Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys

Magnesium alloys have superior mechanical property for industry applications as structural materials, but their poor corrosion resistance is still a bottleneck problem. Mg-Al-Zn alloys are one of the most extensively used Mg alloys. In order to study the effects of the second phases on corrosion property systematically, Mg-xAl-(15-x) Zn (x = 12.5, 5.6, 3.3, 1.0 wt%) two-phase alloys containing a certain amount of the different second phases, two binary compounds gamma, MgZn, and two ternary second phases Phi, q, were prepared based on the Mg-Al-Zn phase diagram. On this basis, corrosion property of the alloys in the 3.5 wt% NaCl solution was studied by corrosion morphology observation and electrochemical tests. The role of the different second phases in the corrosion processes was investigated. It was revealed that the second phases precipitated both in the Mg matrix and along the grain boundaries and acted as microcathode, accelerating corrosion dissolution of the Mg substrate. The acceleration effect of the second phases is in the order of gamma-Mg17Al12 > q-Mg44Zn41Al1 > Phi-Mg-21(Zn, Al)(17) > MgZn. The alloys were also investigated for their diversity in corrosion product characters. The corrosion mechanism was discussed terminally by equivalent circuit of the electrochemical impedance spectrum. (C) 2016 Elsevier B.V. All rights reserved.