Fundamental factors determining the magnitudes of electric fields causing anodic oxide growth on film-forming metals

Abstract

It is found that the fundamental factor determining the magnitude of the electric field, E ox, observed for the anodic oxide growth on a valve metal is the value of the electric displacement, D, given for the present case by D = ϵ ox E ox⋍ 1·10 8 V cm −1. Here D is also called the dielectric displacement vector and ϵ ox is the dielectric constant of the oxide. The inverse correlation between 1/ϵ ox and E ox suggested by this equation is indeed observed for the valve metals for which the data are available. It is also shown that the equation provides a possible new way to interpret the effect of the electrolyte solution on E ox, since incorporation of the anions of the electrolyte into the oxide during the anodic growth causes changes in ϵ ox and thence the E ox. In the context of a general discussion of these matters, a brief recapitulation is given of the previous work on other fundamental questions such as: why do only a few metals undergo thick oxide formation and why does the anodic oxide growth cease on every metal after a certain thickness has been achieved?