Estimation of Some Parameters in the Point Defect Model (PDM) for the Passivity of Metals

In this paper, we justify that the potential drop at the barrier layer/solution (bl/s) interface of the barrier layer of the passive film on a metal, phi f/s, is a linear function of the applied voltage V and pH in the case of sufficiently thin barrier layer. Furthermore, we show that a relationship exists between parameters alpha (the dependence of phi f/s on V) and beta (the dependence of phi f/s on pH), that is, those parameters are not independent, in the general case. It is also shown that some dependence of parameters alpha and beta on temperature can exist. However, this dependence in the practical region (0 degrees C-100 degrees C) is weak and, in many cases, it is possible to neglect the dependence of parameters alpha and beta on temperature. Finally, we show that it is possible to estimate, theoretically, the parameter beta (and accordingly parameter alpha) by using models for specific systems. For the system considered in the current report (carbon steel in concrete pore water), the estimated value of parameter beta is confirmed experimentally. These findings are discussed in terms of the electronic and point defect structures of the barrier layer.