BCl3 and H2 were

used for deposition of TiB2. The partial pressure of TiCl4 was 11.3 mbar

and the H2/BCl3 ratio was 35:1. The deposition of TiB2 occurs according to

the following reaction.

TiCl4 (g) + 2BCl3 (g) + 5H2 (g) → TiB2 (s) + 10HCl (g) (3-2)

BCl3 N2 H2 Evaporator

Vacuum Pump

Neutralizer

Reactor

Exhaust

Gases

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Chapter 3: Experimental Work and Methods

c. Titanium boronitride (TiBN): The deposition of TiBN was carried at the

same condition as the TiN but with addition of different flow rates of BCl3.

The TiBN-1 was obtained with BCl3 flow rate of 0.16 Nl/min, and TiBN-2

with BCl3 flow rate of 0.32 Nl/min.

Based on the mass balance, all coatings had a thickness of approximately 2 µm.

3.3.2 Coating of low carbon steel

Based on the mass balance, the deposition time was adjusted according to the

growth rate to reach a coating thickness of approximately 3 µm. For deposition of

TiN precursor system of TiCl4 /H2 / N2 was used. The flow rate of H2 was 12

Nl/min and the ratio of H2 to N2 was 1:1. The TiCl4 was introduced to the reactor

as vapor by bubbling hydrogen through the evaporator; the flow rate of TiCl4 was

adjusted to 0.65 Nl/min. The deposition of TiB2 takes place from a system TiCl4/

BCl3/ H2. The flow rate of the precursor TiCl4 was adjusted to 0.65 Nl/min and the

flow are of the H2 gas was 12 Nl/min, the H2 to BCl3 ratio was 35:1. TiBN

deposition was performed from TiCl4/ BCl3/ H2/ N2 at the same process

parameters as TiN but with variable BCl3 flow rate. The TiBN-1 was deposited at

a BCl3 flow rate of 0.16 Nl/min and TiBN-2 at BCl3 flow rate of 0.32 Nl/min.

Thicker TiBN-3 coating of 6- 8 µm was achieved by increasing the deposition

time to have a free pin holes coating. Process gases of H2 / N2 / BCl3 and TiCl4

vapor were used for coating of TiBN. The flow rate of H2, BCl3 and N2 was 12

Nl/min, 0.32 Nl/min and 12 Nl/min, respectively. The TiCl4 was introduced to the

reactor as vapor by bubbling hydrogen through the evaporator; the flow rate of

TiCl4 was adjusted to 0.65 Nl/min.

The deposition temperature 900°C was chosen, since, the simultaneous

deposition of TiB2 and TiN takes place only between 750°C and 950°C [104].

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Chapter 3: Experimental Work and Methods

A full screening study on the dependence of TiBN deposition on the process

temperature and the concentration ratio of BCl3 and TiCl4 was carried out by the

authors is shown in Figure 3.2 [104].

Additional to low carbon steel samples, tantalum samples were pretreated in the

same way and coated in the same patch, to be dealt as references for better

comparison. Since Ta is an inert metal and thus the contribution of substrate in

the electrochemical tests can be minimized or even excluded.

Figure 3.2: Dependence of TiBN deposition domain TiBN on the temperature

and the BCl3 /TiCl4 ratio.

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Chapter 3: Experimental Work and Methods

3.4 Coating characterization

Different analytical techniques were used to characterize the deposited coatings

before and after the electrochemical measurements and corrosion tests.

3.4.1 X-ray Photoelectron Spectroscopy (XPS)

The elemental composition and chemical state of the deposited layers were

analyzed by using X-ray Photoelectron Spectroscopy (PHI 5600 XPS

Spectrometer) with Al Kα radiation at an incident angle of 45°. In the

measurements, the XPS spectra of the coatings were referenced to the C 1s

peak at 284.6 eV. Meanwhile sputtering by argon of (TiBN-3 with 6-8 µm

thickness) was carried out, the chemical state of the composition was analyzed

up to 500 nm.

XPS is a surface analytical technique; it provides detailed chemical information

from the top 1-10 nm of a sample surface. In this technique, electrons are

emitted from the sample upon its irradiation with X-ray beam to a detector that

finds out their energies [157]. The kinetic energy of the emitted electrons Ekin is:

Ekin= hν- EB (3-3)

Where EB is the binding energy of the ejected electrons, hν is the energy of

incident X-ray beam.

3.4.2 Scanning Electron Microscopy and Energy Dispersive

X-ray (SEM & EDX)

The coatings surface morphology was characterized by HITACHI FE-SEM S4800

Scanning Electron Microscopy (SEM). The SEM is a technique uses a highly

focused electron beam that scans the surface of the solid specimen [158].

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Chapter 3: Experimental Work and Methods

The produced signals by electron-sample interactions include secondary

electrons (that produce SEM images), backscattered electrons and characteristic

X-rays. The desired signal is captured and converted to electron pulse that are

then used to reconstruct the image [159].

Additional compositional chemical analysis was conducted by Energy Dispersive

X-Ray analyzer (EDX) equipped with the Hitachi FE-SEM S4800 to determine the

atomic ratio of the different deposited coatings.

EDX technique is based on the interaction of the electron beam with atoms within

the sample, exciting the emission of X-ray with energies characteristic of the

atomic number of the atoms involved [160]. These X-rays are collected and

analyzed according to energy, and counted using the technique of Energy

Dispersive Analysis of X-rays (EDX). Depending on the electron energy and their

absorption by the solid, the spatial resolution is around 1-3 µm.

3.4.3 X-ray Diffraction (XRD)

The crystallographic structure was investigated by X-Ray Diffractometer (XRD)

from Philips using X’Pert PRO diffractometer with monochromatic Cu Kα

radiation. XRD is a non-destructive analytical technique which provides detailed

information about the internal lattice of crystalline substance, including unit cell

dimensions, bond-length, bond-angles, and details of site-ordering [161]. The

reacted sample is simply placed in the specimen holder of a diffractometer, so

that the X-ray beam falls on the flat scale surface, and the intensity of diffracted

beams measured. The resulting diffraction pattern is then matched with tabulated

standards to get phase identifications.

Additional techniques were used to characterize TiBN-3 since it showed a highly

promising corrosion resistance in test solutions.

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Chapter 3: Experimental Work and Methods

3.4.4 Glow Discharge Optical Emission Spectroscopy (GD-OES)

A quantitative depth profiling of the chemical composition was investigated by

Glow Discharge Optical Emission Spectroscopy (GD-OES) using an ARL

spectrometer. The diameter of the glow discharge lamp was 8 mm and argon

plasma was used for surface sputtering.

GD-OES is an analytical technique used to measure the elemental

concentrations of solid materials; it does not require complex sample preparation

and can be used for bulk, surface and depth profile analysis of metals, oxides

and semiconductive materials. Its ability to perform elemental depth profiling with

a fine spatial resolution is particularly useful for the characterization of surfaces

[162]. In this technique, a stream of argon ions mills materials from the sample

surface. The sputtered material is then excited in a low pressure plasma

discharge and the resulting light emission is used to characterize and quantify the

sample’s composition.

3.4.5 Focused ion Beam Microscopy (FIB)

Focused ion beam microscopy (FIB) using dual-beam SEM/FIB 1540 EsB (Zeiss-

Company- Germany) was used to prepare and image precise cross-sections of

TiBN-3 before and after electrochemical investigations. These measurements

were conducted at the institute of Metals Science and Technology (WTM) of the

Friedrich-Alexander University of Erlangen-Nuremberg. The focused ion beam

milling is a technique uses an energetic beam of gallium (Ga+) ions to selectively

sputter regions of a material, whilst also functioning as a scanning ion

microscope. The milling accuracy is of order of the beam size allowing very

precise sectioning to be carried out [163, 164].

Cross-sections with dimensions of 13-15*25*15 µm (respectively width, length

and depth) were milled with beam current of 10 nA for initial cuts, 2 nA for the

final cleaning.

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Chapter 3: Experimental Work and Methods

The aim was to investigate the possible presence of interfacial corrosion or

coating detachment that might probably resulted from long exposure times in test

solutions. Direct SEM images from the cross-sections were captured. Information

on the distribution of the chemical composition across the sections was obtained

from line-scan analysis. Line-scan analysis gives the possibility to analyze the

sample composition along a line by scanning a fine beam of the exciting particles

across the sample [165].

3.4.6 Metallographic microstructural study

In order to observe the microstructure and the thickness of the different deposited

films on different steels, cross-sections were prepared. For this, coated samples

were firstly cut with Accutom 5R (Struers- Germany), Figure 3.3 shows the

cutting device.

Figure 3.3: Cutting- off machine

Cross-sections, of the samples used to examine the microstructure, were cold

embedded, ground with SiC-paper up to 1200 grit and then were polished for 5

min with diamante paste of 3&1µm particle size. After each step, samples were

cleaned in ethanol in an ultrasonic bath.

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Chapter 3: Experimental Work and Methods

This procedure was followed with chemical etching of the prepared cross-

sections; low carbon steel in (2% HNO3) electrolyte and stainless steel in

(glycerin, HCl and HNO3, with the ratio 3:2:1, respectively).

Etched samples were rinsed in ethanol and then were used for optical

microscoping.

The TiBN-3 coated LCS sample used for SEM was embedded in Epo-Black at

temperature of 180°C and pressure of 70 bar, ground up to 2400 grit and then

polished for 5 min with diamond paste of 3µm particle size followed with OPU-10

N for another 5 min with ultrasonication in ethanol between different steps.

The polished cross-section was first examined by optical microscopy, using

different magnifications to estimate the thickness of the coating. Higher

magnification images were obtained using scanning electron microscopy (SEM).

3.5 The electrochemical measurements

In this study, conventional electrochemical techniques were employed to

evaluate the corrosion behavior of the coated stainless steel and low carbon steel

samples in different test solutions. As the corrosion reactions are initiated at the

coating-substrate interface, measurements of the porosity are essential in order

to estimate the corrosion resistance of the whole coated system. The

determination of porosity is possible by means of optical methods but difficult

because of the small defect size. By using electrochemical measurements,

oxidation and reduction reaction rates on the sample surface can be measured

and porosity can be estimated from these values.

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Chapter 3: Experimental Work and Methods

All electrochemical measurements were carried out under atmospheric conditions

(i. e., the electrochemical cell was not deaerated). The experiments were

conducted in a three-electrode compartment cell (Figure 3.4) using IM6

electrochemical system (Zahner Company, Germany) for data acquisition.

Figure 3.4: Electrochemical cell [166].

A saturated Ag/AgCl connected to a Luggin capillary with 3 M KCl and a platinum

foil served as reference electrode (RE) and counter electrode (CE). The working

electrodes were the different coated steel samples with 1 cm2 test surface area.

The corrosion behavior of coated stainless steel was studied in 0.5 M NaCl by

measuring the electrochemical impedance spectroscopy (EIS) and the

polarization curves. For EIS measurements, an alternating current signal with the

frequency range from 100 kHz to 10 mHz and amplitude of ± 10 mV was applied

to the working electrodes at the corrosion potential. EIS spectra were collected at

0, 3 and 6h of immersion in test solution.

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Chapter 3: Experimental Work and Methods

At the end, polarization measurements were conducted in the potential range

from 100 below the corrosion potential to 2000 mV with a potential scan rate of 1

mV/s.

The electrochemical and corrosion behavior of the coated low carbon steel were

extensively studied in a variety of test solutions.

The first stage of the study was the evaluation of the different coatings (TiN, TiB2,

TiBN-1 and TiBN-2) in simulated soil solution (SSS) to simulate the real case of a

coated steel structure in contact with soil environment; the chemical composition

of SSS is presented in Table 3.3.

The succession of each experiment started by measuring open circuit potential

OCP for 15 min followed with EIS (EIS at 0h), then the OCP was further recorded

for 3 h in the test solution and again EIS was measured (EIS at 3h); this

consequence was repeated and (EIS at 6 h) was obtained. Electrochemical

impedance spectroscopy test was conducted, in the frequency range from 5 mHz

to 100 kHz applied to the electrode at its corrosion potential. Finally, polarization

tests were carried out at the end of each measurement series with a scan rate of

1mV/s in the potential rage from -1 to 1.5 Vvs Ag/AgCl.

Relaying on the fact that TiBN-2 has showed the best corrosion behavior among

the different tested coatings and after increasing the coating thickness to get

TiBN-3, an extensive electrochemical study in different test solution was

performed. In addition to the previously mentioned simulated soil solution, a

simulated seawater electrolyte (SSW), resembling one of the most corrosive

environments for steel structures, was used (see Table 3.3). 1M HCl was used

as a corrosive acidic media.

The regular electrochemical test of TiBN-3 was measuring EIS at the corrosion

potential in test solutions for 48 h at constant time intervals of 3 h.

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Chapter 3: Experimental Work and Methods

The perturbation signal of the applied potential and frequency range were the

same as previously mentioned for coated low carbon steel (LCS) samples.

At the end of each experiment potentiodynamic scanning or cyclic

potentiodynamic polarization measurements were conducted in the same test

solution in the potential range -1 to 4 Vvs Ag/AgCl at a scanning rate of 1 mV.s-1. For

the comparison, bare LCS samples were tested in the same way and taken as

reference samples.

The same regular test of the TiBN-3 coated LCS was carried out at different

temperature i.e. 15, 25, 35 and 45 °C in both SSS and SSW.

Long-time experiments of the TiBN-3 coated LCS were performed in SSS and

SSW. Coated samples were soaked in test solution for 90 days where OCP was

recorded and EIS was measured at corrosion potential in the frequency range 1

mHz to 50 kHz at constant time intervals of 3 to 5 days. After 90 days cyclic

potentiodynamic polarization curves were performed in the potential range -1 to 4

Vvs Ag/AgCl.

Porosity and corrosion parameters (corrosion potential Ecorr, corrosion current

density Icorr, passivity current density Ipass) were obtained from the polarization

curves by Tafel’s method. EIS results were used to determine the total

impedance (|Z|) and phase angle and to model the corrosion process, using a

simple equivalent circuit.

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Chapter 3: Experimental Work and Methods

Different type of electrochemical tests was carried out at cathodic polarization

potential of -1 Vvs Ag/AgCl. Two different measurements were set up to study the

effect of cathodic polarization on the TiBN-3 coated LCS, the first one was by

measuring EIS for 48h under applied cathodic potential (CP) at frequency

ranged from 100 kHz to 5 mHz. The second test was performed by interrupting

the applied CP, where EIS was measured at applied cathodic potential for 24h

with time intervals of 3 h, after that the applied CP was cut and the EIS was

measured at OCP in the same solution for another 24 h with same time intervals.

These measurements were alternately repeated for 10 days, i.e., 4 days on/ 4

days off. At the end, potentiodynamic polarization curves were recorded and the

samples were further analyzed with the same previously mentioned techniques.

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Chapter 4: Results

4 Results

4.1 Characterization of different coatings on stainless

steel

4.1.1 Surface Morphology and optical metallographic cross-

sections of different coatings

To obtain information of the different coatings deposited on stainless steel,

different characterization techniques were applied. Scanning Electron Microscopy

(SEM) was used to examine the morphologies of the different deposited coatings;

results are shown in Figure 4.1.

While the TiN coating shows a faceted microstructure (4.1.a), which has usually

defects in the structure providing good paths for the electrolyte onto the substrate

surface, the TiB2 has uniformly distributed fine grain morphology (4.1.b). Adding

a small amount of boron about 11% to the TiN layer leads to crystal refinement

(4.1.c). By increasing the boron content to 20%, the TiN showed a dome

structure with needles like shape (4.1.d). The competitive reaction between TiB2

and TiN on the steel substrate results in structure refinement, the TiB2 formed

dome like structure in the presence of TiN, and enhances the crystal refinement

of the faceted TiN. The fine-domed surface and the roughness of the surface can

be adjusted by the growth rate [39].

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Chapter 4: Results

Figure 4.1: SEM micrographs of coated stainless steel, a) TiN, b) TiB2

c) TiBN-1 d) TiBN-2

Details of the microstructure of the coated stainless steel can be observed on

microscopic micrographs in Figure 4.2. It can be seen that boron containing

coatings deposited on stainless steel have a compact and smooth morphology

with different thicknesses. Cross-sections of coated stainless steel samples

showed a relative good adhesion at the interface between stainless steel and

boron containing coatings.

