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Ductile Iron Pipe Failure Analysis

Ductile Iron Pipe Failure Analysis



Areas of Expertise

Michael Fox, PhD.



Ductile Iron Pipe Failure Analysis

An underground water supply line to a fire suppression system in a restaurant was discovered to be seriously leaking. The leak was discovered upon investigating damage to the building due to settling. The underlying soil was the type that would undergo severe settling upon exposure to water.

Original Pipe Leak Discovered

Building Damage

Once the leak was uncovered, the building damage was discovered to be massive, requiring the restaurant to be shut down for over a year. There were not only repair costs but considerable lost business revenue as well. The water supply line was made from cement lined ductile iron pipe (DIP), but the DIP only extended about 6 feet from the building and was connected to the water main by a 30-40 foot length of PVC pipe. The corroded DIP is seen below.

Corroded Ductile Iron Pipe

The through-wall leak was located adjacent to a 90-degree elbow. A close-up of the leak with a dime next to it is seen below.

Close-Up of Leak

The failure in this DIP was a classical case of graphitic corrosion. One of the characteristics of graphitic corrosion is that the metal gives every appearance of being physically intact, but actually contains little to no mechanical strength whatsoever. The entire lower half of the pipe was pretty well corroded away (but looked very normal), leaving no mechanical support for the cement lining, which eventually cracked.

An interesting aspect of this corrosion failure was the fact that this underground supply line had been repaired and replaced just 7 years earlier. Therefore, a brand new pipe lasted less than 7 years. The original pipe, which had been replaced, lasted about 25 years, but failed by corrosion in almost the exact same location. Hence, this specific location and soil had a history of corrosion. The 0.6-inch diameter bolt connecting the PVC length to the DIP section was completely corroded away, as seen above. That gave 43 mils/yr lower limit to the rate of corrosion. Applying that corrosion rate to the DIP wall thickness meant that the pipe wall could have been compromised in 5.5 years or more, which would leave 1.5 years or less for active water leakage and the soil settling and building damage to occur. The conductivity and pH of the city water from the supply main was compared to water that had been in contact with the soil. The pH was slightly lower for the soil water and the conductivity was significantly higher, hence more corrosive, as seen below.

Water in contact with the cement lining (from inside an exemplar pipe) was also tested for conductivity and pH. There was little difference in conductivity, but a significant difference in pH, as seen above. The alkaline pH shift caused by the cement lining is well known and, along with other considerations, helped eliminate corrosion from the inside out as a cause for the failure.

Typically, ductile iron pipe (DIP) in this type of service will last 50-100 years when properly installed. Chemaxx opined that the cause of the early (7-year) failure was caused by failing to consider the corrosion history of the site and the subsequent lack of any form of corrosion protection, such as polyethylene wrap, asphalt coating or cathodic protection. When there is a history of corrosion or evidence of corrosive soil, protection against corrosion is required by National Fire Protection Association (NFPA) codes for private water lines to fire suppression systems. Hence, there were also code violations.

There was a lawsuit between the restaurant and the company that repaired the pipe seven years earlier that settled prior to trial.

Dr. Fox is a nationally recognized metallurgy expert, corrosion expert and failure analysis expert who has published numerous peer-reviewed, scientific papers in these fields. He worked for years as a bench scientist and a research manager in the fields of corrosion, metallurgy and failure analysis. While a research manager, Dr. Fox managed over $100 million worth of research in metallurgy, corrosion and failure analysis.