Inhibiting effects of some quinolines and organic phosphonium compounds on corrosion of mild steel in 3M HCl solution and their adsorption characteristics

Abstract

The inhibiting effects of quinoline, 8-hydroxyquinoline, benzo(f)quinoline, quinoline-2-thiol, triphenylbenzyl, and tetrabenzyl phosphonium chloride on the corrosion of mild steel (0.26 wt-%C) in deaerated 3M HCl solution have been studied using the determination of polarisation curves as well as linear polarisation measurements. Adsorption isotherms and thermodynamic parameters for the adsorption process of the inhibitors were also determined and are discussed. Except for benzo(f)quinoline, which acts as an accelerator of corrosion, the other quinolines act as corrosion inhibitors. The inhibition was found to be predominantly anodic with quinoline and 8-hydroxyquinoline, while quinoline-2-thiol is a mixed inhibitor. The increase in inhibition efficiency with temperature, the resulting Langmuir adsorption isotherm and the high negative values of the standard free energy of adsorption ΔG_a° denoted chemisorption. Triphenylbenzyl and tetrabenzyl phosphonium compounds were found to be inhibitors of the mixed type. High negative values of the standard free energy of adsorption and the resulting Temkin isotherm indicated chemisorption of the phosphonium compounds. The positive values of the standard enthalpy and entropy of adsorption indicated that adsorption of the inhibitors is associated with the desorption of H₂O molecules from the electrode surface. High values of the standard energy of activation E_a* of the inhibited corrosion process were interpreted in terms of deactivating coverage.