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Passivation of stainless steel

Passivation is a natural mechanism that makes stainless steel. When austenitic steels rich in chromium come into contact with air, a film of chromium oxide forms on the surface. This film is called passive or inactive, because it does not react with oxygen and thus protects the lower layers of the alloy.

Natural passivation is not always satisfactory, and it takes a long time to form. Moreover, operations on stainless steel (machining, pickling, polishing) can damage this protective layer and reduce the strength of the metal.

It is therefore sometimes necessary to redo the passivation of the stainless steel, by chemical process, with a forced oxidation carried out by phosphoric, nitric or citric acid.

The chromium/iron ratio on the metal surface determines the quality of the passivation. Nitric acid gives a ratio of 1.4 to 1.6 Cr/Fe, while citric acid gives 1.7 to 2.0Cr/Fe, and therefore a better quality passive layer.

Formation of a passive layer of chromium oxide (image Astro Pack)

ASTM A380 and ASTM A967 standards govern the passivation process on stainless steels. A cleaning/stripping step is necessary before the passive layer is applied by chemical oxidation.

Our production workshop offers this operation on all types of stainless steel parts, to improve their robustness and longevity. Our sales team is at your disposal for further information.

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