Corrosion of outside of copper pipes.
The copper pipes in our base developed a black coating on the outside of pipe. We have asump pump that constatntly works and the sump water brings significant amount of silt which is contains iron oxide and the sump water has rotten egg smell (probably hydrogen sulhide(). What is the cause of black colouring on the surface of the pipe and how to prvent it? We put the house for sale and a got an offer. But the housing inspector from a compahy scared the potential buyers that teher is a disaster going to happen any time. They backed off. We lived in the house for 33 yeras. House was built in 1977.