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I have to agree with RickM. The surface treatment otherly known as chemical conversion is a Dichromate treatment (also known as pickling). There are many different treatments depending on desired results that may be used. It is used to prevent corrosion. There are several MIL or AMS specs that apply to these processes.
One type of treatment put in simple form is an acid solution (usually Nitric) mixed with Sodium Dichromate, and a small percentage of either Sodium, potassium or ammonium acid fluoride. The acid etches the surface and the dicromate bonds to the surface leaving a protective coating.
Depending on the required end result sometimes these treatments are preceeded by an Anodic treatment. This is accomplished as a one step treatment employing an aqueous ammonium bifluoride bath in which alternating high voltage currently generally produces a uniform clean white or yellowish appearence. The color can vary based on the chemical composition of the alloy and the hardness. Usually after this type of treatment the parts are dipped in a boiling chromic acid pickle solution for 1 to 15 min and then rinsed in cold water.
I do not work with steel parts so I do not know the exact application for steel. However this works on Magnesium and the process for steel varies but is similar.
I hope this helps.
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