Properties of dawsonite conversion film on AZ31 magnesium alloy

An environmentally friendly method for synthesizing a dawsonite conversion film was developed to improve the corrosion resistance of AZ31 Mg alloy. The film was prepared by two steps: the AZ31 alloy was first immersed in an Al2(SO4)3 solution venting CO2 gas to form a precursor film, and then the precursor film was treated in a Na2CO3 solution dissolved with Al to obtain the dawsonite film. The surface morphology of the conversion film was observed with an environmental scanning electronic microscope. The chemical composition of the conversion film was analyzed by energy dispersive X-ray spectroscopy and X-ray diffractometry. Electrochemical and immersion tests were carried out to evaluate the protection effect of the conversion film on AZ31 alloy. There are some network-like cracks on the surface of the film. The conversion film is mainly composed of dawsonite NaAlCO3(OH)2, Al(OH)3 and Al5(OH)13(CO3)·5H2O, which can increase the corrosion potential and reduce the corrosion current density of the Mg substrate. After immersion tests, the film almost keeps intact, except for the localized narrow areas with several corrosion pits, while the bare material undergoes serious general corrosion. It is indicated that the dawsonite film can provide good protection to the magnesium alloy.