 Solution

Clean and degrease the nickel surface, remove its inhibiting oxide by an appropriate brief surface treatment, rinse, and immerse it in an aqueous bath containing reducible gold(III) species. Control concentration, temperature and immersion time, then rinse after the desired deposition. A solution of a gold complex is practical, but its actual reduction potential must be evaluated for that complex and bath: it need not equal the supplied free-ion standard potential.

At anodic sites nickel dissolves as $\mathrm{N}{\mathrm{i}}^{2+}$, releasing electrons into the conducting substrate. Those electrons reduce gold ions at cathodic sites, nucleating and growing metallic gold. The nickel itself is the reducing agent, so no external electrical supply or separate reducing reagent is required. The coupled electron balance is three nickel atoms dissolved for two gold atoms deposited for the stated oxidation states.

The process can slow or stop when a sufficiently continuous gold coating prevents the electrolyte from reaching nickel. This is not an absolute thickness rule: pores, cracks or exposed edges preserve nickel dissolution and allow continued deposition, sometimes with undercutting. nickel passivation, depletion of gold ions near the surface, diffusion through a porous layer, bath speciation and competing reactions also affect the rate. Dense gold coverage and porous gold coverage therefore have very different limiting behavior.