Especially good was TiB2, it has as well the highest thickness.

Titanium nitride layer can hardly be seen after cross-section preparation; it is

completely detached from the surface and is very thin. It is also interesting to

note that the coating-substrate interfaces show no readily apparent intermediate

phase.

c d

ba

5µm

5µm

5µm

5µm

c d

ba

5µm

5µm

5µm

5µm

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Chapter 4: Results

The microstructure of the substrate shows a two-phase structure resulting from

furnace cooling to room temperature. This structure is known as spheroidite, it is

a dispersion of cementite particles in alpha ferrite, partly lamellar and partly

spheroidal cementite in a ferrite matrix.

Figure 4.2: Optical micrographs of the cross- sections of TiN and TiB2 coatings

deposited on stainless steel

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Chapter 4: Results

Figure 4.2: Optical micrographs of the cross- sections of TiBN-1 and TiBN-2

coatings deposited on stainless steel

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Chapter 4: Results

4.1.2 Chemical composition of the deposited layer

Results of survey scans of the coatings using the X-ray Photoelectron

Spectroscopy (XPS) are listed in Table 4.1.

The carbon signal mostly stems from surface contamination, due to the high

surface sensitivity of XPS, and therefore will not be further discussed. Presence

of oxygen indicates that oxidation of the surface of the coatings has taken place.

This effect seems to be the strongest for the TiN coating. Apart from surface

contamination and oxidation, the composition of the titanium nitride layer

corresponds to Ti-N with at-% ratio of 1:1. The boron to nitrogen ratio in TiBN-1

was 1.1 and in TiBN-2 the ratio was 2.4, about two times higher. The oxidation of

the surface may suggest that not all titanium has reacted to nitrides and carbides,

and some metallic Ti was present on the surface, which upon exposure to air

oxidized. As the amount of oxygen detected on the surface decreases with the

boron content, the amount of not reacted titanium seems to be minimized by

boron.

Table 4.1: XPS elemental composition in at% of the deposited coatings on

stainless steel

1.02.133.238.65.4----19.7TiB2

0.10.720.157.212.68.21.1TiBN-2

0.21.111.759.315.510.51.7TiBN-1

0.21.4----42.423.216.716.1TiN

FeSiBCONTi

Element

Layer

1.02.133.238.65.4----19.7TiB2

0.10.720.157.212.68.21.1TiBN-2

0.21.111.759.315.510.51.7TiBN-1

0.21.4----42.423.216.716.1TiN

FeSiBCONTi

Element

Layer

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Chapter 4: Results

4.1.3 Crystal structure of the deposited coatings (XRD)

The X-ray diffraction pattern of the coated stainless steel is presented in Figure

4.3.

Figure 4.3: XRD patterns of (TiN, TiB2, TiBN-1 and TiBN-2) coatings deposited

on stainless steel

The XRD pattern of the TiBN coating indicates the presence of two main phases,

titanium nitride and titanium diboride (TiB2), in addition to a phase of titanium

boride (TiB). The intensity of TiB2 to TiN increases as the flow rate of BCl3

increases. The ratio of TiB2 to TiN was 1:1 in TiBN-1 and it was increased to 4:3

in TiBN-2. The peak of the boron also results in broadening of the TiN 200.

Position (2Ө), [°]

TiN

TiBN-1

TiBN-2

TiB2

TiB2

101

Position (2Ө), [°]

TiN

TiBN-1

TiBN-2

TiB2

TiB2

101

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Chapter 4: Results

The preferred orientation of the TiN coatings was evaluated by the texture

coefficient (TC) according to the following equation :

𝑻𝑪(𝒉𝒌𝒍) = 𝑰𝒎(𝒉𝒌𝒍)

𝑰𝒓(𝒉𝒌𝒍)/

𝟏

𝒏∑

𝑰𝒎(𝒉𝒌𝒍)

𝑰𝒓(𝒉𝒌𝒍)

𝒏

𝒊=𝟏

Where: Im(hkl) is the measured X-ray relative intensity of the (hkl) plane, and

Ir(hkl) is the relative intensity in the powder pattern.

From the texture coefficient results (Table 4.2) can be seen that the TiN single

layer had a preferred orientation of 200. Also, in TiBN layers the preferred

orientation of the TiN was 200 as found for the single TiN coatings. The TiB2

coating layer had a crystal-preferred orientation of the 100.

Table 4.2: Texture coefficient of TiN in the presence of boron

1.130.441.161.780.49TiBN-2

1.050.491.141.310.95TiBN-1

0.810.861.081.171.08TiN

222311220200111Thkl

Layer

1.130.441.161.780.49TiBN-2

1.050.491.141.310.95TiBN-1

0.810.861.081.171.08TiN

222311220200111Thkl

Layer

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Chapter 4: Results

4.2 Characterization of different coatings on low

carbon steel

4.2.1 Surface Morphology and optical metallographic cross-

sections of different coatings

The morphologies of the same coatings on the low carbon steel are shown in

Figure 4.4. The TiN coating has a star shaped crystal intermixed with lenticular

crystals (4.4.a); this type of microstructure is usually associated with high stress

in the coatings. On the other hand, TiB2 coating shows a needle like morphology

(4.4.b) with paths to the substrate, in addition, this coating was not uniform and

continuous on the whole surface showing many defect spots where the coating

seemed to be flaked off from the surface (4.4.e), and this was confirmed by EDX

measurements where a large difference in atomic ratio of Ti/ Fe was detected

between the different surface sites.

Adding boron (about 34.5 at%) to the TiN layer leads to crystal refinement (4.4.c)

and disappearance of the star-shaped crystals, but the microstructure still has

many grain boundaries. Further increase of the boron content to 40 at% leads to

finer and denser crystal size (4.4.d).

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Chapter 4: Results

Figure 4.4: SEM images of coated low carbon steel coated with, a) TiN, b) TiB2,

c) TiBN-1, d)TiBN-2, e) TiB2 with defects

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Chapter 4: Results

The etched cross-sections of low carbon steel coated with (TiN, TiB2, TiBN-1,

and TiBN-2) were observed under the light metallographic microscope, a cross-

section of received low carbon steel sample is shown for comparison. The

micrographs are shown in Figure 4.5.

As in SEM micrographs, TiB2 cross-section shows a rough non-uniform coating

with many defects. TiN coating has almost a uniform thickness but few cracks

and defects were detected. The structure of TiBN-1&2 is almost free of defects

and the coating became thicker after increasing the flow rate of boron, since the

deposition rate of titanium boron nitride is higher here than the deposition rate of

titanium nitride or titanium boride by themselves.

An interlayer is found between the coating and the steel in every case. This layer

seems to be formed before the coating develops, in terms of diffusion interaction

between the substrate and the coating material.

At the steel interface between the steel and the coating a reaction zone with a

ferritic microstructure is built which resulted from the slow cooling rate. The steel

substrate shows quite clearly that carbon is lost from the steel in the vicinity of

the deposited films i.e. decarburization. The loss of carbon is accompanied with a

decrease in pearlite population of the substrate and a grain growth.

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Chapter 4: Results

Figure 4.5: Optical micrographs of the cross- sections of the as received LCS,

TiN, and TiB2 coatings on low carbon steel

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Chapter 4: Results

Figure 4.5: Optical micrographs of the cross- sections of TiBN-1, TiBN-2

coatings on low carbon steel

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Chapter 4: Results

4.2.2 Crystal structure of the coatings

The crystallographic structures of the deposited coatings on LCS were

investigated by XRD and are shown in Figure 4.6. The deposited titanium nitride

revealed that it is crystalline and exhibited a single phase of fcc NaCl structure

with a preferred orientation 111. On the other hand, the TiB2 has a hcp structure

with a strong 101 preferred crystal orientation. The TiBN-1 and TiBN-2 show

overlapping of the peaks suggesting incorporation of different phases, especially

the peak of TiB2 200 overlaps with the TiN 311 and the peak of TiB2 101

overlaps with the TiN 200. The peak intensities of 111 and 220 of TiN

decrease by increasing the boron content until the boron content reached 40

at%. In TiBN-2 the peak 111 of TiN disappeared. On the other hand, the peak

intensity of TiB2 100 increases with increasing boron content. The TiN 200

overlaps with the TiB2 101, and the peak intensities decrease with increase in

the boron content.

TiBN-1 and TiBN-2 have the same preferred TiB2 crystal orientation of 100. The

TiBN-2 coated steel shows high intensity peaks with a good coincidence with the

line positions of TiB2 100, indicating TiB2 to be the predominant phase in this

layer. A new peak of hexagonal boron nitride (h-BN) appears at 2 of 30o in

TiBN-1, and the peak intensity increases in TiBN-2 as boron content increases.

XRD measurements did not detect any oxides in the coatings. Hence, as already

indicated by the comparison of EDX and XPS data with different information

depths, oxygen is mainly present on the surface of the coatings and therefore the

thin surface oxide layers cannot be detected by XRD.

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Chapter 4: Results

Figure 4.6: XRD patterns of (TiN, TiB2, TiBN-1 and TiBN-2) coatings deposited

on low carbon steel

4.2.3 Chemical composition of the coatings

The elemental composition of the four different coatings determined by XPS is

given in Table 4.3. Oxygen and carbon contaminations on the top surface of the

coatings were expected since the coatings were not cleaned by sputtering before

analysis. The carbon originates mainly from surface contamination and/or

diffusion from the low carbon steel substrate. Presence of oxygen on the surface

is usually associated with spontaneous oxidation of the coatings [167, 168].

Position (2Ө), [°]

TiN

TiBN-2

TiB2

TiBN-1

Position (2Ө), [°]Position (2Ө), [°]

TiN

TiBN-2

TiB2

TiBN-1

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Chapter 4: Results

The TiB2 coating has higher oxygen content than all other TiN based coatings,

hence the oxidation rate of TiB2 seems to be higher than TiN. The oxygen

content in TiBN-2 coating was lower than in the TiBN-1, as the ratio of nitrogen to

boron increases.

Table 4.3: XPS elemental composition of the deposited coatings on low carbon

steel

High resolution XPS spectra of the different coatings are shown in Figures 4.7.1

to 4.7.4. Figure 4.7.1 shows the high-resolution XPS spectra of Ti 2p, N 1s and

C 1s peaks of TiN coated steel. The Ti 2p spectra can be deconvoluted into four

overlapping peaks. The Ti 2p3/2 peak at 455.4 eV and Ti 2p1/2 spin-orbit at

461.4 eV are associated with titanium nitride (TiN) . The peak at 458.2 eV and its

spin orbit at 464.1 eV refers to the oxide component (TiO2) [169]. The C 1s peak

is deconvoluted into three peaks, the dominant peak is located at binding energy

of 284.6 eV corresponding to C-C / graphite, the other peak located at binding

energy of 286.3 eV is associated with C-O and the peak at 288.1 eV is

associated with C-N [170]. The N 1s peak is deconvoluted into three peaks, the

peak at lower binding energy at 395.8 eV is related to the presence of C-N, 397.3

eV to TiN [171] and 399.1 eV corresponds to N-O in TiN coatings [170] .

The O 1s deconvoluted into three peaks at 530 eV, 531.7 eV corresponding to N-

O and TiO2 respectively and at 533.5 eV to C-O or H2O.

11.02.822.645.917.7TiB2

40.534.714.65.64.2TiBN-2

34.527.520.411.26.3TiBN-1

---15.433.627.024.0TiN

BNCOTiElement

Layer

11.02.822.645.917.7TiB2

40.534.714.65.64.2TiBN-2

34.527.520.411.26.3TiBN-1

---15.433.627.024.0TiN

BNCOTiElement

Layer

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Chapter 4: Results

The most intense peak at 530 eV is related to N-O which dominates the other

peaks especially TiO2, which constitute half of the former peak. This indicates

that the nitrogen is found free on the top coating and the reaction between

titanium and nitrogen at this temperature is slow.

Figure 4.7.1: XPS high-resolution spectra of the TiN coated low carbon steel

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Chapter 4: Results

The XPS high-resolution spectra of the TiB2 coated steel are shown in Figure

4.7.2. The B 1s is deconvoluted into three peaks; the peaks at 187.6 eV, 190.7

eV and 192.4 eV correspond to TiB2, BN and B2O3, respectively [172, 173] with

small amount of BN. The Ti 2p spectrum can be fitted into four peaks at binding

energies of 454.7 eV (Ti 2p3/2) and 460.4 eV (Ti 2p1/2) due to the presence of

TiB2, while the peak obtained at 459.1 eV (Ti 2p3/2) and at 465.0 eV (Ti 2p1/2) is

assigned to the TiO2 phase [174, 175]. The peak intensities show that the most

intense peak is characterized by TiO2. The N 1s is deconvoluted into two peaks

at 395.8 eV and 397.9 eV corresponding to C-N and B-N respectively [170, 174].

The TiO2 peak intensity dominate the B2O3 peak intensity, which indicates that

oxidation of boron is slower than titanium.

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Chapter 4: Results

Figure 4.7.2: XPS high-resolution spectra of the TiB2 coated low carbon steel

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Chapter 4: Results

Figure 4.7.3 shows the high-resolution spectra of TiBN-1 coating. The B 1s

spectra deconvoluted into two peaks at 187.4 eV and 190.7 eV correspond to

TiB2 and BN respectively. The TiB2 peak intensity was 1/3 the peak intensity of

BN. The corresponding N 1s shows a main peak at 398.3 eV corresponding to

BN and small peak at 397.4 eV corresponding to TiN. The O 1s deconvoluted

into three peaks at 528.9 eV, 531.0 eV and 533.0 eV corresponding to C-O, TiO2

and H2O respectively. The Ti 2p spectrum was difficult to analyze due to the

strong overlapping of the peaks, however the main constituents were identified

as TiB2, TiN and TiO2.

Oxidation of boron in TiBN-1 coating was not detected. It seems that the

presence of nitrogen prevents the oxidation of boron. By increasing the BCl3 flow

rate to 0.32 Nl/min in TiBN-2, a preferable formation of BN phase is observed

and an absence of the TiN phase. However, the peaks intensities ratio of TiB2/BN

was close to the TiBN-1. The B 1s, N 1s and O 1s spectra deconvolution of the

TiBN-2 coating are shown in Figure 4.7.4.

The binding energy and chemical state of the coatings are listed in Table 4.4.

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Chapter 4: Results

Figure 4.7.3: XPS high-resolution spectra of the TiBN-1 coated low carbon steel

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Chapter 4: Results

Figure 4.7.4: XPS high-resolution spectra of the TiBN-2 coated low carbon steel

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Chapter 4: Results

Coating Binding energy (eV) Chemical state

TiN

Ti 2p

455.4 TiN

458.2 TiO2

N 1s

395.8 C-N

397.3 TiN

399.1 N-O

O 1s

530.0 N-O

531.7 TiO2

533.5 C-O

TiB2

Ti 2p

454.7 TiB2

459.1 TiO2

B 1s

187.6 TiB2

190.7 BN

192.4 B2O3

N 1s

395.8 C-N

397.9 B-N

TiBN-1

B 1s

187.4 TiB2

190.7 B-N

N 1s

398.3 B-N

397.4 TiN

O 1s

528.9 C-O

531.0 TiO2

533.0 H2O

TiBN-2

B 1s

187.4 TiB2

190.7 B-N

N 1s

398.3 B-N

O 1s

528.9 C-O

531.0 TiO2

533.0 H2O

Table 4.4: Binding energy and chemical state of the different coatings obtained

from fitting the main XPS peaks.

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Chapter 4: Results

The chemical analysis performed by EDX of the deposited coatings is presented

in Table 4.5. Oxygen was only found in TiN and TiB2 coatings and could not be

detected in TiBN-1 and TiBN- 2 coatings; hence oxidation of the TiN and TiB2

coatings was stronger than in TiBN-1 and TiBN-2. A comparison of the

EDX and XPS data indicates that most of the oxygen signal stems from the

surface (significantly higher at-% for oxygen determined by surface-sensitive

XPS). No carbon signal was measured by EDX indicating that carbon is present

only as contamination of the top surface. Furthermore, the composition of TiBN-1

and TiBN-2 coatings show that the coatings are not uniform in the depth [176].

