Pickling and passivation are two forms of chemical metal finishing that provide protective properties to metal especially against rust. Actually,pickling and passivation refers metal submerged in a bathing liquid that removes imperfections and rust from the surface of stainless steel.
Where the steel has been heated by welding, or other heat treatments,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 sometimes.Chemical treatments that are correctly performed can clean the surface and ensure the best possible corrosion performance.
Passivation and Pickling are often used together on equipment to remove all contaminates and oxidation, improving the corrosion resistance, extending the life and overall value of your components.
The difference between Pickling and Passivation>>
1,Pickling removes both weld oxide and stainless steel But passivation does not remove weld oxides not attack stainless steel
2,Pickling are typically hydrofluoric, sulphuric (or in steel mills,hydrochloric) acid. It utilizes more aggressive acids than those used in passivation.Passivation are Nitric acid or electropolishing.
3,Pickling Removes surface breaking sulphides and carbon steel.Passivation Removes surface breaking sulphides and, less rapidly, carbon steel smears
4,Pickling Changes appearance as it lowers gloss by etching.Passivation Nitric and chelating treatments should not change appearance.
5,Pickling Usually mixed with nitric acid and with rinsing leaves a passive surface.Passivation will improve the passive film over a simply pickled one.
Standards of Pickling and Passivation>>
ASTM A380, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
ASTM A967 Standard Practice for Chemical Passivation Treatments for Stainless Steel Parts.
These very useful documents give detailed recommendations on many aspects of selection, application and evaluation of these treatments..