Pickling removes a thin layer metallic layer from the steel surface. A mixture of nitric and hydrofluoric acid is usually used to pickle stainless steel. Pickling aims to remove tarnish from welded constructions, in an area where the chrome content of the steel surface is reduced. Even slight scaling, such as occurs after heat treatment, can mostly be removed by pickling.
The passive layer is removed by mechanical processing, and the layer is formed again by an oxidising acid treatment. In contrast to pickling, the passivation treatment does not remove any material, rather the nature and thickness of the passive layer are specifically optimised. The pickling and passivation are not carried out simultaneously, but one after the other by separate acid treatments.
The nitric acid used has only a slight pickling effect on stainless steel and essentially serves to passivate it
For many applications, it is also necessary to electrochemically polish the stainless steel components before passivation (E-polishing). This combination leads to even better corrosion protection.
Stainless steel pickling and stainless steel passivation