So far using the copper as the nagative electrode works just fine with no effect on the copper at all. and I ran 5 AMPS into the 3 little tube cells, with the copper as negative, water stayed clean.
I also ran the cell for over 72 hours between 1 and 2 AMPS. Same thing, water very clean and nothing wrong with the copper.
the only time I saw a problem with copper is when using copper for the positive, or when the water gets very hot. burning hot.
I know, every one says not to use copper because it makes the water green, and the copper corrodes, but I think that is because they used copper for both positive and negative.
Actually when using copper as negative, it gets a white protective coating, I think that is some SS from the positive electrode.