The Ni-ZrO 2 composite coatings were prepared by conventional electrodeposition method in Watts bath containing different amounts of ZrO 2 to be codeposited. Pure Ni coating was also prepared as a comparison. The distribution and amount of the particles in the coatings were studied using an optical microscope, SEM, EDX, microhardness instrument and XRD. The results showed that with increasing ZrO 2 concentration in the bath up to 90 g L 21 , particle content in the coating increases. If concentration of ZrO 2 surpasses 90 g L 21 , ZrO 2 content in the coating decreases, and distribution of the particles in the coating becomes non-uniform. Evaluation of corrosion behaviour showed that with increasing ZrO 2 content in the coating, the corrosion potential shifted towards noble and positive values, and the corrosion resistance increased. The results revealed that the electroplated sample in the bath containing 90 g L 21 ZrO 2 has the maximum particle content and the best corrosion resistance.

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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.