Ellipsometric Study of Film Formation on Iron in Sodium Sulphate Solutions in the pH range 6–12

Abstract

The optical properties of surface films and the kinetics of their growth on iron in de-aerated Na₂SO₄ solutions of pH 6–12 were studied by means of ellipsometry, at open circuit potential and in the range of anodic polarisation potentials. In solutions of pH 6-10, the properties of surface films formed at constant potentials are dependent upon the electrode potential. At potentials below 500 m V_NHE, the films exhibit low complex indices of refraction (n = 1·36–1·50, k = O·002–0·1) and grow linearly with time. The films formed at pH 6–10 above SOD mV_NHE, and those grown at pH 12 in the whole range of anodic potentials, grow according to a logarithmic law and are protective. These films exhibit a complex refractive index, n = 2·4 (1—0·08i), and are composed of iron oxide.