Pickling and passivating of stainless steel systems and installations

Stainless steel systems and installations are cleaned prior to use and in many cases periodically to remove process contaminants such as scaling.

Metal treatment
Stainless steel must prior to use be chemically treated. The corrosion resistance of stainless steel is determined by a chromium-rich oxide film. As soon as this film has formed the material is designated as being passive.

During installation of the stainless steel piping in a system and/or installation the surface is exposed to many mechanical and thermal treatments. This results in local damage to and contamination of the protective oxide film. Then this results in the oxide film losing its corrosion resistant features with the substrate material often corroding more rapidly than would have been the case for the original passive panel material. It is therefore very important that this chromium-rich oxide film is completely sealed and clean before the system/installation is put into use and exposed to chemicals/process and environment.

Corrosion
Stainless steel is not completely resistant to corrosion. It is important that light corrosion products be removed specifically with a view to avoiding deep pit and split corrosion with all their negative consequences and also to prevent contamination of the process/medium in the installation/system.

Precommissioning cleaning
The precommissioning cleaning of Austenitic stainless steel types comprises in general three steps: degreasing, pickling and passivation. Degreasing is required to remove oil, grease and atmospheric pollution and to render the surface optimally accessible to the pickling agent. Degreasing is generally conducted using Vecom Multicleaner/TP-02 or Steamclean HPC-NF. The pickling treatment completely removes iron particles and other surface contaminants to the surface resulting from earlier treatments or otherwise. After the pickling treatment the material is rinsed acid-free using low-chloride water. The pickling treatment renders the surface chromium-rich, the surface being in fact refined. Although the Austenitic stainless steel is, after pickling and when exposed to sufficient atmospheric oxygen, able to passivate spontaneously, closed systems are generally treated chemically in order to passivate their surfaces. The chemical fluids are circulated through the installation/system. This is done using an acid-resistant pump unit. The time occupied by this treatment depends on the type of stainless steel and the temperature. Analyses of the iron content and acid concentrations, determine when the treatment is finished. The treatment is followed by rinsing, first with drinking water and then with demineralized water.