The effects of film thickness and incorporated anions on pitting corrosion of aluminum with barrier-type oxide films formed in neutral borate and phosphate electrolytes

Abstract

The pitting corrosion behavior of high-purity aluminum covered with barrier-type anodic films, which are formed in neutral borate and phosphate electrolytes, has been examined in 0.5 mol dm−3 NaCl solution at an applied potential of −0.6 V versus Ag/AgCl, which is slightly nobler than the pitting potential of −0.64 V in the same solution. The pitting current density, i p, increased with time after an incubation time, t i. The double logarithmic plot of i p and polarization time, t, reveal two straight lines, which are separated at the time, τ. The slope becomes larger after τ for the specimens anodized in the phosphate electrolyte, while it becomes smaller for those in the borate electrolyte. Both the t i and τ increase with the thickness of the anodic films, and at the similar film thickness, they are much larger for the anodic films formed in the phosphate electrolyte than for those in the borate electrolyte. The corrosion process can be divided into three stages: the incubation period up to t i, the pit nucleation period before τ, and the pit growth period after τ. We have discussed the different pitting corrosion behavior of the aluminum specimens covered with the anodic films formed in the borate and phosphate electrolytes in terms of ion selectivity of the anodic films.