Dip-coating of oleic acid-capped NaYbF₄ nanoparticles on mild steel for its corrosion protection in salty water

Abstract Mild steel (MS) has potential applications in marine structures, pipelines, and industrial equipment; however, corrosion in MS severely limits its performance and durability. Effective protection strategies are therefore required to ensure extended service life and reliable use in practical environments. In this direction, the present study encompasses the controlled preparation of oleic acid-capped hydrophobic sodium ytterbium fluoride nanoparticles (NaYbF₄ NPs) via the hydrothermal method, the multilayer coating of as-prepared NPs on MS substrates via dip-coating, and their structural and electrochemical characterization. Besides, two other sets of MS substrates, one coated with bare oleic acid (OA) and the other with hydrophobic NPs incubated with excess OA, have been prepared and tested. The field emission scanning electron microscopy (FESEM) micrograph shows spherical nanoparticles with sizes of ~ 40–80 nm. FTIR spectrum confirms the successful formation of OA-capped NaYbF 4 nanoparticles via -CH 2 stretching vibrations at ~ 2920 and ~ 2850 cm − 1 . The corrosion behaviors of the dip-coated MS (1–3 layers) are inspected in 0.5 M NaCl by polarization tests, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS). The OCPs of the coated MS samples are shifted toward more positive voltages; however, the maximum positive shift in OCP of coated samples compared to bare MS has been observed for MS coated with 2 layers of NaYbF 4 NPs. Bode results further confirm superior protection for MS@NPs-2 L, exhibiting the highest impedance and the lowest corrosion current density (i corr ) of 3 µA cm − 2 (93% protection). Notably, OA-incubated NaYbF₄ nanoparticles showed lower water contact angle (WCA) and higher corrosion current density than oleic-acid-capped nanoparticles, indicating altered surface wettability and electrochemical behavior. Corrosion analysis revealed severe corrosion of bare MS, whereas nanoparticle coatings significantly enhanced corrosion inhibition, with the two-layer coating exhibiting the highest inhibition efficiency among the one-, two-, and three-layer coatings. The overall results provide an effective protective strategy against corrosion, offering a promising approach for advanced surface protection of mild steel.