Case Histories

Insurance Sector

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-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.


 

Microbial Influenced Corrosion of Copper Piping

MIC corrosion perforations in a 1-inch copper sprinkler pipe
Longitudinal cross-section through copper pipe showing 2 corrosion sites covered by “mounds” of bright green-colored corrosion products. Note: Black-colored coating on the inside wall of the copper pipe

Specimen: 1.0-in. OD copper sprinkler line piping

Material: Copper

Environment: Sprinkler line, containing stagnant water. Water was untreated with no corrosion inhibitors or biocides.

Background: After installation, the lines were filled and left wet, with primarily stagnant conditions. Pin-hole leaks had been discovered at multiple locations on multiple floors.

Service Life: The sprinkler line copper piping had been in service for approximately 14 years.

Findings: Through optical and scanning electron microcopy it was found that the copper sprinkler piping had failed prematurely from Microbial Influenced Corrosion (MIC). Strong evidence was found including regions of “biofilm”, large crystals of Cu2O and the base of corrosion pits, micro, bore-like holes adjacent to corrosion pits, and the very random nature of the pitting all indicate failure due to MIC.


 

Microbial Influenced Corrosion of Heating Fuel Oil Tanks

MIC of heating fuel oil tank - corrosion perforation at bottom of tank
Close-up view of the leak site and associated metal loss at the bottom of the tank. The metal loss was limited to a parabolic region at the end of the tank (fuel oil/water interface)
MIC of heating fuel oil tank - metallurgical cross section through tunnelling under corrosion pits
Corrosion pits which were not representative of typical pitting corrosion. Unusual pit morphology and tunnel-like bore holes (encircled) were indicative of microbial activity.

Specimen: Above ground heating fuel oil tank: 935L (approx. 200 Gallons)

Material: ASTM A569 (or equal) sheet steel

Thickness: 2mm

Service Life: The heating fuel oil tank had been in service for approximately 3 years.

Findings: The observed corrosion features were found to be dissimilar to typical corrosion pitting. Pitting was found to be highly localized, and appeared as straight, bore-like holes, in contrast to typical concave pitting. Additionally, tunnel-like features were also observed below the corrosion pits within the wall thickness of the tank. It was determined that the corrosion perforations which caused the above-ground heating fuel oil storage tank to leak were caused by microbial influenced corrosion (MIC). MIC is the corrosion or deterioration of a material which is initiated and/or accelerated by the activities of micro-organisms.


 

Plastic Hot Water Tank Valve

Failure analysis of a plastic valve which was overthreaded into a metal socket
Failure analysis of a plastic valve which was overthreaded into a metal socket
Expelled material on overthreaded plastic valve
Expelled material on overthreaded plastic valve

Specimen: Plastic Hot Water Tank Valve

Material: White Polymeric Plastic

Background: The plastic hot water tank drain valve was originally installed in a hot water tank located in a residential home. The tank leaked into the home and caused approx. $50,000 worth of damage.

Service Life: The water heater would have been in service for less than 5 years

Findings: The valve was poorly installed and had been over-tightened past the “as-manufactured” length of the threads. This action abusively “tapped” further threads in the body of the valve, resulting in undue stresses on the valve, and displaced the plastic of the valve body, creating a “mushroomed” ridge of plastic around the circumference of the valve. This initiated cracking in the valve which led to the failure of the valve and water damage to the residence.