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The question is often asked, what is passivation, and why is it so important to perform on stainless steels?
What is Passivation?
In simplest terms, in order to protect against corrosion of stainless steels, we must remove free-iron ions and other potentially corrosive elements from the surface. In addition, the passivation process provides a thin, transparent oxide film that protects the stainless steel from further oxidation or corrosion.
The type of stainless steel being treated determines the most effective passivation process and bath chemistry. Within the bath, control of process variables such as time, temperature and concentration are paramount. Improper bath and process selection and/or process control will produce unacceptable results, including the potential for pitting and etching of the surface and possible partial or total dissolution of the entire part.
Passivation Process Parameters
Nitric acid is typically used for passivation of the various grades of stainless steel in concentrations of 20-50% (by volume) range. Typical immersion times in the bath are between 20-120 minutes. Typical bath temperatures range between room temperature and 70°C (160°F).
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 critical for success.
How Heat Treating Plays a Role
Perhaps the most overlooked variable in the entire passivation equation is the negative impact of heat-treating practices. Poor machining can also play a significant role. Cross-contamination introduced by these manufacturing steps results in a failure to properly passivate and resultant corrosion.
The following practices will reduce cross-contamination during manufacturing and increase the chances of successful passivation and tests results.
- Thorough cleaning prior to any thermal processing is critical. Stress relieving, annealing, tempering or hot-forming processes can actually draw surface contaminants deeper into the substrate, making them almost impossible to remove during passivation.
- Care should be taken during all thermal processes to avoid the formation of oxides. Passivation is not designed to remove discoloration and will not penetrate heavy oxide layers. In extreme situations, additional pickling and descaling operations are required prior to passivation to remove the discoloration. Controlled atmospheres are highly recommended for all thermal processes to reduce airborne contamination and prevent oxides from developing.
- Use only clean, unused abrasives such as glass beads or iron-free silica or alumina sand for abrasive blasting. Never use steel shot, grit or abrasives that have been used to blast other materials.
- Never use grinding wheels, sanding materials or wire brushes made of iron, iron oxide, steel, zinc or other undesirable materials that may cause contamination of the stainless steel surface.
- The use of carbide or other nonmetallic tooling is recommended.
- Grinding wheels, sanding wheels and wire brushes that have been previously used on other metals should not be used on stainless steel.