My SS hand rail, for the past year or more, it turns black under the water. It’s like a soot. I can cleaned with a cloth impregnated with Muratic acid, it after a week is already a little bit black. Also a year ago, a superficial crack started by the rail. Which I patched. I thought it had to do with grounding, so I run a copper wire to ground about two weeks ago. Any ideas?
I agree. You likely have an electrical leak into your water and the bonding system is doing its job. My guess is a fairly significant issue for it to discolor that fast.