Electrochemical studies of the synergistic combination effect of thymus mastichina and illicium verum essential oil extracts on the corrosion inhibition of low carbon steel in dilute acid solution

Abstract

Inhibition effect of the synergistic combination of thymus mastichina and illicium verum oil extracts (TMAV) on the corrosion inhibition of low carbon steel in 0.5 M H2SO4 and HCl solution was studied by weight loss analysis, potentiodynamic polarization, open circuit potential measurement, ATF-FTIR spectroscopy, and optical microscopy and macroscopic characterization. Results from weight loss shows TMAV performed more effectively in H2SO4 solution compared to HCl with optimal inhibition efficiency of 81.24% and 68.33%. Effective inhibition performance was observed at all TMAV concentration in H2SO4 compared to HCl where TMAV performed poorly until 5% concentration. The optimal inhibition performances from potentiodynamic polarization are 80.85% and 70.43%. The corresponding corrosion current density and polarization resistance are 7.16 × 105 A/cm2 and 8.01 × 105 A/cm2, and 331.73 and 284 Ω. TMAV exhibited mixed type inhibition effect in both acid solutions, strongly influencing the anodic-cathodic plot configurations with respect to concentration. Open circuit potential plots without TMAV were significantly electronegative compared to the plots at 1% and 5% TMAV concentration which were relative electropositive due to decreased thermodynamic tendency of the carbon steel to corrode. The corresponding plots at 1% and 5% TMAV concentration from HCl solution were thermodynamically unstable with significant active-passive corrosion behaviour. TMAV inhibited through chemisorption adsorption according to Langmuir and Freudlich adsorption isotherms in H2SO4 solution, and Frumkin and Freundlich adsorption isotherms in HCl solution with correlation coefficient values between 0.7 and 0.9. FTIR spectroscopic analysis exposed the functional groups and atomic bonds responsible for corrosion inhibition.