The relation between Mn additions, microstructure and corrosion behavior of new wrought Mg-5Al alloys

Three wrought Mg-5Al alloys with different Mn contents (0.5, 1.4, and 3.1 wt%) were examined and compared with a reference wrought Mg-5Al alloy. Additions of Mn decreased the grain size and caused precipitation of various Al-Mn intermetallide with different Al/Mn ratio. The corrosion resistance of the Mn-containing alloys significantly increased in comparison to the reference alloy. This contributed to a reduced harmful effect of the impurities, refined microstructure and formation of corrosion products layer. However, further increasing of Mn content caused greater precipitation of the Al-Mn intermetallics, which in its turn increased the galvanic micro coupling. The alloy with 3.1 wt% Mn additions formed a relatively protective oxide film, which balances the increased number of cathodic particles, and its corrosion rate was very close to that of alloy with 1.4 wt% Mn content.