The acids used in passivation is either citric acid or nitric acid. The chemical solutions in pickling are much aggressive than the acids used in passivation of stainless steel. In pickling, it uses the mixtures of hydrofluoric and nitric acids
ASTM A380 'Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment and Systems' defines pickling as a chemical treatment using strong, corrosive acids that remove metallic contamination, welding and heat-treatment scales
In stainless steel, the passivation process uses nitric acid or citric acid to remove free iron from the surface. The chemical treatment leads to a protective oxide layer, or passivation film, that is less likely to chemically react with air and cause corrosion. Passivated stainless steel resists rust
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