TREATING SOLUTION FOR FORMING BLACK HEXAVALENT CHROMIUM-FREE CHEMICAL COATING ON ZINC OR ZINC ALLOY PLATED SUBSTRATE, AND METHOD FOR FORMING BLACK HEXAVALENT CHROMIUM-FREE CHEMICAL COATING ON ZINC OR ZINC ALLOY PLATED SUBSTRATE

摘要

A treating solution for forming a black hexavalent chromium free chemical coating on a zinc or zinc alloy plated substrate, characterized in that the molar ratio of a phosphate ion to trivalent chromium (NO3-/Cr3+) is less than 0.5, the trivalent chromium is present in the form of a water-soluble complex with a chelating agent, it contains a cobalt ion and/or a nickel ion, the cobalt ion and/or the nickel ion is not precipitated through forming a sparingly soluble metal salt together with the chelating agent, but is present stably therein, and it react with zinc when it is contacted with a zinc or zinc alloy plated substrate, to form a black hexavalent chromium free chemical coating comprising zinc, chromium, cobalt ion and/or nickel, and the chelating agent on a zinc or zinc alloy plated substrate. A black hexavalent chromium free chemical coating formed by the use of the treating agent for plating on a zinc or zinc alloy plated substrate exhibits the corrosion resistance comparable or superior to that of a conventional coating containing hexavalent chromium.