Substantial corrosion resistance improvement in heat-treated Mg-Gd-Zn alloys with a long period stacking ordered structure

The microstructure and corrosion behaviors of Mg-15.24Gd-4.75Zn (in wt.%) alloy exposed to 3.5 wt% NaCl solution were investigated under as-cast, solution (T4), and aging-treated (T6 and T6F) conditions. Results show that the LPSO phase is absent in as-cast and T4-treated alloys but present in T6 and T6F alloys. All lamellar LPSO phases are distributed in α-Mg intragrains, which separates the matrix from the eutectic phase. The lamellar LPSO phase can act as a corrosion barrier, and protect the matrix from being easily corroded by decreasing the surface area of the matrix exposed to the NaCl solution at early stages of corrosion. The LPSO phase can also act as an anode during micro-galvanic corrosion with the W-phase, thereby protecting the matrix and restraining the cathodic reaction of hydrogen evolution. The interior of the LPSO phase can further prevent penetration of corrosive ions during subsequent corrosion processes. Thus, samples with the LPSO phase display uniform corrosion morphologies and considerably improved corrosion resistance.