Tungstate ion-assisted hydrothermal conversion of magnesium hydroxide coatings on AZ31 magnesium alloy

Surface modifications are often employed to improve the corrosion properties of magnesium alloys. In this study, the eco-friendly magnesium hydroxide (Mg(OH)2) conversion coatings were successfully applied on AZ31 substrates through hydrothermal treatment with the assistance of tungstate ions. Phase composition, surface and interface morphologies, surface composition, and adhesion property were characterised using X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy and scanning scratch tester respectively. The coating obtained was fairly dense and uniform. Coating thickness increased with the increase of hydrothermal treatment time, but longer times ( > 6 h) has a detrimental effect on the coating quality. The combined effect of adsorption and complexing of tungstate ions can effectively change the growth of Mg(OH)2 crystal from anisotropic to isotropic. This effect was crucial to produce the dense and uniform conversion coating. Scratch test indicated that the conversion coating adhered to the substrate strongly. Electrochemical testing revealed that the conversion coating caused AZ31 to be of higher corrosion potential with a much lower corrosion current density. This current study provides a promising surface treatment method for magnesium alloys towards their further applications.