Spatiotemporal Evolution of Interfacial pH and H2 Level During Degradation of Mg-Al-Y-Zn-Gd Alloys in Hanks’ Balanced Salt Solution

Understanding the dynamic interfacial microenvironment of degrading Mg alloys is critical for elucidating the evolution of localized corrosion under physiological conditions. Here, spatially resolved microsensors were used to characterize the local pH and H2 concentration at the interfaces of Mg-Y-Al-Zn alloys without Gd (0Gd) and with 3 wt.% Gd (3Gd) during degradation in Hanks’ balanced salt solution (HBSS). Both local pH and H2 concentration rapidly increased at the interfaces of 0Gd and 3Gd immediately after immersion in HBSS, accompanied by pronounced near-surface pH and H2 gradients. The initial local pH and H2 level for 0Gd were comparable to or slightly lower than those of 3Gd. However, 0Gd subsequently exhibited distinct localized high pH and H2 level associated with the filiform corrosion. From 3 to 12 h, the localized pH and H2 features of 0Gd progressively intensified, while 3Gd maintained a relatively weaker and more spatially distributed response. During prolonged immersion from 16 to 18 h, localized H2 accumulation also emerged on 3Gd despite comparatively moderate local alkalization. These findings reveal the dynamic and spatially heterogeneous nature of the Mg corrosion microenvironment and demonstrate that simultaneous mapping of local pH and H2 provides complementary information for resolving its spatiotemporal evolution.