Passivation of materials is needed from time to time due to potential contamination of processes, aesthetic appearance or function within a system. It is used to remove surface contaminants from forming, machining, shot peening and heat treating. This surface preparation generally does not change the tolerances, and should not etch or pit the surface.
For aerospace applications using NITRONC 60, AMS 2700 Method 1 or 2 can be employed to clean the surfaces. Usually just a small amount of nitric will clean it up nicely in Method 1, such as a Type 1 or 6. We have found the citric acid in Method 2 to be ineffective to sufficiently brighten the appearance, though it may be employed as a process.
Typically, materials being removed are due to a free iron concern. Free iron from tool steels, racks for storage and rollers can leave trace amounts during transfer. The free iron then will oxidize and give the appearance that the stainless is rusting, even when it is only the surface.