the directions that came with mine said if you use the hands free zapper version that uses the conductive pads to use the salt water solution (1 tsp Sea Salt in one cup of boiled water, allow to cool, save in sealed container)
With the copper pipes, "Cover the pipes with wet (plain water) paper towel or cotton cloth."
Also, "When zapping for extended peiords, make sure the paper towel remains wet with the salt solution for the entire time (plain water if using the pipes)"
I have only used my zapper 2 times, but have had no discoloration. Good luck. Let us know if any difference.