Molybdates as Corrosion Inhibitors in the Presence of Chlorides

The effect of the molybdate ion (MoO₄⁼) concentration on the corrosion inhibition of 1010 steel was investigated in aerated chloride/sulfate-containing solutions. The chloride content was 304 ppm Cl⁻, and the sulfate was 352 ppm SO₄⁼. Weight loss measurements and electrochemical methods were employed for the investigation. The pH was controlled at 8.7. Weight loss measurements indicated that about 165 ppm of molybdate reduced the corrosion rate below 10 mg·dm⁻²/day at room temperature. At 160 F (71 C), about 50 ppm of molybdate reduced the corrosion rate to 4 to 5 mg·dm⁻²/day. The potentiodynamic polarization studies and the examination of corrosion potential and immersion time relationship disclosed that the molybdate ion acts as a passivating agent in the presence of oxygen. Passivity was also obtained under potentiodynamic polarization conditions in a nitrogenated solution. No passivation was obtained in the absence of the molybdate ion. For spontaneous passivation, both molybdate and oxygen (oxidizing agent) were needed. Corrosion rates in a passive state according to the polarization curve should be nil.