For the first time, electroless nickel plated ZrO 2 (Ni-NCZ) particles were used for the coelectrodeposition of Ni-NCZ composite coating. Optical microscope, SEM and X-ray diffraction studies showed that Ni-NCZ has a rough surface with a smaller Ni crystallite size than Ni-ZrO 2 due to the conductive properties of NCZ particles, which provide more nucleation sites for Ni clusters. The energy dispersive X-ray spectroscopy measurement results showed that Ni-NCZ has more co-electrodeposited particles than Ni-ZrO 2 . This is due to the more positive zeta potential of NCZ particles compared to that of ZrO 2 . The microhardness measurements demonstrated that the Ni-NCZ composite coating has a higher microhardness than Ni-ZrO 2 due to the higher amounts of co-electrodeposited particles and lower crystallite size. Electrochemical impedance spectroscopy and potentiodynamic polarisation test showed that the corrosion resistance of Ni-NCZ is higher than that of Ni-ZrO 2 .

Electrochemical impedance spectroscopy of Ni and Ni-ZrO 2 composite coatings was studied. Investigation of corroded surfaces showed that the cluster boundaries in pure Ni and weak bonds between Ni matrix and ZrO 2 particles in Ni-ZrO 2 composite coating are the appropriate paths for corrosion to proceed. An equivalent circuit diagram based on blocked and partially corroded surface characterisations was proposed, and good agreement was observed between theoretical impedance spectra obtained on the basis of the equivalent circuit and spectra recorded during the measurements. Changes of microstructure and corrosion proceeding paths were recognised as the reasons for the higher corrosion resistance of Ni-ZrO 2 with respect to pure Ni.

Ni-Al composite coating was prepared by coelectrodeposition technique in Watt's bath containing dispersed Al particles to be codeposited. Pure Ni coating was also prepared as a comparison. Morphology and composition studies showed that Al particles were uniformly distributed in the Ni matrix. Microhardness measurements and microstructure studies revealed that the incorporation of Al particles lowers the microhardness of Ni coating due to the high ductility of Al and weak bonds between Al and Ni. The results of potentiodynamic polarisation tests demonstrated that the corrosion resistance of Ni-Al composite coating is lower than that of pure Ni. The microstructure and corrosion path studies revealed that the lower corrosion resistance of Ni-Al composite coating is resulted from weak bonds between Al particles and Ni matrix and formation of active-passive cells on the surface of the coating.

For the first time, functionally ZrO 2 content graded Ni-ZrO 2 composite coating has been successfully co-electrodeposited from a bath with gradually increasing stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum stirring rate in which the maximum content with uniform distribution of ZrO 2 particles in the coating can be achieved. To produce ZrO 2 content graded Ni-ZrO 2 composite coating, the stirring rate was continuously increased from 0 to optimum value. The results showed that functionally graded Ni-ZrO 2 composite coating has been successfully electroplated with continuous gradient distribution of Zirconia content in the coating using controlling of bath stirring rate. With increasing of ZrO 2 particles content, the microhardness increases from the interface towards the surface of the coating. This could provide a high wear resistance with high adhesion of the coating to the substrate.

Watt's baths containing different amounts of Al powder were used to electroplate the pure Ni and Ni–Al composite coatings. Surface morphology, microstructure of the samples and particle distribution in the coatings were studied using optical microscope, SEM, EDX and XRD. The results showed that the electroplated sample in the bath containing 40 g.L−1 Al has the maximum particle and the best particle distribution. Evaluation of microstructures demonstrated that Al particles change the microstructure of Ni from coarse‐columnar to fine‐granular structure due to providing new nucleation sites and stopping columnar growth of Ni grains. Corrosion measurements revealed that Ni‐Al coatings have lower corrosion resistance than pure Ni probably due to formation of active‐passive cells and weak bandings between Al particles and Ni substrate.