Structural transition in pitting corrosion of binary alloys

A growth model is proposed to simulate the pit formation in pitting corrosion of binary alloys. The metal is modeled by a random binary alloy where one species is easy to dissolve and the other species is hard to dissolve. The pit growth model is described by the anti-diffusion-limited aggregation model. A computer simulation is performed to form the corrosion pit in two dimensions for various composition ratios. It is shown that at the percolation threshold, a structure transition of the corrosion pit occurs between the diffusion-limited corrosion and the dissolution-limited corrosion. Above the percolation threshold of easily dissolving species, the pitting corrosion is controlled by the diffusion of anions and the width w of the pit scales as w≊${\mathit{h}}^{1.86\pm 0.05}$ where h is the depth of the pit. Below the percolation threshold, the pitting corrosion is controlled by dissolution of metal and the width w of the pit is proportional to the depth h of the pit.