  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.
