How to repair pitting corrosion

Pitting corrosion is a non-reversible process i.e. once corrosion has begun you can only deal with the consequences and not return it to its original state.

Given the prevalence of corrosion in aggressive environments, a corrosion allowance, permissible weight loss or component thickness reduction is often incorporated within the original design specification. If a component is expected to operate for 10 years continuously, and corrode at a rate of up to 0.5mm per year, then a corrosion allowance of 5mm extra metal is added to the standard design. This pragmatic approach can serve many applications well, if they are not particularly safety-critical. However, the factors used assume a uniform and predictable rate of corrosion will occur. Changes in operating conditions or the incidence of severe pitting or crevice corrosion can significantly reduce the anticipated working life, so regular inspection is required.

Considering the excess metal that must be incorporated in components, plus the costs of inspection, means that limiting the likelihood and incidence of corrosion through better metal specifications, process design and operating conditions can all be valuable exercises.

If a component is still structurally sound, pitting corrosion can be mechanically removed by grinding or polishing and the missing metal replaced by weld build-up. Care must be taken to prepare and undertake the welding process carefully, to avoid reducing the properties of the surrounding metal.

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