Influence of β-Mg<sub>17</sub>Al<sub>12</sub> Phase on Initial Pitting Process of AZ91D Magnesium Alloy

Abstract

AZ91D magnesium alloy for casting is significantly corroded in chloride environment. In addition, Al-rich-α phase, which is formed during cooling process around β-Mg17Al12 phase, is one of the important factors on corrosion behavior in AZ91D. It is expected that pitting formation is dominant in the corrosion behavior in magnesium alloys. To clarify the influence of the Al-rich-α phase to pitting formation and corrosion behavior in AZ91D alloy, we have investigated the relationship between Microstructure and pitting formation in this study. We have carried out polarization curve measurement and scanning electron microscope (SEM) observation before and after the corrosion-tests. SEM micrograph of as-cast samples showed AZ91D alloy consist of α phase, Al-rich-α phase which shows bright Z-contrast due to high Al contents and β phase. Al-rich-α phase formed around β phase. After the corrosion examination, holes were observed at Al-rich-α phase around the β phase, and it is thought that small pits grow up to be holes, because β phase is remained, while the Al-rich-α phase around the β phase preferentially dissolves with time. The result indicates that Al-rich-α phase around β phase enhance the formation of pitting and preferentially dissolve it in initial stage of corrosion in AZ91D alloy.