Synergistic effect of I- ions on the corrosion inhibition of Al in 1.0 M phosphoric acid solutions by purine

The effect of purine, as a safe inhibitor,was investigated by measuring the corrosion of Al in 1.0 M deaerated stirred H3PO4 solution at 25 degrees C. Measurements were conducted under various experimental conditions using polarization and impedance measurements, complemented with EDX examinations of the electrode surface. According to these results, purine alone showed a poor inhibition effect. Addition of I- ions enhanced the inhibition efficiency of purine. The synergistic effect is attributed to enhanced adsorption of purine by the adsorbed I- ions. Potentiodynamic polarization studies showed that purine alone and the mixture of purine and I- ions act as mixed-type inhibitors for the corrosion of Al in 1.0 M H3PO4 solution. The impedance diagram exhibited three time constants or semi-circles of which the sizes are dependent on inhibitor concentration and immersion time. The capacitive time constant at high frequencies may be related to the oxide film itself. The second capacitive time constant at low frequencies can be attributed to oxide film dissolution. The inductive loop at medium frequencies may be attributed to the relaxation of adsorbed charged intermediates. Inhibition via hydrogen bond formation is also discussed here. (C) 2008 Elsevier B.V. All rights reserved.