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Carbon on stainless

Can anyone explain to me exactly what is happening to stainless when it is contaminated by carbon. I know this causes corrosion, but what is happening on a molecular level. How is the structure of the stainless changing and why?

Brian Francis
- Dallas, Texas
2002

Hi Brian!

I assume that you're referring to what is called 'sensitization' of stainless steels, which can happen during welding, heat treating, or other high temperature processing. If so, remember that stainless steel gets its corrosion resistance from the thin layer of chromium oxide on the surface, and that you need a minimum of about 10.5 % chromium for this to happen. The 'sensitization' mechanism is carbon actively combining with the chromium in the stainless steel to form chromium carbides. When the chromium is tied up as chromium carbide, it cannot form the chromium oxide layer, and the corrosion resistance of the stainless steel is compromised.

Lee Gearhart
metallurgist - E. Aurora, New York
2002

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