Mechanistic evaluation of adsorption and corrosion inhibition capabilities of novel indoline compounds for oil well/tubing steel in 15% HCl

The investigation presents the superior corrosion inhibition capabilities of two novel indoline compounds namely 2-(1-(4-((furan-2-ylmethylene)amino)-5-thioxo-4,5-dihydro-1H-1,2,4-triazol-3-yl)-2-(1H-imidazol-2-yl)ethyl)isoindoline-1,3-dione (FTTI) and 2-(1-(4-((furan-2-ylmethylene)amino)-5-thioxo-4,5-dihydro-1H-1,2,4-triazol-3-yl)-2-methylbutyl)isoindoline-1,3-dione (FTMI) for oil well/ tubing steel in 15% HCl solution. The gravimetric studies confirm corrosion inhibition efficiency of 99.08% and 97.75% respectively at 303 K for significant low dosage of 50 μML^-1. The corrosion inhibition efficiency remained nearly constant at elevated temperatures due to predominant chemisorption of the inhibitors upon the metallic substrate. The chemisorption of the compounds is confirmed by the values of free energy of adsorption (ΔG_ads), which lie within the range of −47.60 kJmol⁻¹ to −55.42 kJ mol⁻¹ for FTTI and −45.95 kJmol⁻¹ to −54.34 kJmol⁻¹ for FTMI. The electrochemical analysis approves that the reduction in cathodic corrosion reaction (metal dissolution) occur through inhibitor adsorption upon the active sites; while the anodic reaction (hydrogen evolution reaction) is reduced due to thickening of the double layer at the metal solution interface. The substrate surface analysis through SEM, AFM, EDX and XPS reveal that the inhibitors generate an organic–inorganic hybrid protective film to protect the surface form aggressive acid solution.