Experimental investigations on the use of Cordia myxa extract as green corrosion inhibitor for carbon steel in hydrochloric acid

The corrosion inhibition of Cordia myxa peel (CMP) and body (CMB) extracts as sustainable green inhibitors for carbon steel (CS) corrosion in 1 M HCl was investigated. The effect of inhibitor concentration (50–250 ppm), immersion time, and temperature (298–328 K) was evaluated. It was found that after 9 h at 298 k, the inhibitory efficacy reached 72.16% and 95.9% with an increasing concentration of CMP and CMB up to 250 ppm. The PDP results showed maximum inhibition efficiencies of 80.29% and 98.06% for CMP and CMB, respectively. EIS analysis demonstrated an increase in charge transfer resistance and a decrease in double-layer capacitance with increasing inhibitor concentration. The thermodynamic analysis revealed that the adsorption process was spontaneous, with ΔG°ads values ranging from −4.76 to +7.76 kJ mol−¹ for CMP and from −4.63 to −9.50 kJ mol−¹ for CMB. The calculated ΔH°ads values were −129.08 kJ mol−¹ for CMP and +43.20 kJ mol−¹ for CMB, indicating an exothermic adsorption process for CMP and an endothermic adsorption behaviour for CMB. Adsorption studies indicated that the extracts followed the Freundlich adsorption isotherm. Surface characterisation by FTIR and AFM further confirmed the adsorption of Cordia myxa on the steel surface.