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Corrosion and surface processes on aluminum in citric acid

The corrosion and surface processes on high purity (5N) aluminium covered by a thin surface oxide film was studied in 0.05 M citric acid solutions (pH = 2-8), by the means of potentiodynamic polarization, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). An EG&G PAR Model 273A potentiostat and EG&G PAR Lock-In Amplifier Model 5210, controlled by PC, were used in electrochemical measurements. The results of potentiodynamic polarization measurements show that in citric acid solutions aluminium exhibits passive behaviour, due to formation of barrier-type of thin 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/citric acid solution interface. More specifically, dissolution of aluminium is essentially controlled by the surface processes (adsorption and surface complexation). Dissolution (corrosion) rate is very small and 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 results of cyclic voltammetry measurements show that during anodic polarization, when the dissolution rate becomes higher than adsorption rate, the adsorbed Al-citrate surface complexes species were removed from the surface into solution. This leads to the effect of large hysteresis when the polarization is reversed. The EIS measurements show that impedance of the system Al/citric acid is characterized by a high-frequency capacitive and a low-frequency inductive behaviour (Fig 1.). The high frequency capacitive loop can be related with the tickness and dielectric properties of the barrier oxide film. The low-frequency inductive loop of the impedance spectra can be attributed to the observed surface processes (especially to the adsorption process). From the experimental impedance data polarization resistance (Rp), capacitance (C) and inductance (L) were evaluated using the appropriate equivalent circuit.

aluminium; corrosion; citric acid

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