Water Dissociation on α-Titanium(0001): Insights into the Role of Water Vapor on Early Stages of High-Temperature Oxidation of Titanium

Understanding the role of water vapor in the high-temperature oxidation of titanium is imperative for the design of oxidation-resistant Ti alloys as jet engine components. As a first step toward developing a microscopic understanding of the effect of water vapor on the early stages of oxidation, using density functional theory and ab initio thermodynamics-based calculations, we have studied the interaction of a water molecule with the α-Ti(0001) surface. In particular, water dissociation and surface to subsurface diffusion of the dissociated species on the α-Ti(0001) surface have been investigated. The latter corresponds to the situation of very low oxygen coverage on the surface. Within the harmonic approximation, our calculations show that not only water dissociation is facile on this surface, but also at higher temperatures, the adsorption is dissociative. Further, we find that the dissociated H atoms prefer to diffuse from surface to subsurface rather than forming a hydrogen molecule. The effect of the presence of atomic H depends on whether these are at the surface or in the subsurface. For the former, they reduce the surface to subsurface diffusion barrier of the O atom, compared to that observed in the absence of water vapor, while for the latter, the diffusion barrier is enhanced, thereby suggesting that at the very early stage of oxidation when the oxygen coverage is low, the presence of subsurface H atoms enhances the O diffusion barrier by reducing the solubility of atomic O in the Ti lattice.