= 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 $\mathrm{Ni^{2+}}$, 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 \b[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.
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