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Effect of surface processes on the dissolution kinetics of aluminium in citric acid

The effect of surface processes on the dissolution kinetics of aluminium in 0.05 M citric acid solutions (pH = 2-8) were studied. Effect of addition of fluoride ions on the dissolution kinetics of Al in citric acid was also studied. Measurements were performed by the mean of controlled potential coulometry using an EG&G PAR Model 273A Potentiostat and rotating disc electrode. Aluminium oxide surface in aqueous solution of citric acid is significantly hydrated. The hydration of aluminium-oxygen bonds on the oxide film and subsequent dissociation of the surface hydroxyl groups leads to the possibility of different surface reactions, such as adsorption and surface complexation. There is a strongly tendency of specifically adsorbed ligands (from the citric acid solution) to form surface complexes at the oxide/solution interface and that to form corresponding solute complexes in the solution phase. Thus, dissolution of aluminium in citric acid is very complex process, in which besides the electrochemical processes, also surface processes play a significant role. In citric acid solutions pH = 3-6, dissolution of Al is essentially controlled by the surface processes. More specifically, dissolution rate depends on the degree of surface protonation and on the concentration of surface complexes. The rate of detachment of surface complexes into solution is the rate-determining step for the process of dissolution. In citric acid solutions pH = 2 and pH = 7-8 dissolution rate increases, especially strongly in solution of pH = 8. In solution of these pH values, dissolution of Al is predominantly influenced by the electrochemical processes, and the dissolution rate is controlled by the mass (solid state) transport. Addition of fluoride ions in citric acid solution increase dramatically dissolution rate of Al, and dissolution kinetics was changed, from surface controlled to mixed controlled processes. Competitive and reversible adsorption of citrate ions displacing fluoride slows down the rate of detachment of the surface complexes (and also the rate of dissolution of Al) which becomes the rate-determining step.

aluminium; citric acid; surface processes; dissolution kinetics

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