Lloyd E. Sponenburgh wrote:

The culprit is H2S - hydrogen sulfide gas. The source???? Who knows.
You
do.


BTW... when I said that "HCl" can do it, too, I should have noted that
chloride corrosion on copper can vary in color from a dark earthen brown to
blue. But the black layer you saw is almost certainly from H2S.

LLoyd