Microstructural characterization and corrosion behaviors of Ni-Cu-Co coatings electrodeposited in sulphate-citrate bath with additives

- Ni-Cu-Co coatings deposited in sulphate-citrate bath with additives were nano-scale materials with F.C.C structure.
- An anomalous co-deposition of Ni-Co was maintained and hardly affected by Cu (II) concentration in additives-contained bath.
- Microstructure of coating was estimated by extrapolation method according to Nelson-Riley function and XRD profile analysis.
- Effects of microstructure and composition on microhardness and corrosion resistance were investigated.

Nanocrystalline Ni-Cu-Co coatings with face centered cubic (F.C.C.) structure were electrodeposited in sulphate-citrate bath with additives. Composition analysis showed that an anomalous co-deposition behavior of nickel-cobalt was maintained and hardly affected by copper (II) concentration in the copper- and additives-contained bath. Use extrapolation method according to the Nelson-Riley function and X-ray diffraction (XRD) profile analysis to estimate microstructure of the coatings. The results showed that the lattice parameter, microstrain and dislocation density increased and the grain size decreased with the increments of copper and cobalt incorporated into nickel. The coatings displayed a sliver-mirror-like appearance and lots of pores formed on surface. Especially, more large pores appeared on the copper-rich (44-49 wt.%) coatings. The coating comprised of 49 wt.% nickel, 13.5 wt.% copper and 37.5 wt.% cobalt attained a maximum microhardness. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were employed to investigate the corrosion behaviors of the coatings. The coating comprised of 50.3 wt.% nickel, 34.7 wt.% copper and 15 wt.% cobalt possessed higher electrochemical impedance than the copper-rich coatings, exhibiting a better corrosion resistance in 3.5 wt.% NaCl solution.