Pickling, Electropolishing, and Passivation
Passivity is the state in which stainless steels are chemically inert due to the chromium oxide film that naturally forms on the surface of the metal. During fabrication, this oxide layer can be broken and the steel can be contaminated.
Passivation is defined by ASTMA380 as "[the] removal of exogenous iron or iron compounds from the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself." Sometimes oxidants are used to promote the regeneration of the oxide layer after the removal of the free iron. Passivation will not restore the corrosion resistance of welded areas or areas where carbon steel has become embedded into the stainless steel.
Picking is a chemical cleaning process that removes the surface layer of the stainless steel. Electropolishing is an electrochemical process for removing the surface layer of the stainless steel. Electropolishing is often thought of as electroplating in reverse. Both pickling and electropolishing can be used to remove the heat scale associated with welding, and will remove the chromium depleted layer under the heat tint. After pickling or electropolishing, the oxide layer can be regenerated naturally or passivation can be specified.