*3.5. Temperature*

Shi et al. [60] postulated that there is a two-fold effect with respect to temperature. When electrolyte temperatures are low, there is an increase in nanoparticle kinetic activity with a rise in the temperature of the bath, and this boosts adsorption of nanoparticles into the metal matrix. Increasing the temperature decreases the density and viscosity of the bath, thereby improving the mobility of ions within the electrolyte. As such, coatings deposited at higher temperatures exhibit superior properties owing to thicker coatings and higher nanoparticle contents than those deposited at lower temperatures [53].

Temperature variation has also been reported to alter the size of crystals in the deposited coatings. Prabu and Wang [53] reported that increasing temperature from 20 to 60 ◦C resulted in larger and

sharper crystals. This was attributed to increase in rate of reducible ion diffusion to the cathode with an increase in temperature which decreased the polarization resistance. Research on the effect of temperature on Ni–Co coatings shows that the deposition rate increases with increases in temperature of the electrolytic bath [4]. Idris et al. [54] reported that, for Ni–Co coatings electrodeposited using high speed jet electrodeposition, an increase in temperature from 55 to 65 ◦C (at 1 A/cm<sup>2</sup> current density) resulted in a subsequent increase in thickness of the electrodeposited coatings from 61.4 to 71.7 μm, respectively. Similar increasing trends of the thickness with rise in temperature were observed at current densities of 0.1, 0.3 and 0.5 A/cm2. This can be attributed to grain growth as a result of a free growth mode of Ni resulting from the temperature rise.

When the temperature exceeds certain limits, thermodynamic ion movement is enhanced greatly, and the nanoparticle's kinetic energy increases. As a result, less nanoparticles become adsorbed into the metal matrix. This conforms to Langmuir's adsorption theory, where temperature increase beyond certain levels has a negative effect on nanoparticle absorbability. As a result, the electric field and the overpotential of the cathode are decreased, making it harder for nanoparticles to be embedded into the coating. As such, lower contents of nanoparticles are observed in the deposited coatings.

### *3.6. Electrolyte pH*

According to past research, it can be seen that the composition and structures of Ni–Co alloys and their nanocomposites can significantly affect their physiochemical properties. The effect of pH on Ni–Co deposits is predominantly dominated by three factors [61]: (i) the acidic environment in the electrolyte dissolving newly deposited metal atoms on the cathode surface, (ii) metal hydroxide formation and adsorption on the surface of the electrode, and (iii) normal electrodeposition of metals. When the electrolyte pH is low, the newly deposited metal becomes dissolved at a faster rate and formation and adsorption of metal hydroxides becomes depressed. In this case, the electrodeposition process is mainly dominated by normal electrodeposition of metals and this results in lower Co2<sup>+</sup> in the bath. At higher pH values however, the formation and adsorption of metallic hydroxides is promoted, and the newly deposited metal dissolution becomes suppressed. For higher pH values, the electrodeposition process is dominated by formation and adsorption of Co hydroxides on the cathode surface and this produces higher Co contents in deposited coatings [62,63]. Tian et al. [61] reported a gradual increase in Co content from 9.4% to 19.6% with an increase in the value of pH from 2.0 to 5.4. pH value in the electrolyte has also been observed to affect current efficiency. An increase in current efficiency from 52.1% to 81.2% was also reported with increase in pH from 2.0 to 5.4. Research linking pH value to hydrogen evolution has also been reported. Increase in pH in Ni–Co-based deposits has been associated with an increase the in hydrogen evolution rate, followed by creation of trace amounts of Ni and Co hydroxides which hinder the growth of crystals [64].