Table 4.5: Atomic percent compositions (by EDX) of different deposited coatings

on low carbon steel- as received

0.5682.022.79----14.63TiB2

0.0967.87----15.2216.78TiBN- 2

0.1262.10----19.9217.68TiBN- 1

0.75----3.6847.6947.61TiN

FeBONTi

Element

Layer

0.5682.022.79----14.63TiB2

0.0967.87----15.2216.78TiBN- 2

0.1262.10----19.9217.68TiBN- 1

0.75----3.6847.6947.61TiN

FeBONTi

Element

Layer

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Chapter 4: Results

4.3 Characterization of thick TiBN-3 coating deposited

on low carbon steel

Previous results showed that increasing the boron content leads to denser TiBN

with smaller grain size, making it an adequate protective agent against corrosion.

Nevertheless considerable amount of defects were still present in the coating,

this might affect the performance of the coating over a long time leading to

unsatisfactory results on corrosion resistance. This problem can be resolved by

increasing the coating thickness and modifying the film microstructure. This is

supposed to reduce the pinhole and improve the corrosion resistance of the

coating.

Thicker TiBN coating of 6-8 µm (TiBN-3) with 75% boron content was deposited

on low carbon steel. Coating morphology was examined by SEM; it has a very

dense and fine structure consisting of leaf shaped crystals as shown in Figure

4.8. When the coating thickness was 3 µm and boron content was 40 at%, the

microstructure had fine crystals, by increasing the coating thickness to about 6-8

µm and the boron content to 75 at%, the shape changed to leaf shaped

assessing the role of thickness and boron content on the microstructure.

The microscopic cross sectional micrograph of the coated sample, Figure 4.9,

confirms the presence of a good adhesive defect-free coating layer.

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Chapter 4: Results

Figure 4.8: SEM micrographs of as-deposited TiBN-3 on low carbon steel with

increased thickness and boron content

Figure 4.9: Optical micrograph of the cross section of TiBN-3 coated low carbon

steel

TiBN-3

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Chapter 4: Results

The XRD patterns of TiBN coated low carbon steel with different thicknesses

(3µm, 6µm) are shown in Figure 4.10. The patterns suggested the formation of

different phases of TiB, TiB2 and TiN. The predominant phase of the TiBN

coating was TiB2 with a preferred orientation of (100). Other diffraction peaks

corresponding to TiN and TiB were also observed.

An overall increase of peak intensities of different detected phases was observed

for thicker TiBN coating in comparison with the thinner one.

Figure 4.10: XRD patterns of TiBN coated low carbon steel, a) 2-3µm thickness-

b) 6-8 µm thickness

The SEM cross sectional view of TiBN-3 coated LCS exhibited a well adhered

and continuous coating with a compact structure, as shown in Figure 4.11. No

observation of detachments or cracks was seen in the layers. The good adhesion

of the TiBN layer and the cracks free structure gives a good indication of the

possibility to use the coating for efficient corrosion protection.

20 30 40 50 60 70 80

0

200

400

600

800

1000

(a)

(b) 110

TiB2

102

200

101200

TiN

220

100

200 201

TiBTiB

2

TiN

TiB2

TiB2

TiB2

TiB2

Y A

xis

Title

Position (2), [o]

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Chapter 4: Results

The concentration depth profiles of the elements Ti, B, N, C and Fe were

measured by Glow Discharge Optical Emission Spectroscopy (GDOES) as

shown in Figure 4.11. Deposition of TiBN results in formation of three different

sublayers: the first sublayer has a thickness of about 3 µm is formed from

diffusion of titanium in the iron substrate. It prevents iron boriding which is a

beneficial as formation of the FeB compound causes embitterment of the steel

sample [177].

On the other hand as seen in the depth-profile, outward diffusion of carbon from

the substrate takes place and increasing the carbon amount in the second

sublayer. The second sublayer consists of iron, carbon and boron with small

amounts of titanium and nitrogen, resulting in formation of complex compounds

of iron boride, iron nitride and iron carbides, with small amount of titanium nitride

and titanium carbide; this sublayer is about 1 µm in thickness. The third sublayer

consists of B, N and Ti which consists of phases of TiB, TiN and TiB2 as

measured by XRD. It has a thickness of 5 µm.

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Chapter 4: Results

Each of these compounds is considered to be stable against corrosion. In this

sublayer, the iron concentration decreases rapidly at the TiBN interface,

supporting the hypothesis that TiBN works as a good barrier for diffusion of iron.

Figure 4.11: SEM micrographs of the prepared cross section of the TiBN coated

LCS and the GDOES elemental composition depth profile in at% of carbon, iron,

titanium, nitrogen and boron of TiBN-3 coated low carbon steel.

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Chapter 4: Results

Cross-sections of the bulk composite coating of original samples using the FIB is

shown in Figures 4.12 (a&b). The coating thickness was measured from SEM

imaging, it is about 8-10 µm, (since the specimen is tilted, the viewing angle, Ө,

is considered when the thickness of the coating is measured). The first notable

observation from the cross-section was the fine grained porous-free TiBN

coating, and the good coating/substrate adhesion with almost no voids.

The several FIB sections revealed the presence of an interfacial non-uniform

diffusion layer. The layer thickness varied in different samples, from few

nanometers (Figure 4.12.b) to 2-3 µm (Figure 4.12.a). EDX line scan

measurements showed that the layer is rich with Ti (Figure 4.13.c). The results

are in a good consistence with those obtained from GD-OES analysis.

Figure 4.12.a: SEM after FIB cut of original coated low carbon steel TiBN-3

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Chapter 4: Results

Figure 4.12.b: SEM cross-sectional view of TiBN-3 coated low carbon steel after

FIB cut

Figure 4.12.c: Line-scan of the interfacial layer in TiBN-3 coated low carbon

steel- FIB cut

5 µm

Ti

Fe

5 µm

Ti

Fe

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Chapter 4: Results

Cross-section milling was performed for coated Ta samples; SEM image is

presented in Figure 4.13.a. The section shows two discrete phases, the darker

phase being the TiBN coating, and the lighter phase the Ta substrate. The

measured coating thickness was similar to the thickness of the film deposited on

LCS. No indication of the presence of interfacial diffusion layer was detected as

in the case of coated LCS. Figure 4.13.b is a line-scan of the cross-section, a

steep drop in Ti concentration at the coating/metal substrate can be observed,

with a rapid increase in Ta concentration. Extensive examination of the cross-

section with EDX mapping was carried out and can be observed Figure 4.13.c. It

proved that there was no inward diffusion of Ti into Ta, in addition, few voids

were observed in Ta substrate underneath the coating.

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Chapter 4: Results

Figure 4.13.a: SEM cross-sectional view of TiBN-3 coated Ta after FIB cut

Figure 4.13.b: Line-scan of the interfacial layer in TiBN-3 coated Ta- FIB cut

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Chapter 4: Results

Figure 4.13.c: EDX-mapping images of the cross-section in the TiBN-3 coated

Ta

The chemical composition of the coating was investigated by X-ray photoelectron

spectroscopy (XPS) and energy dispersive X-ray (EDX).

XPS depth profile of the top 500 nm of the TiBN film has different composition

from the bulk of the coating as measured by GDOES, Figure 4.14. The XPS

results show carbon signal which originates from environment contamination

and/or further outward diffusion of carbon from the TiBN layer which results in

graphite formation. This value decreased when reaching the TiBN film.

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Chapter 4: Results

In comparison to XRD and GDOES results, presence of oxygen indicates that

oxidation has taken place only at the surface of the coatings due to exposure of

the coating to the ambient atmosphere.

The nitrogen concentration decreases by depth, while the titanium and boron

show a slight increase of concentration by depth.

Figure 4.14: XPS depth profile of TiBN coated LCS

High-resolution elemental spectra were recorded during sputter etching of TiBN

film to determine the chemical state and the phase composition of the coating

[174].

Figure 4.15 shows the change in the chemical state of B 1s, N 1s and O 1s with

sputtering depth. The B 1s spectra deconvoluted into two peaks at 187.4 eV and

190.7 eV corresponds to TiB2 and BN, respectively. The peak intensity ratio of

TiB2 /BN was 0.7 at the top of the layer (sputter depth of 0 nm) and increased to

1.2 at sputter depth of 500 nm.

0 100 200 300 400 500 6000

20

40

60

80

100

at

(%)

Sputter depth (nm)

B1s

C1s

N1s

O1s

Ti2p

Sputter depth (nm)

At

%

0 100 200 300 400 500 6000

20

40

60

80

100

at

(%)

Sputter depth (nm)

B1s

C1s

N1s

O1s

Ti2p

Sputter depth (nm)

At

%

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Chapter 4: Results

The N 1s peak deconvoluted into two main peaks, the main peak at binding

energy of 397.4 eV corresponds to TiN and a small peak at 398.3 eV

corresponds to BN. The nitrogen amount decreases in depth, but the ratio of TiN

to BN almost keeps constant at 0.1. BN phase was not detected by the XRD

which emphasizes that BN only formed on the top surface as this phase

decreases and diminish in the bulk coating.

The O 1s spectra deconvoluted into three peaks, the peaks at 529.1 eV and

531.2 eV corresponding to CO and TiO2 respectively, while the peak at 533.2 eV

is assigned to oxygen or B2O3 phase. However, no evidence was found in the B

1s spectra for the formation of the B2O3, indicating that only titanium oxidation

takes place.

Figure 4.15: High resolution XPS spectra of B 1s, N 1s and O 1s of TiBN-3

coated LCS showing the change in the chemical composition as function of

depth.

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Chapter 4: Results

The binding energy and chemical state of the TiBN are listed in Table 4.6.The

elemental concentration was determined from XPS peak areas using the Ti 2p, N

1s and B 1s peaks and appropriate atomic sensitivity factors.

Table 4.6: Binding energy and chemical state of TiBN coated LCS obtained from

fitting the main XPS peaks.

The EDX spectra of the coating showed the presence of B, Ti and N elements

from the phases of TiB2, TiN and TiB as the results obtained by XRD

investigations besides a small amount of carbon and iron. These results are

presented in Figure 4.16.

Figure 4.16: The EDX elemental composition of TiBN-3 coated LCS.

OxygenTiO2

COBNTiNBNTiB2

Chemical state

533.2531.2529.1398.3397.4190.7187.4Binding energy (eV)

O 1sN 1sB 1sElement

OxygenTiO2

COBNTiNBNTiB2

Chemical state

533.2531.2529.1398.3397.4190.7187.4Binding energy (eV)

O 1sN 1sB 1sElement

0.280.91Fe

16.4345.63Ti

6.124.97N

1.641.14C

75.5447.36B

At%Wt%Element

0.280.91Fe

16.4345.63Ti

6.124.97N

1.641.14C

75.5447.36B

At%Wt%Element

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Chapter 4: Results

4.4 Electrochemical investigations on coated metals

4.4.1 Electrochemical characterization of coated stainless steel

4.4.1.1 Open-circuit potentials and potentiodynamic polarization

measurements

The open-circuit potential values (OCP) of bare stainless steel (SS) and the

different coatings are shown in Table 4.7. The open circuit potential values of

coated SS are all higher than that on the bare SS which started relatively low and

decreased further with immersion time, suggesting that the SS is not passive in

chloride containing environment (0,5M NaCl). Initially, the OCP of the TiN was

more positive than TiB2, TiBN-1, or TiBN-2, but after 6 hours of immersion in the

sodium chloride solution, the corrosion potential of the TiN coating almost

reached those of the steel substrate. Adding boron to TiN nitride coating seemed

to enhance the corrosion resistance of the coatings since the OCP values were

relatively nobler than the OCP of bare SS and did not undergo a big change with

elapsing time, in contrary, the OCP of TiBN-2 was even better after 6h immersion

time. The best OCP value was shown by TiB2 coated SS.

Table 4.7: The open circuit potential values of the blank and coated stainless

steel in 0.5 M NaCl after 6 hours immersion

-409-457TiB2

-444-474TiBN-2

-490-460TiBN-1

-524-436TiN

-602-545X46Cr13

OCP2 at 6h (mV)OCP1 at 0h (mV)Type

-409-457TiB2

-444-474TiBN-2

-490-460TiBN-1

-524-436TiN

-602-545X46Cr13

OCP2 at 6h (mV)OCP1 at 0h (mV)Type

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Chapter 4: Results

The completion of the potentiodynamic measurements were performed after 6h

immersion time in test solution, results are presented in Figure 4.17.

Clearly, all coated steel samples show a significantly better corrosion behavior

than the bare steel, in terms of corrosion current density (near the corrosion

potential) and passivity. The uncoated metal undergoes a continuous active

dissolution, as can be expected for this steel in chloride containing medium.

All coated samples show a very similar, improved corrosion behavior in the

vicinity of the corrosion potential. Upon anodic polarization, significant differences

in the electrochemical behavior of the different coatings can be observed. The

TiN coating shows almost no region of passivity, but instead a steady increase of

the current, reaching the values of the bare metal sample at higher anodic

potentials. Boron containing coatings, TiB2, TiBN-1 and TiNB-2 exhibit a passive

behavior over a wide potential range, with low current densities in comparison to

TiN coated steel with a superior behavior for TiB2 which is in a good agreement

with OCP measurements. Table 4.8 presents the results of the electrochemical

measurements of different coatings and their porosity. The calculated porosities,

(Eq. 2-10), of these samples differ, and also do the corrosion behavior. TiB2 and

TiBN-2 have quite similar polarization curves and the lowest porosity and

therefore the best general corrosion resistance. TiN film has the highest porosity

and current density and thus the worst corrosion resistance.

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Chapter 4: Results

Figure 4.17: Polarization curves of the coated steel in 0.5 M NaCl.

Table 4.8: Results of electrochemical experiments for different coatings on

stainless steel

0.7*10-30.00735*103-423TiB2

0.9*10-30.00830*103-396TiBN-2

1.26*10-30.0817.2*103-523TiBN-1

1.04*10-30.726*103-611TiN

icorr

mA.cm-2

Calculated

porosity

%

Rp

Ω.cm2

Ecorr

mV

Coating

0.7*10-30.00735*103-423TiB2

0.9*10-30.00830*103-396TiBN-2

1.26*10-30.0817.2*103-523TiBN-1

1.04*10-30.726*103-611TiN

icorr

mA.cm-2

Calculated

porosity

%

Rp

Ω.cm2

Ecorr

mV

Coating

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94

Chapter 4: Results

4.4.1.2 Electrochemical impedance spectroscopy measurements

Further information on the electrochemical behavior at the corrosion potential

was obtained by electrochemical impedance spectroscopy. Figure 4.18 shows

the EIS spectra as a function of immersion time in 0.5 M sodium chloride for

uncoated/coated stainless steel. The EIS of the coated samples (Figures b, c, d

and e) show different behavior than the uncoated SS (Figure a).

Initially, the coated SS samples show high impedance values in the low

frequency range, indicating a good corrosion resistance. The uncoated steel and

TiN coated steel show low impedance values of about 10 KΩcm2, while, boron

containing coatings show higher impedance values ranging from 40 to 500

KΩcm2.

After 3 hours of immersion, the impedance values at low frequencies decrease to

lower values, which can be attributed to the penetration of the solution through

the coatings onto the steel substrate. Just after immersion in the NaCl solution,

the impedance values decrease rapidly, the impedance value of TiN decreased

to 4 KΩcm2, TiBN-1 decreased to 10 KΩcm2, TiBN-2 to 20 KΩcm2 and TiB2 to 10

KΩcm2, while, the uncoated steel drops to 1 KΩcm2. By increasing the immersion

time to 6 hours, the coatings TiBN-1 and TiN as well as the uncoated steel result

in a small increase of the impedance values at the low frequency range.

TiB2 shows the highest initial value of impedance among the coatings, in the low

frequency range the impedance is ca. 500 KΩcm2, while TiBN-2 shows a value of

100 KΩcm2. However, the decrease in the low-frequency impedance values is

faster for the TiB2 coating than for the TiBN-2 coating. At the end of 6 h

immersion, all boron-containing coatings still show somewhat higher values of

impedance (10-20 KΩcm2) than the uncoated and TiN-coated steel (1-4 KΩcm2),

indicating a decrease of the average corrosion rate by a factor of ca. 10.

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Chapter 4: Results

Figure 4.18: Bode plot of EIS data obtained of the uncoated/ coated stainless

steel as function of immersion times exposed to 0.5 M NaCl measured at the

corrosion potential.

