Synergistic Inhibition of Corrosion of Carbon Steel by the Ternary Formulations containing Phosphonate, Zn (II) and Ascorbic Acid

Studies on corrosion inhibition of carbon steel in low chloride aqueous environment using ternary inhibitor formulations based on phosphonates namely 1-Hydroxyethane-1,1-diphosphonic acid (HEDP) and Nitrilotris(methylenephosphonic acid) (NTMP) are presented. From these studies, an environmentally friendly organic compound namely ascorbic acid (AA) is proved to be an excellent synergist to the binary inhibitor formulations, HEDP – Zn 2+ and NTMP – Zn 2+ in corrosion control of carbon steel. Gravimetric studies infer that the required minimum concentrations of both the phosphonate as well as zinc ions for good corrosion inhibition could significantly be reduced by the addition of ascorbic acid. The minimum concentration of Zn 2+ required for effective inhibition in case of HEDP – Zn 2+ – AA and NTMP – Zn 2+ – AA are 15 ppm and 20 ppm respectively. Further, still lower concentrations of Zn 2+ are sufficient for the maintenance of the protective film in case of both the inhibitor formulations. Studies on effect of pH on corrosion inhibition indicate that both the ternary inhibitor formulations are effective in corrosion control in wide pH ranges. HEDP – Zn 2+ – AA is effective in the pH range 5.0 to 9.0 and NTMP – Zn 2+ – AA is effective in the pH range 4.0 to 10.0.

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