While corrosion pitting is associated with bell splits and circumferential failures, it can also cause failures by itself, by reducing the wall thickness of the pipe until to the point where the internal water pressure removes any remaining wall material. The resulting leaks can be very small in size or quite large.

Not all corrosion pits are as easily identified. Both grey cast iron and ductile iron pipes can experience graphitisation, a process by which much of the iron is removed from the metal, leaving behind a  pipe, leaving the pipe very vulnerable to failure. Ductile iron pipes have often been assumed to be immune to graphitisation, but examples from both Ottawa and Toronto clearly
show graphitisation in this pipe material. Unless they receive some form of corrosion protection, all cast iron pipes can be vulnerable to corrosion. However, human actions can worsen the problem. These pits appear to have been caused by the being pipe dragged on the ground, probably during installation. The surface of the pipe was scratched, promoting local corrosion. When
the pit was deep enough that only a little metal remained (the top edge of the hole in the picture was sharp enough to cut paper when the pipe was removed from the ground), water pressure caused it to blow out.

Conditions that promote pipe corrosion may also mean that simple repairs are inadequate to solve the underlying problem. The pictures below show a gray cast iron pipe that failed four times before it was finally removed from service. The first three failures were circumferential breaks. In each case, a stainless steel clamp was placed over the break and the pipe was returned to service (the two sections of pipe shown were joined together by one of the clamps and the other breaks were at either end of the section). Finally, corrosion blow out holes appeared along the pipe’s length, causing it to be removed from service.