Advanced Deburring & Finishing

Passivation - Nitric Acid and Citric Acid

Stainless steel derives its corrosion resistant properties from its chromium content. The chromium, in the presence of air, expecially oxygen, forms a thin, hard adherent film of chromium oxide on the surface of the alloy. It is this chromic oxide layer that is inert (passive) to the surrounding environment and gives stainless steel its corrosion resistant properties.

 

Passivation, according to the ASTM A380, is "the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film."

 

Simply stated, utilizing a mild oxidant, such as a mineral or organic acid solution, to promote the removal of excess iron from the surface of the stainless steel will enhance the formation of a chromic oxide layer thereby enhancing  its corrosion resistant properties.

 

Typically, passivation is performed with a nitric acid bath from 20 to 50% by volume. Temperatures range from ambient to 160 degrees F. Immersion times can be as long as two hours and even include the addition of chromate salt. Nitric acid is by far the most accepted means by which passivation is performed. Recently however, there has been an increase in the amount of industries that have successfully implemented citric acid into their passivation proceedures.