The aim of cleaning/passivation is to cleanse the surface of any imperfections and to thereby create a more even, more compact, more perfect, thicker and finally Cr-enriched and therefore better passive layer. It is a constant combined process of cleaning and improving the quality of the passive layer. An improvement in the surface quality is achieved by, for e.g., dissolving with oxidizing nitric acid, HNO3. These acids must not attack the material itself; on the other hand they only have a limited dissolving capability for e.g. iron oxide. In such cases additives are used, which support the procedure and thus making the treatment far more effective. Years of experience in the past has shown that well cleaned and passivated systems are far more resistant to surface attack or contamination. In some cases there may be an attack by rouging. Even this phenomenon can be considerably mitigated by a good preventative cleaning or passivation.
Cleaning/passivation stainless steels