Cathodic Protection of Below-Slab Copper Plumbing to Address Slab Leak Problems
By Roger A. Benham, P.E.
Leak Control Systems, Inc.
The occurrence of below-slab leaks (slab leaks) caused by soil-related corrosion of buried copper piping used in buildings is a wide-spread problem. The application of cathodic protection to protect below-slab copper from slab leaks is becoming more widely used as buildings age and owners are looking for cost saving alternatives to repiping or ongoing leak repairs. The application of cathodic protection can achieve a significant reduction or elimination in leak frequency (See Figure 1).
Cathodic protection is an electrical process used for preventing corrosion of a metal in contact with an electrolyte (soil or water). An example of a common use of cathodic protection is the corrosion protection for a typical hot water heater (see Figure 2).
The use of cathodic protection has an important role in our modern industrial society, as exemplified by the following quote from a corrosion science text book authored by a prominent materials scientist.
“The guarantee that no leaks will develop on the soil side of a cathodically protected buried pipeline has made it economically feasible to transport oil and high pressure natural gas across half of the American continent.”
Herbert H. Uhlig, M.I.T., Cambridge Mass
Leaks on gas and oil pipelines are completely unacceptable due to environmental, safety, and cost of product concerns. Unfortunately and inaccurately, water piping leaks are treated as routine repair matters. This mentality has led to a tremendous amount of waste due to the prevalence of plumbing leak problems that could be prevented with cathodic protection.
Cathodic Protection Design and Benefits
Cathodic protection must me properly designed to be effective. The design requirements for each property can vary. Factors affecting the proper design of a cathodic protection system include the amount of metal in the ground, protective current requirements, soil conductivity, piping layout and geologic formations affecting current distribution, piping continuity, building(s) footprint, and electrical power requirements. The improper applications of cathodic protection, the black-box stigma, and an improper association with psuedo-science gadgets and gizmos have damaged the view of some property owners and managers regarding the use of cathodic protection.
Benefits of cathodic protection are its low cost and convenience. Cathodic protection can be installed without any disruption to the building occupants because all the work is done outside of the building(s), Figure 3. Cathodic protection can complement other leak mitigation alternatives and does not prevent or hinder you from using other options in the future such as full or partial repiping, coatings, or alternate pipe materials as these methods and materials become more advanced.
How Cathodic Protection Works
The scientific principal of how cathodic protection works is part of every high school chemistry book. Although it may seem complicated, there is no need to use laymen’s terms when describing how cathodic protection works.
Cathodic protection is achieved when electrical current is passed between two electrodes (anode and cathode) in an electrolyte (soil or water). Under this condition electrochemical reactions must occur on the surface of the electrodes. Oxidation (corrosion) must occur at one electrode (the anode) and reduction (protection from corrosion) must occur at the other electrode (the cathode). In a cathodic protection system the pipe to be protected is made the cathode in the circuit and only a harmless reduction reaction can occur on its surface. The harmful oxidation reaction only occurs on the remotely installed anodes, which are designed to be consumed over a period of time. The bottom line is that you need to have an adequate number of anodes placed in the proper locations to achieve full protection.
Replumbing Overhead and Use of Protective Sleeves
In some cases there is no way to eliminate buried piping. For example, a property with a central boiler that circulates hot water to multiple buildings would require that the piping between the buildings to be routed below-grade. Also, many repipes and coatings jobs only involve selected parts of piping system leaving a lot of buried piping in the ground susceptible to corrosion.
The option of “sleeving” buried piping is no guarantee against leak problems, and in fact, has been associated with accelerated soil-related leak problems at the inevitable “holidays” in the coating. Coatings, whether field of factory applied, cannot be guaranteed to remain perfect through the installation process or life of the piping. The holidays in the coating can become the focal point of existing corrosion cells, resulting in severe localized corrosion attack and leak problems. Piping that is coated should be supplemented with cathodic protection. The combined use of coatings and cathodic protection has become the mainstay course of action for the corrosion control needs in the gas and oil pipeline industry, and again, the typical tank-type hot water heater (Figure 2).
Limitations of Cathodic Protection
From the above description, it is obvious there are some physical requirements that make cathodic protection work. You must have a voltage, a circuit, a cathode, an anode, and an electrolyte to conduct the electrical current between the anode and the cathode. Because air is a non-conductor (non-electrolyte) you cannot cathodically protect the surfaces of above-grade piping or structures exposed to air. Also, you cannot cathodically protect the internal surfaces (water-side) of piping because it is not practical to run an anode inside of the plumbing throughout the entire building. You can protect the internal (water-side) surfaces of a hot water heater because you can physically place an anode inside the tank, and the protection is limited to what the anode can physically “see”; the protective current from the anode will take the path of least resistance to complete the circuit and will not travel down the piping network. In short, the application of cathodic protection is limited to protecting the external (soil-side) surface of buried or submerged pipes and tanks.
For water-related (internal) corrosion of metallic piping in buildings other corrosion control process can be used including water treatment inhibitors, coatings, or other surface treatments.
Cathodic protection is a viable alternative to address ongoing slab leak problems caused by soil-related corrosion of buried piping. Cathodic protection should be used to supplement sleeving or other coatings processes used on buried copper piping. The misunderstanding or misapplication of cathodic protection, and the misdiagnosis of what is truly causing the plumbing leaks to occur, has hindered its more widespread use. A properly trained and experienced engineer or corrosion expert should be used to make recommendations regarding the appropriate application of cathodic protection.
About the Author
Roger Benham is a principal with Leak Control Systems, Inc. He is a degreed metallurgical engineer and a registered professional corrosion engineer. He has twenty-five years of experience dealing with leak problems in buildings. He has designed and installed cathodic protection and water treatment inhibitor systems for hundreds of thousands of residential units. He conducts failure analysis and forensic investigations on corrosion issues in buildings and serves as an expert in construction defect litigation cases. He can be reached at 619-795-7011 or firstname.lastname@example.org.