Influence of trace As content on the microstructure and corrosion behavior of the AZ91 alloy in different metallurgical conditions

The influence of trace As content(0.04, 0.05, 0.06 wt%) on the microstructure and corrosion behavior of AZ91 alloy in different metallurgical conditions(as-cast, as-quenched, peak-aged) was firstly investigated. It is found that the corrosion resistance of AZ91 alloy in all conditions is significantly enhanced by As addition, mainly due to cathodic toxication, melt purification and incidence reduction of local corrosion. As alloying leads to the formation of Mg3 As2 and makes the β-Mg1 7 Al1 2 phase less continuous in the as-cast alloys. The alloy containing 0.06 wt% As obtains a higher corrosion rate than those of the alloy containing 0.05 wt% As owing to a more discontinuity of β phase. The β phase is dissolved into the matrix in the as-quenched alloys and reprecipitates along the grain boundaries after aging. The more continuous β phase distribution in the peak-aged alloys contributes to corrosion resistance. The corrosion rates are in the order of as-quenched > as-cast > peak-aged. The lowest corrosion rate(0.67 mm/y) in 3.5 wt% Na Cl solution is obtained in the peak-aged AZ91-0.05 As alloy, which is over 70% lower than that of as-cast AZ91 alloy(2.22 mm/y).