Effect of ammonium chloride on microstructure, super-hydrophobicity and corrosion resistance of nickel coatings

Nickel coatings were prepared at different concentrations of NH4Cl as a crystal modifier in the electrodeposition bath. Surface morphology, wettability and the preferred orientation of the coatings were characterized by scanning electron microscopy, optical contact angle meter and X-ray diffractometer (XRD), respectively. The results showed that NH4Cl considerably affected the microstructure of the coatings; NH4Cl modified surface morphology towards an appropriate micro-nano cones structure. The contact angle of water droplets on the nickel coatings increased over time, eventually approaching a super-hydrophobic surface. Indeed, the super-hydrophobic surface showed a good long-term durability in air. XRD results indicated that NH4Cl changed the preferred orientation from (220) to (111) by a complex growth mode. The role of NH4Cl was studied by cyclic voltammetry and electrochemical impedance spectroscopy techniques. It was found that NH4Cl inhibited the nucleation of nickel deposits by increasing charge transfer resistance. Electrochemical measurements at long-term immersions indicated that the super-hydrophobic nickel coatings greatly enhanced the corrosion resistance of copper substrate in the neutral 3.5 wt.% NaCl solution. The results revealed that super-hydrophobicity considerably increased the polarization resistance from 21 to 272 kΩ · cm2. After decay of hydrophobic property, the passive oxide layer played the major role to protect the coatings in the corrosive solution.