Investigating the synergism of some hydrazinecarboxamides and iodide ions as corrosion inhibitor formulations for mild steel in hydrochloric Acid: Experimental and computational studies

Abstract

Three hydrazinecarboxamides, namely, 2-cyclohexylidenehydrazinecarboxamide (SCYHEX), 2-(pentan-3-ylidene)hydrazinecarboxamide (SP3ONE) and 2-(pentan-2-ylidene)hydrazinecarboxamide (SP2ONE) were tested as inhibitors of acid corrosion of mild steel both without and with addition of iodide ions. The formulated inhibitor solutions showed excellent corrosion inhibition efficiency as they suppressed steel to the tune of 82% without KI additive, and up to 97% with KI additive. The shifts in corrosion potentials recorded from polarization measurements were anodic and ranged from 25 mV to 55 mV (without KI) relative to the uninhibited electrolyte, suggesting that the Schiff bases are mixed-type corrosion inhibitors with prominent effects on anodic dissolution of steel. The inhibitors adsorb at steel/electrolyte interface as reveled by the AC impedance measurements that showed more than 5 times increase in the charge transfer resistance and equivalence decrease in double layer capacitance of the inhibitor-containing solutions compared to the blank. Theoretical density functional theory (DFT) calculations suggested possible covalent interactions between the Schiff bases molecules and Fe with Fe---N and Fe---C bond lengths ranging from 1.87 Å to 1.96 Å. Experimentally derived synergistic parameters for Schiff bases-iodide ions interactions were generally greater than 1 and a strong H---I bond of ca. 2.3 Å length was observed in what appeared to be first time theoretical modelling of inhibitor-iodide ion synergistic interactions. DFT derived reactivity indices of protonated Schiff bases molecules correlated with the observed strengths of corrosion inhibition. Monte Carlo simulations study also confirmed the tendency of the inhibitor molecules to adsorb on mild steel surface and displace corrosive ions from the steel surface.