I have an in-ground concrete pool (epoxy paint finish) with an Amerlite underwater light. The chrome bezel continuously turns black. The tarnish can be scrubbed off, but always returns. (There is also an ongoing, low-intensity battle with minor metal stains, with no known source for the metal.)
The tarnish probably indicates a bonding problem, but.................
There is a bond wire from the light niche to the bonding lug on the pump. The bond wire is #8 *stranded*, however. The wire connects to the inside of the niche and runs up the conduit with the power cord. The attachment of the bond wire to the niche is slathered in some sort of caulk. Using a wire nut in the J-box, the stranded bond wire is connected to another run of #8 stranded wire that goes back to the pump. It is a 30-40 foot run from the niche to the pump.
While the pool was partially drained I used a VOM to test, and there is continuity from the niche to the bonding lug on the pump. I connected my test wire to the screw hole where the screw holding the bezel in place is inserted, so I can assume that there is continuity from the bezel to the pump.
There are two other bond wires (also #8 stranded) connected to the bonding lug on the pump. One comes from the heat pump. The other comes from the concrete pool deck, with a bond wire from the rebar in a hot tub slab that I poured spliced into it. I have no idea of the quality or condition of the bond in the concrete pool deck.
The only real weirdness that I can see is the bond wire connection inside the niche, possibly in contact with the water if the caulking has failed. Since the goal of bonding is to keep everything, including/especially the water, at the same potential, I'm not sure this is a problem.
Does anybody have any ideas or suggestions on how to diagnose or fix what probably is some sort of current flow causing the tarnish on the light bezel?