Achieving a Self-Densified Conversion Coating Architecture by the Bath Composition Design Towards a Multi-Step Nucleation Pathway

Abstract

In order to obtain the high corrosion resistance conversion coating on magnesium alloy, a new designing idea for the conversion bath was proposed to facilitate the multi-step nucleation process. The select of cation candidates should comply with the following rules: firstly, the solubility product (Ksp) value should be low enough for precipitation; secondary, the Ksp values among different species should not deviates significantly from each other. The phosphate conversion bath was used as an example to validate the idea. Results demonstrates that when Mn- or Ca-containing salt was added, a less compact architecture was obtained due to the lack of secondary nucleation process. The compactness and protectiveness was significantly improved when Mn- and Ca-containing salt were simultaneously added, which was due to the result of a multi-step nucleation process.

Keywords: Mg alloy, nucleation, Corrosion Resistance, chemical conversion coating, bath design