Study of complex formation in Al(III) – Gluconic acid system and the influence of UV light on the dissolution and passive behavior of Al

The passive and dissolution behavior of Al was studied in 0.25 M gluconic acid solution (HG) under the conditions of continuous illumination (300–400 nm) and non-illumination at 25 °C. Measurements were conducted based on cyclic polarization technique, complemented with SEM examinations. Addition of HG induced localized attack, rather than anodizing, via the formation of Al–gluconate soluble complex species. Complexation with gluconate (G−) anion was elucidated using elemental analysis, IR-spectroscopy and UV–vis spectra. The infrared spectral data is in agreement with coordination through carboxylate-to-metal, with G− acting as a monodentate ligand. A little ennoblement in the pitting potential (Epit) was observed for the illuminated electrode (little influence on pit nucleation). On the other hand, the anodic currents at potentials exceeding the pitting potential are greatly reduced upon illumination (significant influence on pit growth and propagation). These findings indicated that the incident photons of the UV light enhanced the resistance of the passive film towards localized attack. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to the onset of attack. The repassivation potential (Erp), however, was found to be independent of the energy of the incident UV light. SEM images revealed that the severity of localized attack was suppressed upon illumination.