Tuning corrosion inhibition of mild steel via positional bromine substitution in antipyrine-derived amides: mechanistic insights from electrochemical and DFT investigations.

Corrosion of mild steel in acidic environments is a major industrial challenge, especially during pickling and descaling, requiring efficient organic corrosion inhibitors with enhanced adsorption and stability. In this study, three novel brominated amide derivatives of 4-aminoantipyrine (M1-M3) were synthesized to investigate the influence of bromine substitution position and effect of steric hindrance caused by alkyl group on corrosion inhibition efficiency. The synthesized compounds were characterized using Fourier Transform Infrared (FT-IR) spectroscopy, Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy, Carbon-13 Nuclear Magnetic Resonance (13C NMR) spectroscopy, Correlation Spectroscopy (COSY), Heteronuclear Single Quantum Coherence spectroscopy (HSQC), and High-Resolution Mass Spectrometry (HRMS) techniques. Corrosion inhibition potentials were determined via weight loss and electrochemical methods. Surface morphology was analyzed using scanning electron microscopy (SEM), while Density Functional Theory (DFT) measurements were employed to determine electronic properties and adsorption behavior. The inhibitors shows concentration dependent protection, with M1 showing the highest inhibition efficiency, reaching 95.91 ± 0.6% from polarization measurements and 95.75 ± 0.6% from EIS measurements at 0.5 mM, respectively. EIS measurements showed a progressive increase in polarization resistance and decrease in double-layer capacitance with increasing inhibitor concentration, indicating enhanced interfacial surface coverage. Potentiodynamic polarization results identified M1-M3 as mixed-type inhibitors, affecting both anodic and cathodic corrosion reactions. Thermodynamic analysis of M1 revealed the ΔE a of 36.86 ± 0.9 to 28.28 ± 0.7 kJ mol-1, ΔH of 34.22 ± 0.6 to 25.64 ± 0.4 kJ mol-1, ΔS of -181.42 ± 2.1 to -214.51 ± 3.2 J mol-1 K-1 and ΔG of -15.56 ± 0.28 to -12.41 ± 0.19 kJ mol-1 on increasing the concentration (0.2-1.0 mM) at 303-333 K, indicating spontaneous adsorption with predominantly physical adsorption. The SEM analysis confirmed a smoother surface of mild steel in presence of inhibitors, while DFT calculations provided molecular level insight into their electronic characteristics and probable interaction sites. Results demonstrate that bromine substitution and molecular structure significantly influence inhibition efficiency. The combined experimental, thermodynamic, and theoretical findings confirm that the bromine substituted inhibitors act as efficient mixed type inhibitors through spontaneous adsorption and stable film formation, making them promising for industrial applications.