The transport of ions and electrons through microscopically inhomogeneous passive films: Breakdown implications

The effects of microscopic non-uniformity (e.g. impurities and other point defects, dislocations and grain boundaries) on current flow in passive layers are examined. The approach is to model inhomogeneity effects by a distribution (spatial and energetic) of activation barriers to simulate the imperfections in question. The response of the resulting system to the driving force provided by the electrochemical potential gradient is obtained numerically by solving self-consistently the microscopic hopping equations for the local currents over the unequal barriers. The results of the calculations show that if the microscopic regions surrounding Cl − (or similar impurity) ions have somewhat lower activation energies for charge motion than the medium itself, the current increases exponentially (to first order) with the Cl − concentration in the passive film. This could be a very important factor for the breakdown of passivity. It is found that lower amounts of Cl − can lead to breakdown if there are grain boundaries and other easy diffusion paths in the passive layer, thus indicating that non-crystalline films can be more effective than crystalline films for maintaining passivity. Plots of the product of conductivity with temperature vs reciprocal temperature, which are exponential for uniform activation energy barriers, are shown to be non-exponential when there are easy diffusion paths in the passive layer.