Effect of NH4Cl concentration on the microstructure, wettability and corrosion resistance of Ni-Co coatings

Super-hydrophobic nickel-cobalt (Ni-Co) alloy coating was fabricated on Q235 steel by single-step electrodeposition, without any post-treatment with low-surface-energy agents. The influence of NH 4 Cl concentration on surface morphology, chemical composition, thickness, crystal structure, wettability, and corrosion performance was systematically investigated. Characterization techniques included scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and a contact-angle goniometer, while corrosion behavior was evaluated by Tafel polarization and electrochemical impedance spectroscopy (EIS). The results showed that the addition of crystal modifier NH 4 Cl did not affect the thickness and phase structure of Ni-Co coating but affected the composition and micro-morphology of the coating. With the increase of NH 4 Cl concentration, the cobalt content of the coatings gradually decreased, the nickel content gradually increased, and the micromorphology of the coatings changed from a prismatic cone-like structure to a sea urchin-like micro-nano structure. The contact angle of the Ni-Co coating prepared by 150 g/L NH 4 Cl was 152.56°, showing stable super-hydrophobicity, the self-corrosion current density ( I corr ) of the coating was the smallest, and the polarization resistance ( R p ) was the largest, which had the best corrosion resistance. After 12 h immersion in 3.5 wt% NaCl the coating retained its morphology and a contact angle above 150°, with only minor localized corrosion products, confirming durable super-hydrophobicity and superior anticorrosion performance.