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

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 Al₂Gd/Al–Mn–Gd particles that consumed Al and reduced the volume fraction of β-Mg₁₇Al₁₂ phase. Meanwhile, the β-Mg₁₇Al₁₂ 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-Mg₁₇Al₁₂ phase and Al₂Gd/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-Mg₁₇Al₁₂ phase, then causing localized serious corrosion. The finer and dispersed rod-shaped β-Mg₁₇Al₁₂ phase and Al₂Gd/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.