Single crystal experiments on grains of polycrystalline materials: oxide formation on Zr and Ta

Passivation on polycrystalline Zr (hcp) and Ta (bcc) depends on the crystallographic orientation of the individual grains, determined by electron back scattering diffraction (EBSD). Microelectrochemical experiments yield characteristic data, such as oxide formation potentials, formation factors, capacities, and thicknesses, of local oxide formation on single grains. Strong differences occur on Zr, for Ta only a small influence of crystallographic orientation is observed. The rate of oxide formation, determined from potentiodynamic and capacitive measurements, and EBSD pattern quality, increases with decreasing surface atom density. The dominating influence can be described by the Euler angle phi, the second angle phi2 has a smaller influence. The differences in surface atom density lead to a specific current-dependent potential drop during oxide growth and therefore to a shift in the oxide formation potential. Furthermore the crystallinity and epitaxy of the oxides can depend on the orientation of the base metal causing a variation in ion conductivity.