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

The pitting corrosion of pure Al in neutral 0.50 M Na2SO4solution in the absence and presence of NaCl, NaBr and NaI under theinfluence of various experimental variables, including X– ions concentration,temperature and anodic step potential (Es,a), has been studiedusing potentiodynamic and potentiostatic techniques, complementedwith Energy Dispersive X-ray (EDX) examinations of the electrodesurface. Potentiostatic measurements showed that the overall processcan be described by three stages. The first stage corresponds to thenucleation and growth of a passive oxide layer. The second and thethird stages involving pit nucleation and growth, respectively. Nucleationof pit takes place after an incubation time (ti). The rate of pitnucleation (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 bedescribed in terms of an instantaneous three dimensional growth underdiffusion control. The effect of Cr2O72–, CrO42–, WO42–, and MoO42– asinorganic inhibitors on the pitting corrosion inhibition of pure Al in(0.5 M Na2SO4 + 0.2 M NaCl) solution has also been studied. Thepresence of these anions results in an increase in the incubation time,and a decrease in the pit growth current density of Al to an extentdepending on the nature and the concentration of the inorganicinhibitors. The inhibition efficiency of these inhibitors decreases in theorder: Cr2O72– CrO42– WO42– MoO42–.