Quantitative relation of discontinuous and continuous Mg17Al12 precipitates with corrosion rate of AZ91D magnesium alloy

Although qualitative investigations have been performed on the effects of discontinuous and continuous Mg17Al12 precipitates (DP and CP, respectively) on the corrosion resistance of AZ91 alloy, it is not clear whether the change in the corrosion resistance (i.e., an improvement or deterioration) is due to the DP, CP, or the different corrosion experimental conditions used. Herein, the effects of the DP and CP produced by an agingtreatment on the corrosion resistance of the AZ91D Mg alloy were systemically investigated in the same corrosion test condition (i.e. 3.5 wt% NaCl). The morphologies of the DP and CP were controlled by changing the aging-treatment conditions. The area fraction of the DP, the number of CPs per unit area, and the average length of the CPs were quantified as microstructural factors. A quantitative correlation analysis between the corrosion rate and the quantified microstructural factors revealed that the corrosion resistance of the aging-treated AZ91D Mg alloy depended on the DP and CP connectivities. DP with low connectivity accelerated the corrosion by acting as a galvanic source, while DP with high connectivity along the grain boundary functioned as a corrosion barrier, resulting in a reduced corrosion rate. Finely precipitated CPs inside the alloy grains always functioned as a corrosion barrier regardless of their amount and distribution. As the CP connectivity increased, the corrosion rate decreased, and the macroscopic corrosion morphology changed from local to uniform.