Technical Information about Stainless Steel - Pickling and Passivation

Stainless steel can corrode in service if there is contamination of the surface. Both picling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assis the formation of a continuous chromium-oxide, passive film.

Pickling

Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means.

Where the steel has been heated, by welding, heat treatment and other means, to the point where a coloured oxide layer can be seen, there is a chromium depleted layer on the surface of the steel underneath the oxide layer. The lower chromium content gives lower corrosion resistance. To restore the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface. Mechanical removal may leave abrasive or other particles embedded or may be impractical, so chemical means are usually employed.

Passivation

Passivation is the treatment of the surface of stainless steels, often with acid solutionsk, to remove contaminants and promote the formation of the passive film on a freshly created surface.

Common passivation treatments include nitric acid solutions or pastes which will clean the steel surface of free iron contaminants. Care must be taken in selecting and using passivation treatmenets to ensure the selected treatmenet will target the contaminant. Passivation will also aid in the rapid development of the passive oxide film on the steel's surface. Passivation deos not usually result in a marked change in appearance of the steel surface.

Both pickling and passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel.