Passivation kinetics of Mg-Nd-Gd-Zn-Zr (EV31A) and Mg-Y-Nd-Gd-Zr (WE43C) in NaOH solutions

Passivation kinetics of two Mg-RE alloys, such as Mg-Nd-Gd-Zn-Zr (EV31A), and Mg-Y-Nd-Gd-Zr (WE43C) were investigated in two different heat treated conditions (solution treated and overaged) in 0.01 - 1.0 M NaOH solutions under potentiostatic conditions. Negative reaction order was observed in dilute NaOH which transitioned to positive values as the passivation time increased and in the 1 M NaOH as well. The passive layers showed platelet morphology and the size of the platelets decreased with increase in the NaOH concentration. The hydrogen evolution reaction (HER) kinetics was not improved on the passive layer covered surface of the Mg-RE alloys in contrast to the improvements reported on the hydroxide covered pure magnesium. The electrochemical impedance increased with increase in the NaOH concentration in the solution treated condition of both Mg-RE alloys, whereas the overaged EV31A alloy showed a reverse trend. The passive layer of EV31A showed almost 100% higher charge carrier density than the film formed on the WE43C in the overaged condition. A better passivation behavior was observed in the solution treated condition than that in the overaged condition which could be attributed to the uniform distribution of the RE elements in the solution treated specimens. The WE43C alloy revealed better corrosion resistance in the alkaline solution than the EV31A alloy.