Metal Dissolution Kinetics in Aluminum Etch Tunnels

Abstract

The potential dependence of the metal dissolution current density of aluminum in etch tunnels was measured, using pulsed increases of current during galvanostatic etching experiments. Etching was carried out in 1 N HCl solution at 65ЊC, and was followed by analysis of the topographic development of the dissolving surfaces using scanning electron microscopy. By manipulation of the current waveform, the dissolving area and the applied current were varied independently. Parallel experiments with current interruptions were used to help identify the area of the dissolving surfaces during the anodic pulse, which were shown to be submicron size patches on the tunnel tip surfaces. After correction for solution-phase potential drops, the current was found to obey an exponential Tafel dependence on potential. Measurements of the dissolved depth vs. time show that the rate of cathodic hydrogen evolution is significantly increased during the anodic pulse, by comparison with that at the more negative potentials during constant current etching. Good agreement of the present current-potential relation was obtained with that derived from the measurements of pit currents in thin films by Frankel et al.

Kinetic Model for Aluminum Dissolution in Corrosion Pits

“…In another kinetic study of dissolution in tunnels, the potential driving force for dissolution was measured within 0.1 ms after step increases of applied current during etching. 21 Contrary to the earlier measurements of pit growth rates, the results indicated a Tafel-type exponential current/potential relationship, with dissolution current densities as high as 100 A/cm 2 . The present work addresses the issue of the potential dependence of the dissolution current density.…”

“…Measurements of dissolution kinetics in etch tunnels were carried out using a current step waveform developed previously in a study of etching in 1 M HCl. 21 That waveform is illustrated in Fig. 1, where symbols for the various current levels and step times are defined.…”

“…Experimental kinetic studies of oxide passivation in aluminum pits were carried out by the present authors, and a model for passivation was presented. [13][14][15][16] Those investigations found evidence that a pit surface is covered by a monolayer of chloride, at potentials near the critical potential for repassivation; chloride desorption from this layer initiates oxide film growth.…”

“…It was shown in Ref. 21 that the current interruption completely passivated the actively dissolving tunnel tip surface; however this surface experienced a high rate of pit initiation during the anodic pulse, resulting in recovery of the dissolution current within 0.5-1 ms. This time agrees with the decay times of the initial peaks in Fig.…”

A Mathematical Model for the Growth of Aluminum Etch Tunnels

“…[4][5][6][7] In these experiments, the step or ramp causes the oxide film to cover part of the tunnel tip. In step experiments, passivation occurred in a time of order 1 ms, comparable to the characteristic time of potential decreases accompanying the step, but much smaller than the time for concentration changes to occur.…”