Passivation is a step often taken to maximize the corrosion resistance which is inherent in stainless steel. When done incorrectly, the process can actually induce corrosion rather than prevent it.
The stainless steel component is placed into a bath of passivation chemicals; passivation is not scale removal nor is it similar to painting. The process induces a protective film of oxide on the stainless steel. The thickness of this film is less than 1/100,000 the thickness of a human hair.
Under ideal conditions, a spotlessly clean, straight from machining and polishing can acquire this passive film from exposure to the air. In practice, however, the part will be contaminated with organic and metallic materials that were transferred to the part while it was being produced. All of these particles must be removed otherwise the film will not be homogeneous and cover every part of the component surface.
What happens during machining?
When the stainless steel is being machined, the tools that are employed are usually high-carbon steel. During the process of manufacture, microscopic particles can be transferred from the tool steel to the surface of the stainless steel component. There are times when a freshly manufactured stainless component picks up a hue of rust, this is not the base metal that is rusting; it is the iron from the tool that was used.
Minute particles of dirt containing iron can be picked up from many surfaces in a machine shop and even though the component is wiped clean, it can still show surface rust once exposed to the air. The sulfides which are the result of chemicals used in the production of the stainless steel will act as sites for the initiation of corrosion if the part is not passivated.
The passivation process is in two steps, cleaning and an acid bath.
Cleaning is important:
Any evidence of coolants, grease or oil that was deposited on the part during manufacture must be removed. A commercial degreasing solution is used either as a bath if the part is small or wiped on when the part is large. It is a misconception that cleaning of the surface contaminates will take place when the part is being passivated in acid, this will not happen. The greases react with the acid in the bath and form bubbles of gas, which collect on the surface of the part and retard passivation.
Once thoroughly cleaned, the part is immersed into a chemical bath. The acids used can be either nitric, nitric acid with sodium dichromate or citric acid. Depending on the type of stainless; the time in the bath, the temperature of the bath and composition of the chemical bath are taken into account.
When passivation chemicals are used for the process the results are not optimum. The best possible way to passivate stainless steel is by subjecting it to Electropolishing at New England Electropolishing.