Tribocorrosion of Copper Water Tubes
Tribocorrosion, also known as impingement damage, is the combined effect of corrosion and erosion caused by rapid flowing turbulent water. It is probably the second most common cause of copper tube failures behind Type 1 pitting which is also known as Cold Water Pitting of Copper Tube.
Source: Wikipedia — Tribocorrosion of Copper Water Tubes (CC BY-SA 4.0)