International standards on corrosion

Objekttyp: AssociationNews

Zeitschrift: IABSE bulletin = Bulletin AIPC = IVBH Bulletin

Band (Jahr): 13 (1989)

Heft B-50: IABSE bulletin

PDF erstellt am: 08.10.2015

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IABSE PERIODICA 3/1989 IABSE BULLETIN B-50/89 31

International Standards on Corrosion

The following article is a slightly shortened versionof the original published in ISO Bulletin, Volume 20,no. 4, April 1989.

Corrosion is a very destructive process involving chemi¬cal and electrochemical as well as physical and mechani¬cal factors. A survey carried out in a chemical plant in theFed. Rep. of Germany showed that 40% of corrosionthere was due to stress-corrosion cracking, hydrogenembrittlement, and corrosion fatigue - all of which havemechanical as well as chemical aspects.

According to the source quoted above, in 1977 lossesdue to corrosion in the USA were estimated at USD 15billion. It is no doubt impossible to estimate the lossesdue to the corrosion of the world's cultural resources -cathedrals, temples, religious shrines, historical build¬

ings, monuments - as a result of air pollution and acidrain.

ISO Technical Committee (ISO/TC) 156 Corrosion ofmetals and alloys held its first plenary meeting in 1976and identified atmospheric corrosion as a priority area forStandards development. In 1978, Working Group 4 wasgiven the assignment of developing a Classification Sys¬

tem for atmospheric corrosion. This system is nowcontained in four documents currently in the Draft Pro¬

posal (DP) stage (DP 9223, 9224, 9225 and 9226). Otherstandardization work relating to corrosion is underway in

Sub-committee 7 (Corrosion tests) of Technical Commit¬tee (ISO/TC) 107 Metallic and other imorganic coatings;Sub-committee 2 (Anodized aluminium) of ISO/TC 79Light metals and their alloys; and in some sub-commit-tees of ISO/TC 35 Paints and varnishes.

As part of the ISO standardization process to developbetter and more comprehensive values of both long- andshort-term corrosion rates. ISO/TC 156 is currently set¬

ting up a worldwide programme involving exposure ofmetals to the environment. The purpose of the Prog¬

ramme is to provide more data on the various climatesof the world and how atmospheric corrosion can varyfrom region to region, continent to continent. Four diffe¬rent metals, with 54 specimens of each metal, will beexposed at each site. Both flat panels and wire helixesare being used so that results from the two specimentypes can be correlated. Time of wetness data will alsobe obtained as well as the amount of sulfur dioxide andChloride deposited on the metals. Short- and longtermcorrosion rates can thus be studied as a function ofatmospheric conditions. The results of this programmeare expected to make procedures for estimating atmos¬pheric corrosion rates more reliable.

Defining terms

The scope of ISO Technical Committee (ISO/TC) 156Corrosion of metals and alloys is standardization in thefield of corrosion of metals and alloys including corrosiontest methods and corrosion prevention methods. Inter¬national Standard ISO 8044: 1986 Corrosion of metalsand alloys - Terms and definitions is designed to providedefinitions concerning corrosion that can be understoodto have the same meanings by all concerned, at interna¬tional level. «Some corrosion terms in present use havedeveloped through common usage and are not always

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logical. It has not, therefore, been possible to definecertain terms in the form they are used in some coun¬tries. Because of the occasional conflicts between tradi¬tion and logic some definitions inevitably represent a

compromise».Traditional usage of the term corrosion itself is cited inthis International Standard as an example of such a

conflict. «Corrosion» has been used to mean the pro¬cess of corrosion and the products of the process, aswell as the damage caused by the process. In Interna¬tion Standard ISO 8044, therefore, «corrosion» meansthe process itself. Any detectable result of corrosion in

any part of a corrosion system is termed «corrosioneffect». The term «corrosion damage» thus Covers anyimpairment of the funtion of the technical System ofwhich the metal and the environment form a part. Con¬

sequently, «corrosion protection» implies that theimportant thing is to avoid corrosion damage rather thanto prevent corrosion, «which in many cases is imposs¬ible and sometimes not necessary».

