Nitric passivation is the cleaning of stainless steel, and should take place immediately after fabrication. The process maximises the inherent corrosion resistance of the steel and acts to removes surface contamination from the piece.
Although often invisible to the naked eye, iron particles can contaminate steel items during or immediately after fabrication. This iron, which can be from machine shop tools or dust, can cause the surface of the steel to rust, and breaks down the very fine, naturally occurring oxidised film which acts as some protection for uncontaminated metal.
The process of nitric passivation is a two-step procedure which can provide the best possible corrosion resistance.
The first stage is cleaning, which is a vital part of the process, and can often be overlooked. Grease, coolant or other shop debris must be thoroughly cleaned from the surface to obtain the best possible corrosion resistance. Machining chips or other shop dirt can be wiped carefully off the part. A commercial degreaser or cleanser may be used to clean off machining oils or coolants. Foreign matter such as thermal oxides may have to be removed by grinding, or by methods such as acid pickling.
Following cleaning, the piece is placed in the nitric passivating treatment, which produces a corrosion resistant oxide layer, thicker than the one already present on the steel.