Studies on Methanolic Extract of Lepidagathis keralensis as Green Corrosion Inhibitor for Mild Steel in 1M HCl

Abstract

The methanolic extracts of the leaves and stem of the plant Lepidagathis keralensis were evaluated for anticorrosion behavior against mild steel in 1M HCl. Corrosion inhibition studies were done by gravimetric method, electrochemical impedance spectroscopy and potentiodynamic polarization methods. Surface morphology of mild steel in the presence and absence of inhibitors were studied using SEM analysis. UV-Vis studies were also done to evaluate the mechanism of inhibition. Both the extracts showed good inhibition efficiency which increased with increase in concentration of the inhibitor and decreased with increase in temperature. The mechanism of inhibition was explained by adsorption which obeyed Langmuir adsorption isotherm. Thermodynamic calculations revealed a combination of both physisorption and chemisorption of the inhibitor on the surface of mild steel. The extracts behaved as mixed type inhibitors as determined by polarization studies. Quantum chemical studies on Phenoxyethene, one of the major components in the leaf extract of the plant was also carried out to support the experimental results.

Fig. 1. Corrosion inhibition efficiencies of different concentrations of the extracts of leaf and stem of Lepidagathis keralensis as determined by weight loss method.

Fig. 2. Nyquist plots obtained for mild steel in 1M HCl at 303K in the presence and absence of a) Lepidagathis keralensis Leaf extract b) Lepidagathis keralensis Stem extract.

Fig. 3. Tafel polarization curves for mild steel in 1M HCl at 303K in the presence and absence of a) Lepidagathis keralensis Leaf extract b) Lepidagathis keralensis Stem extract.

Fig. 4. Langmuir adsorption isotherm at different temperatures for mild steel in 1M HCl with different concentrations of a) Lepidagathis keralensis Leaf extract b) Lepidagathis keralensis Stem extract.

Fig. 5. Plot of ln K_ads against 1/T for a) Lepidagathis keralensis Leaf extract b) Lepidagathis keralensis Stem extract.

Fig. 6. UV-Vis absorption spectra of solutions of A) extracts in 1M HCl before mild steel immersion, B) extracts in 1M HCl after mild steel corrosion and C) 1M HCl without extracts after mild steel corrosion for Lepidagathis keralensis leaf extracts.

Fig. 7. UV-Vis absorption spectra of solutions of A) extracts in 1M HCl before mild steel immersion, B) extracts in 1M HCl after mild steel corrosion and C) 1M HCl without extracts after mild steel corrosion for Lepidagathis keralensis stem extracts.

Fig. 8b. FT-IR spectra of A) stem extract and B) the film formed on the mild steel surface after immersion in 1M HCl solution containing stem extract.

Fig. 8a. FT-IR spectra of A) leaf extract and B) the film formed on the mild steel surface after immersion in 1M HCl solution containing leaf extract.

Fig. 9. SEM images of a) mild steel before corrosion b) mild steel in 1M HCl in the absence of inhibitor b) mild steel in 1M HCl in the presence of 200ppm of Lepidagathis keralensis leaf extract c) mild steel in 1M HCl in the presence of 200ppm of Lepidagathis keralensis stem extract.

Fig. 10. a) optimized geometry b) HOMO and c) LUMO of Phenoxyethene.

Table 1. Corrosion parameters for mild steel in 1 M HCl obtained from weight loss measurements at different temperatures in the absence and presence of inhibitor.

Table 2. Impedance parameters obtained for Nyquist plots for mild steel corrosion in 1M HCl at various concentrations of leaf and stem extracts as inhibitor.

Table 3. Polarization data for mild steel corrosion in 1M HCl at various concentrations of leaf and stem extracts as inhibitor.

Table 4. Thermodynamic parameters calculated for the adsorption of leaf and stem extracts as inhibitor on mild steel surface in 1M HCl at different temperatures.

Table 6. Quantum chemical parameters calculated for Phenoxyethene