Passivation of STAINLESS STEEL - stainless steel surface treatment - Brief Article - Technical

Automotive Finishing, Fall, 2000 by Dan Englebert

Getting the properties you paid for...

The conversation usually begins like this: "Hey, this is Joe from Joe's Machine Shop. We have a job in here and the customer wants us to have some kind of passivate coating something or other. Do you guys do that? How thick is that stuff? Is that like plating, paint or what? What color is it? How much tolerance should I allow for it?" The opening statement usually ends with a phrase like: "I don't even know why they need it. What is the point of using stainless steel if you are going to put some kind of coating on it anyway?"

Joe is not the exception. Many machine shops, purchasing agents and engineers are somewhat in the dark when it comes to the relationship between corrosion resistant (stainless) steel and chemical passivation. Even among the finishing community, there is some disagreement about the theory behind the process of chemical passivation. Some believe it is effective because it is a cleaning process. Others credit the enhanced corrosion resistance properties to the thin, transparent oxide film resulting from chemical passivation.

Regardless, the bottom line is that it works. Verification tests, including copper sulfate immersion, and accelerated corrosion tests, such as salt spray, high humidity and water immersion, undisputedly confirm the effectiveness of chemical passivation. Advanced material engineers in aerospace, electronics, medical and similar high-tech industries have used chemical passivation for years. The applications demand the maximum performance from components manufactured from corrosion-resistant steels, and they realize that passivation is one of the most effective methods of achieving these results.

What is passivation?

According to ASTM A38O, passivation is "the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself." In addition, it also describes passivation as "the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film."

In lay terms, the passivation process removes "free iron" contamination left behind on the surface of the stainless steel from machining and fabricating. These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed. In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation (corrosion). So what is passivation? Is it cleaning? Is it a protective coating? It is a combination of both.

How is passivation performed?

The process typically begins with a thorough cleaning cycle. It removes oils, greases, forming compounds, lubricants, coolants, cutting fluids and other undesirable organic and metallic residue left behind because of fabrication and machining processes. General degreasing and cleaning can be accomplished many ways, including vapor degreasing, solvent cleaning and alkaline soaking.

After removing organic and metallic residues, the parts are placed into the appropriate passivation solution. Although there are many variations of passivating solutions, the overwhelming choice is still the nitric-acid-based solutions. Recently, there has been substantial research performed to develop alternative processes and solutions that are more environmentally friendly, yet equally effective. Although alternative solutions containing citric acid and other types of proprietary chemistry are available, they have not been as widely accepted commercially as nitric-acid-based solutions.

The three major variables that must be considered and controlled for the passivation process selection are time, temperature and concentration. Typical immersion times are between 20 mm and two hours. Typical bath temperatures range between room temperature and 160F. Nitric acid concentration in the 20 to 50% by volume range is generally specified. Many specifications include the use of sodium dichromate in the passivation solution or as a post passivation rinse to aid in the formation of a chromic oxide film. Careful solution control, including water purity, ppm of metallic impurities and chemical maintenance, are crucial for success.

The type of stainless steel determines the most effective passivation process. Bath selection (time, temperature and concentration) is a function of the type of alloy processed. A thorough knowledge of the material types and passivation processes is paramount to achieving the desired results. Conversely, improper bath and process selection and/or process control will produce unacceptable results. In extreme cases, this can lead to catastrophic failure, including extreme pitting, etching and/or total dissolution of the entire component.