Effect of Misch Metal on the Corrosion Behavior of AZ91 Magnesium Alloys under Wet-Dry Cycles

The microstructure and element distribution of AZ91 alloy were changed by adding the mischmetal of lanthanum and cerium (La, Ce). La or Ce element can be found in various phases of AZ91 alloy with mischmetal addition. Part of them can be dissolved in the alpha-Mg solid solution, while the others formed different intermetallic compounds with chemical composition Al-4(La,Ce) or Al10Ce2Mn7. Al-4(La,Ce) is in acicular shape and Al10Ce2Mn7 is in granular shape. The fraction of beta phase in AZ91 alloy decreased obviously with the addition of mischmetal, and the distribution of Al element transferred from the grain boundaries to grain internal area. The results of the corrosion behavior of AZ91 magnesium alloys with addition of mischmetal in snow-melt agent solution under wet-dry cycles show that a shift of the corrosion potential (E-corr) towards more negative values is observed for all the alloys with mischmetal addition, but their corrosion current densities (I-corr) are much lower than that of the AZ91 alloy. The scanning electrochemical microscopy (SECM) test results reveal that the addition of mischmetal could reduce the number of electrochemical active points on the surface. In addition, the compactness of corrosion product film of magnesium alloy is increased with addition of mischmetal.