Chemical blackening (browning) of ferrous alloys

Features of galvanic treatments Chemical blackening

The chemical blackening, also known as browning, is a chemical conversion treatment that can be carried out both on ferrous alloys and stainless steels: it produces a slight surface layer of black oxide that provides a mild protection against corrosion of the base material without altering the size of the treated parts.

This treatment is chosen to confer a better appearance to the objects, for antireflective purposes or for the protection of the base material in closed environments and without exposure to corrosive atmospheres.

Normally the blackening treatment is followed by an oiling process to improve the aesthetic appearance and to provide a better protection from corrosion.
The oiling process must not be carried out in case of use in optical applications, where the main requirement is the antireflection function (in this case it is often suitable to make the surfaces opaque with a peening with glass beads before the treatment).

The resistance to corrosion can be improved by a subsequent passivation treatment that allows to pass the oxalic acid test, as required by MIL-DTL-13924D: this operation is mandatory for military use and any time reference is made to MIL rule. 

A possible corrosion resistance in salt spray test can only be carried out on the blackened steels of AISI 300 series (as specified in MIL-DTL-13924D norm).