Stainless steel castings can exhibit what appears to be rust on their surface under specific conditions.  This type of appearance indicates the presence of iron on the surface of the part.  Contamination is simple with typical sources from dust in the air, contact with a plain carbon container, grinding or lapping.  It usually does not show until after exposure of the parts to moisture when it then looks like the stainless steel castings are “rusting”.

To address this problem, castings are acid passivated.  This term refers to immersion in a strong Nitric Acid solution.  The passivation process not only removes any associated iron but also assures complete oxidation of the surface Chromium that is the source of the corrosion resistance of stainless steels.  Although a foundry can acid passivate before shipment, best practice calls for passivation after machining.

This treatment should not be confused with Pickling which is a term used for Sulfuric Acid cleaning of carbon and low alloy sheet products.  An alternative can be to sand blast to clean a surface but care must be exercised that the blast media is not contaminated from an iron source of previously blasted parts.