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

(Stailess steel)

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

TiN

Imp

ed

an

ce (

.cm

2)

-ph

ase (

o)

Frequency (Hz)

0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

Imp

ed

an

ce (

.cm

2)

-ph

ase (

o)

Frequency (Hz)

TiBN-1 0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

TiBN-2

Imp

ed

an

ce (

.cm

2)

-ph

ase (

o)

Frequency (Hz)

0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

TiB2

Imp

ed

an

ce (

.cm

2)

-ph

ase (

o)

Frequency (Hz)

0 h

3 h

6 h

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96

Chapter 4: Results

4.4.2 Electrochemical characterization of coated low carbon

steel

4.4.2.1 Open-circuit potential and potentiodynamic polarization

measurements

The values of measured open circuit potentials (OCP) of bare and coated steel

(LCS) are shown in Figure 4.19.

While OCP of bare metal in test solution was around -700 mV, which is expected

for steel in soil, all other samples showed higher OCP values. However, a slight

decrease in OCP of TiB2 coating was observed, due to the defects in the coating,

where the solution could penetrate fast onto the substrate and accelerate the

corrosion process.

The TiN-coated steel initially shows relatively high OCP value. However, after ca.

3 hours a drastic decrease of the OCP is observed, indicating onset of substrate

corrosion.

TiBN-1 and TiBN-2 coated samples were much nobler than TiN coated sample;

the TiBN-1 coatings show a high OCP value during the 6 h experiment, however,

the slightly decreasing trend of the OCP together with the cathodic potential

transients indicates that the system is not completely stable. Only the TiBN-2

coated sample shows a stable, high OCP value, which even slightly increases

with time.

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Chapter 4: Results

Figure 4.19: Open circuit potential at 6 h in simulated soil solution.

Potentiodynamic polarization curves were used to estimate the electrochemical

activity of the coating and for comparison of the different types of coatings.

Figure 4.20 presents the polarization curves of bare and coated low carbon

steel. Specimens coated with TiB2 have high current density polarization curves

and their calculated porosity is also high. Compared to the other coatings studied

its corrosion performance was very poor. TiN and TiBN coated samples show

nobler corrosion potential and lower anodic current densities, as shown in Table

4.9. The calculated porosity of TiBN coatings was found to decrease with

increasing boron content and consequently the corrosion behavior was much

better, as TiBN-2 shows the best corrosion behavior amongst all the investigated

coatings. The calculated porosity values correlate with the corrosion current

densities of the polarization curves at low overpotentials. The higher the

calculated porosity, the higher is the current density.

These results are in good agreement with the OCP measurements.

0 5000 10000 15000 20000 25000-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

LCS

TiB2

TiN

TiBN-1

TiBN-2

Po

ten

tial (V

vs A

g/A

gC

l)

time (sec)

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Chapter 4: Results

Figure 4.20: Polarization curves of the coated low carbon steel in simulated soil

solution.

Table 4.9: Results of electrochemical experiments for different coatings on low

carbon steel

-1,0 -0,5 0,0 0,5 1,0 1,510

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (V vs Ag/AgCl

)

LCS

TiB2

TiN

TiBN-1

TiBN-2

3.5*10-31.689.6*103-598TiB2

0.04*10-30.0017480*103-445TiBN-2

0.5*10-30.1384*103-583TiBN-1

1.83*10-30.1928*103-541TiN

icorr

mA.cm-2

Calculated

porosity

%

Rp

Ω.cm2

Ecorr

mV

Coating

3.5*10-31.689.6*103-598TiB2

0.04*10-30.0017480*103-445TiBN-2

0.5*10-30.1384*103-583TiBN-1

1.83*10-30.1928*103-541TiN

icorr

mA.cm-2

Calculated

porosity

%

Rp

Ω.cm2

Ecorr

mV

Coating

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Chapter 4: Results

4.4.2.2 Electrochemical impedance spectroscopy measurements

The EIS measurements of uncoated/coated LCS as a function of time are

displayed in the form of Bode plot, Figure 4.21.

In contrast to bare metal, which exhibits very low polarization resistances after 3

h (1.76 kΩ·cm²) and 6 h (1.55 kΩ·cm²), all coated samples have much better

corrosion resistance in the SSS.

TiN coating showed relative high impedance values after 3 h (38.74 kΩ·cm²) and

6 h (17.76 kΩ·cm²), whereas TiB2 coating revealed only a marginal protection, in

which the lowest values for RP for all coatings were measured. It must be

mentioned here that a higher resistance (5.21 kΩ·cm²) was measured after

longer immersion time than for shorter one (4.57 kΩ·cm²); this could be due to

the accumulation of corrosion products on the delaminated spots of the coating

which hindered the dissolution process. The reported Rp values refer to

impedance values at low frequency.

Contrary to TiN and TiB2, TiBN layers seems to provide a very effective

protection to the underneath substrate i.e. TiBN-1 layer reveals high RP values of

approximately 120 kΩ·cm², with almost no change with time. Moreover, the boron

content strongly influences the deposited layer and its corrosion resistance.

Increasing the boron content from 34.5 at% to 40.5 at%, leads to impedance

increment to several MΩ·cm² (2.19 MΩ·cm²) after 3 h and (4.47 MΩ·cm² after 6

h). These values demonstrate – in agreement with the polarization curves - the

very efficient barrier properties of the TiBN-2 coatings.

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Chapter 4: Results

Figure 4.21: Bode plot of EIS data obtained of the uncoated/ coated low carbon

steel as function of immersion times exposed to simulated soil solution measured

at the corrosion potential at different immersion times.

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

LCS

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

3hr

6hr

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

TiB2

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0hr

3hr

6hr

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90-p

hase (

o)

TiN

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

Imp

ed

an

ce (

.cm

2)

-ph

ase (

o)

Frequency (Hz)

TiBN-1 0 h

3 h

6 h

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

TiBN-2

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0hr

3hr

6hr

-ph

ase (

o)

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101

Chapter 4: Results

4.4.2.3 Surface characterization after electrochemical measurements

The morphology of the coatings after the electrochemical measurements is

shown in Figure 4.22. The observations are in agreement with OCP results.

Clearly the surface of TiN and TiB2 coatings have been attacked by the corrosive

medium and became more porous. Localized corrosion was observed on some

sites of the surface, they were mainly on delaminated spots of TiB2.

On the contrary, TiBN-1 and TiBN-2 did not show visible changes on their

surface morphology, although the OCP of TiBN-1 started to decrease after 4 h

immersion time. Different localized corrosion morphologies of TiN and TiB2 were

observed as shown in Figure 4.22. The pits were initiated under the coating in

defective sites. The pit propagation occurs underneath the coating, as the coating

itself is stable in the solution. After substantial localized dissolution of the

substrate, breaking of the coatings occurs as was observed in the optical

micrograph of the cross-section done after the electrochemical tests, Figure

4.23.

On the TiB2 coated surface the pits look different; they were mostly formed on

sites of local low coating thickness.

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102

Chapter 4: Results

Figure 4.22: SEM micrograph of coated steel after the electrochemical

measurements in simulated soil solution.

Figure 4.23: Optical micrograph showing the pitting corrosion on TiN coated low

carbon steel after electrochemical test in simulated soil solution.

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103

Chapter 4: Results

4.4.3 Summary: The electrochemical and corrosion behavior of

different coatings on different steel substrates

The deposited ceramic thin films on both stainless steel and low carbon steel

have significantly improved the corrosion resistance of the substrate materials in

the different electrolytes used during this study. The corrosion performance was

found to be strongly influenced by the porosity related to the coating defect

density and by the uniformity of the deposited film layers and their adhesion to

the substrate. According to this, the coatings with the best corrosion resistance

were TiB2 on stainless steel and TiBN-2 on low carbon steel.

Galvanic corrosion couple was created between the active base metal and the

coating in the presence of defects and microcracks in the deposited films and led

to sever local attacks in forms of crevice and pitting corrosion. Adding boron to

TiN was found to decrease the grain size and form coating with mixed phases

and mixed orientation with less defects which enhanced further the corrosion

resistance of the coating. These coatings, TiBN, with different boron content

seem to be a promising candidate for corrosion protection of steels in different

environments. Therefore, the second part of the study was focused on improving

further the coating properties to achieve a better corrosion resistance.

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104

Chapter 4: Results

4.4.4 Electrochemical characterization of TiBN-3 coated low

carbon steel

4.4.4.1 The results of 48 hours measurements

Thicker TiBN coating (6-8µm) with higher boron content (75%) was deposited on

low carbon steel samples (LCS) and on tantalum. Coated samples were

electrochemically tested in different electrolytes (simulated soil solution, SSS,

simulated seawater, SSW, and 1 M HCl) for different times (0, 2 and 90 days).

4.4.4.1.1 Open circuit potential measurements in simulated soil solution and

simulated seawater

Figures 4.24 (a, b) show the time variations of free corrosion potentials for

uncoated/ TiBN-3 coated LCS and Ta in SSS and SSW, respectively, in an open

circuit conditions, pure titanium was measured for comparison. The data illustrate

that all tested specimens in both solutions possess nobler potentials than the

uncoated low carbon steel with an overall slight increase with time elapsing. The

law value of the potentials at the beginning of immersion can be explained by the

time necessary for the formation of hydroxide/ oxide passive layer on the surface.

The corrosion potential of the uncoated LCS was less noble and decreased

steeply upon immersion due to the less protective behavior of the formed oxide

layer and due to the presence of the chloride in both solutions which increase the

probability of pitting corrosion to take place.

Moreover, the open circuit potential values of TiBN-3 coated LCS are almost

identical in both test solutions and show a small deviation from the coated Ta in

SSW, this indicates that the measured OCP was of the coating and that the

interference of the substrate could be excluded from the measured reaction. In

SSS, OCP values of uncoated/coated Ta were higher; this might be attributed to

the nature of test solutions and their salt content.

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105

Chapter 4: Results

Figure 4.24: Open circuit potential measurements of different specimens in SSS

and SSW

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106

Chapter 4: Results

4.4.4.1.2 Potentiodynamic polarization measurements in simulated soil solution

and simulated seawater

The potentiodynamic polarization measurements on coated Ta and LCS samples

in SSS and SSW after 48 h immersion in test solutions at ambient temperature

are presented in Figure 4.25, the polarization curves of uncoated metals (LCS,

Ti, and Ta) are shown for comparison.

As can be seen the coated LCS specimens exhibited active-passive behavior,

with much lower anodic current densities about three orders of magnitude and

more noble corrosion potential than the uncoated LCS.

Moreover, no pitting corrosion could be observed with increasing the polarization

potentials.

The behavior of TiBN-3 coating on Ta and LCS substrates is slightly different

from the behavior of uncoated Ti and Ta in both test solutions, with identical

anodic and cathodic current densities. However, the active-passive peak

observed on coated LCS and Ta specimens at potential of 0.6 V did not exist in

the case of bare Ti and Ta specimens, which strongly suggests that it is related

to the coating itself. All coated samples present, as a common feature, a marked

anodic current peak during the anodic polarization with a maximum about 2.0 V.

this anodic peak is not found on the untreated Ti and could be referred to the

oxidation of TiN to TiO2 [47, 178], following the oxidation reaction:

TiN + 2H2O → TiO2+1/2 N2 + 4H+ + 4e¯ (4-1)

Anodic polarization of the coating in different test solutions is accompanied by the

formation of areas of passivation of same length, characterized by very low

corrosion current densities indicating the low rate of dissolution of these

compounds in the passive state. An exception is represented by low carbon steel

in both solutions where it underwent continuous dissolution, represented by high

anodic current densities which were higher in SSW.

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107

Chapter 4: Results

Figure 4.25: Potentiodynamic curves of uncoated / TiBN-3 coated LCS and Ta

and of bare Ti after 48h in: (a) SSS, (b) SSW

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,010

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

uncoated LCS

TiBN-3 coated LCS

uncoated Ta

TiBN-3 coated Ta

uncoated Ti

a

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,010

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

uncoated LCS

TiBN-3 coated LCS

uncoated Ta

TiBN-3 coated Ta

uncoated Ti

a

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,010

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

uncoated LCS

TiBN-3 coated LCS

uncoated Ta

TiBN-3 coated Ta

uncoated Ti

b

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,010

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

uncoated LCS

TiBN-3 coated LCS

uncoated Ta

TiBN-3 coated Ta

uncoated Ti

b

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Chapter 4: Results

4.4.4.1.3 Electrochemical measurements of TiBN-3 coated low carbon steel in

1M HCl

The open circuit potential of TiBN-3 coating on low carbon steel in 1M HCl

corrosion is shown in Figure 4.26. The coating shows OCP values close to

potentials measured in neutral test solutions with markedly continuous increase

over the 48 h immersion time. The observed fluctuations may be due to a

probable break down in the titanium oxide passive layer followed by a

spontaneous repassivation.

Figure 4.27 represents the polarization curve of TiBN-3 measured in 1M HCl; the

measured I/E curves in SSS and SSW were added for comparison. The different

coated samples are characterized by different cathodic and anodic polarization

behaviors with similar Ecorr. The cathodic current density is higher and related to

hydrogen reduction reaction [125]; the high cathodic current density indicates the

high corrosion rate of the coating in acidic solution. At more anodic potentials the

passive oxide layer TiO2 forms again but with higher corrosion current density icorr

which is about two times of magnitude higher than of icorr in SSS and SSW.

A major difference in the anodic polarization part between the samples in acidic

HCl and neutral SSS and SSW solutions is the spontaneous formation of the

passive oxide layer in HCl. This can be explained by the reduction of the oxides

during the cathodic polarization and the slow dissolution of the coating in the

acidic medium which results in quicker oxidation of the freshly exposed coating.

Additional observation is the shifting of the anodic current peak related to TiN

oxidation to higher potential than 2.0 V, as also observed by other authors [48,

175, 179] .

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Chapter 4: Results

Figure 4.26: Open circuit potential of TiBN-coated LCS in 1M HCl

Figure 4.27: Potentiodynamic curves off TiBN-3 coated low carbon steel after 48

h exposure in: SSS, SSW and 1M HCl

0 10 20 30 40 50-240

-230

-220

-210

-200

-190

-180

-170

-160

-150

OCP of coated LCS in 1M HCl

time (days)

Po

ten

tial (m

V v

s A

g/A

gC

l)

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110

Chapter 4: Results

4.4.4.1.4 Electrochemical impedance spectroscopy measurements (EIS) of

TiBN-3 coated low carbon steel in different test solutions:

48h results

The EIS spectra of uncoated LCS, Ti and Ta, measured in SSS, SSW over 48

hours are shown in Figure 4.27. The spectra of the EIS were taken at an interval

of three hours between two consequent spectra, but for data simplicity only a 12-

hour interval was represented by a Bode plot.

The uncoated LCS showed the lowest impedance values with phase angle

around 30o. Compared to LCS, both Ti and Ta exhibited a significant higher

impedance values in both solutions, about 2-3 times of magnitude higher than the

impedance of LCS with phase angle bigger than 80°. This can be attributed to

the passive nature of both metals (Ti, Ta) in SSW and SSS. Nevertheless, this

behavior was more pronounced in SSW. Nevertheless, coated Ta in SSS

showed two time constants. This was attributed, according to optical observation,

to the breakdown in the coating and the penetration of the solution into Ta

substrate.

Figure 4.28 represents the EIS spectra on TiBN-3 coated LCS and Ta in SSS

and SSW. Both coated metals exhibited impedance dispersion steady in time

with very high impedances in low frequency region. No macro-defects were

present and there was no visible sign of local corrosion attack after the exposure.

The same observation was made for coated samples; the impedance magnitude

over wide frequency range was higher in SSW. On the contrary, the changes in

impedance spectra of TiBN-3 coated LCS in 1M HCl were very significant and

occurred after certain exposure time interval (6 hours) indicating higher corrosion

rate in HCl, Figure 4.28. The changes included decrease of impedance and

phase shift in the middle at low frequency range, which indicates a decrease in

the capacitive nature of the film as well as a decrease in the polarization

resistance of the whole coating system.

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111

Chapter 4: Results

Figure 4.27: The impedance spectra of uncoated LCS-Ti and Ta in different test

solutions- exposure 0- 48 hours.

