Adsorption and anticorrosion behaviour of mild steel treated with 2-((1H-indol-2-yl)thio)-6-amino-4-phenylpyridine-3,5-dicarbonitriles in a hydrochloric acid solution: Experimental and computational studies

The corrosion inhibition potentials of some derivatives of 2-((1H-indol-2-yl)thio)-6-amino-4-phenylpyridine-3,5-dicarbonitrile (TAPD) namely 2-(1H-indol-2-ylthio)-6-amino-4-(4-nitrophenyl)pyridine-3,5-dicarbonitrile (TAPD-I), 2-((1H-indol-2-yl)thio)-6-amino-4-phenylpyridine-3,5-dicarbonitrile (TAPD-II), and 2-(1H-indol-2-ylthio)-6-amino-4-(4-hydroxyphenyl)pyridine-3,5-dicarbonitrile (TAPD-III) on mild steel in 1 M HCl solution have been investigated using experimental and computational methods. The –OH substituted compound (TAPD-III) showed highest corrosion inhibition efficiency, while the [sbnd]NO ₂ substituted compound (TAPD-I) exhibited the least efficiency. The inhibition efficiency increased with increase in concentrations of the TAPDs. Weight loss study showed that inhibition efficiencies of the TAPDs obeyed the order: TAPD-III (96.95%) > TAPD-II (93.04%) > TAPD-I (91.30%).Same trend of inhibition efficiencies of TAPDs was observed using experimental and computational techniques. Electrochemical study suggested that TAPDs inhibit corrosion by adsorption on the steel surface and their adsorption behaviour follows the Langmuir adsorption isotherm. The compounds showed mixed-type corrosion inhibition activities. SEM and EDX analyses further confirmed the adsorption of the inhibitor molecules on mild steel surface. Theoretical density functional theory calculations and molecular dynamics simulations gave corroborative results that are in agreement with experimental findings.