Passivating Stainless Steel

Passivated Stainless SteelOne of the primary considerations in the selection of material is the corrosion resistance and one of the best materials to be selected for this reason is stainless steel due to the natural tendency of the metal to be passive. What is passivation? Passivation is the treatment of the stainless steel with an oxidizing chemical (typically nitric or citric acid) which promotes a thin oxidizing layer to form on the surface of the metal. This thin layer will help protect the metal from chemical attack.

Fabrication of the metal can reduce the overall corrosion performance of the stainless steel; therefore, making the passivation process an essential step. If iron particles become embedded in the surface of the metal during processing they must be removed. If they are not removed, these microscopic particles may promote discoloration, rusting or even pitting. By dissolving these particles in an oxidizing bath, it will enhance the corrosion resistance of the stainless steel thus fortifying the surface of the metal against such attacks.

The passivation process should be the last step in the fabrication process. It is generally performed by immersing the parts in either a nitric acid or citric acid bath and then rinsing in a clear de-ionized water and then dried. If due to the size of the part it is impractical to immerse the part in a bath, the acid solution may be applied to part using a swab and rinsed with water and dried. It is important that the surface of the steel is free from scale, oils or other lubricants prior to the chemical treatment in order for the passivation to be effective. There are several tests which can be performed on the metal to determine its passivity such as high humidity test, salt spray test, copper sulfate test or free iron test.
Stainless steel is not stain proof only stain less so by passivating stainless steel you have given the part the best chance of withstanding any corrosion in its life cycle.