Pickle and Passivate are acid chemical treatments for stainless steel, to remove any contaminants, scaling or discolouration. Together they produce a clean and chemically passive oxide surface film ensuring resistance to corrosion.

Stainless steels cannot be passivated unless the steel surface is clean and free from contamination and scale from welding operations.

Scale may therefore need to be removed first by 'pickling' (or mechanical abrasion) and although the surface of freshly pickled stainless steel will normally be immediately passivated once the pickling acid has been washed off, these two treatments are not the same.

Pickling usually involves nitric/hydrofluoric acid mixtures, whereas, traditionally passivation has been done using only nitric acid. Nitric acid alone can also be used to remove any light surface iron contamination after which the acid facilitates the passivation of the cleaned steel surface.