Simultaneous enhancement of mechanical properties and corrosion resistance of as-cast Mg-5Zn via microstructural modification by friction stir processing

Magnesium alloys are ideal lightweight materials; however, their applications are extremely limited due to their low strength, poor ductility, and weak corrosion resistance. In the present study, a friction stir processing (FSP) treatment was employed to optimize the mechanical properties and corrosion resistance of an as-cast Mg-5Zn alloy. The average grain size of the Mg-5Zn alloy was refined from 133.8 µm to 1.3 µm as a result of FSP. Along different directions, FSP exhibited the enhancement effects on different mechanical properties. Furthermore, according to the potentiodynamic polarization results, the corrosion current density at the free-corrosion potential of the FSPed sample, was 4.1 × 10⁻⁶ A/cm² in 3.5 wt.% NaCl aqueous solution, which was significantly lower than that of the as-cast sample. Electrochemical impedance spectroscopy revealed that the polarization impedance, R_p, of the FSPed sample was 1534 Ω/cm² in 3.5 wt.% NaCl aqueous solution, which was 71.4% greater than that of the as-cast sample. The corrosion morphology of the FSPed sample in 3.5 wt.% NaCl aqueous solution exhibited largely uniform corrosion, rather than severe localized corrosion characteristics, which further reduced the corrosion depth on the basis of reducing the corrosion current density. The results presented herein indicate that FSP is a viable technique for simultaneously improving the mechanical properties and corrosion resistance of the as-cast Mg-5Zn alloy.