Corrosion-resistant and superhydrophobic nickel‑phosphorus/nickel/PFDTMS triple-layer coating on magnesium alloy

To improve the corrosion protection of a nickel-based coating to the underlying magnesium alloy, a superhydrophobic nickel‑phosphorus/nickel/fluorinated polysiloxane (PFDTMS) triple-layer composite coating was successfully designed and fabricated on AZ31 magnesium alloy. The composite coating consists of an electroless-deposited nickel‑phosphorus (ELNi) bottom layer, an electrochemically deposited nickel (ECNi) nano-cone array middle layer, and a PFDTMS top layer via a dehydration-condensation reaction of silane. The surface morphology, structure, and chemical composition of the coating are identified by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The static water contact angle (WCA) of the composite coating is 153.0° ± 4.6°, attributed to the synergistic effect between nano-cone-shaped nickel and PFDTMS layer. Compared to the untreated magnesium alloy, the corrosion current density of the coating decreases by four orders of magnitude based on the Tafel curve, and the corresponding impedance modulus at low frequency (|Z|ƒ=0.01 Hz) increases by almost four orders of magnitude based on the electrochemical impedance spectra (EIS), demonstrating the superior corrosion protection to magnesium alloy. Lastly, the wetting states of the water droplets on the nickel‑phosphorus layer with and without electrodeposition of nickel nano-cone array and silane modification are also discussed in detail.