"Corrosion resistance of Stainless Steel"

Question:

"I have a problem with a stainless product which is corroding. It has been suggested that the user may have washed it in an acidic cleaning product thereby "damaging" the steel's corrosion resistance. I have limited background in materials (electronics graduate), but from what I understand, stainless gets it's corrosion resistance from it the constituents of it's alloy rather than from any special surface preparation. I understand that it is the naturally forming oxide layer that gives it the corrosion resistance, but am I correct in thinking that if this layer were removed that a new one would form an the steel's corrosion resistance would remain? I will ask for the full specification of the steel we are being supplied to determine if it is appropriate for the application. Has anyone any suggestions on how I should follow this up ? Does the acidic cleaner theory hold any water."

Answer:

You are correct in assuming that stainless steels obtain their corrosion resistance from alloying content, particularly chromium and to a lesser extent molybdenum for grades containing this elememt. Normally when stainless steel corrodes, it suffers localized corrosion, as it rarely loses its passive film over the entire surface, unless the solution is very aggressive, i.e., a strong inorganic acid like hydrochloric acid. When it is removed from the corrosive environment it will reform the passive film, if there is sufficient oxygen potential, and normally there is. Surface finish can affect corrosion resistance to some degree, with smoother finishes normally increasing corrosion resistance, especially in atmospheric environments. Stainless steel can spot rust, if iron comtamination has been embedded in the component during processing. This is one reason for pickling in nitric acid, to remove the iron and passivate the surface (based on nitric acid's stong oxidizing potential). Based on your description, I doubt that the acid cleaner has permanently damaged the corrosion resistance of the stainless steel.

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