Molybdate and Tungstate as Corrosion Inhibitors and the Mechanism of Inhibition

A comparative study of corrosion inhibition of iron by molybdate, tungstate, chromate, and nitrite was made to evaluate the relative efficiencies of the different ions and to investigate the mechanism of inhibition. It was found that both molybdate and tungstate are efficient inhibitors and comparable in this respect to chromate and nitrite; it was also shown that, unlike chromate and nitrite, they do not oxidize ferrous ion and, consequently, the primary mechanism of inhibition probably does not involve the coprecipitation of oxidation and reduction products of the metal and inhibitor ion to form a protective film of stoichiometric oxides. In principle, therefore, passivating inhibitors need not be oxidizing agents. Inhibition appears to be associated with adsorption at the metal‐solution interface and it depends on the properties of the complex formed by the inhibitor ions and the metal rather than the homogeneous chemical properties of the ions. Since molybdate and tungstate are efficient but nonoxidizing inhibitors, it is suggested that they could be used in systems containing galvanic couples or oxidizable organics where chromate and nitrite are excluded by their oxidizing character.