Thermodynamic calculation and experimental investigation of the surface enrichment of electrochemically activated Al–Me (Sn, In, Zn) alloys

The formula for surface composition of solid solution Al–Me (Zn, Sn, In) has been derived from the Gibbs's adsorption equation. Using the first-principles data of Al and Me surface energies, the values of adsorption xMe, and surface enrichment of alloy xMe/NMe versus concentration in the bulk of alloy NMe, were calculated. The value of xMe/NMe decreases in the order In–Sn–Zn in Al–Me alloy. The surface of Al–1 wt.% Sn alloy was analyzed with SIMS and Auger methods for Al and Sn both prior and post corrosion attack or anodic dissolution in Na2SO4. The maximum experimental magnitude of xSn/NSn = 136 was found to be close to the theoretical one. A large diffusion zone was revealed in the near-surface layer of that alloy. The phenomena of ultra-high rate of atomic diffusion within surface layer at room temperature and high solubility of tin are discussed in terms of thermodynamic vacancy model.