Passivation of Stainless Steels

Passivation of stainless steel parts removes impurities picked up from previous manufacturing operations; fragments of iron or steel from tooling or cutting.

Metal debris such as this will result in nucleation sites for corrosion reactions, and thus their removal is crucial. Before passivation treatment, components get degreased and cleaned which chemically enhances the surface energy of the base material.

AMS 03-2, the successor to the old DEF STAN, lists methods M1 and M2. Method M1 involves immersion in a mixture of nitric acid and sodium dichromate solutions at circa 50 Celsius for a minimum of 20 minutes. In the case of ferritic and martensitic alloys, it is necessary to carry out the further treatment in sodium dichromate at around 65 Celcius.

Method M2 provides a nitric acid-only alternative that does not require the use of sodium dichromate. Dichromate solutions contain hexavalent chromium, which has various health hazards associated with its use. Hard Anodising Surface Treatments Ltd is a member of the UK SEA chromium consortium which seeks to extend the use of hexavalent chromium.

As well as iron and nickel, stainless steel contains chromium. Chromium is far less soluble in nitric acid than iron, especially the loose iron debris mentioned earlier. The passivation solution displaces iron atoms from the substrate, increasing the chromium/iron ratio in the passivated film. What finishes off the process is the atmospheric oxidation of the exposed chromium. The oxidation of chromium is not a chain reaction that continues into the base material, like rust (iron oxide). Given that the abundance of chromium has increased relative to iron in the substrate and given that oxide molecules are twice the size of their base metal atoms, the chromium oxide formed also acts as a barrier film over the adjacent iron.

The specification AMS 2700 lists six processes that are minor modifications to M1 and M2.

Subsequent testing is possible using aqueous copper sulphate to detect any ferrous contamination after treatment.

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