If, in addition to processing oils and/or grease, atmospheric and/or organic process contaminants are also present in a contaminated system, these must be removed prior to the pickling of a system. If these contaminants are not removed before the pickling, it will have an adverse effect on the pickling phase and the desired result will not be achieved.
A pickling step will remove the iron oxides (also mill scale when present, depending on the selected cleaning medium). To protect the base material, an additive (inhibitor) must be added for most cleaning methods.
After pickling, the metal surface is active. Depending on the metal surface, it will immediately rust in the case of carbon steel without treatment (fly rust). In the case of stainless steel, a passive chromium oxide layer will form spontaneously after 24 hours by exposure to the air.
- Passivating of carbon steel
To prevent rust, the carbon steel is temporarily protected by means of passivation: a chemical treatment whereby a stable gamma-iron (III) oxide (γ-Fe2O3) layer is formed. A standard passivation of carbon steel is carried out by means of ammonium citrate and an oxidizer or ammonia and hydrogen peroxide.
- Passivating of stainless steel
To speed up the passivation process of stainless steel, closed systems often require a chemical passivation treatment. By chemically passivating, the chromium oxide skin is built up more quickly and the stainless steel again reaches optimum corrosion resistance in the shortest possible time. A standard passivation of stainless steel is carried out by means of nitric acid.