Process description: Coating oxide(cemical blacking)

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Chemical blacking – oxide coatings

Immersion-type chemical oxidation coatings are used mainly for appearance, as a paint base, or for their oil-retention characteristics.


Steel

Steel may be blackened in a high temperature bath containing 480 – 840 g/l of a mixture of about 75 wt-% sodium hydroxide and 25 wt-% sodium nitrate, or a mixture of 12 wt-% sodium nitrate and 13 wt-% sodium nitrite may also be used. Lower boiling points are obtained by including various additives containing sulphur. The resulting oxide coating is somewhat less corrosion-resistant because of the inclusion of iron sulphide in the film. Some of the sulphide-modified materials may also be used to blacken stainless steel.

Following thorough rinsing, the oxide film is normally coated with an oil, wax, or lacquer.


Stainless steel

Stainless steel can also be blackened in molten sodium/porassium dichromate at about 370°C provided the parts are not adversely affected by the high temperature. The resulting oxide has good corrosion resistance, and the procedure is covered by military specifications.

The “Inox” process, dipping in chromic acid and sulphate at 60 – 90 °C can be used, and chromium-nickel steel can be coloured black by dipping in NaOH/NaNO2 at 125 – 130°C.


Copper

Copper may be blackened in a bath containing 120 g/l of a mixture of 75 wt-% sodium hydroxide and 25 wt-% sodium chlorite at 93 – 100°C. The black copper (II) oxide produced has a fuzzy appearance. On bright finished parts, this fuzzy nap disappears when lacquer is applied, restoring the lustrous appearance, or it may be laid down (nor removed) by gentle buffing or tumbling in sawdust or other soft abrasives.


Brass

Brass may be blackened in the same solution but may not respond, depending on the zinc content and metallurgical history. Activation may be carried out by immersion in a bath containing 120 g/l of a mixture of 85 wt-% sodium hydroxide and 15 wt-% sodium chlorite operated at 93 – 100°C for 5-10 min to dezincify the surface, after which the blackening bath will usually function normally.


Aluminium

Aluminium may be given an oxide coating by immersion by a number of different processes including the Alzac and Juratka processes. Electrolytic processes (anodising) are generally preferred.


Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.



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