Electrochemical and computational studies of benzothiophene compounds as corrosion inhibitors of mild steel in 1 M hydrochloric acid

Abstract

In the study, five benzothiophene derivatives were used as corrosion inhibitors for mild steel in a 1 M hydrochloric acid solution. The molecules have the same benzothiophene ring and only differ in the substituents attached to the ring. The study investigated their anti-corrosion and adsorption capabilities on mild steel in an acidic medium. Electrochemical, quantum chemical, spectroscopic, and adsorption isotherms techniques were used to evaluate the inhibitors for anti-corrosion properties. Careful examination of the Tafel and EIS analyses showed that the inhibitors exhibited good inhibition efficiency, and the following order of the inhibitors was reported: B5 > B2 > B3 > B4 > B1. The Tafel analysis showed that most of the inhibitors functioned as mixed-type inhibitors (interacting with both the anodic and cathodic reactions). The exception was B4, which mainly acted as a cathodic inhibitor. The EIS analysis showed that the benzothiophene derivatives exhibited high corrosion resistance levels by forming a passive film on the mild steel surface to shield the alloy from corrosion. The benzothiophene inhibitors also obeyed the Langmuir adsorption isotherm with their 𝑅2 values near to unity. The Δ𝐺𝑎𝑑𝑠°, values for four inhibitors were less than -20𝑘𝐽 𝑚𝑜𝑙−1, suggesting the inhibitors were interacting with the mild steel via physisorption. The only exception was B5 which exhibited values between the range of -20𝑘𝑗 𝑚𝑜𝑙−1 to 40 𝑘𝐽 𝑚𝑜𝑙−1, which meant the inhibitor interacted with the metal via physisorption and chemisorption. The computational study produced promising highest occupied molecular orbital and lowest unoccupied molecular orbital figures which showed that the benzothiophene ring and the substituents could donate and receive electrons. The calculated parameters showed that the molecules all have the potential to interact with the metal surface. The order of decreasing 𝐸𝐻𝑂𝑀𝑂 was observed to be B1 > B3 > B2 > B4 > B5, which showed B1 with the highest electron-donating ability. The decreasing order of 𝐸𝐿𝑈𝑀𝑂 was determined to be B1 > B2 > B3 > B5 > B4. The trend was not aligned with the experimental inhibition efficiencies, which suggested that the inhibition potentials of the inhibitors were entirely informed by their ability to receive electrons from the occupied orbitals of Fe. The decreasing order of the energy gap was shown to be B1 > B2 > B3 > B5 > B4. B4 had the lowest reported energy gap in the study. However the difference in the energy gap values were not significant, indicating that the inhibitors exhibited high levels of reactivity.