Abstract: Three synthesized piperidin-4-one oximes, 3-ethyl-2,6-diphenyl-piperidin-4-one oxime (A), 1- methyl-3-isopropyl-2,6-diphenyl-piperidin-4-one oxime (B), and 3-isopropyl-2,6-diphenyl-piperidin-4-one oxime (C), were tested at different concentrations to determine their ability to inhibit corrosion of mild steel in 1 mol·L^-1 H₂SO₄ and measured by a mass loss method (at various temperatures), polarization and impedance measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy (EDS), and a quantum chemical method. The synergistic influence of compounds A, B and C with iodides has also been evaluated. All three compounds show good inhibition efficiency in the following order: A>B>C. Compounds A, B and C were found to physically adsorb on the surface of mild steel while obeying the Temkin isotherm. Polarization measurements indicated that these compounds behave as a mixed mode inhibitor. XRD and SEM with EDS studies revealed the formation of a protective barrier on the mild steel surface by these oximes. The electron donating ability of the studied molecules was tested using semi empirical methods. The studies revealed that the oxime nitrogen, the piperidine moiety, and the phenyl ring assist largely in corrosion control. The studies also showed that the steric crowding by the alkyl group in the piperidine ring affects the inhibitor efficiency. Further, it is interesting to note that all of the studied compounds exhibit synergism with iodide ions.

Key words: Oxime, Steric crowding, Polarisation, Impedance, X-ray diffraction, Scanning electron microscopy