Galvanostatic formation of barrier‐type Anodic oxides. Simplified theory of anodization

Abstract

A comprehensive presentation of the theory of galvanostatic anodization has been given in order to enable interpretation and prediction of the anodization behaviour of barrier layers during technical work on various value metals. The following parameters have been considered, and the relations between them have been established: oxide formation rate, reciprocal capacity, electric field strength, pre‐polarization oxide thickness, interfacial potentials, the height and width of energy barrier for ion transport in the oxide phase, the oxide growth rate as derived from the dependence of cd on time under constant potential, and capacity. Furthermore, it has been demonstrated that potentiostatic curves can be derived from galvanostatic results, and the derived curves are similar to those directly measured by the potentiostatic technique. The theory has been satisfactorily applied to the experimental results based on the anodic charging curves and other complementary measurements on a large number of valve metals including Nb, Ta, Zr, Ti, Al, W, Te, Sb, and Bi.

Evidence for the activation‐controlled galvanostatic growth of thin Anode Films on Tin in some neutral media

“…Complications may arise when (i) the phase boundary potentials change appreciably with the polarizing current density during the measurement of the anodic charging curve, and (ii) when the material of the anode film is not homogeneous, but is composed of more than one phase separated by phase boundaries, and each phase is characterized by its own electric field. Experimence from previous work on valve metals [26] shows us that changes in the phase boundary potentials (metaUoxide, and oxide/solution) are relatively small and can, for all practical purposes, be neglected. Under this condition one may write for the experimentally measured potential E:…”

“…The mathematical treatment of ion transport becomes too complicated when: (i) both cation and anion transport are simultaneously considered, particularly since the respective transport numbers can only be determined by tracer techniques [35] which are not available to us, and (ii) the presence of any contribution from the cathodic process which involves the transport of ions in the direction opposite to the effect of the electric field,. Fortunately enough, the latter situation has not been encountered in almost all work on valve metals even for very thin films [26]. Therefore, we shall limit our treatment on the basis that the total anodic current density used (ia) is equal to the ionic current density i consumed for the formation of the anodic film (cf.…”

“…In almost all the literature on the anodization of valve metals, the oxide formation rate is related to i by an empirical relation having the form [26,[42][43][44]: where a and b are empirical constants characteristic of the system (metal/solution). According to (24), the plot of log(dE/ dt)i against log i yields a straight line relation.…”

“…Furthermore, it is anticipated to: (i) reveal the role of the electric field during oxide growth, and (ii) establish the kinetic parameters of oxide growth on Sn and their inter-relations according to the theory of galvanostatic anodization [26].…”