BASICS OF CORROSION - STRESS CORROSION CRACKING (SCC)
What is SCC? SCC is the conjoint action of stress and a corrosive environment which leads to the formation of a crack which would not havedeveloped by the action of the stress or environment alone. Why is it a problem? Because, it can happen ‘unexpectedly’ and rapidly after a period of satisfactory service leading to catastrophic failureof structures or leaks in pipework. Where does it occur typically? Typical SCC failures are seen in pressure vessels, pipework, highly stressed components and in systems when an excursion fromnormal operating conditions or the environment occurs. Where do the stresses come from? The stresses that cause SCC are either produced as a result of the use of the component in service or residualstresses introduced during manufacturing. Where does the corrosive environment come from? The environment is either the permanent service environment i.e. sea water or a temporary one caused by operationssuch as cleaning of the system which can leave a residue, or if the stress is applied during the operation initiate cracking. How is this different from ‘normal’ corrosion? SCC is a corrosionmechanism that requires the pairing of a material with a very particular environment and the application of a tensile stress above a critical value. Corrosion can
occur in otherenvironments without SCC. Examples of well-known material/environment pairs are: MATERIAL Brass Stainless steel High strength steels How can SCC be controlled? By selecting a material that is notsusceptible to the service environment and by ensuring that any changes to the environment caused by cleaning etc are not detrimental. By controlling the service stresses through careful design and minimisingstress concentrations to keep them below the critical value. Residual stresses can be reduced by heat treatments and careful design for manufacturing. By using corrosion inhibitors during cleaning...
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