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Passivation of Stainless Steel

   Passivation describes the treating of a metal with a mild oxidant (such as nitric acid) to remove
surface iron or iron compounds by dissolution.  This action forms a protective passive film on the surface of the metal.  The trace iron left behind from machining and fabrication can provide sites for corrosion if left untreated.
    The process is performed by first cleaning the surface with solvents or an alkaline solution to remove organic or metallic residues.  Next, the parts are placed into passivation solutions.  The choice for this solution is usually nitric acid.  The process is controlled by three key variables:  time, temperature, and concentration.

Typical Range of Key Variables
Time:  20-120 minutes
Temperature: 60 - 160 0F
Concentration: 20 - 50% Nitric Acid by volume

    The key variables are typically determined by the type of stainless steel alloy being processed.  For example, 316 stainless steel would have different conditions than 317L stainless steel.
    After the passivation bath, a sodium dichromate bath is often used to promote the formation of a chromic oxide film.  After this final treatment, the sample is placed into a copper sulfate solution.  Any remaining iron will show up as pink spots on the sheet.  This would be considered unacceptable.  Other testing methods can include a two hour salt spray or a 24 hour high humidity test.

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