1. Stagnant water allowed to sit in horizontal lengths of piping for extended periods can corrode the inside of copper pipe.
2. Water moving through a pipe at unusually high velocity can wear away copper from the interior of the pipe.
3. If the pipe ends weren’t properly reamed smooth prior to soldering, the raised burrs inside the pipe can cause local turbulence, called erosion corrosion, which will corrode the pipe.
4. On cold-water piping, pitting and corrosion is often caused by an excess amount of flux residue. The excess flux gets carried through the pipe and deposited along the way, pitting the interior.
5. Iron deposits from a rusty water heater can also corrode copper pipes. Be sure to change the water heater’s anode rods every three to five years.
6. If corrosion is present on both hot- and cold-water pipes, have your house water evaluated by a water-testing laboratory.