The Passivation Process
What is passivation and what are the key benefits? Passivation is a process to make certain metals (alloys) more resistant to rusting (sometimes called staining) than they would otherwise be on their own.
This is a process, not a coating: it increases the part’s own ability to resist rusting. It works on some grades of steel – those grades commonly called stainless steels.
They are called stainless because they stain (or rust) less than other grades, but they can still stain (rust). What passivation does is to increase their resistance to staining (rusting).
In theory, this process can be applied to almost any metal. In practice, it is normally applied to stainless steels and titanium.
The process consists of aggressively washing, rinsing and then soaking the parts under controlled conditions in an acid solution followed by multiple rinses. It does not apply anything to the surface of the part; instead, it is removing microscopic specs of free iron from the surface.
There are several standards describing the precise details of the process. ASTM International, an international standards organization, publishes several passivation standards including ASTM A380, ASTM A967, ASTM B 600, and ASTM F 86. The first two are frequently used by general industry while the last two are normally used by the medical industry.
SAE International is a professional association and standards developing an organization that was previously known as the society of automotive engineers. They publish a passivation standard, AMS 2700 which is frequently used by the aerospace industry.