A resistometric study of the passive film on an evaporated iron electrode

Cathodic reduction mechanisms of the passive film formed on an evaporated iron electrode in a neutral solution were studied by chronopotentiometry, colorimetric analysis and measurement of the electrical conductance of the film electrode (resistometry). By galvanostatic cathodic reduction, the outer γ-Fe₂O₃ layer was reduced to ferrous ion and dissolved with nearly 100% current efficiency without showing any change in the conductance of the film. The inner magnetite layer was partly reduced to metallic iron and the residual part was dissolved as ferrous ion. The latter reaction was predominant in low pH. From the increase in conductance in the initial part of cathodic reduction, it was concluded that adsorbed oxygen fortified the passivity by lowering the conductance of the passive film.