Can a bright nickel deposit become dull over time? S.K.
The short-and-to-the-point answer is, yes, it can. But first we should distinguish between the nickel layer itself becoming dull and the substrate oxidizing through the nickel layer. If the substrate is oxidizing, depositing a thicker layer of nickel can solve the problem. This strategy obviously does not work if the nickel layer is oxidizing.
When we think about oxidation of surfaces, we usually think of steels and other iron containing materials, but many other metals can suffer the same fate. Electroplaters always talk about nickel surfaces becoming passive. A passive nickel surface is nothing more than a nickel layer with a small amount of oxidation on the surface. If a bright nickel surface has prolonged exposure to the atmosphere, the light oxide layer becomes heavier and the eye now sees this as a dull finish.
In the case of a bright nickel surface, one reason that we apply a thin layer of chromium over the nickel is to slow down this oxidation process.