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

Abstract

Effects of applied potential and solution temperature T. on the pitting corrosion of pure aluminium (AI) were investigated in 0.01 M NaCI solutions containing various sulphate (SO, 2-) 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,, values and a simultaneous increase in anodic current density at potentials much higher than the Ep,, value as the SO42-ion concentration increases. This implies that SO,"-ions impede pit initiation at potentials below Ep, but enhance pit growth above Ep,,. This was confirmed from the larger pit growth rate parameter b values of pure A1 exposed to SO4 ~ ion-containing chloride solutions during the abrading action than those exposed to SO42-ion-free chloride solution. Furthermore, at E=25~ the charge density Q values for the A1 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,=60 ~ and 80~ From the experimental findings, it is suggested that SO42-ions act as inhibitors of pitting corrosion on pure AI below E~,, and at T,=25~ whereas they act as promoters at T,=60 ~ and 80~ This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive C1-ion attack, which occurs at potentials above E,,,,.

Importance of Etch Film Formation During AC Controlled Pitting of Aluminium

“…A suitable sulfate concentration in the etchant can lead to a deep and homogenously etched layer with high surface area [Ono & Habazaki 2009]. It was shown previously that sulfate ions could shift the pitting potential in the positive direction [Tomcsanyi et al 1989, Kim et al 1999, Lee et al 2000, Ono & Habazaki 2009. This was later interpreted as the elimination of the competitive adsorption of sulfate ions by chloride ions [Ono & Habazaki 2009].…”

“…This was later interpreted as the elimination of the competitive adsorption of sulfate ions by chloride ions [Ono & Habazaki 2009]. The rise of the pitting potential indicates the impediment of pit initiation of pure aluminium by sulfate ions [Kim et al, 1999, Ono & Habazaki 2009. It was also observed that an excess of H 2 SO 4 in the electrolyte promotes the formation of an etched layer with uniform thickness, since the pit formation is suppressed on the aluminium surface [Ono & Habazaki 2009].…”