*7.3. Corrosion Behaviour*

The corrosion behavior of electrodeposited Ni–Co alloy can be attributed to: chemical composition, grain size, preferred orientation, and phase structure. In the case of chemical composition, it has been reported that formation of Ni–Co alloy can change the nobility of the materials thereby affecting corrosion resistance [124]. Co is less noble that Ni, that is, it is more reactive compared to Ni. Therefore, increasing the Co content is bound to produce coatings with greater electrochemical activity than that of purely Ni coatings [125]. The polarization resistance of Ni–Co alloys has been reported to increase with increase in Co content up to a given limiting value, beyond which the corrosion resistance decreases with further increase in Co content. This is concurrent with findings reported by Babak et al. [126] where Ni–17Co coatings exhibited better corrosion resistance (10.08 kΩ cm2) compared to Ni–42Co alloy coatings (3.32 kΩ·cm2) as shown in Figure 13.

**Figure 13.** The potentiodynamic polarization curves of the Ni–Co alloy coating as a function of the cobalt content [126]. Reprinted from Applied Surface Science, 307, Babak Bakhit, Alireza Akbari, Farzad Nasirpouri, Mir Ghasem Hosseini, Corrosion resistance of Ni–Co alloy and Ni–Co/SiC nanocomposite coatings electrodeposited by sediment codeposition technique/Pages No. 351–359, Copyright (2020), with permission from Elsevier.

The phase structures of Ni–Co binary alloy coatings have been observed to consist of FCC single-phase solid solutions [12,126]. In the case of the coating's materials microstructure, single phase structures have proven more corrosion resistant than two-phase structures. Corrosion attacks usually occur along grain boundaries (between phases) owing to the galvanic cells that are created between the phases and their higher levels of energy compared to parts located within the crystal itself. Moreover, base centered cubic (bcc) phases have lower corrosion resistance compared to face centered cubic (FCC) phases as a result of their lower packing factor.

As a factor of preferred orientation, it has been reported that Zn–Ni alloys exhibit high corrosion resistance owing to crystallographic planes being predominantly present with higher packing densities [127]. Babak et al. reported that Ni–17Co alloy coatings exhibited high corrosion resistance as a function of predominantly (111) preferred orientation. Lupi et al. [66] found that electrodeposited Ni–Co alloys containing 40%–50% Co content provide the best catalytic properties of the alloy for the reaction leading to evolution of hydrogen in alkaline media.
