Performance evaluation of newly synthetized bi-pyrazole derivatives as corrosion inhibitors for mild steel in acid environment.

• 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]