Kinetic model for oxide film passivation in aluminum etch tunnels

Aluminum etch tunnels are micrometer-wide corrosion pits with large length- width aspect ratios, in which dissolution proceeds from the tip or end surf aces, while the sidewalls are covered by oxide films. The dynamics of oxide film passivation in etch tunnels has been investigated using decreasing cu rrent ramps superimposed on the otherwise constant applied current during a nodic etching in 1 N HCl at 700 degreesC. The ramps cause the dissolving ar ea on the tip to be continuously reduced by passivation around its perimete r. Analysis of potential transients along with tunnel width profiles shows that two additive processes contribute to the passivation rate, expressed a s the rate of decrease of actively dissolving area: a potential-dependent T afel-type kinetic expression and a term proportional to the time derivative of the potential. The potential driving force is the "repassivation overpo tential," the difference between the potential at the dissolving surface an d the repassivation potential there. The kinetic model for passivation is c onsistent with both potential transients and tunnel width profiles, over a range of current ramp rates. The rate-controlling step of passivation is co nsidered to be potential-dependent removal of chloride ions from the dissol ving surface. (C) 2000 The Electrochemical Society. S0013-4651(00)02-013-9. All rights reserved.