GB1388693A - Direct chromium plating - Google Patents

Abstract

1388693 Electro-depositing chromium BRITISH NON-FERROUS METALS RESEARCH ASSOCIATION 6 July 1973 [7 July 1972] 32047/72 Heading C7B Cr metal or alloy is electro-deposited direct on to Al, brass or Zn using an electrolyte comprising an homogeneous liquid mixture of water and a dipolar, preferably aprotic organic compound containing a highly electro-negative oxygen atom i.e. an oxygen atom activated by an adjacent electron-donating atom or grpup (e.g. the nitrogen of an amido group), the ratio by volume of water to dipolar compound being not greater than 95 to 5, with dissolved trivalent Cr and NH 4 ions. The dipolar aprotic compound may be dimethylformamide. The anion of the electrolyte may be halide, e.g. as NH 4 Cl, CrCl 3 and NaCl, or sulphate. One or more stabilizers e.g. boric acid, surfactants e.g. aliphatic alcohol, levellers or brighteners may be included. The anode, e.g. of C or Pt, may be separated from the cathode by a porous diaphragm. The cathode surface may be polished and/or degreased e.g. in trichloroethylene vapour, and treated to remove the oxide layer, e.g. by dipping in HF or cathodic activation in NaCN solution followed by rinsing and dipping in HF, before electro-deposition. Corrosive attack of the surface by the electrolyte is prevented by an initial Cr strike. Prior art Cr plating on Zn uses Na tetrachromate salt with post polishing. Specifications 1368749, 1368748, 1333714 and 1144913 are referred to.