This would result in a buildup of excess vacancies beyond the thermodynamic equilibrium concentration and enhanced diffusion of the less noble element toward the surface and the more noble element away from the surface, owing to compositional gradients created by the selective removal process. This upper-bound calculation, based on a divacancy diffusion mechanism, can support currents as high as 0.2 mA/cm2 (1.3 mA/in.2 ), corresponding to a divacancy diffusivity of 1012 cm2 /s (1.6 1013 in.2 /s). R