Corrosion Inhibition of AZ31-xLi (x = 4, 8, 12) Magnesium Alloys in Sodium Chloride Solutions by Aqueous Molybdate

In this work, corrosion inhibition of lithium-containing AZ31 magnesium alloys AZ31-xLi (x = 4, 8, and 12 wt.%) has been examined in 0.05 M NaCl solution with and without 10–150 mM of sodium molybdate as the corrosion inhibitor. X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive spectroscopy analyses have been used to investigate the phase composition and microstructure of the alloys. The effectiveness of the molybdate inhibitor has been evaluated by a set of electrochemical methods, including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and dynamic electrochemical impedance spectroscopy (DEIS). Instantaneous DEIS measurements allowed to evaluate the effectiveness profile of sodium molybdate, giving high inhibition efficiency (>85%) at concentrations higher than ca. 35 mM. Post-corrosion SEM, Raman, and X-ray photoelectron spectroscopy analyses confirmed the morphology, ionic and valence composition of the formed passive layers on the surface of AZ31-xLi alloys. Based on experimental observations, a two-stage corrosion mechanism of AZ31-xLi alloys in molybdate-containing NaCl solutions has been proposed.