Common Causes of Corrosion Associated with Welded Stainless Steels: Intertek study
Corrosion occurred in a client’s stainless steel products. Intertek metallurgy experts recommended actions to prevent corrosion from happening again.
Following initial visual inspection, Intertek conducted optical microscopy on the suspect samples. Scanning electron microscopy was performed on metallographic sections. Optical microscopy showed structural details associated with sensitisation in the heat affected zone. SEM provided very high magnification imaging and semi-quantitative chemical analysis. SEM detected chromium depletion around carbide features in the microstructure. Recommendations were made to the client which helped resolve the corrosion problem.
Stainless steels obtain their stainless (non-rusting) characteristic as the result of alloying additions of chromium. 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.
http://www.intertek.com/metallurgy/stainless-steel-corrosion
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