Formation of a passive chromium oxide layer (image Astro Pack)
Passivation is a natural mechanism on austenitic steels, rich in chromium: on contact with air, a passive layer of chromium oxide forms on the surface, which protects the steel from oxidation.
This natural passivation is not always sufficient and takes time to form. Furthermore, stainless steel parts can be pickled, machined or polished, which completely removes the passive layer.
It is therefore useful to passivate or repassivate the stainless steel, by chemical process, using different acids: phosphoric, nitric or citric acid.
The quality of the passivation is determined by the chromium/iron ratio on the metal surface. Nitric acid treatment results in a ratio of 1.4 to 1.6 Cr/Fe, while citric acid creates a better quality layer, with a ratio of 1.7 to 2.0Cr/Fe.
The methods used for passivation are governed by ASTM A380 and ASTM A967. They involve a cleaning/stripping step prior to the creation of a passive layer by chemical reaction.
We offer this service on all types of stainless steel parts to improve strength and durability. Please contact our sales team for more information.