Pickling and Passivation

 
Pickling
Pickling is a process where a stainless steel object is subjected to some form of acid treatment. The common type acids are usually a mixture of Hydrofluoric and Nitric acid.
This removes the very thin passive or chromium oxide layer which may have been affected by heat from thermal cutting and welding or other mechanical processes such as rolling, bending, guillotining and punching leaving a fully alloyed surface.
 
Passivation
Passivation is the restoration by chemical means of the thin passive layer or chromium enriched oxide layer. This layer gives stainless steels its corrosion resistance. Passivation products are usually Citric or Nitric acid based, there are other formulas that are non-acid based available in the market place.