Join Date: Dec 2007
Location: Antwerp, Belgium
Bikes: commuter, racer, ATB,
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Most nipples are made of aluminium or brass, they can have nasty colours, but no rust.
The washers on the nipples inside the rim (little rings) usually are made of nickel plated steel. These washers may rust fast if water gets inside the rim and soaks the cotton rim band. (Is that the correct name?) Modern rim band is made of polyesther tape.
The rusting of the washers is not a big problem as long as you do not need to turn the nipples. In case you want to do some wheel truing, the nipples will not move and even if they do, the rusted washers may break and fall apart.
Finally these washers will rust to dust and the spokes will become loose. Such a wheel is a safety hazard.
You will have to rebuild that wheel from scratch; which may be a bad idea if the parts (rim and hub) were poor quality. Or you may prefer to buy (or build) a brand new wheel with good quality parts.