"Chemical Corrosion of 304 Stainless Silo"

Question:

"Due to a failure of our monitoring systems we have recently encountered a chemical corrosion of a 230 M3 304 Stainless steel silo. During the CIP process there appeared to be a failure on the water flushing step between the Triplex Acid step (UN 3264, Hazchem 2PE) and the Sodium Hypochlorite (UN 1791 Hazchem 2R).
The visual outcome of this failure was a dark brown staining of the contact surfaces. We decided that the surface required to be re-passivated. This was carried out with a 35 % Acid solution. On closer inspection after the passivating process we have found that a chemical corrosion has taking place which has resulted in deep pits and in four area's holes in the base. My question to the forum is do you think the parent metal in the general area of these holes will be corroded beyond the point where repairs can take place? ie sensitised."

Answer:

Sensitization is the process whereby chromium carbides in austenitic stainless steels migrate to grain boundaries at temperatures between ~ 800F-1600F. The chromium depleted areas next to the grain boundaries are susceptible to accelerated corrosion in certain corrosive media. Corrosion of your silo had no influence on sensitization of the base metal. Sensitization is simply a function of the thermal history of the SS. It was either sensitized prior to the incident or not. Sensitization also has no influence on whether or not you can affect repairs to the silo to replace the corroded areas.

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