Passivation of stainless steel surfaces
The reconstruction of the microscopically thin passive layer already starts during the pickling process when metal oxides are removed from stainless steel surfaces. The steel stabiliser chrome reacts with oxygen. This chemical reaction depends amongst other things on the degree of activation of the base material, the surrounding temperature and the supply of oxygen.
Test results prove:
- The passive layer is already well developed after the pickling process but it is not consistent.
- The development of the passive layer through contact with oxygen can take several weeks. During this time there is the risk that ferrite particles come in contact with air contaminants (SO2, SO3, NOx amongst others) and form salt crystals or other reactions which create corrosion nuclei in the passive layer.
- If the surface undergoes a passivation process with nitric acid (HNO3) immediately after the pickling process it enables a homogeneous formation of a passive layer and especially its densification.
A high quality passive layer with optimal chemical resistance forms immediately. This is why it is generally recommended to do the passivation process.