✍ Scribed by W. Wilhelmsen; T. Arnesen; Ø. Hasvold; N.J. Størkersen
No coin nor oath required. For personal study only.
The title subject has been studied by weight loss experiments and polarization measurements in 4 M KOH at temperatures between -20 and 60°C and by SEM and energy dispersive X-ray spectrometry of the anode surfaces. The weight loss experiments reveal that addition of 0.1 weight percent
In greatly improves the corrosion resistance of the anode. This finding does not agree with the results of previous papers. This difference may be explained with the longer test time employed in the present work. The present polarization measurements reveal that addition of In to the anode increases the current output at low potentials and causes the current to oscillate at intermediate potentials. The effects of In disappear at high potentials or at low temperatures. The current oscillations were found to exhibit constant period and amplitude. The maximum and minimum current were found to correspond to the continuation of the low potential part of the polarization curve of the Al-In alloy and to the current of super pure Al, respectively. In order to explain these observations we propose a simple model: the active sites on the anode surface are In particles, deposited at the alloy surface at negative potentials. The rapid oxidation of Al at these sites will cause a local pH drop. The active sites become passivated as the local current density exceeds a critical limit, due to growth of aluminium oxide or hydroxide. The low anodic current at the passivated sites will cause the [OH-] to increase again, and the deposit will dissolve. Therefore, the oscillations seem to be connected to deposition and dissolution of Al oxide or hydroxide.
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