Effect of Chloride Ion Concentration on the Corrosion Behavior of Mg-Gd-Y Alloys: An Experimental and...

Chloride-induced film breakdown critically limits the application of rare-earth-containing magnesium alloys, yet the concentration-dependent transition from passivation to localized corrosion remains insufficiently understood. Here, the corrosion behavior of an as-cast Mg-9Gd-3Y-0.6Zn-0.5Zr alloy in 0-5.0 wt % NaCl solutions was investigated using quasi-in situ SEM, SKPFM, cross-sectional SEM/EDS, XPS, electrochemical testing, and first-principles calculations. Increasing NaCl concentration accelerated corrosion, with the i_corr rising from 33.57 μA cm^-2 in pure water to 1156.42 μA cm^-2 in 5.0 wt % NaCl, while the interfacial resistance decreased from 3210.0 to 15.8 Ω cm². Within the examined concentration range, a pronounced corrosion-mode transition occurred around 0.5 wt % NaCl, changing from relatively uniform dissolution with rare-earth-assisted MgO/Mg(OH)₂ film repair to severe microgalvanic pitting around RE-rich second phases. XPS and cross-sectional EDS indicate that Gd/Y oxides contribute to film densification at low chloride levels but become insufficient under high chloride attack. DFT calculations further reveal that increasing Cl^- coverage changes OH^- adsorption from competitive to cooperative behavior, promoting nonprotective Mg-Cl-OH products. These results clarify the chloride-controlled balance between RE-assisted passivation and microgalvanic corrosion.