Isatin Schiff base is an effective corrosion inhibitor for mild steel in hydrochloric acid solution: gravimetrical, electrochemical, and computational investigation

This paper describes the synthesis and characterisation of an isatin Schiff base, namely 2-(2-oxoindolin-3-ylidene) hydrazinecarbothioamide (OHB). The chemical structure of OHB was elucidated through proton-nuclear magnetic resonance ((1)H-NMR), carbon-nuclear magnetic resonance ((13)C NMR), and Fourier-transform infrared (FT-IR) spectroscopic techniques. OHB was evaluated for its corrosion inhibition ability on mild steel specimens in 1 M HCl using gravimetrical methods and electrochemical measurements such as electrochemical impedance spectroscopy (EIS) and potentiodynamic techniques complemented with microscopic analysis. The results indicated that OHB is a mixed-type inhibitor and showed good corrosion inhibition, with a maximum corrosion inhibition efficiency of 96.7% at a concentration of 0.5 mM and 303 K. The inhibition performance increased with an increasing OHB concentration and decreased with increasing temperature. The inhibition efficiency was attributed to the formation of a protective film on the surface of the tested mild steel coupon. The electrochemical impedance studies also indicated that the charge transfer resistance increased with an increase in OHB concentration. The morphological analysis confirmed the inhibition performance of OHB and the protective barrier film conformed to Langmuir monolayer adsorption. The experimental and theoretical corrosion kinetics and thermodynamic parameters were in agreement and revealed that an adsorption film of Fe–N coordination bonds formed on the mild steel surface.