DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and...
Transcript of DOKTOR-INGENIEUR€¦ · Thin films of titanium nitride (TiN), titanium diboride (TiB 2), and...
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
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
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.
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
v
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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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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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
1
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.
2
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.
3
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].
4
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.
5
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.
6
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.
7
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
8
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
9
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.
10
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.
11
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.
12
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).
13
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.
14
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].
15
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].
16
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.
17
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.
18
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].
19
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].
20
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)
21
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)
22
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.
23
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.
24
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.
25
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
26
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.
27
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.
28
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.
29
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.
30
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.
31
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.
32
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.
33
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.
34
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).
35
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.
36
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].
37
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].
38
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.
39
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].
40
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
41
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)
42
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.
43
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
44
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].
45
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.
46
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].
47
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.
48
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.
49
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.
50
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.
51
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.
52
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.
53
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.
54
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.
55
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].
56
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
57
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
58
Chapter 4: Results
Figure 4.2: Optical micrographs of the cross- sections of TiBN-1 and TiBN-2
coatings deposited on stainless steel
59
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
60
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
61
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
62
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).
63
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
64
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.
65
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
66
Chapter 4: Results
Figure 4.5: Optical micrographs of the cross- sections of TiBN-1, TiBN-2
coatings on low carbon steel
67
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.
68
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
69
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
70
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
71
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.
72
Chapter 4: Results
Figure 4.7.2: XPS high-resolution spectra of the TiB2 coated low carbon steel
73
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.
74
Chapter 4: Results
Figure 4.7.3: XPS high-resolution spectra of the TiBN-1 coated low carbon steel
75
Chapter 4: Results
Figure 4.7.4: XPS high-resolution spectra of the TiBN-2 coated low carbon steel
76
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.
77
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
78
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.
79
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
80
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]
81
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.
82
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.
83
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
84
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
85
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.
86
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
87
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.
88
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
%
89
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.
90
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
91
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
92
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.
93
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
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.
95
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
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.
97
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)
98
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
99
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.
100
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)
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.
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.
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.
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.
105
Chapter 4: Results
Figure 4.24: Open circuit potential measurements of different specimens in SSS
and SSW
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.
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
108
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] .
109
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)
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.
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
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)
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.
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
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
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.
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
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
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.
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
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
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
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
124
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
125
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.
126
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#
127
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.
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
129
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)
130
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)
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
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.
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]
134
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.
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)
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.
137
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
138
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.
139
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)
140
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
141
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.
142
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
143
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.
144
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
145
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.
146
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.
147
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.
148
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
149
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].
150
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.
151
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.
152
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.
153
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.
154
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.
155
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.
156
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.
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.
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.
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.
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
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.
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
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.
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.
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.
166
Chapter 8: Bibliography
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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
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