Types of corrosion

Stress-corrosion cracking results in cracks which makestructures useless for supporting or containing. Stresscorrosion is defined as «a process involving conjointcorrosion and alternating straining of the metal», inInternational Standard ISO 8044. These types of corro¬sion are not so directly related to the environment asother forms. Corrosion fatigue results in the formation ofcracks affecting such moving parts as gears. propellors,turbines and shafts and their housing and attachments.Stress corrosion is particularly damaging to high-qualitymaterials: stainless steel, nickel-base alloys, alloy steels,aluminium-base alloys, copper-base alloys and titanium-base alloys. Stress corrosion is the subjeet of a seven-part International Standard presently being developed,ISO 7539-1 Corrosion of metals and alloys - Stresscorrosion testing. Part 1: General guidance on testingprocedures has already been published. The remainingparts of this International Standard are in the final stagesor preparation.Other related standardization work currently underwaydeals with determination of resistance to exfoliationcorrosion of high strength aluminium alloys, determina¬

tion of resistance to intergranular corrosion of Solutionheat treatable aluminium alloys, stress corrosion testson welded specimens, aqueous corrosion testing ofzirconium alloys for use in nuclear power reactors, deter¬mination of resistance to stress corrosion cracking ofaluminium alloys, and determination of dezineificationresistance of brass.

Electrochemical corrosion involves at least one elec-trode reaction and applies to stainless steels and somealloys but has little to do with stress except for «servicestress» of the material. Iron, mild steels, alloys steelsand stainless steels can also suffer from «hydrogendam-age» caused by the entry of hydrogen. In InternationalStandard ISO 8044 this is called «hydrogen embrittle¬ment.

Fretting corrosion occurs between «mated surfaces»which are stressed. In ISO 8044 this is defined as a

process involving conjoint corrosion and oscillatory slipbetween two surfaces in contact. The Standard alsopoints out that fretting corrosion may occur at mechani¬cal joints in vibrating structures.Other types of corrosion include chemical corrosion,gaseous corrosion, atmospheric and microbial corrosion,and pitting corrosion, which is corrosion resulting in pitsor cavities extending from the surface into the metal.This type of corrosion is the subjeet of a future Interna¬tional Standard now in the preliminary stages and enti¬

tled Work Item (Wl) 6-026 Corrosion of metals and alloys

- Evaluation of pitting corrosion.

Corrosion testingA corrosion test is carried out to assess the corrosionbehaviour of a metal, the environmetal contamination bycorrosion products, the effectiveness of corrosion pro¬tection, or the corrosiveness of an environment. A Ser¬

vice corrosion test is a test carried out while the equip¬ment or metal is in use («in service»). There are alsosimulated corrosion tests conducted under simulatedservice conditions.Some International Standards in existence or beingdeveloped which have to do with corrosion testing havealready been mentioned. An International Standard oncorrosion fatigue tests is currently in process, as areStandard dealing with corrosion tests in artifical atmos-pheres at very low concentration of polluting gases,corrosion tests in salt spray atmospheres, and generalrequirements for field tests for atmospheric corrosiontesting, as well as general requirements for corrosiontests in artificial atmospheres.Much is obviously being done in the area of InternationalStandards concerning corrosion protection, as defined inISO 8044 Corrosion of metals and alloys - Terms anddefinitions. To conclude by repeating a key passagefrom this Standard, the important thing is to avoidcorrosion damage rather than to prevent corrosion,which in many cases is impossible and sometimes notnecessary.ISO work in corrosion and corrosion testing is part of theon-going international effort to, conserve and protect theenvironment.For further information: ISO Central Secretariat, P.O.Box 56, CH-1211 Geneva