Halogen engineering of 4-aminoantipyrine amide derivatives for the corrosion protection of mild steel: experimental and DFT insights

Corrosion of mild steel in acidic environments is a major industrial challenge, driving the synthesis of efficient and tunable organic inhibitors. Herein, twelve halogenated 4-aminoantipyrine amide derivatives (M1-M12) were synthesized and characterized by FT-IR and NMR (1H-NMR, 13C-NMR, COSY, and HSQC) spectroscopy and HRMS. Their ability for the corrosion inhibition of mild steel in 1 M HCl was evaluated by weight loss, PDP, EIS, and SEM analyses and DFT calculations. At 1 mM, M5, M12 and M7 exhibited inhibition efficiencies (IE) of 96.46 ± 1.9%, 95.58 ± 1.9% and 94.69 ± 1.8%, respectively; M5 also exhibited the highest electrochemical efficiencies at 0.5 mM (PDP 96.51 ± 0.9% and EIS 95.70 ± 1.1%). Efficiency increased with concentration and decreased with temperature. Negative ΔG° values (-15.53 to -10.86 kJ mol-1) and the Langmuir fit indicate spontaneous, predominantly physical adsorption, possibly with specific donor-acceptor contributions. PDP showed mixed-type behaviour, and EIS showed increased charge-transfer resistance. Halogen type, substituent position and alkyl-chain length strongly influenced performance.