Pitting corrosion is a localised form of corrosion which leads to the formation of cavities or holes on a metal’s surface. Unfortunately, it is a non-reversible process. This means that once corrosion has begun, you can only deal with the consequences and it cannot be returned to its original state. In saying that, approaches may be taken to repair it.
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. This is therefore typically limited to carbon steels for instance, where corrosion leads to an overall thinning of the metal.
Changes in operating conditions or the incidence of severe pitting or crevice corrosion can significantly reduce the anticipated working life. Pitting corrosion is non-uniform, sometimes more difficult to fully inspect for, and will increase at an accelerated rate once initiated – the conditions within a corrosion pit tend to become more and more concentrated, encouraging even more rapid corrosion to occur.
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.
To prevent pitting corrosion occurring in the future, it is worth looking into corrosion resistance alloys. At Langley Alloys, we have a wide range of high strength and corrosion resistant alloys such as Ferralium 255 and Hiduron 130. If you have any more questions about corrosion resistant alloys, please contact us today. A member of our team will be more than happy to help and can also advise you on the best option for your application.