Spatially Resolved Local In Operando Techniques Visualize the Interface of Biodegradable Metals

摘要

The degradation of biodegradable metals typically initiates locally and evolves variably, which is difficult to be captured by general experimental methods. The advancement of spatially resolved local in operando techniques enables a powerful in situ approach to study the degradation mechanism of biodegradable metals at the meta l interface, especially following the evolution of interfacial pH and O 2 concentration, which are substantial factors during the metal biodegradation. The degradation behavior of biodegradable Mg, Zn, and Fe alloys are systematically characterized by spatially resolved local techniques that measure local pH, O 2 levels, and H 2 concentration at the metal interface. The local pH indicates anodic/cathodic process and formation of pH-dependent degradation products at metal interface. The local O 2/H2 concentrations demonstrate typical cathodic reactions accompanying metal degradation, including hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The results reveal interfacial electrochemical processes of biodegradable Mg, Zn, and Fe alloys. Coun terintuitively, ORR, a secondary cathodic process for Mg, contributes higher to the corrosion of slowly degrading ultra-high-purity Mg (16.5%) than in fast corroding commercially-pure-Mg (1.3%). The dissolution of Mg typically induces high local pH (10.4 -10.6 in NaCl), which yet gets relatively low and stable (7.5 -8.0) in Ca 2+-containing Hanks’ balanced salt solution (HBSS, pH=7.4) without synthesis pH buffers at 37 ° C under hydrodynamic conditions. In contrast, Zn and Fe alloys experience significant O 2 consumption and localized acidification because dissolved metal cations hydrolyze in HBSS at 37 ° C. The interfacial pH variation is buffered in Ca2+-containing HBSS due to Ca-P-containing precipitates. The addition of synthetic pH buffers stabilizes the interfacial pH for Zn alloys but not for Mg. The Ca-P-containing products layer protects Mg, Zn, and Fe alloys from water and oxygen in Ca 2+-containing HBSS, slowing ORR as the layer densify. These findings emphasize the importance of understanding the interac tions at metal -fluid interface during metal biodegradation.