Electrochemical behaviour of aluminium in electrocoagulation processes

• Electrochemical behaviour of Al electrodes in electrocoagulation processes studied.
• Important role of solution chemistry and electrode surface roughness on passivation.
• Humic acid attenuated the depassivating effect of chloride ions.
• Freshly roughened electrode surfaces exhibited spontaneous depassivation.
• Reactor model predicted lower pH (5.5) at anode surfaces than in bulk solution (7).

Passivation of aluminium electrodes is a common problem in electrocoagulation reactors used for the purification of natural waters, leading to high cell voltages and compromising the viability of such industrial processes. In order to identify possible mitigation strategies, cyclic voltammetric, potentiostatic and galvanostatic measurements were made to investigate the effects of electrode surface topography and solution composition on the electrochemical behaviour of pure aluminium and Al 1050 in neutral electrolyte solutions. In addition, electrochemical impedance spectroscopy was used to estimate thicknesses of passive layers before and after dissolution. Chloride ions, well-known pitting promoters, were found to facilitate dissolution of passive surfaces, though attenuated by the presence of the naturally-occurring contaminant humic acid. Under galvanostatic conditions, a smooth aluminium electrode showed a constant rise in electrode potential due to passivation. However, even in the absence of pitting promoters electrodes with a ‘rough’ surface finish (Ra-values>1 µm) exhibited an initial potential increase, followed by a dramatic decrease to relatively low steady state values due to ‘spontaneous de-passivation’. The precipitation of dissolved aluminium(III), releasing H+ ions, was identified as the probable cause of this effect, leading to acidic pH values, locally dissolving the passive layer and enhancing dissolution rates.