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• Bi-pyrazole derivatives are tested for corrosion protection of mild steel in HCl. • The adsorption of tested compounds follows Langmuir isotherm model. • DFT calculations show a widespread distribution of reactive sites in both molecules. • MD simulations suggest a nearly parallel adsorption of molecules over iron surface. Two new synthesized bi-pyrazole-carbohydrazides, namely 1,1′-(propane-1,3-diyl)bis (5-methyl-1H-pyrazole-3-carbohydrazide) (P2PZ) and 1,1′-(oxy bis (ethane -2,1-diyl)) bis (5-methyl-1H-pyrazole-3- Carbohydrazide) (O2PZ) were evaluated as corrosion inhibitors for mild steel (MS) in 1.0 M HCl media. Comparative study of inhibitory action of P2PZ and O2PZ was conducted first using mass loss and electrochemical measurements (potentiodynamic polarization curves, PPC, and electrochemical impedance spectroscopy, EIS) and it had shown that P2PZ outperforms O2PZ. The electrochemical studies showed a corrosion inhibition efficiency of 95% and 84% at 308 K and 10−3 mol/L of P2PZ and O2PZ, respectively. Their efficiency augmented with augmentation of concentration and decreased with augmentation of temperature. Electrochemical impedance spectroscopy showed that the corrosion inhibition followed a charge transfer process. Polarization curves suggested that the P2PZ and O2PZ acted as mixed-type inhibitors and followed the Langmuir adsorption isotherm. The free energy of adsorption (ΔG ads) values, which lie within the range of -39.94 to -37.36 KJ mol−1, confirmed a physicochemical adsorption process. Scanning electron microscope (SEM) images indicated the formation of a protective layer on the surface of the metal in the presence of both inhibitors. Density Functional Theory (DFT) and molecular dynamics (MD) simulation have been introduced to carry out a theoretical investigation of the inhibitors' reactivity and interaction with the iron surface, respectively. The comparative study based on experimental and theoretical results showed that the trend of inhibition potentials of tested inhibitors depends on their electron-donating/accepting abilities. The trend of reactivity of molecules P2PZ and O2PZ matched well with experimental results. [Display omitted] [ABSTRACT FROM AUTHOR]

• Studied corrosion inhibitors, M2PyAz and B2PyAz act as efficient inhibitors for MS. • Tafel results revealed that M2PyAz and B2PyAz as a mixed-type inhibitor. • Adsorption of both inhibitors obeyed Langmuir isotherm. • Theoretical calculations results were correlated to the inhibition efficiency. New bi-Pyrazol-Carbohydrazides, N,N-bis (3-Carbohydrazide-5-methylpyrazol-1-yl) methylene ( M2PyAz ) and 1,4-bis (3-Carbohydrazide-5-methylpyrazol-1-yl) butane (B2PyAz), were synthesized and their inhibitor effect against mild steel (MS) corrosion in 1 M HCl solution has been evaluated. Comparative study of inhibitory action of M2PyAz and B2PyAz was conducted first using weight loss measurements (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and theoretical computational methods. The inhibition efficiencies of M2PyAz and B2PyAz can reach 98.6% and 87.8% respectively when the optimal concentration is 10–3 M. This result indicates that M2PyAz behave as excellent corrosion inhibitor towards MS because of its molecular size giving a denticity of electron donor atoms. EIS results indicate that the charge transfer resistance increases with augmentation concentration for both inhibitors and the MS corrosion in this system is controlled by a charge transfer process. PDP curves suggest that the M2PyAz and B2PyAz act as mixed type corrosion inhibitors. Free energy of adsorption obtained from the Langmuir isotherm model for both M2PyAz and B2PyAz implies that both molecules inhibit the acid attack mainly by chemisorption. WL-measurements were carried out in temperature range of 308–348 K, illustrating approximately 98.5% and 89% inhibitory efficiency respectively for M2PyAz and B2PyAz after 6 h immersion at 308 K in 1 M HCl solution. SEM images indicates the protective layer growth on the surface of metal in the presence of both inhibitors. SEM images analysis in the presence of both inhibitors confirmed the electrochemical outcomes. Density functional theory (DFT) method and molecular dynamics simulations (MDs) application suggest that M2PyAz and B2PyAz can show excellent corrosion inhibition character. The trend of reactivity of both molecules M2PyAz and B2PyAz is in agreement with experimental study. [ABSTRACT FROM AUTHOR]