Effects of SO 4 2? , S2O 3 2? and HSO 4 ? ion additives on the pitting corrosion of pure aluminium in 1 M NaCl solution at 40–70 °C

Effects of sulphate (SO₄^2–), thiosulphate (S₂O₃^2–) and hydrogensulphate (HSO₄^–) ion additives on the pitting corrosion of pure aluminium in 1 M NaCl solution have been investigated at various solution temperatures T_s, 40–70 °C using potentiodynamic polarisation experiment, double current step experiment and scanning electron microscopy (SEM). From the analysis of the galvanostatic potential transients obtained from the double current step experiment, it was suggested that both SO₄^2– and S₂O₃^2– ions retard the initiation of the etch pits, and that they also inhibit the passivation of the etch pits during the current interruption to promote the subsequent re-activation of the etch pits over the whole range of T_s. In contrast, it was found that HSO₄^– ions promote the initiation of the etch pits and at the same time, they enhance the passivation of the etch pits during the current interruption with rising T_s. From the SEM micrographs, it was revealed that as T_s increased the pit morphology changed from circular shape to irregular shape with rough surface in the presence of SO₄^2– or S₂O₃^2– ions, but it changed to strip-like shape in the presence of HSO₄^– ions. The beneficial effects of anion additives on the increase in surface area were discussed on the basis of the morphological change of the etch pits in the presence of anion additives.