What We Can Learn From Mass Underground Water Pipe Corrosion
Water main breaks cost the United States and Canada a whopping $1 billion USD every year. Corrosion prevention for cast iron and ductile iron pipes located in corrosive soils may be overdue.
In North America, underground water pipe break rates were up 27% in the six years between 2012 and 2018, and the increase in break rates for cast iron pipes is much higher at 43%. Some 28% of failures are directly attributed to corrosion. However, the failure modes such as circular and longitudinal cracks in cast iron and ductile iron pipes could also be due to corrosion because water main failures correlate to the severity of the corrosive soil conditions. Some 75% of utilities are aware of corrosive soil conditions in the regions where some of their water mains are located.
There is a pressing need for substantial effort to reduce failure rates by adopting catholic protection (CP) and in some cases by providing additional coating and lining for the external pipe surfaces.
The Magnitude of Underground Water Pipe Breaks
Water main breaks cost the United States and Canada a whopping $1 billion USD every year. On average there are 700 water mains failures each day and failures could reach 250,000 annually. A lack of effective corrosion prevention could be the main cause for the failure of the ferrous pipes that are predominantly used for the water system.
Years of neglect has left the underground water pipes in a state where they may be leaking 10% of their water into the ground, while also causing severe health issues due to drinking water contamination. A report based on a survey conducted by Steven Folkman, a researcher at Utah State University’s Buried Structure Lab, examined the critical condition of the underground water distribution system, a vital public asset. (Steven Folkman, Water Main Break Rates in the USA and Canada: A Comprehensive Study, 2018).
Utah State University had previously issued a similar report in 2012, and after six years this new report should act as an eye opener — for it finds that underground pipe breakages increased 27% in a six-year period. (Average deterioration rates increased from 11 breaks per 100 miles to 14 breaks per 100 miles.)
Table 1. Pipe break rate trends (2012 to 2018).
|Type of Pipe||Change in Pipe Break Rate|
|Asbestos cement pipes||46%|
|Cast iron pipes||43%|
|Ductile iron pipes||13%|
|Concrete steel cylinder (CSC) pipes||- 43%|
|PVC pipes||- 10%|
The pipe break rate (Table 2) is the critical measurement used to evaluate the condition of water distribution piping systems, which are the pipes that distribute drinking water to homes and businesses. Because most of the water pipe network is underground, it doesn’t receive the attention it deserves until disruptions become unmanageable. (For a widely reported example, read The Role of Corrosion in the Flint Water Crisis.) Wastewater infrastructure is also in a similar situation.
Table 2. Water main basic break rates per 100 miles of pipe length per year.
|Type of Pipe||Basic Break Rate per 100 Miles of Pipe Length per Year|
|Cast iron pipes||34.8|
|Ductile iron pipes||5.5|
|Asbestos cement pipes||10.4|
|Other material pipes||12.4|
What We Can Learn from the Data on Pipe Breaks
These figures indicate that the predominant failure of metallic pipes can be attributed to corrosion either directly or indirectly. A pipe structure weakened by corrosion can develop cracks on the pipe's surface and cause failure.
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Regarding piping material, break rates of cast iron and asbestos cement water mains increased the most. Pipe break rates of cast iron pipes increased 43% in six years from 24.4 breaks to 34.8 breaks per 100 miles.
Cast iron is the one of most common piping material used in North America, and along with ductile iron piping these could be the primary contributors for the overall rise in break rates.
Ductile iron and cast iron pipes have been used in the eastern and upper northwest parts of the United States, and total 56% of all pipes. Asbestos cement, ductile iron, cast iron and polyvinyl chloride (PVC) pipes account for 91% of total pipes. Because the pipe break rate for ductile iron pipes are comparatively lower than for cast iron pipes, cast iron pipes should be given a top priority, as these can bring down the overall break rates substantially.
