Electrochemical and DFT/MDS investigation of the synergistic corrosion inhibition of XC48 carbon steel by a benzodiazepine–Zn2+ system in acidic media

Investigations were conducted on the synergistic inhibitory effect of 2,3-dihydro-1H-1,5-benzodiazepine-4-yl-4-hydroxy-6-methyl-2H-pyran-2-one (DMABD) and Zn²⁺ ions on the corrosion of XC48 carbon steel in a 0.5 M HCl solution. Several techniques were used, including weight loss measurement, electrochemical impedance spectroscopy, potentiodynamic polarization, scanning electron microscopy, and atomic force microscopy. This study shows that XC48 steel is more effectively inhibited by increasing the concentration of DMABD, and an enhancement in inhibition efficiency was observed with the addition of 10⁻⁵ M Zn²⁺ due to synergism. The highest inhibition efficiency of 82% was observed for 5 × 10⁻⁵ M DMABD alone, while the addition of 5 × 10⁻⁵ M DMABD + 10⁻⁵ M Zn²⁺ to improved the inhibition efficiency to 92% for 24 h of immersion at room temperature (298 K). Electrochemical test data demonstrate that they are mixed inhibitors and exhibit remarkable corrosion prevention ability. Furthermore, the inhibition efficiency of DMABD/Zn²⁺ depends on temperature. The adsorption of DMABD alone or with Zn²⁺ ions on the metal surface followed the Langmuir adsorption isotherm. This is supported by a negative standard free energy of adsorption ( \(\Delta {\text{G}}_{{\text{ads }}}^{^\circ }\) = −39.45 kJ mol⁻¹) and a high adsorption equilibrium constant (K_ads = 149.47 × 10³ M⁻¹), indicating mixed adsorption (chemisorption and physisorption). The morphological analysis confirms that the synergistic effect arises from the adsorption of the DMABD/Zn²⁺ complex. The formation of this protective layer was validated by observed changes in UV–Vis spectroscopy, surface roughness (via SEM and AFM), and the disappearance of corrosion product peaks in XRD patterns. These findings were further supported by density functional theory (DFT) and molecular dynamics simulations (MDS).