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In years gone by, the process specified was to "pickle and passivate"
- a two step method - in order to create a surface on stainless steel
that would be resistant to corrosion. Today, we differentiate between
the two. It is two separate processes. Pickling (or Chemical Descaling) is done to remove scale. Stainless passivation is done to
make the surface more passive and corrosion resistant.
 Stainless
Passivation
Passivation is a process performed to make a surface passive, i.e., a
surface film is created that causes the surface to lose its chemical reactivity.
Passivation unipotentializes the stainless steel with the oxygen absorbed
by the metal surface, creating a monomolecular oxide film. Passivation
can result in the very much-desired low corrosion rate of the metal.
Passivation is performed when free iron, oxide scale, rust, iron particles,
metal chips or other nonvolatile deposits might adversely affect the metallurgical
or sanitary condition or stability of the surface, the mechanical operation
of a part, component or system, or contaminate the process fluid.
Passivation is performed on clean stainless steel, providing the surface
has been thoroughly cleaned or descaled. Since the term "passivation"
is used to describe distinctly different operations or processes relating
to stainless steels, it is necessary to define precisely what is meant
by passivation.
- Stainless passivation is the process by which stainless steel will spontaneously
form a chemically inactive surface when exposed to air or other oxygen-containing
environments.
Steels containing more than 11% Chromium are capable of forming an invisible,
inert or passive, self-repairing oxide film on their surface. It is
this passive layer that gives stainless steels their corrosion resistance.
If a stainless steel surface is scratched, then more Chromium is exposed
which reacts with oxygen allowing the passive layer to reform. However,
if a particle of carbon steel is embedded in the scratch then the passive
layer cannot reform and corrosion will occur when the metal is wetted
or exposed to a corrosive environment.
- Stainless passivation is the chemical treatment of a stainless steel surface
with a mild oxidant such as citric acid passivation solution. This process is to
accelerate the process noted above in No. 1.
- 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 citric acid passivation solution that will remove the
surface contamination but will not significantly affect the stainless
steel itself.
This is necessary because during handling and processing such as rolling,
forming, machining, pressing, tumbling, and lapping, particles of iron
or tool steel or abrasive particles may be embedded in or smeared on
or into the surfaces of stainless steel components. If allowed to remain,
these particles may corrode and produce rust spots on the stainless
steel. This is due to the formation of a galvanic couple between two
dissimilar metals that can promote a corrosive reaction. To dissolve
the embedded or smeared iron and prevent this condition, as well as
restore the original corrosion-resistant surface, semi-finished or finished
parts are given a 'passivation' treatment.
- Passivation also is accomplished by electropolishing. Electropolishing
is an electrochemical process that is a super passivator of stainless
steel and results in a more passive surface than the other methods mentioned
above.
Stainless Passivation Specifications in use at Delstar:
- ASTM A 967-01, Chemical Passivation Treatments for Stainless Steel
Parts
- ASTM A 380-99, Standard Practice for Cleaning, Descaling, and Passivation
of Stainless Steel Parts, Equipment and Systems
- Federal Specification QQ-P-35C, Passivation Treatments for Corrosion
Resistant Steels
- ASTM B 912-00, Passivation of Stainless Steels Using Electropolishing
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