Corrosion behavior and mechanism of Mg–Er–Zn–Zr alloys in different states

The corrosion behavior and mechanism of as-cast, solid-solution treated, and as-extruded Mg-14.4Er-1.4Zn-0.3Zr (wt.%) alloys are investigated. The microstructure characteristic of as-cast alloy, i.e., the semi-continuous 18R-long period stacking ordered (LPSO) phase as cathode with the potential difference (PD) of 83 mV, is the main reason for the relatively strong tendency of micro-galvanic corrosion. The lower micro-galvanic tendency by the decreased size, number, and PD (30 mV) of LPSO-phase particles, and the existence of a few Er3+ in corrosion film, are mainly responsible for the improved corrosion resistance of solid-solution treated alloy. The as-extruded alloy exhibits a superior corrosion resistance (corrosion rate: 1.11 mm y−1 in 3.5 wt.% NaCl solution) as compared to many reported Mg alloys, which mainly attributed to the formation of nano-spaced basal plane solute-enriched stacking faults (SESFs) within fine dynamic recrystallized (DRXed) grains. The nano-scale SESFs as the weak anode with PD of 26 mV weaken the galvanic tendency to form a relatively homogeneously electrochemical microstructure. Moreover, the preferential corrosion of SESFs releases sufficient Er3+, promoting the quasi-passivation state of corrosion film. The construction of weak anodic nano-lamellar SESFs structure within fine grains is a feasible method for the synergetic improvement of strength and corrosion resistance of Mg alloys.