Effects of adsorption of O2 and H2O molecules on the corrosion behavior of the NiTi alloy surface: a DFT investigation

Abstract

The influence of O₂ and H₂O adsorption significantly affects the electrochemical corrosion of the NiTi alloy, with unresolved corrosion disparities between the NiTi-B₂ and NiTi-B19′ phases. Density functional theory (DFT) calculations are utilized in this investigation to explore the adsorption of O atoms at varying coverages on the NiTi-B₂(110) and NiTi-B19′(010) surfaces. The goal is to elucidate their oxidation behavior differences. Subsequently, the effect of O adsorption on the dissolution trends of these phases is assessed by inducing Ni/Ti vacancies to simulate alloy dissolution thermodynamically. Additionally, interactions between H₂O molecules and O-pre-adsorbed NiTi alloy surfaces are examined to simulate the atomic evolution of the oxidized surface under exposure to humid air and corrosive solutions. The findings indicate a propensity of the NiTi-B19′ phase to react with O, forming an oxide film more readily than the NiTi-B₂ phase. O adsorption facilitates Ni dissolution and retards Ti dissolution on the alloy surface. Higher O coverage promotes easier dissolution of Ni and Ti atoms on the NiTi-B₂(110) surface compared to the NiTi-B19′(010) surface, underscoring the greater corrosion resistance of the NiTi-B19′ phase. Both clean and O-pre-adsorbed NiTi alloy surfaces physically adsorb H₂O molecules. Notably, an O monolayer substantially mitigates the detrimental effects of H₂O molecules on the corrosion resistance of alloy surfaces. This research contributes to a deeper comprehension of the corrosion mechanisms in NiTi alloys.