Case Histories » Insurance

Graphitic Corrosion

Graphitic corrosion of an cast iron water 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.

Specimen

8.25-inch ID cast iron water pipe.

Material

Ferritic-pearlitic gray cast iron.

Environment

Buried in soil.

Background

The 8.25-inch 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 of the pipe wall (shown above) revealed several regions of severe localized corrosion. The iron constituent of the cast iron had dissolved away, leaving only a layer of brittle and weak graphite. This 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. As a result, slight loading imposed upon the pipe could result in the fracture of the pipe.