Molybdate and nitrite as corrosion inhibitors for copper-coupled steel in simulated cooling water

The use of molybdate (MoO42- and nitrite (NO2-) as corrosion inhibitors for mild steel galvanically coupled to copper was investigated. In simulated cooling water (SCW), galvanic contact of copper is harmful to mild steel in acidic and alkaline pH ranges from 3 to 12. Galvanic contact is very harmful when MoO42-, NO2-, or both are present at low pH, but it becomes less harmful or even beneficial as pH increases toward the alkaline region. MoO42- alone in SCW was shown to inhibit corrosion of copper-coupled mild steel at pH greater than or equal to 6, and nitrite was shown to do the same at pH greater than or equal to 4.5. Below these values, both accelerated galvanic corrosion. Synergistic corrosion inhibition by the inhibitors was obtained at pH > 6. Galvanic contact of copper was shown to be beneficial to mild steel when the system was inhibitive.