Modeling of electrochemical oxide film growth -- a PDM refinement

TL;DR

This paper refines the Point Defect Model (PDM) for electrochemical oxide film growth by incorporating physically valid boundary conditions and charge distribution equations, revealing limitations of the original assumptions and enabling more realistic modeling.

Contribution

Introduction of a Refined PDM (R-PDM) that accounts for finite defect dimensions and potential drops, improving the physical accuracy of oxide film growth modeling.

Findings

Original PDM assumptions are valid only under specific conditions.

R-PDM shows electric field and potential drop depend on external potential.

Refined model enables more realistic passive layer formation descriptions.

Abstract

The Point Defect Model (PDM) is known for over 40 years and has brought deeper insight to the understanding of passivity. During the last decades it has seen several changes and refinements, and it has been widely used to analyze growth kinetics of different alloys. Nevertheless, the model has been based on still unconfirmed assumptions, as constant and potential independent electric field strength. To overcome this limitation, we introduce a Refined PDM (R-PDM) in which we replace those assumptions by using additional equations for charge distribution including new physically valid boundary conditions based on considering finite dimensions for the defects by introduction of two defect layer at the film boundaries and by calculating the potential drop at the surface of the film towards the solution over the compact double layer. The calculations by the R-PDM show that the original PDM assumptions are only valid for very specific parameter combinations of oxide film growth and vacancies transport and cannot generally be taken for granted. We believe our findings of electric field and potential drop dependency on the external potential to pave the way for a more realistic description of passive layer formation.