The Corrosion Inhibition Effect of Salpn Schiff Base on Low-Carbon Steel in a Hydrochloric Acid Environment: An Integrated Study Combining Laboratory Experiments and Computational Modeling

The N,N′-Bis(salicylidene)-1,3-propanediamine Schiff base (Salpn) was synthesized, characterized, and assessed as a corrosion inhibitor for low-carbon steel (LCS) in a 0.5 mol L−1 HCl solution. The study included chemical, electrochemical, and quantum mechanical methods to provide a comprehensive assessment. Experimental results revealed that the inhibition efficiency (IE) of Salpn increased with concentration, reaching a maximum of 69.1% at 300 ppm and 298 K, while a slight decrease to 64.3% was observed as the temperature increased. Tafel plot identified Salpn as a mixed-type inhibitor, while electrochemical impedance spectroscopy (EIS) revealed that the double layer capacitance decreased while the charge-transfer resistance increased as the concentration of Salpn increased. The thermodynamic study revealed that the adsorption of Salpn on the LCS surface follows the Langmuir isotherm model. The calculated standard free energy of adsorption (ΔG°ads) values ranged from −27.53 to −30.17 kJ mol−1, confirming that the inhibition process occurs via a mixed mechanism involving both physisorption and chemisorption. The presence of a protective film on the LCS surface was suggested by SEM observations, while EDX analysis showed an increase in C, O, and N signals, providing further indication of the inhibitor’s integration into the surface layer. Density functional tight-binding (DFTB+) calculations supported the high inhibitory performance by showing a low hardness value (0.091 eV). The compound’s high global softness (σ = 10.989 eV−1) suggested that it is an effective corrosion inhibitor. The Monte Carlo (MC) simulations demonstrated a strong interaction with a highly negative adsorption energy of −654.145 kJ mol−1. These findings collectively validate Salpn as an effective and strongly adsorbing corrosion inhibitor.