I came acroos a 27 year old home with copper supply lines, I noticed that there was long stretches of lines that were actually black…Any explanations and/or verbage for my report… No leaks, no cross connections, no dissimilar material contact.
Oxidation. If you want to know more than you need to then read this.
Initially, bare Cu metal atoms react with air to form the pink oxide, cuprite, Cu2O, which has Cu+1 cations. This gradually oxidizes further to the black oxide, tenorite, CuO, with Cu+2 ions. The black sulfide CuS also sometimes forms. In the presence of moisture, the blackish layer slowly reacts with sulfur dioxide and carbon dioxide from the air to eventually form the patina, which is a mixture of 3 minerals:
brochantite, a green, hydrated copper sulfate, Cu4SO4(OH)6
malachite, the green, hydrated copper carbonate Cu2CO3(OH)3
azurite, the blue, hydrated copper carbonate Cu3(CO3)2(OH)2
In these compounds, copper atoms from the metal surface have been fully oxidized into Cu+2 (cupric) ions. The rate of patina formation and the proportion of the components depend upon humidity and the amount of air pollution.
And oxidation is helped along many times by low voltages running along the copper water pipes.
At an inspection a few years ago, every bit of copper was greenish black. Having read about low voltages on copper lines and the resulting coloration, I explained it to my Clients. My follow-up found that the electrician had, indeed, found stray electricity of up to 11 volts on the copper water pipes.
There was a very good article around here somewhere about stray electricity, but I’ll be darned if I know where it is right now. Perhaps Ms Margarita and Dr Cuervo can find it when they get in this morning.