Kinetic Model for Oxide Film Passivation in Aluminum Etch Tunnels

Abstract

Aluminum etch tunnels are micrometer‐wide corrosion pits with large length‐width aspect ratios, in which dissolution proceeds from the tip or end surfaces, while the sidewalls are covered by oxide films. The dynamics of oxide film passivation in etch tunnels has been investigated using decreasing current ramps superimposed on the otherwise constant applied current during anodic etching in 1 N HCl at 70°C. The ramps cause the dissolving area on the tip to be continuously reduced by passivation around its perimeter. Analysis of potential transients along with tunnel width profiles shows that two additive processes contribute to the passivation rate, expressed as the rate of decrease of actively dissolving area: a potential‐dependent Tafel‐type kinetic expression and a term proportional to the time derivative of the potential. The potential driving force is the “repassivation overpotential,” the difference between the potential at the dissolving surface and the repassivation potential there. The kinetic model for passivation is consistent with both potential transients and tunnel width profiles, over a range of current ramp rates. The rate‐controlling step of passivation is considered to be potential‐dependent removal of chloride ions from the dissolving surface. © 2000 The Electrochemical Society. All rights reserved.