| Passivation 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." In addition, it
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."
What is passivation?
In lay terms, the stainless steel passivation process removes "free
iron" contamination left behind on the surface of the
stainless steel from machining and fabricating. These contaminants of stainless steel
are potential corrosion sites that result in premature corrosion
and ultimately result in deterioration of the component
if not removed. In addition, the passivation process facilitates
the formation of a thin, transparent oxide film that protects
the stainless steel from selective oxidation (corrosion).
So what is passivation? Is it cleaning? Is it a protective
coating? Passivation is a combination of both.
How is passivation performed?
How is passivation performed? The passivation process typically begins
with a thorough cleaning cycle. It removes oils, greases,
forming compounds, lubricants, coolants, cutting fluids
and other undesirable organic and metallic residue left
behind because of fabrication and machining processes. General
degreasing and cleaning can be accomplished many ways, including
vapor degreasing, solvent cleaning and alkaline soaking.
Typical immersion times are between 20 min and two hours.
Typical bath temperatures range between room temperature
and 160F. Nitric acid concentration in the 20 to 50% by
volume range is generally specified in the passivation process. Careful solution control,
including water purity, ppm of metallic impurities and chemical
maintenance, are crucial for stainless steel passivation success.
Effective Passivation Process
The type of stainless steel determines the most effective
passivation process. Bath selection (time, temperature and
concentration) is a function of the type of alloy processed.
A thorough knowledge of the material types and passivation
processes is paramount to achieving the desired results.
Conversely, improper bath and process selection and/or process
control will produce unacceptable passivation results. In extreme cases,
this can lead to catastrophic failure, including extreme
pitting, etching and/or total dissolution of the entire
component. |
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