Mitigating steel corrosion in cooling water by molybdate based inhibitors

Abstract

SODIUM molybdate is a passivating inhibitor (passivator) which can inhibit the corrosion of steels in neutral or near-neutral aqueous-environment solutions in the presence of dissolved oxygen. Because of its non-toxic character, molybdate-based inhibitors are considered a good alternative for inhibiting the corrosion of steels in the cooling towers used in refineries and chemical plants. In this paper, the corrosion behaviour of molybdate used alone or in combination with other chemicals was studied by using electrochemical techniques and weight-loss tests. The results indicate that only a few parts per million (ppm) of molybdate are needed to passivate the surface of steel in aerated pure water. However, in the presence of corrosive ions, it is necessary to add a much larger quantity of molybdate in order to achieve the same inhibitory efficiency as in the pure-water system. The corrosion-inhibition efficacy and cost-effectiveness can be improved by a synergistic formulation of silicate and molybdate. The optimum ratio for the molybdate/silicate concentration was found to be 1:4.