Understanding the concept of Stainless Steel corrosion and how it occurs in stainless steel installations is important in the problem investigation of this nature for any metallurgy expert to recommend remedial actions to prevent corrosion from happening again.
An initial visual inspection, always suffices followed by an optical microscopy on the suspect samples. Scanning electron microscopy can then be undertaken on metallographic sections. The aim of the optical microscopy is to show if there is any structural details associated with sensitization in the heat affected zone. SEM on the other hand is useful in helping analysis at high magnification imaging and semi-quantitative chemical analysis. SEM can help detect chromium depletion around carbide features in the microstructure. If these are identified then recommendations to help resolve the corrosion problem can easily be adduced.
Stainless steels derive their resistance to corrosion characteristics from their chromium alloying. Chromium promotes the formation of a stable self-healing oxide layer on the surface of the steel. A common cause of corrosion associated with welding of stainless steels is the precipitation of chromium carbides known as sensitisation. Processing can leave the steel with regions with depleted chromium, thus allowing rust formation. The carbides precipitate favourably at grain boundaries depleting the surrounding area of chromium, thus reducing the corrosion resistance.
For stainless steel to resist rusting it requires a minimum amount of chromium. Marine environments accelerate corrosion in these cases.