The growth mechanism of pits in NaCl solution under anodic films on aluminum

Pitting behaviour and pit morphology on anodized pure aluminum in NaCl solution were studied with an electrochemical method and scanning electron microscope (SEM). Severe pitting happened at high potential for anodized pure aluminum. Polarization curves show that the pitting process was diffusion controlled. Many pits with various sizes were observed on sample surfaces after polarization. Some pits developed to several hundreds of microns in diameter, but with the anodic films remaining as covers of the pits. Small openings were seen on the film covers as the diffusion channels for dissolved species within the pits, which reveals that the pits developed in a highly occluded state. The pits show different shapes from hemispherical to oblate. The ratio of pit radius to pit depth, R/t, is proportional to the ratio of pit opening radius to pit radius, r/R. The bigger the ratio r/R is, the more oblate the pit shape is. A modified diffusion controlled model is suggested to explain the formation of oblate pits.