An Investigation into the Mechanism of Inhibition of a Synergistic Dianodic Corrosion Inhibitor

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Abstract
A synergistic nitrite/N,N-di-(phosphonomethyl) methylamine corrosion inhibitor for ferrous metals in aqueous media has been developed which is an effective corrosion inhibitor when compared to nitrite, zinc chromate, or zinc phosphonate. The protection given to ferrous metals in aerated neutral solutions by this corrosion inhibitor is a direct result of the action on the anodic dissolution/passivation processes. The inhibitor formulation has little or no effect on the cathodic oxygen reduction reaction. It is suggested that inhibition of corrosion is due to the repair of the oxide film by anodically deposited ferric aminophosphonate, which forms a better resistant barrier to corrosion than the γ-FeOOH produced by nitrite alone. The passive film formed by the anodically deposited material in the presence of the nitrite/N,N-di-(phosphonomethyl) methylamine equimolar mixture is thinner, more protective, and less prone to pitting corrosion than the passive films formed in the presence of nitrite alone.

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