Effects of applied potential and solution temperature on the fitting corrosion of pure aluminium in sulphate ion-containing chloride solution

Effects of applied potential and solution temperature T-s on the pitting corrosion of pure aluminium (Al) were investigated in 0.01 M NaCl solutions containing various sulphate (SO42-) ion concentrations using a potentiodynamic polarisation experiment, the potentiostatic current transient technique, ac impedance spectroscopy and atomic force microscopy (AFM). The potentiodynamic polarisation curves showed a rise in the pitting potential E-pit values and a simultaneous increase in anodic current density at potentials much higher than the E-pit value as the SO42- ion concentration increases. This implies that SO42- ions impede pit initiation at potentials below E-pit but enhance pit growth above E-pit. This was confirmed from the larger pit growth rate parameter b values of pure Al exposed to SO42- ion-containing chloride solutions during the abrading action than those exposed to SO42- ion-free chloride solution. Furthermore, at T-s=25 degrees C, the charge density Q values for the Al metal dissolution in the presence of SO42- ions were smaller than the value in its absence. By contrast, as validated by the capacitance values and the AFM images of the re-anodized specimens, an enhanced metal dissolution was observed in SO42- ion-containing chloride solutions at T-s=60 degrees and 80 degrees C. From the experimental findings, it is suggested that SO42- ions act as inhibitors of pitting corrosion on pure Al below E-pit and at T-s=25 degrees C, whereas they act as promoters at T-s=60 degrees and 80 degrees C This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive Cl-. ion attack, which occurs at potentials above E-pit.