Mineral build up on flux joints is not uncommon. Pitting of copper can be associated with the presence of flux that has not been properly cleaned up. This tends to be more pronounced on the cold water supply piping. The purpose of flux is to create and maintain an oxide free surface. Many fluxes, especially in some older fluxes, contain chemicals that are aggressive to copper. Proper cleaning of the piping after soldering needs to be performed. Flux residues from the soldering may continue to corrode the copper.
