Pickling and Passivation
Pickling can remove the oxide layer on the surface of the workpiece. Passivation is the formation of a new chromium-rich protective layer on the surface of the workpiece. If it is not treated after pickling, it tends to be more prone to rust. After passivation, the anti-oxidation ability can be greatly improved.
Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove grease or oil.
