Cleaning/Passivating stainless steels

Stainless steels (SS) with a chrome content of approx. 12% are able to spontaneously build up passive layer under humidity conditions. Stable passive alloys can be recognized by the fact that they have only a very low corrosion current density thereby ensuring that there can’t be any notable corrosion. Under these conditions the material forms a resistant, protective, but very thin chromium oxide layer on the surface. This barrier ensures that the external current conduction to the surrounding medium practically grinds to a halt. The material behaves in a similar manner as would a precious metal.

The naturally-formed passive layer is only a few nanometers thick (approx. 2-7nm) and always has a few weak points. Furthermore certain ions (e.g. chloride ions) are able to destroy the passive. This localized process leads to pinholes and is called pitting corrosion. The preferred areas of attack are imperfections, cracks or flaws, which are always present, e.g.: manganese sulphide, non-metallic enclosures, carbides, scaling residues, rings and scratches, Fe – impurities etc. Added to these are imperfections which are the result of fabricating or installing a component, e.g. fats or oils, non-alloyed Fe and stainless steel shavings,  heat tint, dusts or strong internal stresses (welds, scratches).

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.

Electropolished surfaces already possess a  passivated surface, although residuals from elctropolisching bath can’t  be completely removed. If such surfaces are being used without additional processing e.g. welds or mechanical changes, then theoretically it would be possible to forgo an initial cleaning/passivation. Experience shows, however, that contamination which occurs whilst constructing a system or in some cases even some residuals from the electropolishing can lead to surface problems during operation!

It is impossible to totally avoid typical constructional contamination such as fats, shavings, rust film or building material deposits. Even the fine surface layer formed during welding, which is practically invisible to the naked eye, is known to impair the passive layer and should be removed!

To sum up: Systems and components which have been cleaned and passivated from the beginning have proven themselves better in the application and have better surfaces. Repeated follow-up treatments are, however, important and maintain the functional characteristics of the stainless steel.