Unraveling the relationship between severe plastic deformation and corrosion responses of AZ31 Mg alloys

Despite renowned for high specific strength and electromagnetic shielding properties, Mg alloys still suffer from localized corrosion due to the active chemical nature of Mg and the potential difference between second phases and Mg matrix. In this paper, we mediated fine-grained microstructures of AZ31 Mg alloys towards enhanced corrosion resistance via severe plastic deformation process. The stability of the passive films was promoted with refined average grain sizes from 12.25 μm to 5.58 μm. Pitting corrosion was suppressed through the fragmentation of band-like Al₈Mn₅ phases into dispersed fine particles. Significant amount of Mg₁₇Al₁₂ phases were dissolved into the matrix, leading to an increase in their corrosion potential by 0.06 V. In-situ stress corrosion potential was improved from −1.36 V vs. SCE to −1.26 V vs. SCE under external tensile stress equal to 100 % yield stress. A novel mechanical-electrochemical modeling with stress and time-dependent exponent was established to evaluate the effect of microstructural factors on the corrosion responses of fine-grained Mg alloys. The formation of the surficial passive films was accelerated due to the homogenization and grain refinement to suppress the corrosion rate, calibrated by the time-dependent exponent from 21 to 60 which characterizes the passivation film formation rate. The equivalent corrosion depth of these specimens under the external tensile stress decreased to one tenth, attributed to the stress-enhanced adhesion of the passive films to the Mg alloy surfaces.