Processes on aluminium on the negative side of the open-circuit potential

Cathodic current—time transients were recorded on high-purity aluminium immersed into a neutral 2 M NaCl solution, when subjected to a potential pulse from a potential around the open-circuit potential, to different negative values. Hydration of the oxide layer occurs at potentials more negative than −1700 mV vs. SCE, whereupon hydrogen is evolved at the metal—hydrated oxide interface. A linear Tafel function is obtained for the maximum current attained in that potential region with a slope of 110 mV dec⁻¹ indicating that the Volmer reaction is the rate-determining step, and the exchange current density for hydrogen evolution at aluminium is estimated at 5 × 10⁻¹¹ A cm⁻². Anodic current responses to the return of the potential to the rest values were also recorded at three different time scales. The presence of some substances, formed at the more negative potentials, other than gaseous hydrogen, was detected and their dependence on potential and cathodic pulse duration is discussed.