Theory states that the non-rusting properties of stainless steels are due to a very thin, invisible oxide film which completely covers the surface of the parts thereby preventing corrosion. A freshly machined, polished or pickled article will acquire this film rather quickly from contact with the atmosphere. However, such fabricated parts may be contaminated with small particles of foreign matter, such as iron particles, which should be removed so that full stainless properties of the surface are obtained. An example of this is the slight amount of steel worn off the cutting tools and transferred to the stainless during machining. Under certain circumstances, a thin coating of rust may appear on the part. This is corrosion of the embedded tool steel and not the parent metal.
The primary purpose of a passivating treatment is to remove surface contamination so that the optimum corrosion resistance of stainless steel will be maintained. Passivation is not a scale removal treatment and it is generally desirable to pickle fabricated stainless steel prior to placing it in service. Pickling may also provide sufficient passivation. ASTM A380 provides an authoritative source of information on pickling and passivation. Passivation should be preceded by degreasing followed by a thorough water rinse.