Experimental and theoretical investigations of the corrosion protection of mild steel by methanolic Triumfetta rhomboidea J. leaf extract

Triumfetta rhomboidea leaf extract (TRLE) was used as a natural and benign corrosion inhibitor of mild steel (MS) in HCl (1 mol/L) by gravimetric, electrochemical and theoretical techniques. Identification of the major compounds in TRLE was conducted on a Gas Chromatography-Mass Spectrophotometer (GC-MS). Theoretical calculations were done and the relationship between the experimental inhibition efficiencies and quantum chemical parameters were obtained. The parameters of thermodynamic equations (K_ads; E_a; $\Delta G_{ads}^o$; $\Delta S_{ads}^o$; $\Delta H_{ads}^o$) were calculated and explained. The potential of inhibition of TRLE increased as the concentration of the TRLE increased but reduced with temperature increase. The adsorption of TRLE on the MS surface followed the Langmuir isotherm model. It was revealed that corrosion inhibition may be due to the spontaneous physical adsorption of the TRLE molecules on the surface of the MS. The respective polarisation and impedance data indicated that TRLE is a mixed-type inhibitor and the inhibition mechanism is due to charge-transfer. Scanning electron microscopy-energy dispersive X-ray analytical methods confirmed that TRLE protected the surface of MS and reduced the dissolution of the MS in the acidic solution. The GC-MS data revealed that octadecanoic acid and 9-actadecenoic acid were the most abundant compounds in TRLE and thus, were selected for quantum and molecular dynamics simulation. The theoretical predictions were in good agreement with experimental results.