In terms we can all understand, passivation removes “free iron” contamination left on the surface of stainless steels collected during machining and fabrication.  Free iron contamination left on the surface is a potential site for corrosion, ultimately resulting in the pre-mature deterioration of the component if not removed.  During the passivation process not only is the surface cleaned, but the process facilitates the formation of a thin chromium rich, transparent oxide film that protects the surface from corrosion.    With all this said, passivation is not cleaning, and it is not a protective coating, but it is actually a combination of both, enhancing the corrosion resistance of stainless steel.The three major variables that must be controlled during passivation are time, temperature, and concentration.  For nitric acid based solutions, immersion times range from 20 minutes to two hours.  The typical bath temperature ranges from ambient to 160° F and the nitric acid concentration is generally in the range of 20-50% by volume.  The solution must be monitored on a regular basis to control water purity, ppm of metallic impurities, and for maintaining the required chemical balance in order for success.  For citric acid baths the typical time required for passivation has been measured up to five times faster allowing passivation baths to be run at room temperature or using low heat to achieve positive results.  In addition, citric acid efficiently removes iron from the surface requiring much lower concentrations than nitric acid.  The typical solution for citric acid ranges from 4-10% citric acid by weight. 

Passivation

There are generally two opinions as to why passivation works; one credits its effectiveness because it is a cleaning process, the other because of the development of a thin transparent oxide film, which results from chemical passivation.
What is passivation? ASTM A380 describes passivation as “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.”  Additionally, it 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.”