Galvanostatic formation of barrier‐type Anodic oxides

Abstract

Experimental Factors Affecting Oxide Growth Results of galvanostatic anodization of various metals with varying valve‐metal behaviour including Nb, Ta, Zr, Ti, Al, W, Te, Sb, and Bi are compiled and compared with respect to the effect of various experimental factors such as surface pretreatment, stirring, solution composition, temperature of the bath, on the efficiency of oxide growth relative to that for other anodic processes. It is concluded that all of the above factors play an important role in the anodization of the non‐typical valve metals, particularly Sb, Bi and W. Typical anodic charging curves associated with the process of oxide growth are given for several valve metals. Experimental data relevant to the phenomena of oxide breakdown are recorded and compared. The results indicate that the mechanism of breakdown varies for the same metal with variations in the experimental conditions.

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

“…Experience shows that chemical polishing invariably leaves a chemical film on the metal surface, and both the composition of this film and its thickness depend on the chemical used and the duration of the chemical pretreatment [27]. In order to avoid complications that may arise as a result of the occurrence of pre-immersion chemical film, careful mechanical pre-treatment has been found to lead to fairly reproducible results, and this procedure has therefore been preferred in many studies on valve metals [25]. The mechanical pre-treatment adopted in the present work involved polishing with metallurgical papers of various grades, with intermittent washing with conductivity water and drying with filter paper between each two successive grades.…”

“…The test Sn anode was placed outside the cathode holder, and its surface was made to approach as far as possible the tip of the Luggin capillary. Since C1-jons were found to hinder the oxide growth on a number of valve metals [25], and because of the fear that this may also happen with Sn particularly in phosphate solutions, precautions were made to minimize the diffusion of C1-ions from the sat calomel halfcell into the electrolytic cell. This was achieved by joining the Luggin capillary to a long salt bridge carrying a tap and containing the same solution under investigation.…”

“…The free end of the salt bridge was dipped into a beaker containing sat KCl in which a dip-type sat.calome1 was immersed. Since aerated and deaerated solutions gave very nearly the same results [25], experiments were made here in aerated unstirred solutions. The Sn electrode was lowered into solution under polarization at the required current density, such that current immediately flowed when the electrode contacted the solution.…”

Investigation of the Passivity of Tin in Neutral Media by cyclic galvanostatic polarization

“…The electrode surface was mechanically polished using metallurgical papers of various grades, washed with distilled water, and was finally rubbed with tissue paper for sufficient time until the surface became bright and smooth [31]. The open-circuit potential was measured using an x-y recorder.…”

“…It appears from the literature that among the primary factors which affect the nature of the passivating films formed on Sn are the nature, composition, and pH of the solution [31]. In order to gain more information on the passive behaviour of Sn in neutral solutions, the present study has been undertaken in order to report the results of open circuit potential, and those of cyclic galvanostatic polarization in solutions which (i) produce thin transparent passive films (eg phosphate buffer), and (ii) those which produce thick anodic films (chloride, and sulphate), and compare the results in both cases.…”