Formation Mechanism and Properties of Titanate Conversion Coating on AZ31 Magnesium Alloy

Abstract

A titanate conversion coating was made on AZ31 magnesium alloys in a titanium chloride and hexafluorosilicic acid solution. The titanate conversion coating formation proceeded via the formation of a porous magnesium hydroxide and fluoride layer. Compact silicon hydroxide and titanium hydroxide were then formed locally on top of the porous layer. The fluorine ion dissociated from hexafluorosilicic acid contributed to the incorporation of magnesium fluoride in the porous layer. Moreover, a pH rise due to hydrogen discharge caused the precipitation of silicon hydroxide and titanium hydroxide. With continued immersion, the porous layer thickened and the coverage of the compact silicon hydroxide and titanium hydroxide proceeded to a larger extent, which, in turn, reduced the corrosion area fraction and enhanced the polarization resistance of the AZ31. However, prolonged immersion resulted in more cracks with larger openings, which reduced the corrosion resistance of the titanate conversion coating. As a result, the titanate conversion coating formed at immediate immersion times provided the best corrosion protection on the AZ31.