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

uncoated LCS in SSS

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

bare Ti in SSS

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)10

-210

-110

010

110

210

310

410

1

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

-ph

ase (

o)

bare Ta in SSSImp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

-ph

ase (

o)

bare Ta in SSSImp

ed

an

ce (

.cm

2)

Frequency (Hz)10

-210

-110

010

110

210

310

410

1

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

bare Ta in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

at 0h, at 12h, at 24h

at 36h, at 48h

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112

Chapter 4: Results

Figure 4.28: The impedance spectra of TiBN-3 coated LCS and Ta in different

test solutions- exposure 0- 48 hours.

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSS

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

at 0h, at 12h, at 24h

at 36h, at 48h

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

coated Ta in SSS

-ph

ase (

o)

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated Ta in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

at 0h, at 12h, at 24h

at 36h, at 48h

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in 1M HClImp

ed

an

ce (

.cm

2)

Frequency (Hz)

0h

6h

12h

18h

24h

30h

36h

42h

48h

-ph

ase (

o)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in 1M HClImp

ed

an

ce (

.cm

2)

Frequency (Hz)

0h

6h

12h

18h

24h

30h

36h

42h

48h

-ph

ase (

o)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in 1M HClImp

ed

an

ce (

.cm

2)

Frequency (Hz)

0h

6h

12h

18h

24h

30h

36h

42h

48h

-ph

ase (

o)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in 1M HClImp

ed

an

ce (

.cm

2)

Frequency (Hz)

0h

6h

12h

18h

24h

30h

36h

42h

48h

-ph

ase (

o)

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113

Chapter 4: Results

4.4.4.2 The results of long time (90 days) measurements

Long time immersion tests were carried out to examine the corrosion resistance

of the deposited TiBN-3 coating on LCS. This type of experiments enables the

detection of initiation and growth of the corrosion pits if they exist in the coating.

Moreover it helps to clarify the nature of the occurrence of the corrosion.

The corrosion potential of coated samples for long immersion time in SSS and

SSW are presented in Figure 4.29; the starting potential for both samples in both

test solutions was around -320 mV. In SSS the potential was increased

constantly up to 25 days immersion time where it reached a value of about -175

mV. Then it decreased to -200 mV in the 40th day, followed by increase and so

on. Whereas in SSW, a steep increase was observed up to 10 days immersion

where the potential value was about +100 mV followed with a decrease in the

next 10 days measuring about +20 mV. After that the potential underwent

constant changes up to the 70th day where the changes became less and more

stable potentials values were measured.

This stable behavior was earlier established in SSS where both samples reached

almost the same OCP value after 90 days immersion.

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114

Chapter 4: Results

Figure 4.29: The corrosion potential as function of time of TiBN coated LCS

measured in SSS and SSW over 90 days.

Impedance spectra for TiBN-3 coating in SSS and SSW are given in the Bode

plot representation in Figures 4.29. The time evolution of the spectra gives a

clear picture of the changes in corrosion behavior of the CVD/low carbon steel

system during 90 days exposure. In both figures, log |Z| is linear with log f and

phase angle ө has values close to 90°, with high impedance magnitudes at very

low frequencies indicating that the TiBN-3/LCS coating system was in passive

state in the simulated environments. Slight changes in impedance values were

recorded with time elapsing. Comparing EIS spectra and Ecorr values measured

at different time indicate that EIS impedance increased/decreased slightly when

Ecorr changed [180]. Both solutions possess different resistances, which were

also slightly changed with immersion time. This can be due to the change in salt

concentrations after adding fresh solution to the running experiment.

0 10 20 30 40 50 60 70 80 90-400

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

Po

ten

tial (m

V v

s A

g/A

gC

l)

time (days)

in simulated soil solution

in simulated seawater

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115

Chapter 4: Results

Figure 4.30: Electrochemical impedance spectra of TiBN-3 coated LCS

presented as Bode plot as function of immersion time for a period of 90 days in:

(a) SSS, (b) SSW.

10-3

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0days

10 days

25 days

51days

64days

79days

90 days

10-3

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0days

10 days

25 days

51days

64days

79days

90 days

10-3

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0days

10 days

25 days

51days

64days

79days

90 days

10-3

10-2

10-1

100

101

102

103

104

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0days

10 days

25 days

51days

64days

79days

90 days

10-3

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 days

9 days

13 days

25 days

33 days

38 days

55 days

67 days

78 days

90 days

10-3

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 days

9 days

13 days

25 days

33 days

38 days

55 days

67 days

78 days

90 days

10-3

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 days

9 days

13 days

25 days

33 days

38 days

55 days

67 days

78 days

90 days

10-3

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

108

0

10

20

30

40

50

60

70

80

90

ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0 days

9 days

13 days

25 days

33 days

38 days

55 days

67 days

78 days

90 days

a

b

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116

Chapter 4: Results

4.4.4.2 Potentiodynamic cyclic voltammograms of TiBN-3 coated low carbon

steel at different immersion times in simulated soil solution and

simulated seawater

In order to evidence differences in the passivation capability of coated samples at

different exposure times (0, 2 and 90 days), potentiodynamic cyclic voltammetry

tests were performed Figures 4.31 (a, b). The corresponding corrosion

parameters Ecorr and icorr values were determined by Tafel extrapolations for the

measured curves, results are summarized in Table 4.9.

The anodic current densities at different immersion times were almost identical,

the characteristic peaks at 0.6 V was vanished in SSW after 2 days while it can

still be seen on samples in SSS, moreover, the second peak at 2.0 V was

present in both solutions after different exposure. After the first potential sweep

in the noble direction, the anodic current densities in the reverse scan were less

indicative the formation of a robust protective passive film which was not

damaged although exposed for long time to the test mediums. The cathodic

current densities remain almost unchanged after different immersion times.

Collectively the results indicate the high corrosion resistivity of the TiBN-3 coating

in both solutions.

At the end of the test, the surface of the coated samples appeared slightly pale

gold but no sign of corrosion attack were visible; the surface of the untreated Ti

resulted as well with the same color. This change in color is an indication to the

formation of titanium oxide layer on the surface of the coated samples, with

thickness similar to the oxide layer formed on bare Ti upon anodization in test

electrolytes. It is well known that the thickness of titanium oxide is directly

proportional to the potential applied [181]; with growing potential, the oxide layer

show different colors, i.e. yellow- purple-blue- light blue- silver- yellow… etc.

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117

Chapter 4: Results

The voltammogram of untreated Ti in SSS and SSW carried out after 48h

immersion are shown in Figure 4.32.

From the figures it can be seen that Ti has the same active-passive behavior as

in the coating system in same solutions but with lower anodic current densities

and absence of characteristic peaks observed for the coated samples. The

reverse scan curves proceeded towards the low current density region. This type

of the cyclic polarization curve is known to resist localized corrosion.

Figure 4.31: Cyclic voltammograms of TiBN-3 coated LCS at different immersion

times in a) SSS

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

at 0 days

at 2 days

at 90 days

a

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

at 0 days

at 2 days

at 90 days

a

Page 132: DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on

118

Chapter 4: Results

Figure 4.31: Cyclic voltammograms of TiBN-3 coated low carbon steel at

different immersion times in b) SSW

Figure 4.32: Cyclic voltammograms of untreated bare Ti at 48h in SSS and SSW

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

at 0 days

at 2 days

at 90 days

b

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

at 0 days

at 2 days

at 90 days

b

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

pure untreated Ti in

SSS

SSW

Page 133: DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on

119

Chapter 4: Results

Corrosion potential and corrosion current density measurements gave

information on the reactions around the corrosion potential. The corrosion current

densities were determined by measuring Tafel slops of potentiodynamic curves

of different studied TiBN-3 coated systems in different solutions at different

immersion times. Those values were used to estimate the porosity of the different

systems after electrochemical tests; results are listed in Table 4.9.

TiBN-3 coated LCS samples showed low current densities and porosity in SSS

and SSW although after 90 days immersion. The increase in porosity between 2

and 90 days was very minimum in SSS while in SSW it increased by a factor of

10, this can be due to the high Cl‾ concentration in SSW. Compared to SSS and

SSW, the corrosion performance of TiBN-3 coated LCS in 1M HCl was poor, this

indicates the high corrosion current density and the high calculated porosity after

only 2 days immersion. It can be concluded that the current density higher the

higher is the calculated porosity,

Porosity determination of TiBN-3 on Ta from the polarization data is difficult

because the response of the substrate is very small [125]. Thus, the correlation

between the calculated porosity and current density could not be applied on

TiBN-3 coated Ta, as almost no change in the current densities was observed

after electrochemical measurements.

Page 134: DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on

120

Chapter 4: Results

Table 4.9: Results of electrochemical experiments for uncoated/ TiBN-3 coated

LCS in SSS, SSW and 1M HCl, and for uncoated/ TiBN-3 coated Ta in SSS and

SSW

6.383.8*10-3-3902TiBN-3 coated

in 1M HCl

1.18*10-5

1.5*10-6

1.3*10-7

4.7*10-5

2.1*10-5

4.4*10-6

-

-

Calculated

Porosity

%

49.1*10-3-46090

22.1*10-3-3682

22.6*10-3-3290

TiBN-3 coated LCS

in SSW

52.3*10-3-40090

51.1*10-3-4052

50.5*10-3-3310

TiBN-3 coated LCS

in SSS

6-8582Uncoated LCS

in SSW

2.05-7892Uncoated LCS

in SSS

icorr

(µA.cm-2)Ecorr

(mV)time (day)Sample name

6.383.8*10-3-3902TiBN-3 coated

in 1M HCl

1.18*10-5

1.5*10-6

1.3*10-7

4.7*10-5

2.1*10-5

4.4*10-6

-

-

Calculated

Porosity

%

49.1*10-3-46090

22.1*10-3-3682

22.6*10-3-3290

TiBN-3 coated LCS

in SSW

52.3*10-3-40090

51.1*10-3-4052

50.5*10-3-3310

TiBN-3 coated LCS

in SSS

6-8582Uncoated LCS

in SSW

2.05-7892Uncoated LCS

in SSS

icorr

(µA.cm-2)Ecorr

(mV)time (day)Sample name

Page 135: DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on

121

Chapter 4: Results

4.4.4.4 The electrochemical behavior at different temperature

Figures 4.32 represents the polarization curves of TiBN-3 coated low carbon

steel after 48 hours immersion in SSS and SSW at different temperatures 15, 35

and 45°C. It is seen that the corrosion potentials of the coated samples were not

appreciably affected with the increase in temperature. Moreover, while the

cathodic polarization current density increased slightly, the increase in the anodic

polarization current density was more significant with the increasing temperature;

this increase can be seen in the active-passive transition part around the

corrosion potential. This trend, coupled to the marked stimulation of the anodic

process at the end of the exposure time, induces a correspondent shift of the two

characteristic peaks at (0.6 and 2.0 V), respectively, towards lower potentials,

accompanied with higher anodic current density for the peak at 0.6V. This trend

was clearer for the coated samples in SSS.

Figure 4.32: Potentiodynamic curves of coated low carbon steel at different

temperature after 48h in: (a) SSS, (b) SSW

-1 0 1 2 310

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(3) (2)

(1)

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

a

-1 0 1 2 310

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(3) (2)

(1)

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

a

Page 136: DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on

122

Chapter 4: Results

Figure 4.32: Potentiodynamic curves of coated LCS at different temperature after

48h in: (a) SSS, (b) SSW

4.4.4.5 Pitting corrosion

Potentiodynamic cyclic voltammogram measurements were used to evaluate the

pitting corrosion resistance of TiBN-3/LCS system.

Figures 4.37(a-c) represent the curves of bare/TiBN-3 coated LCS, bare/ TiBN-3

coated Ta, and of TiBN-3 coated LCS and Ta, respectively; measurements were

conducted in SSW. When a defect exists in the coating, the coated samples

underwent a transition course: from passivation to pitting.

-1 0 1 2 310

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(3)

(2)

(1)

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

(1)-at 15°C

(2) at 35°C

(3) at 45°C

b

-1 0 1 2 310

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(3)

(2)

(1)

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

(1)-at 15°C

(2) at 35°C

(3) at 45°C

b

-1 0 1 2 310

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

(3)

(2)

(1)

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

(1)-at 15°C

(2) at 35°C

(3) at 45°C

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123

Chapter 4: Results

The initiation of pitting corrosion is confirmed by the steep increase in the anodic

current density at high anodic potential and the corresponding reverse curve

which was about the same as that of the low carbon steel substrate Figure

4.37(a).

Figure 4.37.a: Potentiodynamic cyclic voltammetry curves off TiBN-3 coated LCS

showing pitting corrosion behavior in SSW.

Coated LCS and Ta show identical anodic behavior on the positive going scans

Figure 4.37(b); pitting initiation starts at anodic potentials higher than 3 VvsAg/AgCl.

On the reverse potential scan, the coated LCS remains in active dissolution state

with high anodic current density, whereas the coated Ta show a repassivation

behavior, which indicated by the gradual decrease of current reaching values

cathodic with respect to the forward positive scan.

Moreover, the characteristic trend of the reverse scan is rather identical with

those presented in Figure 4.31 of pitting-free coated samples, and not with that

of the bare Ta Figure 437(c).

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

|Cu

rren

t D

en

sity| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

bare LCS

TiBN-3 LCS

a

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Chapter 4: Results

Figure 4.37(b, c): Potentiodynamic cyclic voltammetry curves off TiBN-3 coated

LCS& Ta showing pitting corrosion in SSW

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

TiBN-3 coated Ta

TiBN-3 coated LCS

-1 0 1 2 3 410

-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

|Cu

rren

t D

en

sit

y| (A

.cm

-2)

Potential (Vvs Ag/AgCl

)

bare Ta

TiBN-3 coated Ta

b

c

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Chapter 4: Results

4.4.4.6 The electrochemical behavior of TiBN-3 coated low carbon steel

under applied cathodic potential

Cathodic protection is a very important electrical mean of mitigating corrosion; it

is applied mainly to buried and submerged metallic structure (primarily steel).

Coatings of more noble metals than the substrate metal like titanium based

ceramic coatings on steels are only protective when there are no pores. In other

cases local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic

protection is theoretically possible, but this protection combination is not very

efficient since the coating usually consumes more protection current than the

uncoated steel. Nevertheless, in some special cases a high protection is of a

great importance, where good coatings have to be accompanied with cathodic

protection despite its high costs (e.g. pipelines in industrial installations, cables

and storage tanks in power stations, refineries and tank farms).

Therefore, in this study we tried to simulate most real field practical conditions,

including cathodic protection test and their influence on this type of coatings.

TiBN-3 coated LCS samples were cathodically polarized at potential of -1 V for

48 h in aerated SSS and SSW. Figure 4.33 (a & b) shows the Bode diagrams

measured on TiBN-3 coated low carbon steel under -1 V, EIS of coated LCS at

OCP are shown for comparison. The effect of cathodic potential on the AC

impedance behavior of the coated specimens can be obtained by comparing the

figures. It can be seen that the total impedance of the specimens under cathodic

potential of -1 V is about two times of magnitude lower than those EIS measured

at OCP Figure 4.33 (c & d). Nevertheless, the impedance increased after 8 hour

immersion and reached a stable value over the time period of cathodic

polarization.

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Chapter 4: Results

Figure 4.33: Bode plot of cathodically polarized bare/TiBN-3 coated low carbon

steel:

(a&b) bare low carbon steel samples in SSS and SSW respectively

(c&d) TiBN-3 coated low carbon steel in SSS and SSW respectively

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

ph

as

e (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

0

10

20

30

40

50

60

70

80

90

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90

ph

as

e (

o)

Imp

ed

an

ce

(.c

m2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSS

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

at 0h, at 12h, at 24h

at 36h, at 48h

10-2

10-1

100

101

102

103

104

105

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

coated LCS in SSW

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

at 0h, at 12h, at 24h

at 36h, at 48h

b a

c#

d#

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Chapter 4: Results

4.4.4.7 Interrupted cathodic polarization measurements

Cathodic protection system might fail while operating due to different reasons,

e.g. cut-off of the power supply which can result from human mistakes and/or

nature external influences. If these failures happen consequently protected

structures will gradually corrode, and when they suddenly fail, it can lead to

consequences ranging from annoying to potentially catastrophic. In these terms,

titanium-based ceramic coatings are still not well studied and a comprehensive

work must be done in this field.

In this experiment the aim was to study the effect of interrupting applied cathodic

current on the coating and to screen the changes in the corrosion resistance.

