Stainless steels are corrosion-resistant, but they are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter allow water molecules to oxidize some of the iron in those spots despite the alloying chromium. The pickling and passivation process thus help to remove those impurities and prevent further damage to the metal.
Pickling: Pickling is a process used to remove weld heated tinted layers from the surface of stainless steel fabrications. It provides protection to the metal surface against corrosion, pitting, and surface damage.
Passivation: Passivation process improves the corrosion resistance of the stainless steel. It removes the surface contaminants like iron and carbon impurities