Effect of antimony(III) on carbon steel corrosion inhibition by molybdate in citric acid solution

Molybdate is known as a good corrosion inhibitor of carbon steel (CS). But it cannot inhibit CS corrosion in citric acid solution at 85 °C. It has been observed that the presence of small concentration of Sb(III) along with MoO₄^2- inhibits CS corrosion efficiently. The corrosion inhibition by MoO₄^2- have been studied extensively by varying the concentration of Sb(III) and MoO₄^2-. A critical concentration of MoO₄^2- is required to passivate CS in acid medium in the presence of Sb(III). The study shows that molybdate forms a thin protective layer on CS surface in presence of Sb(III) which provides the corrosion inhibition. Inhibition property and the layer composition on CS surface have been studied by electrochemical and surface analytical techniques. The protective layer is found to be composed of both Mo and Sb and appears to be formed due to cathodic reduction of Mo⁶⁺ to Mo⁵⁺ and Mo⁴⁺ and anodic oxidation of Fe and Sb. (author)