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 70ЊC. 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.

Thermodynamic descriptors to predict oxide formation in aqueous solutions

“…This approach omits key aspects of the oxidation problem at the nanoscale. Initial (≤10 −7 s) and kinetically-controlled film growth can be described by decoupled kinetic models, such as percolation and diffusion models [1][2][3], however, these rely often on experimentally-derived parameters that are challenging to extract. On the other hand, thermodynamic phase diagrams have demonstrated success as predictive tools for understanding scale growth [4][5][6][7][8][9].…”

The electrical double layer in a nanopore in a barrier surface film

“…For example, it is important to know whether chloride ions can penetrate into the pores and contact the exposed metal surface. According to various models of pit initiation, the presence of chloride ions at the metal surface would determine whether breakdown or repassivation of this surface should be expected [10][11][12][13][14].…”

Thermodynamic Descriptors to Predict Oxide Formation in Aqueous Solutions

“…This technique omits key aspects of the oxidation problem at the nanoscale, such as diffusion, kinetics, and the relative energetic stabilities of competing phases. Initial ( s) and kinetically controlled film growth can be described by decoupled kinetic models, such as percolation and diffusion models; − however, these often rely on experimentally derived parameters that are challenging to extract. On the other hand, thermodynamic phase diagrams have demonstrated success as predictive tools for understanding scale growth. − Predominance diagrams, including Pourbaix , and stability diagrams, − compare how the environment, described by pH, potential, concentration, and temperature, changes the stability regions of ions and solids.…”