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Journal of Commerce, 89, August 64, pp. 4-5, 2000-08-14
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Cathodic protection: knowing when and where it will be most effective and important for extending system lifespans
Koehl, S.
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Cathodic Protection: Knowing When and Where It Will Be Most Effective Important for Extending System Lifespans
Koehl, S.
A version of this paper is published in / Une version de ce document se trouve dans : Journal of Commerce, August 14, 2000, pp. 4-5
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Cathodic Protection: Knowing When and Where It Will Be
Most Effective Important for Extending System Lifespans
By Stefan Koehl
Published in Journal of Commerce, August 14, 2001, pp. 4-5
Abstract: Knowing when and where cathodic protection is worth installing could help
water utilities extend the life of their distribution systems at reduced cost. Such a guideline would be especially valuable in the next several decades as repair and replacement of old cast and ductile iron mains begins to pose a significant expense for North American water utilities.
When steel or iron rust underground, it is usually an electrochemical process.
Change the electrical flow and you avoid unwanted corrosion. That is the idea behind cathodic protection. But how well does it work in practice on buried iron water mains?
Cathodic protection as a means to control galvanic corrosion was used as early as 1824, where zinc plates were used to protect the copper cladding of the hulls of British ships from corroding in the sea water.
As scientists increased their understanding of electrochemistry, they realized that corrosion could be redirected to another nearby metal or an electrical current could counter the natural corrosion reaction. In the 1930s some U.S. utilities began applying these ideas to water main protection. Over the past couple decades an increasing number of Canadian water utilities have installed cathodic protection in efforts to slow down increasing failures in aging iron water pipes. In some locations it seems to be more effective than in others. Many factors seem to affect its efficacy. Knowing when and where cathodic protection is worth installing could help water utilities extend the life of their distribution systems at reduced cost. Such a guideline would be especially valuable in the next several decades as repair and replacement of old cast and ductile iron mains begins to pose a significant expense for North American water utilities.
To that end, Dr. Yehuda Kleiner with the National Research Council's Institute for Research in Construction is conducting a three-year research project to measure and compare the success of cathodic pipe protection efforts by 11 Canadian water utilities. Other utilities or organizations with an interest in cathodic protection are welcome to join the project, said Kleiner.
installed, Kleiner will look for patterns and correlations that indicate its effectiveness in different situations and at different points in the life span of the mains.
Untangling-within the data-all the factors that can influence corrosion and the ability of cathodic protection to combat it will be a difficult task. The rusting process can be galvanic, electrolytic or even bacteriological with different factors having greater or lesser influence with each type. Most corrosion of iron water mains is galvanic: two metals of different electrical potential-an anode and a cathode-together with an electrolyte-such as soil moisture-generate an electrical current.
That can happen when copper service pipes are connected to iron or steel mains, when new pipe is connected to old pipe of the same metal or even when new pipe is laid near old abandoned metal pipe.
It can also occur between stressed areas and unstressed areas of a pipeline or between ferrous sections and graphitised sections, that is, where earlier corrosion has left mainly the carbon flakes originally mixed with the iron or steel.
Galvanic corrosion can also happen when metal comes in contact with electrolytes-such as soil solution with dissolved minerals-of different composition or concentration. This too generates an electrical current as the metal corrodes. Such situations can arise in non-uniform soil conditions. Likewise, differing concentrations of oxygen along the pipe surface can drive the galvanic corrosion process. For example, an iron main might pass through sand that offers more oxygen and then clay, which provides less. Cuts, scrapes and abrasions or crevices on the pipe that create small environments shielded from the surrounding soil can also have less oxygen.
Then there is electrolytic corrosion. In that case a stray electrical current from an outside source causes the process. It might come from dc powered transit systems, from industrial motors, from railway signals or even an adjacent cathodic protection system, such as for gas mains.
Finally, there is bacteriological corrosion. How certain microbes bring about corrosion is not yet fully understood, but it is probably by creating chemical conditions that support galvanic
corrosion. One type that has been identified is sulfate-reducing bacteria.
These various types of pipe corrosion all share a common characteristic: the pipe is the anode or positive terminal in the electrochemical cell. Since it is always the anode that corrodes, cathodic protection works by making the pipe the cathode or negative terminal.
That can be achieved in two ways and each of the two may confer varying degrees of effectiveness in the different corrosion situations.
The most common method of cathodic protection is to provide a sacrificial metal and make an electrical connection between it and the pipe. Zinc or magnesium, depending on conditions, work best with iron and steel pipe. Since the galvanic process always corrodes the less noble of two
different metals, the iron remains protected until all the zinc or magnesium is depleted. Less common, but more effective in some circumstances, is impressed current cathodic
protection in which an electrical current is sent into the pipe and through the soil in the opposite direction of the electrical flow of the corroding pipe.
Kleiner hopes to gather data on as many combinations of cathodic protection and corrosion situations as possible. Drawing conclusions from analysis of the information, his research has six objectives:
• Find the best way to measure the effectiveness of cathodic protection,
• Find which variables significantly influence the effectiveness of cathodic protection, • Find which design variables most increase the expected life of water mains,
• Find which theoretical model best predicts water main failures before and after installation of cathodic protection,
• Find how effective cathodic protection changes the life-costing of a water main, and • Find a method for choosing the best times to put in cathodic protection for mains in
different environments.
To what extent these objectives can be met will depend on the quality, detail and breadth of the data, said Kleiner. "This is unlike other research which can be designed beforehand. With this project, we'll have to take what is available."
While most utilities have some type of form for recording information about the repair of main failures, there is no standardization, he said. "We'll just have to see what we get."
Besides supplying the historical data and communicating their experiences, the participants are also helping to support the research with financial contributions. They include:
• Calgary, • EPCOR (Edmonton), • Ottawa-Carleton, • Markham, • Halifax, • Niagara Falls, • Gatineau,
• Hamilton-Wentworth, • Peterborough,
• Toronto, • Windsor, • Saskatoon, and
• anode producer Corrpro Canada Inc.
Most of the utilities are currently using cathodic protection, but some have only begun recently. Kleiner said the first use of cathodic protection for water mains in Canada was probably in the early 1960s mainly in large transmission mains. In the smaller distribution mains use of cathodic protection has been reported since the early1980s.
"In the water distribution industry water main failures have received much attention only in the last 20 to 30 years, when water mains installed in the early 1900s and after the Second World War started failing in significant numbers," he explained.
"Consequently, not a lot of research has been published on cathodic protection for water mains." The project began last summer and the first wave of data has recently come in, said Kleiner. Completion is expected in 2002.
