Synergistic effect of KI on the corrosion inhibition efficiency of polynuclear Schiff base, anthracene-9(10H)-one-3-aminopropanoic acid (A9O3AP), on mild steel (MS) in 0.5？M sulphuric acid solution has been investigated using weight loss measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization studies. The results show that inhibition efficiencies on MS increase with increase in concentration of the inhibitor and enhancement in inhibition efficiency was observed on addition of potassium iodide due to synergism. The adsorptions, of inhibitor and inhibitor + KI on the surfaces of the corroding metal obey Freundlich and Langmuir isotherms, respectively. Polarization studies revealed that A9O3AP acts as a mixed type inhibitor. Thermodynamic parameters ( ; ) were calculated using the adsorption isotherms. A probable synergismtic mechanism is proposed. 1. Introduction The use of certain organic compounds as inhibitors is the most practical method to prevent corrosion of the mild steel (MS) in acidic media [1, 2]. Compounds with π-bonds generally exhibit good inhibitive properties due to interaction of π-orbital with the metal surface [3]. Schiff bases are organic molecules possessing azomethine linkage and many of them act as effective potential corrosion inhibitors [4–7]. The addition of halide ions to sulphuric acid solutions containing organic inhibitors has been found to stabilize the adsorption of organic cations, leading to improved inhibition efficiency. The synergistic effect of the halides have been observed to increase in the order Cl-< Br-< I-. Due to large size and ease of polarizability, iodide (I-) shows the highest synergistic effect [8–11]. The present investigation was undertaken to examine the corrosion inhibition behavior and synergism mechanism [12, 13] with I- of a novel polynuclear Schiff base (A9O3AP) derived from anthracene-9 (10H)-one and 3-aminopropanoic acid in 0.5？M H2SO4 solution on MS at 303？K. 2. Experimental 2.1. Inhibitor Polynuclear Schiff base (A9O3AP) was obtained by the condensation of equimolar mixture of anthracene-9(10H)-one and 3-aminopropanoic acid in ethanol. The reaction mixture was refluxed for 5 hours, cooled by keeping overnight to obtain yellow coloured compound, filtered, washed, and dried. Figure 1 shows the molecular structure of polynuclear Schiff base A9O3AP. Anal.calcd for C17H15NO2: C, 76.88; H, 5.65; N, 5.28%. Found. C, 75.96; H, 6.13; N, 4.95%; m.p. = 240°C; IR (KBr): ？cm？1, ？cm？1. 1Hnmr: 12.19, 13Cnmr: ？ppm. Figure 1: Molecular structure of A9O3AP. 2.2. Solutions The
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