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.
Passivation, according 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 itself.” ASTM A380 also describes passivation as “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.”
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.
The following Case History is of a Nitric Acid Passivation System for precision aerospace components. The system consists of five stages comprising five RAMCO MK Immersion Washers. The line runs from right to left. Ventilation ductwork is provided for all wet stages. Roller conveyor sections are on each platform and in between stages as required to provide smooth lateral transfer of baskets from stage to stage. The materials of construction include 304 stainless steel , 316L stainless steel, Alloy 20Cb-3 heating elements and Nema 4X control panels (see below).
WASH – The parts are batch loaded into 18”L x 18”W x 6”H stainless steel baskets and placed on the first transport elevator in Stage One. Using a high alkaline detergent at 170 degrees F the parts are washed using vertical agitation.
RINSE – The basket is laterally transferred over special platform roller conveyor sections onto Stage Two. Using hot water at 150 degrees F the parts are rinsed using vertical agitation.
PASSIVATION USING NITRIC ACID – The basket is laterally transferred over special platform roller conveyor sections on to Stage Three. Using a 25% solution of nitric acid at 135 degrees F the parts are passivated using vertical agitation.
RINSE – The basket of parts is then laterally transferred over platform roller conveyor to Stage Four. Using DI water at 150 degrees F the parts are rinsed using vertical agitation.
HOT AIR BLOWOFF DRYING – The basket of parts is then laterally transferred over platform roller conveyor to Stage Five. Using hot air at 180 degrees F the parts are dried. RAMCO Hot Air Knife Blow-Off Dryers get maximum evaporation providing rapid drying without excessive heat.