process for coating of a thin protective layer of chromium, a metal base, consisting of cadmium or copper

摘要

In a process for the surface protection of copper or cadmium or alloys having a major content of these metals (e.g. brass, bronze or Cd-Zn alloy) the surface to be protected is exposed to a solution containing a compound of hexavalent chromium combined with oxygen, and a compound capable of binding ions of the metal to be protected to form a soluble complex, the pH of the solution (which lies in the range 6.5-13) and the amount of complex-former being such as to lower the concentration of metal ions in the solution to a value at which, at the pH of the solution, a protective chromiferous layer is formed on the surface. Suitable chromium compounds are chromic acid, potassium chromate, and dichromates. Specified complex-formers are potassium cyanide, triethanolamine, ethylenediaminetetraacetic acid, and NH4-ions. The pH of the solution may be adjusted by the addition of sodium or potassium carbonate, sodium or potassium hydroxide, or phosphoric acid/alkali mixtures.