Electrochemical assessment and DFT/DFTB insights of Melastoma candidum leaf extract as a corrosion inhibitor for carbon steel

The superior mechanical properties contribute to the use of carbon steel for various purposes. However, the weak corrosion resistance presents a challenge for users, showing the need for appropriate prevention methods such as natural corrosion inhibitors from Melastoma candidum leaf extract (MCLE). Therefore, this study aimed to investigate the role of MCLE inhibitors in protecting carbon steel in 3.5% NaCl solution. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests were applied to examine the role of inhibitor molecules by mixing at concentrations of 0, 200, 400, 600, 800, and 1000 ppm. The results showed that the highest inhibition efficiency of 83.89% (PDP) and 85.07% (EIS) was achieved at the maximum concentration (1000 ppm), with MCLE classified as mixed inhibitors. To analyze the inhibition mechanism, adsorption isotherm theories were applied, with the Frumkin isotherm providing the best fit. This suggested that the molecules of the organic inhibitor were adsorbed on the steel surface based on physisorption. DFT calculations revealed favorable electronic properties of MCLE phytochemicals, with oxygen-containing groups serving as active adsorption sites. SCC-DFTB simulations confirmed strong adsorption on the Fe(110) surface, with adsorption energies of −39.54 to −206.83 kcal/mol, indicating the formation of a stable protective film. Surface characterization further validated the corrosion protection ability of MCLE, highlighting its potential as a sustainable inhibitor for carbon steel in chloride environments.