engleski

The effect of fluoride ions on the electrochemical activity of aluminum

Fluoride ions were found to shift the dissolution potential of aluminium in aqueous electrolyte in the negative direction by about 1 V compared with the dissolution potential in chloride solutions. There is, however, a relatively low limiting current density region (of the order of 1 mA cm-2) leading to passivity (or to the dissolution potential characteristic of chloride solutions in mixed fluoride + chloride electrolytes) at some transition time dependent on the current density. All the phenomena are found to depend on the concentration of fluoride in the solution. The open-circuit potential (OCP) was found to follow a concentration dependence interpretable by Langmuir adsorption isotherm-type function. The polarization curve as well as the time dependence of the potential in the limiting current density region could be interpreted as due to consumption of fluoride ions in the oxide film by aluminium ions emerging at the metal/oxide interface, in the process of formation of uncharged aluminium fluoride. No negative difference effect with respect to hydrogen evolution was found. The latter is relatively high at OCP and subsides with increasing current density. It appears to be a process arising at isolated "active centers" and unrelated to general anodic dissolution.

aluminium ; chloride solutions ; fluoride solutions ; Langmuir adsorption isotherm

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