PVC Pipes Have Low Failure Rates
Twenty two percent of drinking water piping is made from PVC, which is not prone to corrosion and has a very low failure rate (break rate). Regions that have used PVC pipes have reported low break rates, with break rates decreasing from 2.6 in 2012 to 2.3 in 2018. Overall usage of PVC pipes has increased since 2012 due to its low corrosion potential.
Break Rates Analyzed by the Size of the Water Utility
The break rates for cast iron pipes were higher for smaller utilities than for larger ones (Table 3).
Table 3. Cast Iron pipe break rates per 100 miles of pipe length by size of utility.
|Size of Utility||Pipe Break Rate|
|Small (less than 200 miles in pipe length)||60.8|
|Medium (200 - 1000 miles in pipe length)||43.2|
|Large (greater than 1000 miles in pipe length)||27.4|
Only 45% of the utilities adopted condition assessment measurements for water mains, although some of the large and medium size utilities do preventive and condition based maintenance. (Read more about surveys in The Benefits of Timely and Effective Reporting When Conducting Pipeline Close Interval Surveys.)
Smaller and rural utilities have reported double the break rates compared to their larger counterparts. Larger utilities have adopted improved preventive measures that helped them to keep break rates at a lower level. There is room to adopt corrosion prevention best practices in the medium and smaller utilities.
History of Piping Material Selection and Corrosion Protection
Steel pipes installed in the 1930s had welded joints and lacked internal or external protection. Those installed in the 1950s and later were provided with a cement lining of internal surface for corrosion protection. The latest data indicates a break rate of 7.6 per 100 miles of pipe, which is higher than the break rate of PVC pipe at 2.3 per 100 miles and higher than the break rate of ductile iron pipe. This data also demonstrates the feasibility of reducing corrosion failure by providing corrosion protection on external surfaces.
Cast iron pipes with lead joints adopted between 1900 and the 1940s without corrosion protection on the external surfaces and with cement coating on the internal surfaces in some cases had the risk of lead joint leaching.
Cast iron pipes installed with leadite joints during the 1940s had the possibility of experiencing pitting corrosion due to sulfur present in leadite joint, in spite of the cement coating/lining on the inner surface of the pipe. However the cast iron pipes and ductile iron pipes installed after 1950 used rubber joints and cement lining for internal surfaces.
In the case of ductile iron pipes installed after 1970, polyethylene (PE) linings or coatings were used for external surface protection. This could be one of the reasons why the break rates of ductile iron pipes has been considerably lower than the break rates of cast iron pipes.
The historical choice of piping material is also corroborated by pipe age data:
- 21% of pipes are less than 20 year old and predominantly made of PVC and ductile iron
- 43% of pipes are between 20 to 50 years old and are dominantly PVC, ductile iron and asbestos cement and to a lesser extent cast iron
- 28% of pipes are over 50 years of age and are dominated by cast iron, asbestos cement and concrete steel cylinder (CSC) pipes.
The main lesson from this is that while the internal surfaces of metallic pipes were protected from corrosion by the cement lining, the external surfaces remained unprotected.
The Impact of Corrosive Soils
Ductile iron pipes in corrosive soil have a 10-fold increase in break rate as compared to the same pipes located in non-corrosive soils. Metallic pipe break rates have been very high in areas where the soil was found to be corrosive. Break rates were directly correlated to the corrosion index of the underground soil. (Learn about soil corrosion in the article An Introduction to Soil Corrosion.)
Analysis of the failure mode indicates that circular cracks were the most frequent failure mode observed (37%), whereas corrosion was the second most frequent mode of failure (27%). Circular cracks and longitudinal cracks together accounted for 59% of failures. These cracks could be indirectly related to a weakening of the pipe's structure due to corrosion in the case of cast iron and ductile iron pipes.
Corrosion has been found to be the most important mode of failure for ductile iron pipes. This failure has occurred in spite of the fact that a polyethylene (polywrap) type of corrosion protection was provided in the majority of piping systems. Ductile iron pipe failure is aggravated by the progressively lower wall thicknesses used in newer pipe installations. Both internal and external surfaces of ductile iron pipes have been failing due to corrosion.
