Corrosion inhibition by molybdate/polymaliate mixtures

Abstract

The inhibition of the corrosion of mild steel in continuously aerated potable water by the addition of sodium molybdate, Na 2MoO 4, was examined by two independent techniques. The first involved the measurement of open circuit potentials of coupons as a function of time till steady-state, constant potentials, E st, were established. The plot of E st as a function of the logarithm of [MoO 4 2−] disclosed consecutive domains of active corrosion, localized (pitting) corrosion and full passivation. The curve did not allow, however, the evaluation of corrosion rates. This information was ascertained from weight loss determination. The rate of corrosion/log [MoO 4 2−] was mirror image of the potential curve. BELGARD EV® (BELEV), a polymeric maleic anhydride preparation used as an anti-scalant, acted in the neutralized state as a moderate adsorption inhibitor affecting anodic sites. Complete surface coverage was ensured in the presence of ca 100 ppm inhibitor in solution. Mixtures of molybdate and BELEV exhibited inhibition synergism. The combined effect of the mixture exceeded the sum produced by the two individual agents. Differences in behaviour was noted between solutions containing low (30 ppm) and high (≥50 ppm) BELEV. The former solutions failed to inhibit corrosion completely whatever the concentration of MoO 4 2− was. With higher BELEV additions inhibition was achieved at definite [MoO 4 2−]/[BELEV] ratios. These were the compositions which raised the metal potential to or above that characterizing a pitting attack in MoO 4 2− solutions only. Visual observation of the metal coupons and test solutions confirmed the above conclusion. Successful mixtures of MoO 4 2− and BELEV could be worked out which ensured complete inhibition and substantial cost savings.