Pipe corrosion claims from electric current
A common cause of corrosion-related failure is electric current traveling through a pipe and discharging.
A typical failure mode of steel or cast iron water pipes is a result of corrosion. A common cause of corrosion-related failure is electric current traveling through the pipe and discharging. Figure 1 is a diagram showing how electric current traveling along a steel pipe causes localized corrosion. How does electric current find its way into a water pipe?
It is not unusual for a variety of electrical equipment to be connected to steel piping, assuming it is at ground potential — i.e., zero volts — often called earth. Induced current from nearby electrical equipment — including electric transit systems, cathodic protection systems and welding equipment — is one of the sources of current flowing through steel piping, and is often called stray current. When conductive soil conditions are encountered by the pipe, the current can leave the pipe and flow to a lower potential. This is called the point of discharge where the corrosion occurs.
Figure 2 is an example of stray current-related corrosion at the point of discharge. The current leaving the pipe at the point of discharge results in an electrolysis process causing accelerated corrosion of the pipe. There is no corrosion along the remaining length of the pipe provided there is no additional point of discharge. Over time, water pressure ranging from 60-75 psi is sufficient to cause the pipe to fail at the thinned wall and possibly cause severe damage to a building, especially if the water pipe is under the floor slab of the building.
Some ways of preventing stray current corrosion are providing a more resistive soil around the pipe, providing insulators in the pipe circuit and utilizing shielding or collection systems.
Charles C. Roberts, Jr., Ph.D., P.E., (ccr@croberts.com) is president of C. Roberts Consulting Engineers, Inc., which provides professional engineering services in accident reconstruction, failure analysis, fire causation, explosion analysis, and biomechanics.
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