What is passivation? ASTM A380 describes passivation as “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.” Additionally, it 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.”
There are generally two opinions as to why passivation works; one credits its effectiveness because it is a cleaning process, the other because of the development of a thin transparent oxide film, which results from chemical passivation.
In terms we can all understand, passivation removes “free iron” contamination left on the surface of stainless steels collected during machining and fabrication.
“Free iron contamination left on the surface is a potential site for corrosion, ultimately resulting in the pre-mature deterioration of the component if not removed. During the passivation process not only is the surface cleaned, but the process facilitates the formation of a thin chromium rich, transparent oxide film that protects the surface from corrosion.”
Passivation is not cleaning, and it is not a protective coating, but it is actually a combination of both, enhancing the corrosion resistance of stainless steel.
Before passivation the process begins with a general cleaning of the product removing all oils, greases, forming compounds, lubricants, and cutting fluids left behind from machining and fabrication. After removal of organic and metallic residue, the parts are put into the appropriate passivation solution. Although there are many variations, the over-whelming choices are narrowed to two, nitric acid and citric acid base solutions.
The three major variables that must be controlled during passivation are time, temperature, and concentration. Let us explain why.
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