Microstructure and microgalvanic corrosion of an extruded Mg-10Gd-2Y-0.5Zr magnesium alloy

In the present study, the microstructure and corrosion behaviour of an extruded Mg-10Gd-2Y-0.5Zr alloy (noted as GW102) in 3.5 wt.% NaCl for different times have been investigated using a combination of computed phase diagrams, optical microscopy, high resolution scanning electron microscopy (SEM) and scanning Kelvin probe force microscopy (SKPFM). It was revealed that the extruded alloy is composed of fine recrystallized, equiaxed Mg solid solution grains. Additionally different second phases of varying sizes and shapes are present, including a large square-shaped Mg₅(Gd,Y) particles which solidified from the melt and are located within the Mg grains and/or at grain boundaries, fine spherical zirconium-rich particles which are located in the Mg grains and fine needle-like precipitates of Mg₅(Gd,Y) and Mg₂₄(Gd,Y)₅ which are uniformly distributed within the Mg grain interior. The latter was formed during thermal extrusion processing. Further, a number of deformation bands or twins were found existing in the magnesium grains. SKPFM potential maps showed that Zr-rich particles and Gd(Y)-rich particles have positive potentials relative to the Mg matrix and the highest potential was recorded on Zr-rich particles. Immersion testing revealed that microgalvanic corrosion initiated at the periphery of Zr-rich particles due to their strong cathodic activity, thereby forming microgalvanic couples with the adjacent Mg grains.