Ionic motion during field-assisted oxidation of aluminium studied by molecular dynamics simulations

Molecular dynamics simulations with the REAX force field were performed to model the growth of an oxide layer on (1 0 0)-oriented aluminium in three different oxidation environments: atomic oxygen gas, liquid water, and an aqueous ammonium tartrate solution. The anodizing behaviour was studied at two different temperatures (300 K and 600 K), in the absence of an external electric field and with field strengths of 0.5–2.0 V/Å applied over the growing oxide. The aim of this study is to determine how the ion motion in the oxide layer is affected by an electric field and by the oxidizing medium. The principal findings are that (1) O and Al ions move through the oxide via interrupted sequences of short replacement steps rather than as a continuous motion across the oxide, (2) dissociation of H2O molecules at the surface is crucial, and (3) the dependence of the ion motion on the electric field strength exhibits steps in the growth behaviour.