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The effect of some prepared compounds, namely 3,5-dimethyl-1 H-pyrazole (P1), 3(5)-amino-5(3)-methylpyrazole (P2), and 1′,3,5,5′-tetramethyl-1′ H-1,3′-bipyrazole (P3), on the corrosion behaviour of steel in 1.0 M hydrochloric acid solution as corrosive medium has been investigated at 308 K using weight-loss measurement, potentiodynamic polarisation, linear polarisation, and impedance spectroscopy (EIS). Generally, inhibition efficiency of the investigated compounds was found to depend on the concentration and nature of the inhibitor. P3 was a better inhibitor than P1 and P2, and its inhibition efficiency increased with increasing concentration of inhibitor, attaining 94% above 10 M. Potentiodynamic polarisation studies clearly reveal that P3 acts essentially as a cathodic inhibitor. E (%) values obtained from different methods are in reasonably good agreement. EIS measurements show an increase of transfer resistance with inhibitor concentration. Partial π-charge on atoms was calculated. Correlation between the highest occupied molecular orbital energy E and inhibition efficiencies was sought. The temperature effect on the corrosion behaviour of steel in 1.0 M HCl without and with different concentrations of inhibitor P3 was studied in the temperature range 308 to 343 K. Thermodynamic data, for example heat of adsorption ( $$ \Updelta H_{\text{ads}}^{^\circ } $$), entropy of adsorption ( $$ \Updelta S_{\text{ads}}^{^\circ } $$) and free energy of adsorption ( $$ \Updelta G_{\text{ads}}^{^\circ } $$) were calculated by use of thermodynamic equations. Kinetic activation data, for example E, Δ H*, Δ S* and pre-exponential factor, were calculated, and are discussed. The inhibiting action of P3 on the corrosion of steel in 1-10 M hydrochloric acid was also studied by weight-loss measurement. The rate constant and reaction constant were calculated for the corrosion reactions. Adsorption of P3 on the steel surface in 1.0 M HCl follows the Langmuir isotherm model. [ABSTRACT FROM AUTHOR]

[Display omitted] • SAP1, SAP2, and SAP3 performance towards the corrosion inhibition for mild steel in 1 M HCl using weight loss, EIS and PP measurements revealed excellent results. • The inhibition efficiency showed a slight decrease with the rise of temperature. • The surface characterization SEM/EDX justified the adsorption of SAP molecules on mild steel surface. • Quantum chemical calculations and Monte Carlo simulation confirmed the experimentally obtained results. Controlling corrosion of metals and their alloys in acidic environments has recently received increasing scientific attention. Therefore, our study investigated three newly synthesized Schiff base (imine) derivatives, namely (3Z,3'Z)-4,4′-(ethane-1,2-diyldiimino)bispent-3-en-2-one (SAP1), (4′E)-4,4′-(pyridine-2,6-diyldinitrilo)dipentan-2-one (SAP2), and (4′E)-4,4′-(benzene-1,3-diyldinitrilo)dipentan-2-one (SAP3) to understand their adsorption on mild steel (MS) surface effects in 1 M HCl solution using electrochemical and theoretical methods. The obtained results show that the three inhibitors have good protective effects. Electrochemical techniques show that the presence of these compounds significantly increases the polarization resistance and influences both the anodic and cathodic reactions. This means that these inhibitors have mixed properties. The stability of these molecules at higher temperatures, from 298 K up to 328 K, was examined by polarization potentiodynamic measurements. This study revealed that these compounds resist temperature by adsorbing on the MS surface to form a protective barrier. Scanning electron microscopy with Energy Dispersive X-ray Analysis (SEM-EDX) was also implemented to understand the adsorption phenomenon. In addition, density functional theory (DFT) and Monte Carlo simulation (MC) were performed to confirm the experimental results. [ABSTRACT FROM AUTHOR]