Effects of sulphate ion additives on the pitting corrosion of pure aluminium in 0.01 M NaCl solution

Effects of sulphate (SO42-) ion additives on the pitting corrosion of pure aluminium (Al) have been investigated in aqueous 0.01 M NaCl solution as a function of SO42- ion concentration using potentiodynamic polarisation experiment, ac impedance spectroscopy, electrochemical quartz crystal microbalance technique and abrading electrode technique. The addition of SO42- ions to NaCl solution raised the pitting potential (E-pit) of pure Al in value and simultaneously the anodic current density at potentials much higher than the E-pit on the polarisation curves. This implies that SO42- ions impede the initiation of pit on pure Al surface below the E-pit, but enhance the growth of pre-existing pits, which is validated by optical microscopy, It was found that the values of the Cl- ion-incorporated outer film resistance R-out,R-ox in SO42- ion-containing chloride solutions were much lower than those in SO42- ion-free solution, obtained from the impedance spectra measured at potentials below the E-pit. The chloride peak disappeared from the Auger spectra in SO42- ion-containing solutions. The mass decay rate and pit growth rate b were observed to increase in values once the pits were formed in SO42- ion-containing chloride solutions. Based upon the above experimental results, it is suggested that SO42- ions retard the oxide film breakdown by Cl- ion incorporation into the film, while they accelerate the Al metal dissolution through the instantaneous formation of tunnels at the bottom of the pre-existing pits after the exposure of bare surface above the E-pit. (C) 2000 Elsevier Science Ltd. All rights reserved.