Film properties on magnesium in Na₃PO₄ solutions

Abstract

Frequency response of films formed at various potentials and corrosion potential of magnesium electrodes in various concentrations of Na₃PO₄ solution was investigated. An electronic potentiostat with a signal generator was used to control both D. C. and superposed A. C. potentials of the electrode. The impedance at the frequency from 10 to 2 × 10⁴Hz was determined by measuring the amplitude and the phase shift of A. C. and A. C. voltage. The capacitance of the film on the electrode in 0.25M Na₃PO₄ increases with time, probably indicating dissolution or deterioration of the film. The capacitance of the films on the electrode in the Na₃PO₄ solution decreases with increase in the concentration. The fact shows that the film is easily formed. The dissolution and formation of the film on magnesium simultaneously occurs in 0.25M Na₃PO₄ at -1.40V to -1.20V. The deterioration of the film occurs in the potential region over +1.60V (vs. Hg/HgO•0.5N NaOH).