Page 29 - 2016 Professional Signage Catalog

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Stainless Steel Passivation
Stainless steels are corrosion-resistant by nature, which might suggest that passivating them would be unnecessary. However, stainless steels are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter (such as grinding swarf) allow water molecules to oxidize some of the iron in those spots, despite the alloying chromium.
What is passivation?
Passivation of stainless steel 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. Stainless steel is already known as being corrosion-resistant, however the passivation process further strengthens its natural coating by improving the exterior surface of the overall part. Passivation creates a hard, monomolecular oxide layer on the stainless steel.
Passivation processes are generally controlled by industry standards, the most prevalent among them today being ASTM A 967 and AMS 2700. These methods use a nitric acid bath (Method 1), or a citric acid based bath (Method 2.)
The passivation process removes exogenous iron, creates/restores a passive oxide layer that prevents further oxidation (rust), and cleans the parts of dirt, scale, or other welding-generated compounds (e.g. oxides).
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