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

Abstract

Abstract Nickel coatings were prepared at different concentrations of NH 4 Cl 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 NH 4 Cl considerably affected the microstructure of the coatings; NH 4 Cl 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 NH 4 Cl changed the preferred orientation from (220) to (111) by a complex growth mode. The role of NH 4 Cl was studied by cyclic voltammetry and electrochemical impedance spectroscopy techniques. It was found that NH 4 Cl 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 · cm 2 . After decay of hydrophobic property, the passive oxide layer played the major role to protect the coatings in the corrosive solution.