| abstract |
(A.) A process for forming a cobalt conversion coating on a metal substrate, thereby imparting corrosion resistance and paint adhesion properties. The invention was developed as a replacement for the prior art chromic acid process. The process includes the steps of: (a) providing a cobalt conversion solution comprising an aqueous solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III hexavalent complex being from about 0.01 mole per liter of solution to the saturation limit of the cobalt-III hexavalent complex, and acetic acid; and (b) contacting the substrate with the solution for a sufficient amount of time, whereby the cobalt conversion coating is formed. The substrate may be aluminum or aluminum alloy, as well as Cd plated, Zn plated, Zn-Ni plated, and steel. The cobalt-III hexavalent complex may be present in the form of [Co(NH₃)₆]X₃ wherein X is Cl, Br, NO₃, CN, SCN, PO₄, SO₄, C₂H₃O₂, or CO₃. (B.) A chemical conversion coating solution for producing the cobalt conversion coating on a metal substrate, the solution including an aqueous solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III hexavalent complex being from about 0.01 mole per liter of solution to the saturation limit of the cobalt-III hexavalent complex. The cobalt conversion solution may be prepared by a bath makeup sequence including the steps of: (a) dissolving a cobalt-II salt; (b) then dissolving a metal nitrate salt; and (c) then adding ammonium acetate. (C.) A coated article exhibiting corrosion resistance and paint adhesion properties, the article including: (a) a metal substrate; and (b) a cobalt conversion coating formed on the substrate, the cobalt conversion coating including aluminum oxide Al₂O₃ as the largest volume percent, and cobalt oxides CoO, Co₃O₄, and Co₂O₃. |