Thursday, February 14, 2008

Black and Color Chromium Deposits

Many attempts have been made to produced "black" chromium plate, and such deposits have been used commercially. They are usually dark grey rather than black, however, and all dull or mat chromium deposits seem to turn bright metallic rather readily if exposed to much abrasion or wear, in spite of the hardness of the metal. Oiling and other final treatments give grey or dull deposits a blacker appearance. An early "black" chromium plate was produced by using a high current density in a cold bath containing principally chromic and acetic acids.

Modification of this process for "black" chromium plating has been proposed. Current interruption and sulfate-free chromic acid with fluoride or complex fluoride catalyst have been used.

Trivalent Chromium Baths

Historically, trivalent chromium baths have been the first and favorite approach to chromium plating, but they have always failed to match the reproducibility and dependability of chromic acid solutions, as has already been discussed in the section on theory.

The United States Bureau of Mines developed a process for the Electroplating of pure chromium metal from the ore from ferrochromium. Using a mixed bivalent and trivalent chromium sulfate solution, but the process has not been adopted successfully to plating purposes. A two-compartment cell is used at the Union Carbide Corporation, Marietta, Ohio, plant, and continuous operation on a large scale seems necessary for good result. In another review of the operation that low efficiencies always prevail during the startup of a cell.

Success was very limited in effort to utilize such baths for electroplating. A chromium ammonium chloride solution was used for brush plating. Better results in brush plating were obtained with Gregory's salt, ammonium chromium oxalate.

More recently there has been an effort to commercially the use of chromium chloride solution in or containing, organic solvent. The deposits are darker than those from chromic acid solution, and some chlorine gas is generally evolve at the anode. Only decorative deposits are being considered.

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