The formation of metal hydrides, specifically the beta phase, occurring from the surface to the bulk can be attributed to several factors, including the diffusion of hydrogen atoms and the reactivity of the metal surface.
Diffusion of Hydrogen: When hydrogen gas comes into contact with a metal surface, it can diffuse into the metal lattice. Diffusion is the movement of atoms or molecules from an area of high concentration to an area of low concentration. At the surface of the metal, there is a higher concentration of hydrogen atoms, while the bulk of the metal has a lower concentration. Consequently, hydrogen atoms tend to migrate from the surface into the bulk of the metal, driven by this concentration gradient.
Reactivity of Metal Surface: The reactivity of the metal surface with hydrogen plays a crucial role in the formation of metal hydrides. The metal surface provides sites for hydrogen atoms to react and bind with the metal atoms. This can occur through various mechanisms, such as adsorption and absorption, where hydrogen atoms adhere to the metal surface and then penetrate into the metal lattice.
At the surface, the metal atoms are readily accessible for hydrogen adsorption and subsequent reaction, leading to the initial formation of a hydride layer. As more hydrogen atoms diffuse into the metal lattice, they encounter metal atoms further away from the surface. These deeper metal atoms are shielded and less reactive due to the presence of other metal atoms surrounding them. As a result, the rate of hydrogen diffusion slows down, and the formation of the hydride progresses from the surface to the bulk.
Overall, the diffusion of hydrogen and the reactivity of the metal surface drive the formation of metal hydrides from the surface to the bulk. As hydrogen atoms continue to diffuse and react with the metal atoms deeper within the material, the metal hydride phase gradually extends throughout the bulk of the metal.