Effect of iron content on the corrosion rate of a new Mg-Zn-Al-Ca-Ce-Mn alloy system

Abstract

There is a Fe tolerance limit in pure magnesium or Mg-Al based alloys, and it is critical to control Fe impurity within such tolerance to avoid accelerated corrosion in Mg alloys. In this paper, the effect of Fe concentration (20–204 ppm) on the corrosion rate of a new ZAXEM11000 (Mg-1.0Zn-1.0Al-0.4Ca-0.2Ce-0.6Mn) alloy is investigated. Surprisingly, the corrosion rate of the ZAXEM11000 alloy increases linearly with the Fe concentration, rather than exponentially, as would typically be expected after surpassing the Fe tolerance limit. In particular, the corrosion rate of the ZAXEM11000 alloy with 204 ppm Fe is only five times higher than that of the alloy with 20 ppm Fe. At low Fe contents, the Fe-containing secondary phases tend to segregate near grain boundaries. As the Fe content increases, a more dispersive distribution of Fe-containing secondary phases is observed throughout the entire Mg matrix, likely due to the increased amount of Al(Mn,Fe) phase formed during solidification. The high density of Fe-containing secondary phases, which causes trenching of the surrounding Mg matrix, accelerates the overall corrosion rate of the Mg alloys by enhanced galvanic corrosion with high density of noble secondary phases. The results indicate that the corrosion behavior of this alloy system may be dominated by the distribution and morphology of secondary phases, particularly Al-Mn-Fe phases, that develops as the Fe content increases. This study provides a pathway of creating Mg alloys with a high tolerance limit of Fe through alloying.