Anodic polarization behavior of high purity aluminum in acidic and neutral solutions.

The cause of anodic polarization behavior of high purity aluminum in deaerated acidic and neutral solutions was studied by electrochemical measurement. Anodic polarization curves showed limiting current density depending on pH in pH range 4-7. But in the pH range 1-4, anodic polarization curves showed no clear limiting current density, and the values of current density at -900 mV vs. SCE were identical. These behaviors can be explained in the following way. In neutral solution, film thicknese is large relatively, and the potential difference across the Helmholtz layer at the film-electrolyte interface is independent of the electrode potential. In this case, limiting current density depends only on pH. But in acidic solution, film thickness is very small, and the electrode potential affects the potential difference across the Helmholtz layer on which current density depends. The dissolution of film is attributed to the electrochemical reaction depending on inner potential difference at the film-electrode interface.