Cathodic Protection Program

The Prevention of Corrosion on Ductile & Cast Iron Water mains

Objective

The Region of Durham initiated hot-spot and reactive Cathodic Protection programs in 1983. The hot-spot program was initiated to combat the most seriously affected corrosion areas in the Region. Since its conception, the program has progressed from hot-spot protection to a preventative maintenance program of all existing ductile and cast iron water mains. The reactive program consisted of the installation of sacrificial magnesium anodes at water main repair locations. This procedure has been adopted as part of the repair protocol.

Due to the continual corrosion problems and high failure rates associated with ductile and cast iron pipe, the Region of Durham issued a moratorium on the use of ductile iron pipe on May 30, 1985. In its place, we are now using Polyvinyl Chloride pipe (PVC) for water mains under 400 mm in diameter. PVC will not corrode but all appurtenances connected to the water main must be cathodically protected to assure their durability. Larger water main are normally constructed from concrete pressure pipe.

Description

Cathodic Protection is a scientifically proven method of corrosion control for multi-metallic water distribution systems. It minimizes the effects of external corrosion on existing ductile and cast iron water mains, thus reducing water main breaks and extending their useful life. Cathodic protection consists of installing sacrificial anodes (magnesium or zinc) underground, ideally at water main depth and connecting them to the water mains with insulated copper wires. The anodes are also attached to above ground test stations so that their effectiveness, over time, can be measured. The basis of Cathodic Protection is such that the attached anode will corrode instead of the water main. To achieve adequate protection on a water main, installation of several anodes along the pipe is necessary. Each anode is effective over a short range of pipe length. The spacing of the anodes is dependent on the condition of the pipe, pipe size, soil resistivity and strength of stray electric ground currents.

Water main Corrosion Fundamentals

The previously stated factors induce currents between sections of the water main. This current is produced in the same way that a DC battery produces current (by sacrificing one metal to another). In the case of our water distribution system; the ductile/cast iron is sacrificed to itself or to copper connections, brass or bronze fittings, resulting in pinhole depressions in the ductile/cast pipe surface. Ultimately, these develop until the pipe is perforated and a leak appears.

Corrosion of Cast Iron Water mains

A unique characteristic of the corrosion of cast iron is the resultant graphitization of the material. As the corrosion progresses, the cast iron becomes a composite of cast iron and graphite. The graphite retains the shape of the pipe thereby camouflaging the true extent of the corrosion.

Method

The actual installation of Cathodic Protection on site is as follows:

Locating: Usually, the first indication that there is going to be some sort of construction taking place on your street is a person painting the grass, sidewalks, curb or street. There are different colours and sometimes they even stick little flags in the ground. All the neighbours gather around after supper and try to determine what is going on. No need to panic. These marks are necessary for us to determine what is in the ground and where it is so that we can stay a safe distance away from them when we excavate. The red marks are hydro, the yellow ones are natural gas, orange are cable television or fibre-optics and blue is the water service coming into your house or the water main. The white marks along the curb are where the contractor will look at excavating first, but other factors must be looked at before he actually excavates.

Notification: Residents on the selected street will be notified within 48 hours of excavation by form of a letter delivered to your door. All we ask is that you do not park on the street for the short time that the work is being performed. Your water service will not be affected unless there is a water main break, in which case the depot staff will come to your door and give you sufficient notice.

Distribution of Anodes: The contractor will usually drive along the street and place the anodes on the boulevard at intervals of 8 - 12 meters apart on the side of the street that the water main is on. Normally, the water main is on the same side of the street as the Fire Hydrants.

Excavation: A 300 mm - 400 mm diameter piece of sod will be removed where the anode is going. The contractor will usually have a train of three vehicles. The first vehicle will be either an Auger truck or a Vacuum truck. A 250 mm diameter hole is excavated approx. 1.5 m - 2.5 m deep down to the water main. The Auger truck is used when there is no risk of hitting other infrastructure in the ground like hydro, gas or fibre-optic cables. The Vacuum truck is very loud and is like a giant vacuum cleaner that uses a jet stream of water to loosen the soil and consumes the mud.

The Anode: The second vehicle will be the welding truck. At this stage, the stud at the end of the blue wire attached to the anode, is inserted into the end of a long rod and inserted into the hole and spot welded to the water main. The inspector will tug on the wire to make sure the weld is sufficient and the anode is then placed over the hole if the hole was vacuumed. If it was drilled, the anode can be dropped into the hole, back-filled and tamped.

Clean-up and Restoration: The third truck will replace the sod, rake and clean-up the excess dirt from the street. If the hole was vacuumed, back-fill must be brought in and shovelled into the hole from the truck since the original dirt is now inside the Vacuum truck as mud. The sod can now be replaced. The sod is intentionally left high to allow for settlement. If it settles too much, the contractor must lift the sod and install more dirt to restore properly. It is asked that homeowners water the sod so it remains healthy.

Test Stations: Every 150m to 200m the contractor will install a surface mounted test station. The 150mm yellow disk cap will have a Durham Region sticker on it with a telephone number to call if it becomes damaged. The purpose of these test stations is so we can monitor the performance of the anodes by conducting a survey.

The Survey: By hooking on to the test station and using copper sulphate, we take potential readings every meter above the water main. From these readings, we can determine if the anodes are doing their job by corroding instead of the water main.

Summary

The Program Summary shows the locations in Durham Region where Cathodic Protection has been installed since 1983. If the water mains were left unprotected and continued to corrode at the rate being experienced, total replacement of the mains would be necessary at a large capital cost. If this replacement can be delayed or avoided, a greater return can be achieved on the Region's investment.

Conclusion

The maintenance of our water distribution systems and plants ensures that our high quality standards of product and service to the public are maintained. Based on our continual preventive maintenance program it can be concluded that Cathodic Protection is a very successful, reliable and cost effective method of minimizing the effects of external corrosion, thus, extending the life expectancy of those water +mains. Consumers can be assured that our potable water distribution system is safe and reliable with minimum inconvenience of water shut-offs and roadwork resulting from water main breaks.

We have been very encouraged by the results of this program and anticipate similar results in the future.

For a hard copy of the yearly report on the Status of the Cathodic Protection Program within the Region of Durham, please contact the Works Dept. at 905-668-7711 or 1-800-372-1102 or email TechSupport@durham.ca