Extend life of hygrometer? (Prevent breakdown over time)
Those sensors are manufactured by regular PCB etching/milling processes (cheapest way of manufacturing something like this), and I'd bet that they aren't nickel plated (not a commonly offered finish), more likely tin, but in any case they won't 'rust', as the underlying traces are copper, but will corrode over time, or at least develop an oxide layer that reduces the conductivity indicating that the soil is dryer than it really is.

Gold plating would help, and they are available. Gold plated boards are substantially more expensive than tin.

Here's the first one I found.