On the Pitting Corrosion Behaviour of Pure Al by Halide Ions in Neutral Sulphate Solutions and the Effect of Some Inorganic Inhibitors

Abstract

The pitting corrosion of pure Al in neutral 0.50 M Na2SO4 solution in the absence and presence of NaCl, NaBr and NaI under the influence of various experimental variables, including X– ions concentration, temperature and anodic step potential (Es,a), has been studied using potentiodynamic and potentiostatic techniques, complemented with Energy Dispersive X-ray (EDX) examinations of the electrode surface. Potentiostatic measurements showed that the overall process can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involving pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (ti). The rate of pit nucleation (ti –1) increases with increasing halide concentration, temperature, and applied potential. The pit growth current density (jpit) increases linearly with t1/2, indicating that the pit growth can be described in terms of an instantaneous three dimensional growth under diffusion control. The effect of Cr2O7 2–, CrO4 2–, WO4 2–, and MoO4 2– as inorganic inhibitors on the pitting corrosion inhibition of pure Al in (0.5 M Na2SO4 + 0.2 M NaCl) solution has also been studied. The presence of these anions results in an increase in the incubation time, and a decrease in the pit growth current density of Al to an extent depending on the nature and the concentration of the inorganic inhibitors. The inhibition efficiency of these inhibitors decreases in the order: Cr2O7 2– > CrO4 2– > WO4 2– > MoO4 2–