Importance

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Abstract

It is an object of the present invention to provide a processing solution used for forming a hexavalent chromium free, black conversion film, which is applied onto the surface of zinc or zinc alloy plating layers, and which has corrosion resistance identical to or higher than that achieved by the conventional hexavalent chromium-containing conversion film. According to an aspect of the present invention, there is provided a processing solution for forming a hexavalent chromium free, black conversion film on zinc or zinc alloy plating layers, the processing solution comprising: nitrate ions and trivalent chromium in a mole ratio (NO.sup.3-/Cr.sup.3+) of less than 0.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with a chelating agent; and cobalt ions and/or nickel ions, wherein the cobalt ions and/or nickel ions are stably present in the processing solution without causing any precipitation by forming a hardly soluble metal salt with the chelating agent; wherein the solution reacts with zinc when it is brought into contact with the zinc or zinc alloy plating to form a hexavalent chromium free, black conversion film containing zinc, chromium, cobalt and/or nickel, and the chelating agent on the plating.

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First Claim