Nitric Acid Passivation
Stainless steel derives its corrosion resistant properties from its chromium content. The chromium, in the presence of air, especially 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.
ASTM A380 describes passivation as “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 itself” ….. and “the chemical treatment of 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 a chromate salt. Nitric acid is by far the most accepted means by which passivation is performed.
This Case History is of a Nitric Acid Passivation System consisting of four stages comprising four MK Series units as shown above. The line runs right to left. The Nitric Stage is isolated with connecting conveyor section to the right and left and includes a compatible ventilation system that prevents the migration of nitric fumes to the adjacent and surrounding areas.
Due to the nature of Nitric Acid and its fumes the electrical and pneumatic systems are built for corrosive environments. This includes Nema 4X panels and electrical components as well as pneumatics specifically designed for this application. A containment pan for the Nitric Stage is also included