Graphitic Corrosion in Grey Cast Iron

Water Pipe Inspection

These SEM images show cross sections of a grey cast iron water pipe. The cross section surfaces were ground and polished to reveal the continuous network of flake-like graphite peculiar to this form of iron.  A grey cast iron pipe that has undergone graphitic corrosion often visually appears to be fine other then some general surface corrosion. However, due to possible subsurface attack a substantial portion of a pipe’s wall thickness can be converted to a weak and brittle graphite network with dramatically reduced mechanical strength. Graphitic corrosion can lead to catastrophic failure in grey cast iron pipes carrying water at relatively high pressures.

Metallographic Microscopy The free surface at the left side of the image was the outside surface of the previously buried pipe, which had been in contact with moist soil. A damaged area is plainly visible penetrating the pipe wall from the outer surface at the left. This form of attack, known as graphitic corrosion, is specific to grey cast iron. It occurs when the more noble graphite promotes the accelerated attack of the nearby iron metal through galvanic action in a corrosive environment such as a damp soil.
The free surface at the right side of the second image was the inner surface of the same pipe, which had been in contact with potable water. The inner surface clearly suffered corrosive attack resulting in roughening and loss of wall thickness. Additional evidence of graphitic corrosion is visible here. Metal loss due to galvanic attack is obvious around several of the graphite flake clusters visible in this cross section plane. This subsurface damage is possible because of the continuous graphite network and would not have been identified through a surface-based visual inspection. Metallographic Microscopy