Graphitic Corrosion
Specimen: 8.25-in. ID cast iron water pipe
Material: Ferritic-pearlitic Gray Cast Iron
Environment: Buried in soil
Background: The 8.25-in. ID water pipe was part of a riser serving a sprinkler
system. The pipe, which was buried underground, had fractured
into two sections.
Service Life: The cast iron piping had been in service for approximately 30
years.
Findings: Examination of a longitudinal cross-section cut through the pipe
wall (shown above) revealed several regions of severe localized
corrosion in which the iron constituent of the cast iron had
dissolved away leaving only a layer of brittle and weak graphite.
This type of cast iron deterioration is known as graphitic
corrosion. A metallographic examination of a cross-section
transverse to the fracture surface revealed the presence of a wall
thickness composed entirely of graphite. The remaining graphite is
extremely brittle and weak. Thus, slight loading imposed upon the
pipe could result in the fracture of the pipe. |
Longitudinal cross-section through the wall of a cast iron pipe, exhibiting severe graphitic corrosion, i.e. dissolution of the iron from the cast iron pipe leaving behind layers of intact graphite. |
