The corrosion inhibition of mild steel in 1.0？M H2SO4 solution by ethyl hydroxyethyl cellulose has been studied in relation to the concentration of the additive using weight loss measurement, EIS, polarization, and quantum chemical calculation techniques. The results indicate that EHEC inhibited corrosion reaction in the acid medium and inhibition efficiency increased with EHEC concentration. Further increase in inhibition efficiency is observed in the presence of iodide ions, due to synergistic effect. Impedance results reveal that EHEC is adsorbed on the corroding metal surface. Adsorption followed a modified Langmuir isotherm, with very high negative values of the free energy of adsorption . The polarization data indicate that the inhibitor was of mixed type, with predominant effect on the cathodic partial reaction. The frontier molecular orbitals, HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) as well as local reactivity of the EHEC molecule, were analyzed theoretically using the density functional theory to explain the adsorption characteristics at a molecular level. The theoretical predictions showed good agreement with experimental results. 1. Introduction Organic compounds containing polar functional groups such as nitrogen, sulphur, and/or oxygen in a conjugated system have been reported to be effective as corrosion inhibitors for steel [1–8]. Some of the organic compounds are polymeric in nature and therefore possess multiple active centres. The study of corrosion inhibition by polymers has been on the increase in recent times. Polymers are employed as corrosion inhibitors because the presence of many adsorption centres helps them form complexes with metal ions. The formed complexes were adsorbed on the metal surface to form a barrier film which separated the metal surface from the corrosive agents present in the aggressive solution [9–14]. The effectiveness of inhibition by the adsorbed inhibitor system will be determined by the energy released on forming the metal-inhibitor bond compared to the corresponding changes when the pure acid reacts with the metal [15]. Some authors have reported on the effectiveness of polymeric corrosion inhibitors [16–20]. In their accounts, the inhibitive power of these polymers is related structurally to the cyclic rings and heteroatoms which are the major active centres of adsorption. In order to support experimental studies, theoretical calculations are conducted in order to provide molecular-level understanding of the observed experimental behaviour. The
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