The influence of iodide in corrosion inhibition by organic compounds on carbon steel: Theoretical and experimental studies.

The corrosion inhibition by cinnamic, p-coumaric and caffeic acid of carbon steel in 1.0?M HCl and its synergistic combination with KI was investigated using electrochemical and quantum chemical methods. A synergistic effect was observed when 2 mM potassium iodide was added to 0.1 mM inhibitor, where the corrosion inhibition efficiency (IE%) increased from 70% to 80%. In addition, the participation of iodide in the Fe surface was investigated during the adsorption of corrosion inhibitors such as cinnamic, p-coumaric and caffeic acids on Fe(110). The formation of an iodide monolayer on the Fe(110) surface was studied by density functional theory (DFT) periodic slab calculations. The DFT results revealed that cinnamic, p-coumaric and caffeic acids were adsorbed effectively on the metal surface through aromatic rings and carboxylic acid groups, as the bond distances with the Fe surface were 2.09 Å for Fe?C and 1.97 Å for Fe?O. Low adsorption energies were obtained; ?52.90, ?44.29 and ?33.39 kcal/mol for cinnamic, p-coumaric and caffeic acids, respectively. Additionally, the magnetic moment of the Fe plane was changed significantly if the aromatic carboxylic acid adsorbed onto the metal surface had coating strengths as follows: cinnamic > p-coumaric > caffeic. The present study therefore provides a mechanistic understanding of the role of iodide in the corrosion inhibition process.