28% of failed pipes were more than 50 years old, and around 43% were more than 20 and less than 50 years old. 29% percent of pipes failed even before they reached age of 20 years. This points to the feasibility of enhancing longevity of ductile iron and cast iron pipes through effective corrosion prevention measures in the regions where soils are corrosive. Pipes that are more than 50 years old are predominantly cast iron. Ductile iron pipes are generally more than 20 years but less than 50 years old. Those less than 20 years old are either PVC or ductile iron. The largest diameter pipes were predominantly concrete steel cylinders, whereas those less than a 12-inch diameter are cast iron, ductile iron, asbestos cement and PVC.
Corrosive Soils in Canada
For metallic pipes the break rates were generally higher in Canada than in the US, except for steel pipes (Table 4).
Table 4. Comparison of break rates per 100 miles of pipe length in Canada and the United States.
|Type of Pipe||Canada||United States|
In general more corrosive soils were found in Canada. For example, the corrosive conditions of soil in Edmonton, Canada, forced the utilities there to switch from cast iron pipes to asbestos cement pipes and then later from asbestos cement to plastics such as PVC pipes. Break rates for PVC pipes in Canada were reported to be lower than the figures reported for the US, indicating the prevalence of better overall installation and preventive maintenance practices.
Cast Iron Pipes with Leadite Joints
Cast iron pipes and other metallic pipes with leadite joints installed during the period from 1920 to 1960 were found to be causing water main breaks because the pipe material and joint material have different coefficients of thermal contraction and expansion that caused stress changes during extreme temperature variations.
Furthermore, the sulfur content in the leadite material would have initiated pitting corrosion in adjacent pipe material, resulting in water leaks and mains failures. The lead joints used on cast iron pipes from 1900 to 1940 could have resulted in lead leaching into the water, causing an increase in the water's lead content.
Methods of Corrosion Prevention and Awareness
The corrosion prevention measures used for some water mains were the polyethylene wrap method followed by cathodic protection. Dielectric coatings were less frequently applied. Cement lining was used internally for corrosion prevention in some cases for pipes installed between 1920 and 2010.
Some utilities lacked an awareness of the appropriate corrosion prevention methods for a given pipe material in the corrosive soil conditions. There is an opportunity to improve maintenance by creating awareness about corrosion prevention methods. Best practices being adopted by larger utilities could be shared with other utilities.
The presence of corrosive soils and the circular cracks, longitudinal cracks and corrosion observed in metallic pipes indicates that industry action to improve corrosion detection and corrosion prevention for cast iron and ductile iron pipes located in corrosive soils may be overdue.
Taking a cue from utilities in Edmonton, Canada, utilities with assets in corrosive soils may consider replacing metallic pipes with PVC pipes. Alternative action plans can include protecting internal and external pipe surfaces with new, cost-effective coatings.
Apart from corrosive soils, other factors that cause pipeline corrosion include the bimetallic coupling of different pipe joint materials, stray currents, deicing chemicals used on roads, soil temperature changes and soil pollution.
Cast iron and ductile iron pipes are expected to last around 50 to 100 years without failure if effective corrosion prevention techniques are adopted. Cathodic protection has been successfully used for underground oil and gas pipes for 45 years. Sacrificial galvanic anodes connected to cast iron and ductile iron pipes can prevent pipeline corrosion because the anode (which can easily be replaced) will corrode first.
Coatings and polyethylene wraps can also be used in addition to cathodic protection. Coal tar epoxy, polyethylene and fusion bonded epoxy (FBE) are some of the coatings and linings used to electrically insulate pipes from the soil, which acts as an electrolyte. Polyethylene is being used for the external surfaces of ductile iron pipes installed since the 1980s; this could be one of the reasons why the break rate for ductile iron pipes is lower than for cast iron pipes.