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According to ASTM A380, passivation is “the removal of exogenous
iron or iron compounds from the surface of stainless steel by means
of 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.”
In lay terms, the passivation process removes “free iron”
contamination on the surface of stainless steel from machining and
fabricating. These contaminants are potential corrosion sites that
result in premature corrosion, and ultimately result in the deterioration
of a 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?
It is a combination of both.
This invisible free iron film can be the result of improper cleaning
from the mill, contamination from storage, or, in the case of finished
components, traces originating from grinding and machining tools
used in manufacturing. As these tools wear, metal particulate are
being transferred to, or embedded in the surface of stainless steel
parts. Although the iron is not visible to the naked eye, when exposed
to adverse atmospheric conditions, such as high humidity, it results
in detrimental oxidation of stainless steel components. Sulfides
are another contributing factor for corrosion. Sulfur is added to
some alloys to increase machinability. If not removed prior to service,
the Sulfur can initiate sites for localized corrosive attacks.
To remove free iron or sulfides from the surface of a part, surface
finishers passivate (immunize) the component using a variety of
different solutions containing nitric acid and oxidizing salts.
By adjusting the concentration, dwell time, and temperature of these
cleaning baths, the iron or sulfides are dissolved without stock
removal of the substrate alloy. The 300 Series, 400 Series, and
Precipitation-Hardening (P.H.) stainless steels all require different
bath conditions for optimal removal of these trace elements.
Passivation (immunization) is used in a variety of industries to
include aerospace fuel system controls, medical implants, marine
fasteners, and drive components for computers. In any application
where corrosion resistance is important to function, passivation
should be considered. The use of passivation is relatively inexpensive
and enhances the life of the component.
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