scope:

General

Stainless steel surfaces naturally passivate because of the protective passive metal oxide film forming spontaneously on exposure to air. The presence of exogenous surface contamination, including dirt, grease, free iron from contact with steel tooling, and so forth, can interfere with the formation or reformation of the passive metal oxide film. The removal of these contaminants from the stainless steel surface facilitates the spontaneous passivation by allowing the oxygen uniform access to the surface. The removal of iron from the alloy at the metal surface also enhances the passive metal oxide film that is formed.

In this standard, passivation is defined as the chemical treatment of a stainless steel surface with an acid solution, including any necessary precleaning, for the purpose of the removal of surface free iron and foreign matter. Nitric acid and citric acid are generally used for this purpose. Due to the environmental and personal hazards associated with nitric acid, this standard includes the use of citric acid only.

Tanks should be passivated upon initial fabrication or installation, and again as needed, based on use. The transport of corrosive cargos generally leads to passivation being performed sooner or more often. Stainless steel surfaces should also be passivated after welding, removal of corrosion, or other maintenance operations that cause damage to the passive metal oxide film.

The passivation treatment chemistry and process described in this standard is applicable to iron removal with all grades of stainless steel and many nonferrous metals and alloys. The method described in this standard is applicable specifically to the interiors of tanks, but other stainless steel surfaces may be treated in accordance with the process parameters described, using appropriate methods.

Users of this standard shall be responsible for specifying applicable safety, health, and environmental practices to ensure compliance with all applicable regulations, including compliance with insurance requirements and directives.