Effect of Manganese Content on Microstructure and Corrosion Resistance of As-Cast AZ91 Magnesium Alloys

The effects of Mn content on the microstructure and corrosion resistance of as-cast AZ91 magnesium alloys were investigated by static weight loss method, electrochemical method and salt spray test. The microstructure and corroded surface morphology of the alloys were observed with an optical microscope and a scanning electronic microscope. The composition of the alloys was analyzed by energy dispersive spectroscopy and X-ray diffraction. Results showed that most Mn in as-cast AZ91 magnesium alloy existed in the state of granular Al(2) Mn phase which had lower electrochemical activity and could hinder the corrosion process of the Mg alloy. In the meantime, Mn was able to effectively reduce the content of beta -Mg(17)Al(12) phase that was liable to corrosion in the Mg alloy and enhance its dispersive distribution thereby increasing the corrosion potential, reducing the corrosion current and corrosion rate, and improving the corrosion resistance. As-cast AZ91 Mg alloy containing 0.8% Mn exhibited the best corros