Effect of alloyed Ca on the microstructure and corrosion behavior of extruded Mg-Bi-Al-based alloys

Microstructural characteristics and corrosion behavior of extruded Mg-1Bi-1Al-xCa (x = 0, 0.6 and 1.2 wt%) alloys were investigated. It was found that the average grain size, texture intensity and corrosion resistance of the present low-alloyed alloy system initially increased and then decreased with increasing Ca content. Intergranular corrosion was observed in the present alloy system, which was related to the higher energy of the grain boundaries. Furthermore, the corrosion mode changed from pitting corrosion in the Ca-free alloy to the mixed pitting and filiform corrosion in the Ca-containing alloys in the initial stage of immersion based on the analysis in terms of micro-galvanic corrosion and the formation of protective film. As the immersion time increased, the filiform corrosion dominated in the Mg-1Bi-1Al-0.6Ca alloy. However, the corrosion product film of the Mg-1Bi-1Al-1.2Ca alloy was quickly destroyed due to the shedding of the coarse Mg2Bi2Ca2 phase, which resulted in severe corrosion.