These measurements are divided into two parts:

a) Current-on potential measurements, in which a cathodic potential of -1 V

was applied on the coated LCS for 24h.

b) Current-off potential measurements, in which the applied potential was

cut-off for 24 h. During this period OCP and EIS was measured.

Figure 4.34 shows the open circuit potential measurements during cut-off

periods. OCP values in SSS (left diagram) remained almost unchanged during

the 4 cut-off days and reached the value of ~ -50 mV which was even nobler than

the measured value under normal condition ~ -300 mV, (Figure 4.23).

In SSW, the measured OCP values were also nobler than those shown in (Figure

4.23), but this ennoblement was seen in the 3rd and 4th cut-off days. This

increase in OCP after removing the applied potential could be attributed to the

repassivation of the coating surface and the reformation of titanium oxides which

were reduced during cathodic polarization period.

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128

Chapter 4: Results

A possible local increase in pH value during cathodic polarization might also

enhance OCP after interrupting the applied cathodic potential. This was not

extensively tested.

Figure 4.34: Open circuit potential values measured during current-off potential

periods.

Experimental impedance spectra collected during the current-off potential periods

are presented in the Bode plots in Figures 4.35 and 36 for SSS and SSW

respectively. Generally, two time constants are observable; the one appearing at

high ω represents the dielectric characteristic of the CVD coating, while the one

at low ω corresponds to the low carbon steel in pores.

Two maxima could be distinguished in the phase angle curves. The high

frequency maximum decreased with time, while the low frequency maximum

increased with time. This revealed that some change related to the coating and to

the steel substrate in the pores occurred in the corrosion process. A better

polarization resistance can still be seen for the specimens in SSW.

0 4 8 12 16 20 24-400

-350

-300

-250

-200

-150

-100

-50

0

50

Po

ten

tial (m

V v

s A

g/A

gC

l)

time (hours)

OCP- off1

OCP- off2

OCP- off3

OCP- off4

0 4 8 12 16 20 24-400

-350

-300

-250

-200

-150

-100

-50

0

50

Po

ten

tial (m

V v

s A

g/A

gC

l)

time (hours)

OCP- off1

OCP- off2

OCP- off3

OCP- off4

SSS

SSW

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Chapter 4: Results

Figure 4.35: Bode plot spectra of coated samples during current-off potential

period in SSS: a) day1-off, b) day2- off, c) day3-off, d) day4-off

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90a

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

a

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90a

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

a

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90b

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

b

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90b

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

b

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90c

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

c

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90c

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

c

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

d

10-2

10-1

100

101

102

103

104

101

102

103

104

105

106

0

10

20

30

40

50

60

70

80

90d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

d

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

at 0h, at 8h

at 16h, at 24h

d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

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Chapter 4: Results

Figure 4.36: Bode plot spectra of coated samples during current-off potential

period in SSW: a) day1-off, b) day2- off, c) day3-off, d) day4-off

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90a

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

a

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90a

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

a

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90b

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

b

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90b

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

b

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90c

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

c

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90c

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

c

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

day1 off, day2 off

day3 off, day4 off

d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

d

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

day1 off, day2 off

day3 off, day4 off

d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

d

10-2

10-1

100

101

102

103

104

105

100

101

102

103

104

105

106

107

0

10

20

30

40

50

60

70

80

90

at 0h, at 8h

at 16h, at 24h

d

-ph

ase (

o)

Imp

ed

an

ce (

.cm

2)

Frequency (Hz)

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131

Chapter 4: Results

4.4.4.8 Surface analysis of TiBN-3 coating after different corrosion tests

4.4.4.8.1 SEM and FIB-cut analysis

The coating surface was examined by SEM after different immersion times. In

comparison to the original coating, the tested coated samples did not indicate the

presence of visible corrosion attacks such as localized attacks, i.e. pitting

corrosion. Figure 4.38 shows the SEM micrographs of the coated specimens

after 90 days immersion in SSS and SSW.

FIB cross-sectional analysis was done for the coating after 90 days immersion to

look at the bulk and coating after long-time exposure; images are presented in

Figure 4.39. It confirms the excellent adherence of the coating to the substrate,

and it also shows that there was no sign of detachment or separation of the

coating from the steel substrate. Moreover, no microcracks were formed within

the coating bulk during the immersion time.

Figure 4.38: SEM images of TiBN-3 coated low carbon steel after 90 days

immersion in: (a) SSS, (b) SSW

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132

Chapter 4: Results

Figure 4.39: FIB images of the cross-sections of TiBN-3 coated LCS after long

immersion test: (a) SSS, (b) SSW

4.4.4.8.2 X-ray diffraction analysis

The X-ray diffraction patterns recorded after 48h immersion on the samples

exposed to SSS and SSW test solutions did not show a significant difference

from the one recorded from the original sample. It seems, therefore, that the

possible transformations of the coatings induced by the corrosive conditions at

this immersion time did not affect the bulk of the materials and occurred only at

the surface. On the other hand, this was not the case for the samples tested in

same solutions after 90 days; hence, XRD spectra showed increase/decrease in

some peak intensities as presented in Figure 4.40. A drastic change in TiB2 100

and TiB2 201 peak intensities was observed, the formers was decreased while

the later was increased. Moreover a new peak at 2θ= 30° was observed, this

peak is related to c-BN. The peak at 43.69° and 44.43°, related to TiN and TiB2

respectively was almost vanished after the immersion test.

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133

Chapter 4: Results

Figure 4.40: XRD patterns of TiBN-3 coated low carbon steel after long term

immersion test a) as- received, b) in simulated soil solution, c) in simulated

seawater

4.4.4.8.3 XPS surface analysis

For more precise information, XPS was used to follow all transformations of the

coated samples after the long-immersion experiment. Figure 4.41 represents a

comparison of XPS spectra between the original TiBN-3/LCS and the samples

immersed in SSS and SSW for 90 days. Both samples show remarkable changes

in peak sizes in both test solutions with comparison to original specimen. For

both surfaces titanium displays two main peaks with highest intensities at 458.6

[182] and 464.5 eV, respectively, those peaks are attributed to TiO2.

20 25 30 35 40 45 50 55 60 65 70 75 80

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

TiNTiB

(c)

(a)

(b)

TiBTiN

TiB2

TiB2

TiB2

TiN

TiB2

c-BN

Y A

xis

Tit

le

Position (2), [o]

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Chapter 4: Results

On the sample in SSS Ti 2p peaks shows as well a single peak at 453.8 eV

related to elemental Ti [174, 183], and a small peak at 454.6 eV corresponds to

TiB2. Those peaks did not exist on the sample in SSW where the main

components of Ti 2p were TiO2. The increase in TiO2 intensity was accompanied

with an increase of O 1s peak intensity; and it was more significant in SSW,

indicating that the oxidation rate of the coating is higher in SSW than SSS.

The O 1s spectra in SSW is formed by a contribution of N-O at 530.1 eV, TiO2 at

531.2 eV and H2O at 532.2 eV [54]. In simulated soil solution the main

contribution in O1s was from TiO2 and H2O at the same binding energies, no

signal from N-O was observed; instead a small peak at 529.1 eV related to CO,

this can be due to the low concentration of the oxynitride.

In the N 1s spectra a decrease and broadening on the tested specimens was

observed, the peak corresponding to TiN disappeared completely, and the mean

components of N1s were the peak centered at 397.9 eV in accordance with BN

bonding [184] and N-O at 400 eV [170].

The electron binding profile of B 1s in SSS shows a peak at 187.1 eV which

corresponds to TiB2; this peak was completely disappeared for the sample

soaked in SSW. The second peak of B 1s at binding energy 190.7 eV

corresponds to BN still existed on two tested specimens in both solutions but with

lower intensities.

The conclusions derived from the XPS database show good agreement with the

XRD results and allow the identification of the relevant changes in the deposited

coating after the immersion test.

It is important to mention that no iron was detected on both samples after the

long immersion time. These results indicate that the coating is very corrosion

resistive and that no microcracks were formed during immersion time where

substrate corrosion might have taken place.

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135

Chapter 4: Results

Figure 4.41: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS

before and after long-immersion test in SSS and SSW

468 464 460 456 4520

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

Ti2p

Y A

xis

Title

Binding Energy (eV)538 536 534 532 530 528 5260

2000

4000

6000

8000

10000

12000

14000

16000

18000

O1s

Y A

xis

Title

Binding Energy (eV)

196 194 192 190 188 186 1840

500

1000

1500

2000

2500

3000

3500

B1s

Y A

xis

Title

Binding Energy (eV)404 402 400 398 396 394

0

2000

4000

6000

8000

10000

12000

N1s

Y A

xis

Title

Binding Energy (eV)

468 464 460 456 4520

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

Original TiBN-3

after 90 days in SSS

after 90 days in SSW

Ti2p

Y A

xis

Title

Binding Energy (eV)

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136

Chapter 4: Results

4.4.4.9 XPS analysis after measurements at different temperatures

XPS analysis was performed on TiBN-3 samples tested at different temperatures

in simulated soil solution. For chemical state determination high resolution

spectra of Ti 2p, O 1s, B 1s, and N 1s were recorded and presented in Figure

4.42. The main observation is the increase in O 1s peak intensity accompanied

with a decrease in N 1s and B 1s peak intensities with increasing temperature.

The N 1s distribution reveals three contributions; the major peak is centered at

398.2 eV with two shoulders at 396.1 and 399.9 eV. The peak at 398.2 eV is

attributed to BN; the other two peaks are due to N-C and N-O bonds in oxynitride

compounds respectively [185]. The peaks observed on B 1s spectrum peaks at

187.1 eV, 190.3 and around 191.7 eV binding energies correspond to TiB2, BN,

and the last one may be related to boron oxide [62]. The O 1s distribution

contains three contributions situated at 530.2, 532 and 533.1 eV, and are

assigned with N-O, TiO2 and O2 or a combination of O2 and B2O3 [174].

The Ti 2p spectrum is a contribution of four peaks; two peaks with fixed positions

at binding energies of 454.2 and 464.6 eV, these peaks are attributed to TiB2 and

TiO2, respectively, with increasing temperature the first peak decreases and the

second one slightly increases. The other two peaks, a contribution at 455.1 eV is

assigned to TiN [54], it disappeared when increasing the temperature to 45°C

and a new peak at 458.8 eV appeared, it is related to TiO2 indicating the full

oxidation of TiN to TiO2 with increasing test temperature. The peak at 459.5 eV is

a contribution of TiO2, its intensity increased with increasing temperature.

The results indicate that elevating temperature enhances the oxidation of the

coating components, particularly; TiN oxidizes and induces TiO2 on the top of the

coating, which is partially covered the surface.

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Chapter 4: Results

The other coating components, BN, TiB2, seem to be more resistive to elevated

temperature. Both seem to have a slower oxidation rate at these conditions than

TiN.

Figure 4.42: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS after

electrochemical measurements at different temperature in SSS.

470 468 466 464 462 460 458 456 454 452 4504000

5000

6000

7000

8000

9000

10000 at 15°C

at 35°C

at 45°C

Ti2p

Y A

xis

Title

Binding Energy (eV)540 538 536 534 532 530 528 526

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

O1s

Y A

xis

Title

Binding Energy (eV)

194 192 190 188 186 184

2000

3000

4000

5000

6000

7000

B1s

Y A

xis

Title

Binding Energy (eV)404 402 400 398 396 394 392

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

N1s

Y A

xis

Title

Binding Energy (eV)

at 15°C

at 35°C

at 45°C

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Chapter 4: Results

4.4.4.10 XPS analysis after measurements at 48h in 1 M HCl

High resolution spectra of Ti2p, B1s, N1s and O1s peaks of the XPS

measurements performed on TiBN-3/LCS samples tested in 1M HCl are shown

in Figure 4.43. Generally, Ti 2p, B 1s and N 1s peak intensities decreased

drastically after 48h in 1M HCl. The main component of Ti 2p was titanium oxide

at binding energy of 458.8 eV [171]. N 1s was deconvoluted into two components

located at binding energies of 398.3 eV and 400.3 eV. The peak at 398.3 eV is

characteristic for BN [186] and at 400.3 for N-O or N-C [53]. Deconvolution of B

1s spectrum reveals two components at 190.3 eV and 192.1 eV. The first one is

assigned to BN [186] while the second one is probably associated to B2O3 [187].

The XPS spectrum of O 1s is fitted to two components, the first one at 529.4 eV

corresponds to titanium oxide [170] and at 532.7 eV corresponds to oxygen in

adsorbed H2O [188]. It can be concluded that the coating in acidic medium is

less resistive due to the dissolution of formed oxide layer; the main remaining

phase of the coating surface was BN.

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Chapter 4: Results

Figure 4.43: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS after

electrochemical measurements in 1M HCl.

4.4.4.11 Analysis after interrupted cathodic protection measurements

Similar characterization of cathodically polarized TiBN-3 coated LCS was carried

out. SEM micrographs in Figure 4.44, show peel-off of theTiBN-3 layer at

different spots of the cathodically polarized coated LCS specimens.

466 464 462 460 458 456 454 452 450

0

1000

2000

3000

4000

5000

6000

7000

Ti2p

Y A

xis

Title

Binding Energy (eV)

Original TiBN-3

At 48h in 1M HCl

538 536 534 532 530 528

0

2000

4000

O1s

Y A

xis

Title

Binding Energy (eV)

196 194 192 190 188 186 184

1500

2000

2500

3000

3500

4000

4500

5000

B1s

Y A

xis

Title

Binding Energy (eV)402 400 398 396 394

0

2000

4000

6000

8000

10000

12000

N1s

Y A

xis

Title

Binding Energy (eV)

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Chapter 4: Results

Figure 4.44: SEM micrographs of TiBN-3 coated LCS tested under interrupted

cathodic polarization.

XPS analysis confirmed the presence of iron in form of iron oxides on the surface

of the specimen tested under interrupted cathodic polarization, Figure 4.45.

Figure 4.45: XPS high resolution peak of Fe before and after interrupted cathodic

polarization measurements in SSW

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Chapter 4: Results

4.4.5 Summary: The electrochemical and corrosion behavior of

TiBN-3 deposited on low carbon steel

The electrochemical data obtained indicate that the ternary TiBN coatings on low

carbon steel exhibit corrosion properties better than those exhibited by binary TiN

and TiB2 coatings.

Electrochemical results give the experimental evidence that he TiBN-3 is very

dense and present less defects compared to the other coatings (TiN, TiB2, TiBN-

1, and TiBN-2), thus, the electrolyte did not penetrate onto the substrate though

long immersion times in the aqueous solution.

Localized corrosion is the corrosion mechanism expected for the failure of the

coated samples.

Over the frequency range applied, the equivalent circuit employed for the

description of the EIS spectra for the coated samples provides the best fit of the

experimental data. The electrochemical behavior of the materials can be depicted

as a metal covered with a porous film. Charge transfer values obtained from this

provide a quantitative basis for the monitoring of corrosion of substrate covered

by a ceramic film. A standard procedure is to consider that a protective surface

is exhibiting Rct values well above 1*106 Ω cm2 [189]. As the Rct values of (TiN,

TiB2, TiBN-1, and TiBN-2) coatings were found to be smaller than 1*106 Ω cm2

after 6 h exposure to SSS, the progress of corrosion at pinholes or pores must be

considered. In contrary, the Rct of TiBN-3 was found after 90 days immersion in

SSS and SSW to be still higher than 1*106 Ω cm2 indicating the unique corrosion

resistance of the deposited coating. The corrosion protection of the coating in

acidic medium was found to be not sufficient; this was attributed to the dissolution

of titanium oxides upon exposing to 1M HCl.

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Chapter 4: Results

4.5 Equivalent circuit for CVD coated steels

For the interpretation of the electrochemical behavior of the system from

impedance data interpretation, an equivalent circuit depicted in Figure 4.43 was

used. The proposed model suggests that the physical behavior is equal to metal

coated with porous films. This widely accepted scheme has been deduced to

represent the electrochemical behavior of a metal covered with unsealed porous

layers [145, 190-192]. The equivalent circuit consists of the following elements: a

solution resistance Rs of the test electrolyte, a charge transfer resistance Rct and

a capacitance Cdl for defects in the coatings, and a capacitance Cc and a

polarisation resistance Rp for the reminder of the coating layer regarded as intact

(non-defective). During the fitting process, the capacitances were represented by

a constant phase elements to account the deviations from ideal dielectric

behavior related to surface inhomogeneities [193].

