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 beta-Mg17Al12 phase. Meanwhile, the beta-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 beta-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 beta-Mg17Al12 phase, then causing localized serious corrosion. The finer and dispersed rod-shaped beta-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 mu m thickness. (C) 2019 Elsevier B.V. All rights reserved.