CORROSION INHIBITION BY MOLYBDATE POLYMALIATE MIXTURES/

The inhibition of the corrosion of mild steel in continuously aerated potable water by the addition of sodium molybdate, Na2MoO4, was examin ed by two independent techniques. The first involved the measurement o f open circuit potentials of coupons as a function of time till steady -state, constant potentials, Epsilon(st), were established. The plot o f Epsilon(st) as a function of the logarithm of [MoO42-] disclosed con secutive domains of active corrosion, localized (pitting) corrosion an d full passivation. The curve did not allow, however, the evaluation o f corrosion rates. This information was ascertained from weight loss d etermination. The rate of corrosion/log [MoO42-] was mirror image of t he potential curve. BELGARD EV(R) (BELEV), a polymeric maleic anhydrid e preparation used as an anti-sealant, acted in the neutralized state as a moderate adsorption inhibitor affecting anodic sites. Complete su rface coverage was ensured in the presence of ca 100 ppm inhibitor in solution. Mixtures of molybdate and BELEV exhibited inhibition synergi sm. The combined effect of the mixture exceeded the sum produced by th e two individual agents. Differences in behaviour was noted between so lutions containing low (30 ppm) and high (greater than or equal to 50 ppm) BELEV. The former solutions failed to inhibit corrosion completel y whatever the concentration of MoO42- was. With higher BELEV addition s inhibition was achieved at definite [MoO42-]/[BELEV] ratios. These w ere the compositions which raised the metal potential to or above that characterizing a pitting attack in MoO42- solutions only. Visual obse rvation of the metal coupons and test solutions confirmed the above co nclusion. Successful mixtures of MoO42- and BELEV could be worked out which ensured complete inhibition and substantial cost savings.