Figure 4.43: The equivalent circuit used in modelling the electrochemical

impedance spectroscopy results for the different coatings on low carbon steel.

Co

ati

ng

Electrolyte

Rct Cdl

Rs

Cc

Rp

LCS

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Chapter 4: Results

The same circuit was applied to model the EIS data for different coating systems.

Tables 4.10 and 4.11 include the EIS parameters obtained from fitting the

experimental EIS data with the equivalent circuit for coatings with 3 μm in SSS,

and for the thicker TiBN-3, respectively. The data in Table 4.12 summarizes the

parameters at certain times of immersion in SSS.

In Table 4.10, TiBN-1 and TiBN-2 coatings show higher Rp than TiN and TiB2

which indicates that the coatings are denser and compacter, with less pores and

defects. The simulation is in correlation with data measured after 6 h immersion.

Table 4.10: Impedance parameters of different coatings deposited on coated

steel in SSS after 6h immersion in SSS

The simulated data of TiBN-3/ LCS confirm the EIS measurements, showing

again the behavior of a typical porous coating on a metallic substrate.

Nevertheless, the very high polarization resistance values and the pure

capacitive of the coated samples indicate that the coatings porosity (i.e., number

and size of defects) is very low. Furthermore, the time-dependence of the data

showed an increase in the polarization resistance Rp as well as the capacitive Cc

followed by a decrease, reflecting that progressively the pores in the coating are

filled with precipitates of corrosion products followed by dissolution of these

corrosion products in the SSS.

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Chapter 4: Results

However, the impedance resistance and the capacitive values are still high after

90 days of immersion; similar to the values in the beginning of immersion.

From the results, one can conclude that these small micro-cracks did not reach

the LCS substrate; otherwise, the impedance and phase angle would be

expected to drop to the uncoated LCS values. The high values of α obtained for

the TiBN-3/LCS system should also be noticed, which indicate a smooth surface

of the coating film.

Table 4.11: Electrochemical parameters obtained with equivalent circuit

simulation for TiBN-3 coated low carbon steel in SSS at different immersion times

2.22510.954.211.20.871.026.890

3.53480.954.218.80.871.225.479

4.42220.953.823.00.881.228.764

3.02790.953.920.20.881.229.851

2.61740.943.113.80.831.318.425

3.32980.953.115.40.851.219.410

2.33470.953.08.90.811.37.90

Error (%)R

s

Ω cm2α2C

c

µF cm-2

Rp

MΩ cm2α1C

dl

µF cm-2

Rct

MΩ cm2

time

days

2.22510.954.211.20.871.026.890

3.53480.954.218.80.871.225.479

4.42220.953.823.00.881.228.764

3.02790.953.920.20.881.229.851

2.61740.943.113.80.831.318.425

3.32980.953.115.40.851.219.410

2.33470.953.08.90.811.37.90

Error (%)R

s

Ω cm2α2C

c

µF cm-2

Rp

MΩ cm2α1C

dl

µF cm-2

Rct

MΩ cm2

time

days

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Chapter 4: Results

The experimental and simulated spectra of TiBN-3 coated LCS in SSS for two

exposure times are presented in Figure 4.44. Both figures show that the

equivalent circuit satisfies well the electrochemical behavior of the coated sample

measured by EIS.

Figure 4.44: The measured and the simulated EIS data of TiBN coated sample in

SSS at: a) 79 days and b) 90 days.

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Chapter 5: Discussion

5 Discussion

The corrosion protection of steels by hard coatings is one of the most important

and versatile means of improving component performance. It is well known that

the constitution of material systems and the fabrication parameters determine the

coating properties and the microstructure, consequently, their protection

behavior. Therefore, for better understanding of the corrosion behavior of the

various coating systems, the influence of the deposition parameters must be

considered.

5.1 CVD process parameters

Surface morphology and microstructure of CVD deposited coatings are controlled

by many factors that are often interrelated, such as substrate, temperature,

deposition rate, impurities, temperature gradients, and gas flow.

In this study the variable parameters were the input gas composition, gas flow

rate, deposition time, and the used substrates i.e., stainless steel and low carbon

steel, while maintaining the total pressure and the temperature constant during

the deposition.

5.1.1 The effect of substrate microstructure and chemical

composition on different deposited coatings

The different coatings deposited at same deposition conditions show different

morphologies on different substrates. TiN coating morphologies were a mixture of

lenticular plate shape on stainless steel and icosahedral on low carbon steel, with

five-fold symmetry crystallites. TiB2 on the other hand, were relatively smooth

and coherent on stainless steel, whereas, on low carbon steel it showed larger

and more clearly defined crystal facets with spallation.

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Chapter 5: Discussion

Since the initial surface preparation was identical the differences in the

morphologies of different deposited coatings were thought to be due to the

different chemical composition of the used substrate materials. The different

microstructures of different steels, i.e., martensite and ferrite-pearlite, in addition

to the presence of chromium and chromium oxide film on stainless steel, which

had probably re-formed on the previously polished surface, had affected the

initial growth pattern, consequently, the texture of the deposited films, hence, the

growth on the deposited first layer of each deposited film proceeds easily [87,

131]. Among other things, the efficiency of a CVD hard coating depends on the

adhesion to the substrate and indirectly on the properties of the substrate-coating

interfacial region [41]. Generally, all studied coatings on two different steels

showed good adherence onto metal substrates except TiB2 on low carbon steel,

it was failed by spalling at the interface. The spallation of deposit TiB2 from low

carbon steel substrate is probably due to the different thermal stresses: thermal

expansion mismatch, thermal stresses and blistering. These stresses induce

microcracks in the coating with the subsequent weakening of the final product.

TiB2 has a thermal expansion coefficient of (5.5-5.6) which is less than that of the

low carbon steel (10.0) [194]. Thus, the combination low carbon steel-TiB2 faces

high stresses due to the effect of thermal expansion differences, therefore,

tensile stresses in the steel and corresponding compressive stresses in TiB2

layer reach high values and lead to cracking and/or spalling. On the other hand,

chipping was not observed on TiB2 coated stainless steel, although it has a

thermal expansion coefficient two or three times higher than that of TiB2, hence,

this could not be the only reason for chipping. Blistering could also appear when

coating and substrate interact to form intermediate layer. Bad adhesion of TiB2

on low carbon steel was observed by Takahashi et.al and Pierson et al. [87, 194]

and this behavior was attributed to the formation of iron- boride interlayer on iron

substrate prevented the good adhesion of the deposited coating on low carbon

steel substrate.

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Chapter 5: Discussion

The better adhesion between TiB2 and stainless steel can be due to the better

structural match between the deposited coating and the metal substrate [195].

5.1.2 The influence of boron flow rate on the and morphology of

TiN

In TiNB coatings, even small amounts of BCl3 added to the reactant gas phase of

TiN resulted in a pronounced grain refinement. By increasing the BCl3 flow rate

from 0.16 to 0.32 0 Nl/min grains decreased in size even more. Compared to the

nitrides, the surface of borides and boronitrides of Ti showed more metallic luster.

The atomic concentrations as function of the boron flow rate and deposition time

are listed in Table 5.1. Results showed that the increase of the boron flow rate in

the deposition atmosphere results in an increase of the boron content in the

deposited films (TiBN-1&2), furthermore, increasing the deposition time while

maintaining the boron flow rate constant leads to thicker coating with higher

boron content (TiBN-3). The increment of boron content is accompanied with the

reduction of titanium and nitrogen content, and of the oxygen contamination upon

boron incorporation.

Table 5.1: Atomic percent chemical compositions (by EDX) of different deposited

TiBN films as function of the boron flow rate and deposition time during

deposition

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Chapter 5: Discussion

Optically, TiN which initially presents a golden color, becomes silver gray upon

boron addition, this is in a good correlation with XRD results where TiB2 is the

predominant phase in the deposited coating.

Contrary to TiN and TiB2 coating layers which generally had a preferred

orientation, TiBN coatings was revealed to have multi-oriented structure by XRD

analysis. BCl3 incorporation into TiN process caused the coating layer to be a

composite of TiN, TiB, TiB2, and BN crystallite with more pronunciation of TiB2.

Boron has a higher reactivity in comparison to nitrogen and is able to react faster

with titanium forming the titanium-boride composites whereas nitrogen

evaporates [196].

It is also evident from the x-ray diffraction (XRD) broad diffraction peaks that the

average grain sizes of TiBN films are in the nanometer range, which could also

be seen from SEM micrographs of the different deposited coatings, where

increasing the boron content lead to finer crystal size, denser and compacter

structures. Additionally, FIB cross-sections of the bulk coating prepared did not

show the presence of more than one phase within the Ti-B-N matrix. The

individual grains in the polished cross-section of the coating could not be

discerned, this is a typical feature for nanocrystalline coatings [197, 198].

The change in microstructure was associated with an increase in deposition rate

by adding more BCl3. Increasing the flow rate, increases the super saturation and

nucleation rate, while decreasing the crystal size [49]. Moreover, boron interrupts

the columnar coating growth of TiN, which yields a fine-grained structure and

smooth surface at the highest content [199].

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Chapter 5: Discussion

Adhesion is mainly influenced by the resulting substrate-coating interface which

is developed at the early stages of the deposition [200]. The adhesion of different

deposited coating layers onto different steel substrates was not mechanically

actuated.

Optical micrographs of cross-sections and conducted FIB cuts of TiBN-3/LCS

indicated a good adhesion which is supposed to be resulting from the

interdiffusion of carbon and titanium between the substrate and the coating

during CVD coating [201]. Furthermore, the presence of mixed-phase layers in

TiBN prevent the adhesion problems that may result in carbon diffusion from the

steel, since the freed carbon can be absorbed by the cubic TiN lattice without

causing adhesion problems [202].

5.2 Corrosion and electrochemical behavior of different

coatings on different steels

Under most conditions, corrosion of chemical vapor deposited hard titanium

based ceramic layers on steel usually takes a localized form, due to the

establishment of an elecropotential difference between the coating material and

the less noble steel substrate. At the defects, localized galvanic corrosion can

occur, leading to accelerated attack at the coating/substrate interface [125, 203-

206].

Electrochemical measurements were used to evaluate the corrosion behavior of

each different coating system. As known, open-circuit potential measurement is

good to assess any existing through-porosity (or open porosity) in a coating

structure.

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Chapter 5: Discussion

Because Ti based ceramic coatings are applied on stainless steel and low carbon

steel substrates, any through-porosity (allowing the test solution to penetrate the

coating into the interface of coating and substrate) makes the open-circuit

potential of the coating approach that of the substrate. On the other hand, if the

coating is dense (no existing cracks or through-porosity), the measured open-

circuit potential is supposed to introduce the behavior of the coatings material in

the used solution and the surrounding environment.

Additional polarization measurements give information on the reactions around

the corrosion potential. Moreover, the corrosion phenomena (pitting, crevice

corrosion) taking place during the corrosion attack can be evidenced.

In acid solution the cathodic reaction is hydrogen evolution, and in neutral

solution is oxygen reduction. The electrons consumed there must be supplied by

an anodic reaction, i.e. the dissolution of the steel substrate. Because anodic

reaction on the coatings themselves is slow, measured corrosion currents

indicate porous coatings [207]. The corrosion current densities and polarization

resistance are inversely proportional. A high polarization resistance therefore

indicates a low corrosion current density, consequently, low coating porosity.

5.2.1 The electrochemical and corrosion behavior of different

coatings on stainless steel and low carbon steel

On low carbon steel the corrosion resistance of the TiN and TiB2 coatings was

insufficient, probably due to the insufficient film thickness for the coatings and the

unsatisfactory structure and high porosity. Open circuit potential of TiN coated

specimen descended to a potential of the substrate, that is, the specimen

became an activated state at certain time.

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Chapter 5: Discussion

It is known that an activation time, which is a period until OCP reaches that

potential, can be used for evaluation of the corrosion resistance of ceramic films

coated on the steels [13, 208].

On low carbon steel, TiB2 coatings have high current densities after polarization

which is attributed to the bad adhesion of the coating on low carbon steel

substrate and the chipping of the coating at some spots. In contrast, the best

corrosion behavior of coated stainless steel was introduced by TiB2 with the

lowest open circuit potential values and anodic current density, whereas TiN was

found to improve the corrosion resistance very slightly.

The behavior of TiBN films deposited onto both different steel substrates was

found to improve the corrosion resistance in neutral test solutions. TiBN-2

showed the best performance for coated LCS and was equal to TiB2 for coated

stainless steel.

The nature of the chemical bonding character may affect the corrosion behavior

of the single layer films. According to Holleck [209], all titanium-based hard layers

tested here are metallic hard materials since metallic bonding is predominant.

However, besides primary metallic bonding localized metal-non-metal bonds can

also be exist. The level of metallic bonding increases when going from films

based on the group IV to group VI of the transition metals. In the same way it

increases when going from nitrides to carbides and borides. For the coating

tested, this means that TiB2 has the lowest and TiN has the highest amount of

direct metal-non-metal bonds. According to this stability the free enthalpy ΔG is

lowest for TiB2 and highest for TiN, whereas TiBN fall somewhere in between.

This correlation could not be proven in this study.

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Chapter 5: Discussion

The corrosion resistance of the investigated titanium-based coating systems was

found to be mainly influenced by the structure, thickness, and the porosity of the

deposited layers [58, 210, 211], enhanced by the formation of a passive oxide

layer on the deposited films as shown in the electrochemical results and surface

analysis carried out after measurements.

The effect of structure and porosity was confirmed from the calculated porosity

and the polarization resistance (Rp) for each coating system. It was found that Rp

values were inversely proportional to the calculated porosity, and those coatings

with the highest Rp values and lowest porosity showed the best corrosion

resistance. According to many studies [39, 68, 212], TiN and TiB2 themselves are

not very stable in water solutions but it can be oxidized to a more stable

compound Ti(OH)3 and further to TiO2.H2O at pH higher than 2 by the hydrogen

evolution reaction.

The formed TiO2 oxide layer has a passive nature and very resistive to localized

attack. This phenomenon was studied for coating deposited on low carbon steel,

where XPS analysis of the original deposited coatings before the electrochemical

measurements confirmed the presence of TiO2 on the surface of all deposited

coatings.

TiN: consists of TiN, TiO2, NO and small amount of CN.

TiB2: partially oxidized to TiO2 and B2O3, contains also a small amount of BN and

CN.

TiBN-1: consists of TiB2 and BN with small amount of TiN; oxide phases are

detected in a very small extend.

TiBN-2: similar to TiBN-1; much lower TiN content; much lower amount of oxide

phases (TiO2 and B2O3). The BN phase seems to enhance the stability to

oxidation.

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Chapter 5: Discussion

These results indicate that titanium based coatings oxidize once they are

objected to the surrounding atmosphere and build a thin oxide layer, the

composition of this layer varies according to the existing phases in the coating

film.

The role of these oxides was excessively investigated for TiBN-3/LCS system.

5.2.2 The electrochemical behavior of TiBN-3 coating on low

carbon steel

TiBN-3, the coating with its fine size crystallites, mixed orientations, and dense

structure showed the best corrosion performance between all studied coatings on

low carbon steel. The development of the open circuit potential of TiBN-3 coating

in both solutions SSS and SSW over 90 days immersion indicate the occurrence

of insoluble corrosion products on the film surface; a passive thin film was

formed.

This film is mainly composed of TiO2 and NO as shown in XPS analysis after

immersion tests. The passivation is correlated with the relatively high positive

values of the potential. The fluctuation in the measured potential may be due to

the slow rate of the accumulated corrosion products of titanium and/or substrate

in the pinholes or the microcracks followed by dissolution in test solution. One

reason for the local dissolution could be due to the boron resulting from TiB2

oxidation/dissolution will form boric acid, which locally prevents the full

passivation of the coating surface [68].

However, no breakdown events are seen during the 90 days of immersion, as

those would be expected to lead to a sudden significant decrease of the

potential.

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Chapter 5: Discussion

This behavior was further confirmed by potentiodynamic polarization curves

(Figures 4.25).

Coated samples exposed to the atmosphere or the test solutions after coating

process are covered spontaneously by an oxide film. As soon as

potentiodynamic polarization was performed in test solutions between -1.0 and

+3.0 V, the process of dissolution of natural oxide film TiO2 begins first, with

transfer of Ti3+, TiO2+ ions or/and H3BO3 into the solution. Simultaneously, self-

passivated film formation of TiNxOy or TiO2 also begins.

