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Phosphonic acids are noted for their hydrolytic stability, scale inhibiting property and ability to form complexes with metal cations. Hence they have been widely used as corrosion inhibitors [1][2][3][4][5][6][7][8][9][10][11]. In our earlier study [4], we reported on the synergistic effect of sodium salt of phenyl phosphonic acid (PPA) and Zn 2+ on the corrosion inhibition of mild steel in a neutral aqueous environment containing 60ppm Cl -. It was observed that the formulation consisting of 300ppm PPA and 50ppm Zn 2+ has 95 per cent corrosion inhibition efficiency. The addition of a higher concentration of Zn 2+ decreased inhibition efficiency. The reason for this decrease is investigated in this paper. The inhibition efficiencies of various other PPA-Zn 2+ combinations have also been studied. This study also aims to find out whether phenyl phosphonic acid coordinates with metal cations such as Zn 2+ and Fe 2+ , through the phenyl group or phosphonate group of both, in solution and in solid state. ExperimentalPreparation of the specimens Mild steel specimens (0.02 to 0.03 per cent S; 0.03 to 0.08 per cent P; 0.4 to 0.5 per cent Mn; 0.1 to 0.2 per cent C; and the rest iron) with dimensions of 1 × 4 × 0.2cm were polished to a mirror finish, degreased with trichloroethylene and used for the weight-loss method. Weight-loss methodMild solution specimens, in triplicate, were immersed in 100ml of the solutions containing various concentrations of the inhibitor in the absence and presence of Zn 2+ , for a period of seven days. The weights of the specimens before and after immersion were determined using a Mettler balance AE-240.The UV-visible spectra were recorded using a Hitachi U-3400 spectrophotometer.

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