Finishing: Passivation

pressure-water-line-509870_1280What is Passivation?

The passivation process is used to produce a thin layer on the outside of metal, usually stainless steel, in order to “passivate” (or protect) it against environmental factors which can cause corrosion. Passivation can occur spontaneously in nature, or can be expedited by applying a protective material, such as metal oxide.

The Passivation Process

Parts are cleaned to remove any debris and oils left by previous machining. Thorough cleaning is important in order to avoid the formation of gas bubbles which can interfere with the passivation process. The part is then placed in a passivating acid bath, usually using nitric or citric acid, for a specified length of time (usually ranging from 20 minutes to 2 hours) and temperature (ranging from room temperature to 160 degrees F). The acid removes free iron from the surface and forms a thin, transparent oxide layer. When exposed to air, a chromic oxide layer is formed offering passive properties. The part is then rinsed, dried, and tested using various methods, commonly applying high humidity and heat.

 

Illustration of two industrial pipes crossing each other

Benefits of Passivation

The passivation process offers benefits such as:

  • Corrosion resistance
  • Smooth, polished finish
  • Product durability