Determination of kinetic parameters in the point defect model (PDM) for iron using electrochemical impedance spectroscopy and first-principles calculations

Abstract

Unveiling the specific corrosion kinetics is of great interest to the corrosion assessment and service life prediction of metals. Herein, the impact of electric field on point defects migration, the concentration profile in-depth of defect densities, and the kinetic parameters of passivity for iron are studied, in combination of DFT calculations and experiments. The migration energy barriers of point defects within the passive film on iron are found to reduce linearly with increasing electric field strength. The exponential relation between the fluxes of point defects and the electric field is employed rather than Nernst-Plank relation. Prior to EIS optimization, some key parameters (i.e., α, β,${\alpha }_5$ and ${\alpha }_6$) in the PDM are determined from Mott-Schottky analysis on the passive film in steady state. Values of parameters, such as transfer coefficients, standard rate constants, diffusion coefficient, electric field strength, steady-state thickness of the bl, and reaction order for H⁺, are obtained by optimizing EIS data, some of which are well validated by DFT calculation and XPS results. A combination of DFT calculations with electrochemical tests will be a possible way to improve the EIS optimization procedure in the PDM.