A quantitative
theory is developed for better understanding of the potentiostatic growth
of passivating films on metals following a previously proposed general
treatment for voltammetric and galvanostatic transients. The theory is
based, in this case, on an ohmic model for the current/overpotential relation
inside the film and a Tafel relation for current/overpotential at the
metal/film interface. The equations are applied to the potentiostatic
growth of the passivating film on Zn in 0,3 M H3BO3 with 0,15 M Na2B4O7
solution. It is shown that the initial charge, after electrode polishing
and before potentiostatic growth, can be disregarded. It is then observed
that, during potentiostatic growth, the film ionic specific resistivity
decays, passes through a minimum value, and then increases, giving rise
to an aged film. The time for the occurrence of this last increase is
proposed to be called "aging time". As a result, there is the
appearance in the potentiostatic experiments of a maximum charge for film
formation, which depends on the growth potential. This maximum charge
presents, with the increase of the growth potential, a maximum followed
by subsequent incrase. As a consequence, it is proposed that the results
can be interpreted through the existence of two kinds of films present,
each appearing in two diffrent potential regions. Finally, a general explanation
is proposed for the evolution of the ionic specific resistivity in terms
of the existence of defect injection at the metal/film (interstitial cations)
and film/solution (cation vacancies) interfaces, the migration of the
defects and their recombination inside the film.
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