The inhibition mechanism of pitting corrosion of pure aluminum by nitrate and sulfate ions in neutral chloride solution

Abstract

The inhibition mechanism of pitting corrosion of natural oxide film-covered pure aluminum by NO− 3 and SO2− 4 ions has been examined in 0.1 M NaCl solution using potentiodynamic polarization experiments, a.c. impedance spectroscopy, Auger electron spectroscopy and a combination of the potentiostatic current transient method and optical microscopy. It was found that NO− 3 ions can be incorporated into the natural oxide film on pure aluminum at open-circuit potential, but the incorporation of SO2− 4 ions into the film hardly occurs. The incorporation of NO− 3 ions lowered the pitting susceptibility of pure aluminum in 0.1 M NaCl solution. Based upon the experimental results, it is suggested that the pitting corrosion inhibition mechanism by anions can be classified into two different groups: inhibition by competitive adsorption of anions (SO2− 4) with Cl− ions and inhibition by the incorporation of anions (NO− 3) into the film rather than competitive adsorption. Both cases may impede the incorporation of Cl− ions into the film, thus inhibiting pitting corrosion of natural oxide film-covered pure aluminum in chloride solutions.