Effects of grain size and second phase on the corrosion behavior of degradable magnesium alloys

Grain size and second phases have been proven to be key factors affecting the corrosion performance of degradable Mg alloys. However, complex alloys failed to quantify the contribution of a single factor and reveal the underlying mechanism during practical corrosion processes. In this study, high-pure Mg (HPM), Mg-4Zn (MZ) and Mg-0.5Ca (MX) were employed to systematically investigate the influences of grain size (5 ∼ 60 μm) and second phase characteristics on the corrosion behavior of Mg alloys in Hank's solution. The results show the corrosion rate (CR) of HPM increases gradually, while the CR of MZ increases first and then decreases with increasing the grain size. The corrosion behavior of fine-grained MZ alloy is dominated by grain size effect, higher content of MgZn₂ phase and potential difference (25.5 mV), resulting in a fast corrosion and pronounced pitting. Coarse-grained MZ shows a slow and uniform corrosion. The CR of MX gradually decreases with increasing grain size, which is completely opposite to that of HPM. The relatively high content of Mg₂Ca phase and its large potential difference compared with the matrix are responsible for the high CR and severe pitting in the fine-grained MX alloy. Moreover, Ca-P deposition densifies the corrosion product layer, resulting in a low and uniform corrosion. In addition, Hall-Petch relationships between the grain size and the CR, the volume fraction of second phase and the CR were obtained with different slopes for the materials. STATEMENT OF SIGNIFICANCE: This work successfully clarify the individual functions of grain size and second phases on the corrosion behavior of degradable Mg alloys in Hank's solution through alloy design (high pure Mg (HPM), Mg-Zn (MZ) and Mg-Ca (MX) alloys) with different grain sizes (5 ∼ 60 μm) and second phase characteristics. (1) The corrosion rate (CR) of HPM increases gradually, while the CR of MZ increases first and then decreases with increasing the grain size. The CR of MX exhibits a opposite trend compared to HPM. (2) The content of the second phase exhibits a positive correlation with the CR. However, variations in the second phase content exert a more pronounced influence on the CR of the MX alloy.