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

Effects of applied potential and solution temperatureTs 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 potentialEpir values and a simultaneous increase in anodic current density at potentials much higher than theEpit value as the SO42~ ion concentration increases. This implies that (SO42-) ions impede pit initiation at potentials belowEpit but enhance pit growth aboveEpit. This was confirmed from the larger pit growth rate parameterb 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 7s=25°C, the charge densityQ 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 7s=60° and 80°C. From the experimental findings, it is suggested that (SO42-) ions act as inhibitors of pitting corrosion on pure Al belowEpit and at 7s=25°C, whereas they act as promoters at 7s=60 ° and 80°C. This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive Cl ion attack, which occurs at potentials aboveEpit