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Corrosion and passive behaviour of aluminium in citrate solutions

The corrosion and passive behaviour of high purity (5N) aluminium covered by a thin surface oxide film was studied in 0.05 M citrate buffer solutions (pH=2-8), by the means of potentiodynamic polarization, cyclic voltammetry, transient techniques and electrochemical impedance spectroscopy. Electrochemical measurements were performed using an EG&G PAR Model 273A Potentiostat and EG&G PAR Model 5210 Lock-In Amplifier, remotely controlled by PC. The results show that in citrate solutions aluminium exhibits passive behaviour (Fig.1.), due to formation of barrier-type of passive film. The dissolution (corrosion) of aluminium is intimately connected with properties of its passivating surface oxide film and the processes occurring at the oxide/citrate solution interface. Aluminium oxide surface in aqueous citrate solutions is significantly hydrated. The hydration of aluminium-oxygen bonds on the oxide film and the 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 citrate solution) to form surface complexes at the oxide/solution interface and that to form corresponding solute complexes in the solution phase. Thus, dissolution (corrosion) of aluminium in citrate solutions is a very complex process, in which besides the electrochemical processes, also surface processes play a significant role. The results show that concentration of H+ ions, i.e. pH of citrate solution, determined which of these processes prevailed in the process of dissolution of aluminium. In citrate solutions pH=3-6, dissolution of aluminium is essentially controlled by the surface processes (adsorption and surface complexation). 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. The establishing of steady state on aluminum in citrate solution (i.e. attainment of adsorption-desorption equilibrium) is very slow process, as shown at polarization curves recorded on aluminium electrode after different stabilization time (Fig.2.). During anodic polarization, when the aluminium dissolution rate becomes higher than adsorption rate, the anodicaly dissolving substrate removes the adsorbed particles from the surface. This leads to the effect of large hysteresis when the polarization is reversed (Fig.3.). In citrate solutions pH=2 and pH=7-8 dissolution rate increases, especially strongly in solution of pH=8. Aluminium corrodes predominantly by the electrochemical processes (adsorption peak was not observed), and the dissolution rate is controlled by the mass (solid state) transport.

aluminium; corrosion; passive behaviour; citrate solution

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