Passivation

Passivation improves the corrosion resistance properties of components made of precipitation-hardened, austenitic, ferritic and martensitic steels. Passivation treatments improve the surface condition of stainless steel by dissolving iron that has been embedded in the surface during forming or machining. If allowed to remain, the iron can corrode and give the appearance of rust spots on the stainless steel.

Passivation, which consists of immersing stainless steel components in a solution of nitric or citric acid without oxidizing salts, will dissolve the imbedded iron and restore the original corrosion-resistant surface by forming a thin, transparent oxide film.

After passivation processing, parts are submitted for inspection. Depending on the specification, one of the following methods will be used:

Water Immersion
Parts are immersed in a specialized water test tank for 28 hours to verify the passivation process on all grades of stainless steel, with exception of 440C. After testing, speciments shall show no signs of rust or corrosion.

Copper Sulfate
Verifies the passivation of austenitic stainless steel in the 200 and 300 series. The parts shall show no signs of copper deposit after the testing solution is swabbed onto the parts and allowed to dwell for 2 minutes.

Salt Spray
Parts are placed into a closed chamber into which a solution of sodium chloride is atomized, creating a corrosive environment. The test is performed for 2 hours, and after testing, parts shall show no signs of rust, stain or corrosion.

If parts are deemed unacceptable by the inspector, NTP customer service is notified to alert the customer.

After testing, parts are moved to the next operation. Typically, finish application or final inspection.