*3.7. Pulse Frequency*

Increased pulse frequency has been reported to achieve good Ni–Co films with smooth surface morphology and high microhardness, as well as better corrosion resistance of deposited coatings [5,37]. Pulse frequency has a significant influence on the morphology of the deposited coatings as well as the content of nanoparticles. At higher pulse frequencies, smaller grain sizes are obtained. Bigger grains tend to be more thermodynamically stable than smaller ones, and as such, an increase in OFF-time causes the grain size to increase [65]. Similar findings were reported by Yang and Cheng [32]. This was attributed to enhancement of the nucleation process by the SiC nanoparticles by providing electro crystallization nucleation sites and retarding crystal growth. Furthermore, the content of SiC in the Ni–Co/SiC nanocomposites increased with increasing pulse frequency. When pulse frequency levels are higher, the overpotential generated is much higher, and this provides more energy for nanoparticle adsorption into the coatings. As such, it can be concluded that higher pulse frequency offers better properties for deposited coatings as a factor of increased nanoparticle content.

### *3.8. Duty Cycle*

When low duty cycles are used, the resulting coating is characterized by finer and more compact structures than if higher duty cycles had been used. It has been reported that a low duty cycle increases the microhardness and corrosion resistance of electrodeposited Ni–Co/SiC nanocomposite coatings. This can be related to the increase in SiC nanoparticle content in the coatings at lower duty cycles [32]. With the increase in duty cycle, there is a transformation of the surface morphology from a branched, acicular structure to a more nodular structure. It can therefore be concluded that lower duty cycles offer the best properties for deposited materials.

#### **4. Mechanism of Ni–Co–Nanoparticle Electrodeposition**

The process starts with complete combination of Ni and Co in the electrolyte to form a solid solution [66]. At the same time, the surfaces of nanoparticles suspended in the electrolyte adsorb positive and negative ions (particle charging). The nanoparticles are then transferred by electrophoresis force and gravitational force to the growing Ni–Co matrix where they become embedded into the deposit. Ni–Co deposition is believed to be anomalous deposition where the content of the less noble element (Cobalt in this case) is much higher than the concentration of the same element ions in the electrolyte [16,67,68]. This could be attributed to the relatively fast kinetics related to cobalt deposition. Moreover, the inhibition of nickel deposition in presence of cobalt ions is less likely as a result of evolution of metal hydroxides [67]. Figure 6 shows the cobalt content in deposited Ni–Co coatings as a function of Co concentration in the electrolyte [16].

**Figure 6.** Alloy composition as a function of Co2<sup>+</sup> concentration in the electrolyte [16]. Reprinted from Surface and Coatings Technology, 201, Meenu Srivastava, V. Ezhil Selvi, V.K. William Grips, K.S. Rajam, Corrosion resistance and microstructure of electrodeposited nickel-cobalt alloy coatings/Pages No. 3051–3060, Copyright (2020), with permission from Elsevier.

Many models have been proposed concerning co-deposition of nanoparticles. Celis [69] suggested that it was a five-step process which took into consideration the creation of an ionic cloud that engulfed the nanoparticles, movement of the nanoparticles through the electrolyte and the diffusion layer. These steps can be classified as: (i) formation of an ionic cloud on the surface of the nanoparticles from the adsorbed ions; (ii) the charged nanoparticles are then transferred through the electrolyte bulk until they reach the hydrodynamic boundary layer; (iii) through diffusion, the nanoparticles are transferred en-masse to electrode surface; (iv) adsorption of electroactive ions and the free ions occurs on the particles on the cathode; and (v) electroreduction of adsorbed ions occurs followed by incorporation of particles into the growing metal matrix [70]. Guglielmi's model proposed that the adsorption mechanism onto the cathode followed a two-step process for the charged nanoparticles. Firstly, the charged nanoparticles are loosely adsorbed while still being engulfed in a film of adsorbed cations [69].
