Effect of temperature on CO2 corrosion inhibition by black tea extract: A combined experimental and molecular modelling study

Abstract

The temperature impact on the inhibitory characteristics of black tea extract was examined in a 1 wt.% sodium chloride solution under CO₂ saturation. The evaluations were conducted in solutions with pH 5.5 at 20, 40, and 60 °C. The interaction of black tea extract (BTE) with L80-1Cr carbon steel, focusing on its adsorption and chelation properties, was examined using ultraviolet–visible spectroscopy (UV–Vis), electrochemical measurements, and density functional theory (DFT) modelling. Additionally, scanning electron microscopy (SEM), computed tomography (CT) scans, focused ion beam (FIB) and scanning transmission electron microscopy (STEM) were employed to study the morphology and cross-section of the film formed on the steel surface. BTE exhibited significantly improved corrosion inhibition properties with temperature, as a maximum polarization resistance of 800 Ω .cm² and a higher inhibition efficiency of 88 % was observed at 60 °C after 300 h of immersion. Moreover, the inhibition efficiency did not decrease over time; on the contrary, it showed a gradual increase. Density functional theory (DFT) calculations showed that various BTE components have a strong adsorption tendency on the Fe (110) surface and Fe₃C (001), with delphinine presenting the greatest adsorption with −104 kJ/mol and the ability to displace 2 water from the surface. UV–Vis spectroscopy showed a shift to lower wavelengths in peak positions, indicating stronger interactions between BTE molecules and Fe²⁺ ions. Cross-sectional FIB imaging confirmed the formation of Fe²⁺–BTE chelate layers on top of the corrosion products. As the temperature increased, the thickness of this protective layer grew from 215 nm to 406 nm, while the underlying corrosion layer decreased, highlighting improved protection at higher temperatures. 3D and cross-sectional CT showed a smoother surface of the inhibited sample, consistent with the dual action of BTE, adsorption and chelation.