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

Abstract

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, φ_(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 α (the dependence of φ_(f/s) on V) and β (the dependence of φ_(f/s) on pH), that is, those parameters are not independent, in the general case. It is also shown that some dependence of parameters α and β on temperature can exist. However, this dependence in the practical region (0 -100 oC) is weak and, in many cases, it is possible to neglect the dependence of parameters α and β on temperature. Finally, we show that it is possible to estimate, theoretically, the parameter β (and accordingly parameter α) 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 β is confirmed experimentally. These findings are discussed in terms of the electronic and point defect structures of the barrier layer.