Experimental, theoretical modeling and optimization of inhibition efficiency of pigeon pea leaf extract as anti-corrosion agent of mild steel in acid environment

This study is on the application of pigeon pea leaf (PPL) extract as anti-corrosion agent for mild steel in acid environment (HCl solution). The work involves investigation of the corrosion inhibition process using combination of experimental, theoretical modeling and optimization studies. The chemical compositions and functional groups of the extract were identified using gas chromatography mass spectrophotometer (GC-MS) and Fourier transform infrared (FTIR) spectroscopy respectively. For the response surface methodology (RSM), Design Expert software was used for the optimization of the inhibition efficiency. Scanning electron microscopy (SEM) was used to inspect the surface morphology of the mild steel. The inhibition efficiencies of 87.13%, 91%, 92.1% and 90.7% were obtained from the experimental studies of thermometric, gravimetric, potentiodynamics polarization and electrochemical impedance spectroscopic techniques respectively. A quadratic model showing the relationship between inhibition efficiency and inhibition variables was generated. The model was scrutinized with various criteria, which include coefficient of determination (R-2 = 0.9733), p value (<0.0001), adequate precision (22.44), adjusted R-2 (0.9485) and coefficient of variation (4.98). The RSM model adequately predicted the optimum inhibition efficiency of 91% at inhibitor concentration of 0.9 g/L-1. The theoretical modeling reveals that 1-Tridecene and Luteolin were the most active constituents of pigeon pea leaf responsible for the inhibition process. Polarization study revealed that pigeon pea leaf extract acted as a mixed-type inhibitor. Furthermore, the inhibition process was in accordance with Langmuir isotherm. Spectra of the corrosion particles and SEM analysis confirmed the physical attachment of the extract species on the mild steel surface.