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.
