Anodic Behaviour of Iron in Sodium Sulphate Solutions Containing Fatty Acids

Abstract

The effect of some fatty acids (caproic, caprylic, capric) on the anodic behaviour of iron in de-aerated 0·05 M Na₂SO₄ solutions of pH 6–12 was studied by ellipsometry and by measurements of the double layer capacitance (dlc). At certain concentrations of added fatty acids (e.g., caprylic acid at a concentration of 2 × 10^?2 mole/l) the rate of anodic dissolution of iron and the dlc are both decreased.

The most efficient inhibition is observed at pH 6. The surface films formed in solutions at pH 6 exhibit complex indices of refraction, n = (2·6 ± 0·2) {1—(0·16i ± O·02i)} and grow according to a logarithmic law. Similar optical properties and film growth kinetics are found for films formed in similar solutions at pH 12. These films are composed of iron oxide. At potentials below 500 m V_NHE, an increase of pH in the range 6–10 causes a decrease in inhibitive efficiency of the fatty acids, an increase of dlc, and the formation of non-protective films with low refractive indices. At potentials above 500 mV the surface films are protective irrespective of the pH of the solution.