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Black chromate plating

Surface coating process where the workpiece through an anode-cathode reaction is coated with a thin decorative layer of hexavalent chrome with a black tone.

Illustration of Black chromate plating

The process

Before processing can begin the work should have been pre-treated to achieve a clean surface, which is usually done through electrolytic cleaning followed by pickling and rinsing.

The workpiece acts as a cathode and the surface coating agent acts as the anode when connected to a power source and immersed in a solution.

The solution used in this method is a mixture of chromic acid and a catalyst that is dependent on the substrate to be coated. Having the sulfur solution kept at around 30°C the characteristic gray-black chromium coating is achieved.

An anode-cathode reaction occurs which drives the chromium ions to the workpiece and create a layer of hexavalent chromium on the surface of the workpiece and increases its wear resistance and corrosion protection.