Passivating Stainless Steel Parts

Procedures for commonly used stainless steels can use citric or nitric acid solutions.

Maximizing the natural corrosion resistance of parts and components machined from stainless steels. It can make the difference between satisfactory performance and premature failure. Incorrectly performed, passivation can actually induce corrosion.

The process of passivation is a postfabrication method of maximizing the inherent corrosion resistance of the stainless alloy from which the workpiece was produced. It is not a treatment to descale or coat the piece. While there is no consensus on how the process works, a protective oxide film occurs naturally on the surface of passive stainless as the result of exposure to oxygen in the atmosphere. Any clean, freshly machined, polished, or pickled stainless steel part automatically acquires this protective film. Under ideal conditions, it covers all surfaces of the part. The film is extremely thin — just 0.0000001" thick.

During the machining process, contaminants such as shop dirt or particles of iron from cutting tools can be transferred to the surface of the stainless steel parts. Under certain conditions, this contamination can produce a thin coating of rust on the part. Sometimes the crevice where a particle of tool steel is embedded in the surface of the finished part can propagate a corrosive attack on the part itself. Similarly, small particles of iron-containing shop dirt may adhere to the part surface. If not removed, these particles can reduce the effectiveness of the original protective film.

Exposed sulfides also can be a problem. Sulfides improve an alloy’s ability to form chips that break away cleanly from a cutting tool. Unless the part is properly passivated, sulfides can act as initiation sites for corrosion on the surface of the fabricated part.

In both cases, passivation is needed to maximize the natural corrosion resistance of the stainless steel. It can remove both surface contamination and sulfides. A two-step procedure — cleaning, followed by an acid bath — provides the best possible corrosion resistance.

Cleaning should always come first. Machining chips or other shop dirt can be wiped carefully from the part. A commercial degreaser or cleanser can remove machining oils or coolants. Grinding or acid pickling can remove thermal oxides and other foreign matter.