1- Poor Design
2- Poor workmanship in installation
3- Excessive velocity ESPECIALLY on water over 180 degrees should be liminted to no more then 2-3 FPS
4- Hard water
5- acidic water
6- Using a self cleaning (ACID TYPE) flux and not properly flushing it out (See 2)
7- Non reaming of tubing ends causing turbulent flow (see 2
8- Under sized piping (see 1)
9- Chemicals added to water supply