The latter oxide inhibits and slows down the dissolution of the composites [46].

An anodic current peak can be observed at ≈2.0 V in each case, independent

from the chemical composition of the neutral solutions or from the substrate

material. A slight decrease of anodic current as the potential becomes more

positive is noticed, most probably due to a decrease of the real surface area as

the film thickness increases.

Furthermore, cyclic potentiodynamic polarization curves carried out after different

immersion time (Figure 4.31) showed that whether the measurements were

performed directly or after long immersion time, the results were very identical.

The region of constant current with increasing potential suggested that TiBN

surface was passive.

Moreover, the negative hysteresis observed on the reverse scan as the current

decreased suggested that the passive layer did not break down under the

conditions used in the present study. The corrosion in the first stage could be

assumed as follow, when test solution reached LCS substrate through coating

defects, e.g., micro-cracks and pinholes.

The impedance spectra in SSS and SSW do not change during the immersion

time which indicates that the corrosion rate is very low.

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Chapter 5: Discussion

The positive corrosion potential, the high polarization resistance and the low

fraction of anodic current flowing through the pores indicate that the substrate is

passive and the measured response describes the coating.

Thus electrochemical and corrosion stability of the coating can be attributed to:

The increase in coating thickness which reduces the possibility of

through-coating defects (e.g. pores).

The presence of different phases with different crystal orientations which

decreases the opportunity of pore formation due to the discontinuous in

crystallite boundaries in the structure.

The different compositions of the different phases in the coating with

redirected current flow between coating and substrate due to the different

electrical behavior.

The different preferred crystal orientations in the coating texture which

commonly affects its properties (microhardness, adhesion) [213, 214],

consequently its corrosion resistance. As a result, it could be concluded

and according to surface analysis performed after electrochemical

experiments, that TiB2 with 201 and 200, TiN with 311 orientation in

addition to c-BN has better corrosion resistance when exposed to chloride

containing neutral solutions.

Moreover the electrochemical potentials between the phases are different,

thus the corrosion penetration towards the substrate can be reduced

owing to the current flowing within the coating body.

The presence of the oxide and oxynitride components on the surface

which form a barrier protects the surface from corrosion.

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157

Chapter 5: Discussion

The shielding effect of nitrogen anions (N3-) layer at the surface, inhibiting

the oxidation of the underlying titanium ions [215].

It is possible that the iron initially absorbed by the coating may reduce its

corrosion resistance; especially at low layer thicknesses.

Titanium oxides have a strong basic character and dissolve easily in

acidic solution [216, 217] without acting as a barrier. Therefore the

protection was insufficient in acidic HCl.

5.2.2.1 The influence of test solution

The OCP shift in the noble direction for the TiBN-3 coated samples suggests the

formation of a passive film that acts as a barrier for metal dissolution and reduces

the corrosion rate. The potential increase shows that the coating becomes

thermodynamically more stable with time.

The distinguishing feature of the anodic behavior of the titanium boronitride in

sea-water and soil-water in comparison with the acid media 1M HCl is the high

resistance to the oxidation and pitting processes; the passivation range of the

coating exceeds 4 VAg/AgCl. The presence of long passivation regions on the

anodic polarization curves indicates that the coating examined is in the passive

state in SSS and SSW. Thus the corrosion behavior of TiBN-3 in these solutions

can be compared with the behavior of the transition metals Ti, Ta, not susceptible

to pitting corrosion.

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158

Chapter 5: Discussion

Generally, the pH value of a solution essentially influences the kinetics of the

anode and cathode processes and the corrosion rate. The corrosion process in

the SSS and SSW solutions proceeds with the oxygen depolarization. It means

that the corrosion rate is controlled by the oxygen concentration in the solution

and by the rate of the reaction. The generation of oxygen at higher anodic

potential is prevented by the oxide film formed on the coating surface; this is due

to its low electron conductivity.

Additionally, it is confirmed that the anodic polarization curves practically do not

change at different pH values, but the onsets of oxidation reaction [47].

Only at pH=0 for 1M HCl solution the oxidation peak at +2.0 V was shifted to

higher potential. Nevertheless in acidic medium at open circuit potential

conditions, the formed titanium oxides and due to their strong basic character

dissolve exposing the underlying coating directly to the acidic solution; the EIS

measurements and XPS characterization supports the dissolution argument.

The concentration of major ions present in simulated natural water solutions, Na+,

K+, Ca+, Mg+, Cl‾, SO42-, and HCO3‾, influences strongly the corrosion behavior,

since the water aggressiveness is closely related to the concentration of either of

these ions. HCO3‾ has the same concentration in both solutions (SSS and SSW)

and thus both of them have the same pH value. The screening of pH changes

while experiments did not show any changes over test periods indicating that no

increase in solution acidity took place.

The different concentration of Cl‾ in test solutions, i.e. SSW, was found not to

play a role in the corrosion resistance of the applied coating layer. This can be

attributed to the superior corrosion resistance of titanium oxides to chlorides.

The cations Ca+, Mg+ might deposit from corrosion as a calco-magnesium which

has a barrier protection effect, traces of Ca was detected by XPS but it was

difficult to define the chemical bonding.

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159

Chapter 5: Discussion

5.2.2.2 The effect of test temperature

Generally, the increase in temperature is favorable for diffusivity of oxygen and

the various interfacial reactions. The slight increase in cathodic current density

observed in Figure 4.32, indicates that the oxygen diffusion was slightly

enhanced when the test temperature increases from 15 to 45 °C, and

consequently enhanced the oxidation of the TiBN-3 coating. The anodic current

densities were also slightly stimulated by the increased temperature but the

corrosion potentials did not show a remarkable change. The stimulation of the

cathodic and anodic processes in the vicinity of the corrosion potential, suggests

a stimulation of the whole electrochemical process. Nevertheless, increasing

temperature to 45°C was not sufficient to offset the effect resulting from the

enhanced reactivity of the coating with increased temperature. A previous study

carried out on titanium diboride showed that the coating behaves like a passive

metal in NaCl solution due to the formation of a surface oxide film, whose

protectivity decreases with the temperature and disappears at 65°C [69]. No

signs of corrosion could be observed. XPS analysis after electrochemical

measurements showed that the oxidation rate increases with increasing test

temperature, and that the oxidation is mainly of TiN phase where the peak

intensity of TiO2 increased with temperature.

5.2.2.3 Passivity and localized corrosion

Titanium based ceramic films having nobler potential than the steel substrates

are classified as cathodic coatings. These coatings will provide significant

corrosion protection when they are free of pinholes and cracks. Thus, in the

presence of defect; the substrate is subject to galvanic corrosion in the coating

defect as shown in Figure 5.1. The corrosion is rather intensive because the ratio

between the cathodic coating and the anodic spot of bare substrate is very high.

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160

Chapter 5: Discussion

Figure 5.1: Localization of corrosion at a defect in cathodic coating on steel

A part from the galvanic action, small pores can also increase the corrosion by

the mechanism of crevice corrosion or pitting corrosion. The area of a pore will

form an occluded corrosion cell, where the electrolyte will become more

aggressive because of the increased acidity and increased concentration of

aggressive ions, e.g. Cl‾.

There are several reasons for the defect formation in the coatings, i.e. columnar

growth, pores, microscopic cracks. In the experiments of the present study, it has

been shown that when pitting corrosion takes place on coated low carbon steel, a

very high dissolution rate of the substrate material was observed followed with pit

propagation, the coating was not able to sustain the substrate from further

dissolution. On the other hand, on the coated Ta, repassivation was observed

when the applied potential was reduced in the reverse scan.

Hence, each of the analyzed coated materials, i.e. low carbon steel and Ta, show

a different passivity breakdown, the chemical stability of the substrate seems to

play a basic role in the pitting propagation. Galvanic coupling which is supposed

to be the reason for the pitting corrosion in TiBN-3/LCS system could be the

reason as well for the pitting corrosion of TiBN-3/Ta, owing to the fact Ta is more

stable than the coating at the applied anodic potential.

Coating

Steel substrate

Cathode

Anode

Coating

Steel substrate

Cathode

Anode

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161

Chapter 5: Discussion

5.2.2.4 The effect of interrupted cathodic polarization

SEM micrographs of the cathodically polarized samples showed that the coating

underwent blistering and deterioration. This result was also proved by XPS

analysis since Fe was detected in a small amount on the surface.

This lead to the conclusion that the coating system TiBN-3/LCS is not stable

under interrupted cathodic polarization.

Two different mechanisms can explain this damage of the coating:

a. The effect of blistering caused by hydrogen absorption. As can be seen

from cyclic voltammogram of TiBN-3 in SSS (Figure 5.2), hydrogen

evolution takes place at cathodic potentials higher than 0.9 V vs Ag/AgCl. It is

well known that in aerated neutral solutions the oxygen reduction is

stronger than the water reduction as cathodic reaction:

½ O2 + H2O + 2e‾ → 2OH‾ (5-1)

When lowering the potential to achieve cathodic protection, hydrogen

evolution takes place by water reduction reaction:

2H2O+ 2e‾ → H2 + 2OH‾ (5-2)

When hydrogen atoms meet in a trap and combine, they form hydrogen

molecules in the trap. The accumulated hydrogen gas inside the extremely

small cracks will lead to build up of excessive internal hydrogen pressure.

At cretin times, the internal hydrogen pressure will become sufficient to

cause the coating to blister.

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162

Chapter 5: Discussion

b. The reduction and repassivation of the oxide layer on film surface.

At the most negative potential, the oxide layer from the TiBN-3 deposit is

reduced, and the film is directly exposed to the solution. After cutting the

applied potential, and when the coated steel electrode is at OCP, the

anodic reactions occurring at the coating surface and coating microcracks

are the oxidation of titanium to TiO2 and of iron to iron oxides and

hydroxide, in this way, the volume of resulting oxides and corrosion

products will increase, consequently, the size of defects and microcracks

will increase further. Thus and obviously the frequent repeat of on/off

cathodic potential will lead to enlarging the defects in the coating and the

exposed substrate to test solution, increasing the corrosion rate.

Figure 5.2: Cyclic voltammetry of TiBN-3 coated LCS in SSS

-1 0 1 2 3-80,0µ

-60,0µ

-40,0µ

-20,0µ

0,0

20,0µ

40,0µ

60,0µ

80,0µ

100,0µ

120,0µ

2

1

Cu

rren

t D

en

sit

y (

A.c

m-2)

Potential (Vvs Ag/AgCl

)

TiBN-3 in SSS

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163

Chapter 6: Conclusion

6 Conclusion

Different titanium based ceramic films with different chemical compositions were

deposited on two different steels by chemical vapor deposition (CVD).

The electrochemical and corrosion behaviors characterized in different solutions

with different measuring techniques.

The results clearly demonstrate that the corrosion resistivity of a good coating is

related to the impermeability of the protective coating which is in turn depends

on: a) its thickness; b) the absence of defects; c) the adhesion of the layer to the

substrate. In these terms, TiB2 (Titanium diboride) and TiBN-2 (Titanium

Boronitrude) showed the best corrosion behavior on stainless steel, while on low

carbon steel the best corrosion resistivity was obtained by TiBN-2. Generally, the

deposited titanium based ceramic coatings are electrochemically noble compared

with most structural materials and their corrosion is negligible in most of the

ordinary used electrolytes but the existing growth defects in the coating are

detrimental to the corrosion resistance of the coated metal: these defects are

particularly dangerous as they provide direct paths for corrosive electrolytes to

reach the coating/substrate interface, where the localized corrosion can be

initiated due to the potential difference between the coating and the metallic

substrate [218].

Depending on the previous results, CVD process parameters were optimized to

permit the deposition of dense TiBN coating without mechanical defects, such as

micro-porosity, localized cracks or poor local adhesion to the metallic substrate.

Thicker TiBN layers were deposited on low carbon steel substrate, these

coatings showed a superior corrosion resistance in neutral solutions and a

moderate corrosion protection in acidic medium. Increasing test temperature did

not significantly affect the corrosion behavior of the deposited TiBN-3 film.

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164

Chapter 6: Conclusion

The high corrosion protection of TiBN-3 is related to the fact that they are

composed of several phases (TiN, TiB2, and BN). Multiphase systems display

many similarities with composite materials and often display better corrosion

properties than single-phase materials. The coating can also be thought as a

barrier that, by preventing the contact of electrochemically active species with the

metal surface, hinders the occurrence of corrosion processes. In neutral

solutions, TiBN-3 coating behaves like a passive metal because of the formation

of rather protective surface film composed of titanium oxides and oxynitrides.

This passive film was less resistive in acidic medium and dissolved exposing the

coating to the aggressive medium.

Interrupted cathodic polarization in artificial seawater and soil water induces

cracks and leads to coating spalling due to hydrogen evolution and the changes

in oxides volume due to oxidation- reduction processes, where the volume of the

oxide filling the micro-cracks increases and leads to crack broadening and

spalling.

Finally, this study demonstrated that CVD was successfully used to produce a

smooth pore-free coating; giving reliable protection against corrosion to low

carbon (pipeline) steel.

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165

Chapter 7: Outlook (Future work)

7 Outlook (future work)

There are several areas where additional investigations can be an extension of

this thesis and provide valuable information. The recommendations for future

work include:

Investigations of the different microstructures and different adhesion

properties of the different coatings deposited on different steels would be

of a great interest.

A detailed study of the electrochemical oxidation of TiBN and the nature of

formed oxides/oxynitrides should also be very interesting as they influence

the corrosion protection of the film.

Applying multiple layers to enhance the corrosion performance of CVD

deposited Ti-B-N coatings on low carbon steel might also be studied.

Evaluation of the wear-resistance and coating-hardness of the coating

systems would also be useful to develop the optimum balance between

mechanical and corrosion properties for specific applications areas.

The behavior in acidic environments with different pH values should also

be further studied.

To understand the behavior of the different coatings under applied

cathodic protection more experiments should be carried out at different

potentials to define the potential window where cathodic protection could

be applied without damaging the coating and affecting its corrosion

performance.

More analysis techniques, in particular FIB and TEM, should be used to

have better understanding of the formation of cracks and the peeling-off of

the coating.

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166

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198. Pierson, J.F., Tomasella, E., Bauer, P., Reactively sputtered Ti-B-N nanocomposite films: Correlation between structure and optical properties. Thin Solid Films, 2002. 408(1-2): p. 26-32.

199. Wagner, J., Hochauer, D., Mitterer, C., Penoy, M., Michotte, C., Wallgram, W., Kathrein, M., The influence of boron content on the tribological performance of Ti-N-B coatings prepared by thermal CVD. Surface and Coatings Technology, 2006. 201(7 SPEC. ISS.): p. 4247-4252.

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182

List of publications

a) E. Alkhateeb, R. Ali, S. Virtanen, N. Popovska, Electrochemical evaluation

of the corrosion behavior of steel coated with titanium-based ceramic

layers. Surface and Coating Technology 205 (2011) 3006-3011

b) R. Ali, E. Alkhateeb, F. Kellner, S. Virtanen, N. Popovska-Leipertz,

Chemical vapor deposition of titanium based ceramic coatings on low

carbon steel: Characterization and electrochemical evaluation. Surface

and Coating Technology 205 (2011) 5454-5463

c) E. Alkhateeb, R. Ali, N. Popovska- Leipertz, S. Virtanen, Long-term

corrosion study of low carbon steel coated with titanium boronitride in

simulated soil solution. Electrochimica Acta, 76 (2012) 312-319.

d) Abolfazl Motalebi, Mojtaba Nasr-Esfahani, Rania Ali, Mehdi Pourriahi,

Improvement of corrosion performance of 316L stainless steel via

PVTMS/henna thin film. Progress in Natural Science: Materials

International, 22(2) (2012) 392-400

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183

Conference Presentations

Kurt-Schwabe Symposiom 2009, Erlangen, Deutschland- R.Ali,

S.Virtanen, “Corrosion behavior of low carbon steel coated with

polyaniline” - P

EUROCORR 2011, Stockholm, Schweden- R.Ali, E.Alkhateeb,

S.Virtanen, N.Popovska, “Electrochemical evaluation of the corrosion

behavior of steel coated with titanium-based ceramic layers” - O