A high rate of pipe failure due to corrosion could be the result of using various types of metal in a distribution system without fully understanding the chemistry of mixed material combinations.
All metals exposed to soil possess low to high values of electron energy depending on their alloy composition and manufacturing process. When any two dissimilar metals are connected together, the metal having the highest energy level will corrode at an increased rate while the other becomes protected or immune to corrosion.
This process is the same as that found in cathodic protection systems. Cathodic protection is a common form of corrosion control that basically involves sacrificing one metal to protect another. When the two metals are placed in a corrosive environment, a ?sacrificial? direct current flows from the higher energy material to the material with a lower energy. In the process the ?sacrificial? material, typically an anode made of zinc or magnesium, corrodes while the protected material ? steel, iron or copper ? remains untouched.
The same process can occur within distribution systems constructed of dissimilar materials. For example, in a system with cast iron fittings fastened with steel bolts, the steel will corrode first. One the other hand, when cast iron fittings are fastened with stainless bolts, the iron corrodes first.
Water Main Design
Cast and gray iron pipe was the primary material used in distribution systems in the early 1900s. Services connected to these mains were predominantly made of lead, wrought iron and galvanized steel. Many of the cast iron mains installed some 75 years ago are still intact, but the same cannot be said about the services.
Water Main Design
In a paper presented in 1948 to the Water and Sewer Conference, Nat Bubbis, City Engineer from Winnipeg, Manitoba, made the following statement:
Water Main Design
?The original services installed in the city were either galvanized steel or wrought iron pipe. Neither of these materials stood up in our soil and have since been replaced? copper service pipe is now being used as a replacement material.?
Water Main Design
While galvanized steel services seemed to have been an ideal material, because of the galvanic effect between cast iron and galvanized steel, they acted as anodes being sacrificed to protect the cast iron. With the introduction of copper services and direct buried concentric neutrals, the situation was reversed. The cast iron in the Winnipeg system is now subject to more corrosion failure while the copper is protected.
Water Main Design
Since the mid sixties, ductile iron mains with copper services have become a popular choice for new construction. While ductile iron and copper materials have improved significantly from a metallurgical point of view, the galvanic and electrical effects with respect to corrosion have not changed. In fact, the combination of painted ductile iron mains, copper services and electrical grounding has posed a greater corrosion problem than ever before.
Water Main Design
While copper will typically corrode at a rate equal to or faster than steel or iron, copper services have an excellent corrosion free track record over the past 40 years. This demonstrates that copper services are receiving protective current from the sacrificing iron pipe.
Water Main Design
Studies done in other North American cities indicate a dramatic increase in leak rates in the past 25 years, even in higher resistivity soils, substantiating the seriousness of mixed metal systems. Leak rates in Winnipeg started escalating from 250 in 1960 to 2200 leaks per year in 1980. Zinc anodes are now being used in Winnipeg and many other cities with encouraging results.
Fighting back
The trend in water main design is toward corrosion-resistant materials such as PVC, but systems are also being designed to include ductile iron, steel, brass and copper. These materials can be protected from corrosion by the use of coatings and attaching separately connected anodes, or by using pre-engineered factory installed pure zinc modules.
Fighting back
Existing water main systems already attacked by corrosion can be salvaged by cement lining combined with the installation of groups of sacrificial zinc or magnesium anodes at specific locations and at leak excavations. Badly corroded mains may be upgraded by insertion renewal techniques using pressure rated liner pipe.
Fighting back
Other steps that utilities may take to protect their in-ground investment:
Fighting back
New Ductile Iron Systems -- Use polywrap or preferably a factory applied coating on pipe and install a ring type bell end anode. On copper services, install a 12# zinc anode. Fighting back
Existing Residential Areas, Ductile or C.I. ? Install a 24# zinc anode on all copper services at property line and install one on each hydrant. Optional: Banks of anodes connected at street lighting pole neutrals. (Connecting banks of anodes to neutrals increases the grounding capabilities for AC fault current as well as providing corrosion protection to the iron pipe.)Fighting back
Existing Industrial Iron Systems ? Install banks of 10-24# zinc anodes at street lighting poles and connect to neutral ground lugs. Optional: Install banks of anodes at hydrants and one on each industry service.Fighting back
Existing Downtown Iron Systems ? Install 2-24# zinc anodes at each leak repair. Install a 24# zinc anode on each service and hydrant.Fighting back
Existing Rural Iron Feeder Mains ? Install a group of 10-24# zinc anodes at hydrants or at closer intervals along the pipe route and connect header cable to water main. Install a test station. Check for pipe continuity.Conclusion
All metals in contact with soil and moisture are subject to corrosion, even in single metal systems. A variety of factors affect the rate of corrosion including the type of soil and its corrosivity. Corrosion rates can also increase when pipe passes through dissimilar soils or is in contact with concrete thrust blocks. Stray electrical currents, de-icing salts and fertilizers also contribute to corrosion.
Conclusion
All metals in contact with soil and moisture are subject to corrosion, even in single metal systems. A variety of factors affect the rate of corrosion including the type of soil and its corrosivity. Corrosion rates can also increase when pipe passes through dissimilar soils or is in contact with concrete thrust blocks. Stray electrical currents, de-icing salts and fertilizers also contribute to corrosion.
Conclusion
Paint and coatings can form a barrier between metal and soil, slowing corrosion. However, in spots where metal becomes exposed, corrosion can be intensified and failure can actually result sooner than if the metal was completely bare. Therefore paint, epoxies, urethanes and polyethylene wrap should not be considered as the sole source of protection.
Conclusion
Cathodic protection combined with coatings can be a valuable tool for preventing corrosion failure to underground metallic structures. System designers must understand all the parameters involved and design around safe operating levels. Once they have an accurate understanding of how and why failures occur, corrective measures can be easily and economically applied.
About the Author:
Hank St. Onge, Rehabilitation and Corrosion Specialist, is President of Duratron International Inc. of Toronto, Ontario. Duratron provides services to the pipe rehab and cathodic protection industry.
Practices that increase corrosion:
- Concrete encasement or thrust blocks
- Galvanized services off cast iron
- Copper services connected to iron
- Buried concentric neutrals
- Steel bolts with iron fittings
- Stainless bolts with iron fittings
- Copper - brass off PVC
- Misuse of coatings or polywrap
- Stray electrical DC currents
- De icing road salts & fertilizers
- Old pipe connected to new pipe.
