What causes pitting corrosion?

Carbon and low alloy steels will be subject to bulk corrosion, occurring across most of the exposed surface of a component. However, the inherent corrosion resistance of a stainless steel, nickel alloy or copper alloy prevents such gross corrosion, due to the formation of a passive film as soon as these alloys are exposed to oxygen. Therefore, the prevalent form of corrosion in such corrosion resistant alloys (CRA) is pitting corrosion which will initiate at discrete points, usually where there is a weakness in the passive film or where conditions are sufficiently aggressive to attack it.

There are a number of underlying reasons for the particular site of pitting corrosion. It could coincide with the location of an inclusion, which is a compound formed during the steelmaking process. As the inclusion does not contain chromium or molybdenum, it therefore cannot form a protective passive film. Mechanical damage to the component should not ordinarily result in corrosion, as the passive film regenerates almost immediately. But if the working environment has an absence of oxygen, then re-passivation of the surface after damage may not occur or quickly enough.

Pitting corrosion can occur where the conditions are suitably aggressive, such as in acidic solutions or in the presence of chlorides i.e. seawater that encourage the attack of the passive film. Elevated temperatures increase the speed of this corrosion reaction.

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