PASSIVATION USING NITRIC ACID AND CITRIC ACID
Stainless steel derives its corrosion-resistant properties from its chromium content. Chromium, in the presence of oxygen, forms a thin, hard, adherent film of chromium oxide on the surface of the alloy. It is this chromic oxide layer, inert (passive) to the surrounding environment, which gives stainless steel its corrosion-resistant properties.
Passivation, according to 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 number and type of industries that have successfully implemented citric acid into their passivation procedures.