*5.1.1. Electrodeposition of nickel nanocomposites*

Nickel nanocomposite coatings are used in a wide variety of industrial and engineering applications, such as consumer electronics, chemical, computer and telecommunications industries in order to improve corrosion and wear resistance, modify magnetic and other properties. For nickel matrix electrodeposits, a great variety of particles have been used such as oxides i.e. TiO2 [41, 42], Al2O3 [43], CeO2 [44, 45], ZrO2 [46], graphene oxide GO [47], carbon nanotubes CNT; Al2O3/Y2O3/CNT [48], carbides like SiC [49- 51], WC [52] and nitrides such as TiN [53] and Si3N4 [54].

The Ni-TiO2 system was selected because nickel is an industrially important coating for corrosion protection [42]. Generally the volume fraction of co-deposited particles is limited for nanoparticles and usually it is inversely proportional to their size. For example, Shao et al. [43] studied the rate of incorporation of two different sizes of Al2O3 nanoparticles (50 nm and 300 nm) into a nickel deposit. Using similar operating parameters (1000 rpm, 20 mA cm−2), it was found that the percentage volume fraction of the 300 nm Al2O3 in the nickel deposit was much higher compared to the 50 nm Al2O3. The presence of nanosized particles in a metal deposit may induce changes in the crystalline structure of the metallic coating.

Ni−CeO<sup>2</sup> nanocomposite coatings were prepared by co--deposition of Ni and CeO2 nanopar‐ ticles with an average particle size of 7 nm onto pure Ni surfaces from a nickel sulphate. The as-codeposited Ni−CeO2 nanocomposite coatings showed a superior oxidation resistance compared with the electrodeposited pure Ni coating at 800 °C. The co-deposited CeO2 nanoparticles blocked the outward diffusion of nickel along the grain boundaries. However, the effects of CeO2 particles on the oxidation resistance significantly decrease at 1050 °C and 1150 °C due to the outward-volume diffusion of nickel controlling the oxidation growth mechanism [45].

According to Zeng et al [46], increasing concentration of the CeO2 nanoparticles in the bath increased the weight percent of CeO2 particles in the nanocomposite coatings, and improved the micro- hardness, and the friction, corrosion, and wear behaviour of the coatings. However, excessive CeO2 nano-particle loadings were detrimental to the coating properties.

Ni–Al2O3–SiC hybrid composite films with an acceptable homogeneity and granular structure having 9.2 and 7.7 % vol. Al2O3 and SiC nanoparticles, respectively were developed success‐ fully by M. Masoudi et al [50]. Both micro hardness and wear resistance increased owing to dispersion and grain-refinement strengthening of nanoparticles. The oxidation resistance of the Ni–Al2O3–SiC hybrid composite coatings was measured to be approximately 41 % greater than the unreinforced Ni deposit and almost 30 % better than the Ni–Al2O3 composite coatings.

Ni-SiC nanocomposite coatings were applied on AZ91 magnesium alloy from Watts bath with different SiC content. Micro-hardness of specimens was measured and the results revealed a significant enhancement: from 74 Vickers for bare AZ91 magnesium alloy to 523 Vickers for coated specimen. The obtained data showed the superior corrosion resistance for the coated AZ91 magnesium alloy [51].

The effect of incorporation of Si3N4 particles in the Ni nanocomposite coating on the micro hardness, corrosion behaviour has been evaluated by Kasturibai et al. [54]. The micro hardness of the composite coatings (720 HV) was higher than that of pure nickel (310 HV) due to dispersion-strengthening and matrix grain refining and increased with the increase of incorporated Si3N4 particle content. The corrosion potential (Ecorr) in the case of Ni–Si3N4 nano-composite had shown a negative shift, confirming the cathodic protective nature of the coating [54].

According to R. Abdel-Karim et al. [55], Ni–Mo nanocomposite coatings were prepared using a nickel salt bath containing suspended Mo nanoparticles using direct current. The crystallite size (18–32 nm) and the surface roughness increased by raising the current density. A remark‐ able deterioration in the corrosion resistance of Ni–Mo composites was observed with the increase of Mo content. This could be due to crystallite size-refining and surface roughness effect and correspondingly a large surface area. Resulted high surface roughness lead to improved electrocatalytic effect for hydrogen evolution.
