Passivation is when exogenous iron compounds are removed from the surface of stainless steel by way of a chemical process. The process typically involves a treatment of the surface with an acid solution which removes the contamination on the surface but does not affect the underlying material. Nitric acid passivationis often the route taken as it enhances the spontaneous creation of the passive film.
How is nitric acid passivation done?
To begin with, the part as received is given a complete and thorough cleaning. This step is where excessive grease, oil, compounds used in the forming process, coolants and any other unwanted organic or metallic material is removed. These contaminates are the result of the fabrication process. The method used is usually vapor degreasing or high pressure solvent cleaning.
Once the organic and metallic impurities are removed, the part is placed in the solution used for passivation. There are many passivation solutions, but nitric acid passivation is by far the most popular. Nitric acid does carry with it considerable environmental concerns, so there is a lot of research taking place in an attempt to find a more eco-friendly solution, such as citric acid.
During the process, the three variables are time, temperature and acid concentration. Typically, the part is submerged for anywhere between 20 minutes and two hours in a 20 to 50% acid concentration at a temperature between room temp and 160 degrees F. After the passivation is complete, often sodium dichromate is used as a rinse as it aids in the formation of the passive film. With these variables, operator skill and attention are paramount for success.
The bath selection is directly related to the material being passivated. A thorough knowledge of the chemistry in the various grades of stainless steel is mandatory to achieve the desired result. If there is an error in any of the three variables, there is a good chance that the finished product will be pitted, etched or warped.
What equipment is used and what precautions are taken?
Only skilled technicians are employed to perform the passivation process. They know the potential hazards, and they are very familiar with the processes. They wear special boots, gloves and aprons to avoid any splashed acid. The facility in which the equipment is located must be well ventilated,and all the baskets used to carry the parts must be properly designed and maintained. There must never be any carbon steel used in the process, the results would be devastating. Full environmental standards for air and water treatment must be in place.
There are very few standards in the industry and choosing the correct passivation process is left to the skills of the technician. The various processes that have been used are kept, and reference is made to historic variables.
Passivation by Electropolishing is far superior to nitric acid passivation. New England Electropolishing has all the skills and equipment to perform this function.