Effects of the second phases on corrosion resistance of AZ91-xGd alloys treated with ultrasonic vibration

Abstract The effects of the second phases on corrosion resistance of AZ91-xGd alloys treated with ultrasonic vibration were revealed by morphological observation, hydrogen evolution and weight loss methods, potentiodynamic polarization measurement, and electrochemical impedance spectroscopy. The addition of Gd led to the formation of many fine Al 2 Gd/Al–Mn–Gd particles that consumed Al and reduced the volume fraction of β-Mg 17 Al 12 phase. Meanwhile, the β-Mg 17 Al 12 phase morphology in ultrasound-treated AZ91-Gd alloys changed from semi-continuous reticular structure to rod-shaped and granular structure. The ultrasound-treated AZ91–1.0 wt% Gd alloy with fine granular β-Mg 17 Al 12 phase and Al 2 Gd/Al–Mn–Gd particles showed better corrosion resistance than other ultrasound-treated alloys. Micro-galvanic corrosion was formed and rapidly extended to the matrix alloy interior along local coarser reticular β-Mg 17 Al 12 phase, then causing localized serious corrosion. The finer and dispersed rod-shaped β-Mg 17 Al 12 phase and Al 2 Gd/Al-Mn-Gd particles led to micro-galvanic corrosion uniformly distribute on the surface of alloys, and formed a uniform corrosion layer with 18 μm thickness.