2008 Volume 49 Issue 5 Pages 1071-1076
Conversion coating by immersion in solution including Mg(NO3)2 and/or La(NO3)3 was applied to commercial Mg alloys: AZ31, AZ61, AZ91 and AM60. The formed layer was analyzed, and the corrosion resistance of the specimen against salt water was investigated. The influences of the coating conditions on the characteristics of the layer and on the corrosion behavior were discussed. A thin oxide layer was formed on the surface of the specimen by the conversion coating, and La was contained in the layer obtained in the solution including La(NO3)3. The corrosion resistance of Mg alloys wasn’t remarkably improved by the conversion coating using either Mg(NO3)2 solution or La(NO3)3 solution, while it was particularly bettered by the coating using the solution including both Mg(NO3)2 and La(NO3)3. The good corrosion resistance was obtained under the wide concentration ratio of La(NO3)3 to Mg(NO3)2, and the optimum coating conditions for each alloy was specified. The pretreatment, such as the alkaline degrease, the acid pickling and the surface activation, was effective for the uniform coating on large specimen.