Red rust dissolves in water and damages inside water pipes. It is unstable and fragile as it causes damage to human health when ingesting too much. In addition, volume of red rust is about tenfold of that of black rust that often clogs the pipes.
On the other hand, black rust is a harmless insoluble substance that is highly resistant to oxidation and protects inside pipes from corrosion damage. It was also used as surface coating of cooking utensils, such as traditional Chinese frying pan and Japanese teapot, and steam locomotive as well.