STOCHASTIC APPROACH TO ANALYSIS OF PITTING CORROSION OF ANODIC OXIDE FILM ON AL-1WT-PERCENT-SI-0.5WT-PERCENT-CU ALLOY

Abstract

The statistical variation in the pitting potential of anodic oxide films galvanostatically formed on Al-1wt.%Si-0.5wt.%Cu alloy has been discussed as a function of formation current density and formation potential in 0.5 M Na2SO4 + 0.1 M NaCl solution, based upon a stochastic theory. It was found that the distribution of pitting potentials of the anodic films shifted to higher anodic potentials as the formation current density decreased and the formation potential increased. This result indicated that the corrosion resistance is improved for films formed at a lower formation current density and a higher formation potential. Analysis of the distribution of pitting potentials demonstrated that the pitting failure rate function increased with increasing applied potential and that a two-stage variation in pitting failure rate function appeared for the anodic films formed under these conditions. The second stage of the failure rate function exhibits a weaker dependence on the applied potential than does the first stage of the failure rate function. The result seems to arise from an increased crystalline proportion in the anodic films with decreasing formation current density and increasing formation potential. It is suggested from these facts that the enhanced crystalline character accounts for the improved corrosion resistance of the anodic oxide films.