Preparation and Corrosion Resistance of Robust Superhydrophobic Nickel-based Coating

In nature, the corrosion of most metals is universal and spontaneous, so adequate protection must be carried out for metals in use. The coating is one of the most common ways to metal corrosion, such as metal coating, conversion coating, oxidation coating and superhydrophobic coating. Among these protective coatings, the corrosion metal superhydrophobic coating has great application potential in metal protection. The formation of a layer of air as a barrier between a superhydrophobic metal substrate and liquid provides remarkable opportunities in corrosion resistance of metal compounds. However, the poor stability of the superhydrophobic coating limits its wide range of applications. This paper aims to prepare robust superhydrophobic nickel-based coatings on a metal surface to improve corrosion resistance. The brass sheet was cut into a rectangle of 20 mm20 mm as the substrate. A composite coating including a micro/nanostructured porous nickel-plated layer and a polysiloxane layer was prepared on the brass surface via a three-step deposition protocol. In the first stage, the nickel-plated layer with a microporous structure was formed on the brass surface by electroplating in a nickel-plating bath with the addition of ammonium chloride and ethylene glycol. After that, the sample was electrodeposited in another nickel-plating solution containing crystal regulator ethylenediamine hydrochloride to form a sea urchin-like nickel layer. Finally, a polysiloxane layer was deposited on the surface by electrodeposition to obtain a coating with durable superhydrophobic properties. The morphology, composition, hydrophobicity, and corrosion resistance of the coating were characterized with a scanning electron microscope (SEM), an X-ray powder diffractometer (XRD), an X-ray photoelectron spectroscopy (XPS), a Fourier transform infrared spectroscopy (FT-IR), a contact angle tester, and an electrochemical workstation. The mechanical stability of the prepared superhydrophobic coating was characterized by a linear wear test on an 800-grit sandpaper with a 200.0 g weight load. The results showed that the adding ethylene glycol in a nickel-plating bath promoted the evolution of hydrogen in the cathode during electroplating, and a uniformly connected porous nickel coating was formed when the addition amount of ethylene glycol was 50.0-100.0 mg/dm3. After two-step nickel electroplating, a nickel layer with a sea urchin-like structure was formed on the brass surface. A self-cleaning and superhydrophobic layer with a water contact angle of (159±1)° was formed by electrodeposition in the hydrolyzed silane solution under a voltage of -1.5 V for 3.0 min. In the 3.5% NaCl solution, the corrosion current density of the as-prepared composite coating was about 3.6×10-8 A/cm2, reduced by three orders of magnitude compared with the unmodified nickel coating. Additionally, the impedance modulus at a low-frequency (|Z|0.01 Hz) was around 2.0×106 Ω·cm2, increased by three orders of magnitude compared with the unmodified nickel coating. After the wear test, the micro/nanostructured surface existed, which kept the superhydrophobicity of the coating (contact angle above 150°). Besides, the corrosion current density and |Z|0.01 Hz of the composite coating after wear were 5.3×10-8 A/cm2 and 1.3×106 ·cm2, respectively, indicating that good corrosion resistance of the coating was remained. The as-prepared superhydrophobic composite coating by simple electrodeposition and silane modification has a robust superhydrophobic capability and excellent corrosion resistance, which provides good protection for the substrate metal. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.