In this media gallery, learn about the chemistry involved in water pipes and why a failure to protect against corrosion in the water system of Flint, Michigan, tainted city water with lead and a dangerous strain of bacteria, in this media gallery from NOVA: Poisoned Water. Mineral "scale" that builds up inside water pipes serves a protective function: it prevents metal in the pipes from dissolving in water. For the scale to do its job, it must be protected from a corrosive chemical environment. However, when an anti-corrosive chemical has not been used, corrosive water flowing through a metal pipe can cause the scale to deteriorate. This can prompt reactions that leach lead from the pipe into the water and also allow disease-carrying germs to spread. [4:57]