Impact of Molybdate and Nitrite Anions on the Corrosion of Mild Steel

Cooling water systems (CWS) suffer many forms of corrosion and failure. The diversity of corrosion attack is caused by differences in cooling water system design, temperature, flow, water chemistry, alloy composition and operation. Water treatments are extensively used to reduce corrosion in cooling water systems, including anti-scaling, anti-fouling and corrosion inhibition. A range of inhibitors, such as chromate, nitrite, zinc salt, polyphosphate, phosphonate, polycarboxilic acid, silicate, molybdate, are used to inhibit ferrous metals in cooling water systems. Chromate is a highly effective anodic inhibitor; it reacts with ferrous ions to produce a mixture of Cr₂O₃ and Fe₂O₃, which form a protective oxide film over the anodic sites on the metal surface.  Chromate, however, is toxic, and so other inhibitors are preferred.  For example, molybdate, which has long been known as an inorganic corrosion inhibitor, has been shown to function as an inhibitor in CWS, although not as effectively as chromate based inhibitors.  To improve performance, molybdate is often employed in conjunction with nitrite, an oxidising inhibitor.
The paper will summarize work on the synergistic effect of molybdate and nitrite corrosion inhibitors on mild steel in aerated solutions containing chloride.