Effect of yttrium modification on the corrosion behavior of AZ63 magnesium alloy in sodium chloride solution

The microstructures and corrosion behaviors of Mg-6%Al-3%Zn and Mg-6%Al-3%Zn-(0.25∼1.0)%Y in 3.5 wt.% NaCl solution are investigated via morphology observation, phase characterization, immersion and electrochemical methods. The experimental results suggest that yttrium alloying can improve the corrosion resistance of Mg-6%Al-3%Zn throughout the immersion. The initial enhanced corrosion resistance of magnesium alloy is attributed to the Y alloying, which gives rise to the increasing content of Al2O3 in oxide layer. The promoted protectiveness of oxide layer on Mg-6%Al-3%Zn-1.0%Y alloy arouse a filiform corrosion as revealed by in-situ metallographic observation. Furthermore, the Y-containing magnesium alloys still perform higher corrosion resistance compared with that of Mg-6%Al-3%Zn alloy even if the oxide layer is exhausted. This phenomenon is owing to two factors, one is the gradual transformation of intermetallic phases from continuous Mg17Al12 to discrete Al2Y in matrix, by which the micro-galvanic corrosion between α-Mg grains and intermetallic phases is alleviated; the other is the morphology variation of corrosion products, the uniform and compact products layer on Y-containing alloys provide a barrier that effectively prevent the corrosive ions from penetrating into and reacting with Mg matrix.