Abstract

Mild steel plays an important role in many construction industries due to its low cost and outstanding mechanical properties. However, the utilization of strong acid in pickling, construction operation, and oil refining processes adds to a serious corrosion issue on mild steel. 4-nitrobenzohydroxamic acid (4NBHA) and 4-chlorobenzohydroxamic acid (4CBHA) ligands were synthesized to scrutinize the corrosion resistance activity toward mild steel in an acidic medium. Both ligands were characterized by using melting point and Fourier Transform Infrared (FTIR), ultraviolet-visible (UV-Vis) and Nuclear Magnetic Resonance (NMR) spectroscopies. The corrosion resistance of mild steel was studied with different inhibitor concentrations (1 mM, 2 mM, 3 mM, 4 mM, and 5 mM). Results of weight loss method showed that the higher the inhibitor concentration, the lower the corrosion rate.  4CBHA showed a superior corrosion resistance (%E: 80.76) due to the presence of withdrawing groups, which were the chloro group that was less reactive as compared to the nitro- group in 4NBHA (%E: 77.60). Langmuir isotherm calculation showed that the ligands were adsorbed on the mild steel surface by forming a monolayer and there were no interactions between the adsorbed inhibitors. The values of standard free energy (ΔG) were -29.36 kJ mol^-1 for 4NBHA and -28.85 kJ mol^-1 for 4CBHA, which indicated that both ligands were adsorbed spontaneously by chemisorption onto the metal surface by coordinate covalent bonds. Field emission scanning electron microscope (FESEM) analysis proved that the presence of a protective layer on the mild steel had restricted the corrosion process.