Standard potential, nickel couple

The reduction is thermodynamically favored, because the standard potential of the couple Cu2+/Cu is positive (E° = +0.34 V). Metals with negative standard potentials, such as zinc (E° = —0.76 V) and nickel (E° = —0.23 V), cannot be extracted hydrometallurgically. 

It is worth noticing that, according to the Nernst equation, that is related to the equilibrium between cadmium oxide (Cd ) and reduced cadmium, the amount of cadmium oxide (Cd ) remaining in solution at the potential of our system set by the couple (H2/H ), should be theoretically higher than lOOOOppm (Fig. 18.36). From our results, it is obvious that the cadmium concentration in solution is decreased to ppm or even ppb level. Cadmium can then be deposited at a potential more positive than that proposed by the Nernst equation. This phenomenon has already been reported in the past with other metals and has been called underpotential deposition [236]. In fact, when cadmium is reduced, it does not build cadmium-cadmium bonds, but cadmium-nickel bonds. Therefore, the Nernst equation with standard redox potentials (Cd/Cd ) and (H2/H ) does not well represent underpotential cadmium deposition. According to Kolb [237], the potential shift observed in the underpotential deposition of a metallic ion (M +) onto a metal... 

No. Steel is mostly iron whose standard reduction potential is more negative than that of nickel, so it would still be the iron that is oxidized if it is coupled with nickel. 

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