The function of the repassivation process in the inhibition of pitting corrosion on aluminium

Abstract

The pitting potential of aluminium has been measured in aqueous solutions containing 3 × 103 ppm Cl- ion and one of six inhibitors in the range 0.08-0.32 M : acetate, benzoate, citrate, nitrate, phosphate and tartrate, over the pH range 4-9 and in glycerol/water mixtures containing acetate, benzoate, nitrate and phosphate. The quasi-stationary potentiostatic technique was unsatisfactory in benzoate and phosphate solutions which caused visible film thickening since the time of exposure was more important than the inhibitor concentration or scan-rate. A potentiostatic scraping electrode technique was therefore employed, the pitting potential being indicated by incomplete repassivation. The inhibitors were effective in raising the pitting potential in the decreasing order: nitrate ≫ phosphate > citrae = tartrate > benzoate > acetate, with benzoate occupying second place at the highest concentration employed. Current transients decayed exponentially (i = io exp (-βt). β increased linearly with increasingly active potential values. The effect of inhibitors upon β was in the same order as for the pitting potential effect. For all solutions the pitting potential corresponded to the same value of β′ : 5 × 10-3s-1. Pitting occurs when the processes of film growth and metal dissolution at the metal surface are kinetically related in such a way that complete repassivation does not occur. Although the inhibitors act in several different ways, their specific importance lies in the effect of such actions upon the balance of the competing processes at the metal surface. © 1980.