Frequency response of films formed at various potentials and corrosion potential of magnesium electrodes in various concentrations of Na
3PO
4 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 × 10
4Hz 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 Na
3PO
4 increases with time, probably indicating dissolution or deterioration of the film. The capacitance of the films on the electrode in the Na
3PO
4 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 Na
3PO
4 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).
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