Compared to austenitic stainless steel grades, duplex steels show in general a higher resistance to corrosion. However, in a critical environment duplex grades are still susceptible to localised corrosion.

To obtain maximum corrosion resistance the duplex surface should be perfectly clean, not contaminated and without surface defects and imperfections. Welding and processing of duplex influence the composition of the surface and weaken the corrosion resistance of the metal. To fully restore the corrosion resistance of duplex steel, a chemical surface treatment (pickling and passivation) is necessary. Mechanical techniques such as polishing, grinding and shotpeening do not restore the resistance to localised attack upto the level of the base material.

Because of the high chemical resistance of duplex stainless steels, pickling of these alloys in conventional pickle baths which consist of nitric- and hydrofluoric acid requires extremely long pickle times or high temperatures. To overcome this problem VECOM has developed two new pickling procedures which replace the conventional process. In our first new procedure accelerators are added to the conventional pickling bath. The accelerators speed up the pickling process and partially prevent the emission of NO_x. In our second alternative pickling process no nitric acid is used and the emission of NO_x is fully prevented. By using the NO_x-free process significantly less chemical waste is produced. The new developed pickling procedures are also suitable for other stainless steel grades.

In this paper we would like to discuss our new developed pickling and passivation methods. We will also share information on corrosion of stainless steel and duplex grades and how to influence the corrosion resistance by chemical surface-treatment.