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38 TABLE 24 Therefore, anode materials and the configurations in which REASONS FOR FAILURE OF CATHODIC PROTECTION SYSTEMS they are used must be selected, designed, and installed in accor- No. of dance with the best practice and the system operated within safe Reasons for Failure Responses operating ranges. Failure of cathodic protection components 8 resulted in the system being not operational for more than 20% of the time The survey queried the respondents on time to failure of most commonly used cathodic protection systems. The pri- Cathodic protection system not putting out 0 sufficient current owing to improper design mary reason for this question was to try and ascertain the experience agencies have had with cathodic protection. The Cathodic protection system not operational owing 1 to failure of one or more components question did not distinguish between premature failure and Cathodic protection system not putting out 10 end of service life. Therefore, some agencies reported both sufficient current owing to improper settings and some responded only if the systems were considered to Cathodic protection system did not operate owing 2 have failed prematurely. to deficient design Cathodic protection system not installed as 1 In reviewing the durability of anode materials, it is impor- designed tant to understand that whereas some agencies have reported Anode not appropriate for the application 0 failures for certain systems others have very successfully Vandalism damaged system components 5 applied them. Many failures of cathodic protection systems Not identified 5 occurred when (1) experimenting with new systems; (2) agen- Note: Table based on results of Question 47 of the survey. cies installed systems without requisite experience and knowl- edge; (3) systems were not matched to the structure or the environment, improperly designed, or incorrectly installed; the need for monitoring and maintenance. The Technical and (4) systems were not monitored or maintained appro- Committee of NACE reported that "In many cases the way priately. A distinction must be made between designed CP [cathodic protection] is sold today, the client/owner is consumption or weathering of an anode and durability. For not informed of the need for future maintenance of the CP example, when arc sprayed zinc is completely consumed, it System. The client/owner must know up front that an does not signify failure of the system; it is simply time to inspection/monitoring program is required. This must be replace the anode. However, if the anode debonded or became addressed in the scope of work. This area should be passive then it would be considered a failure. addressed in a NACE document" (52). As discussed in chapter four, the ferex anode systems did not exhibit sufficient durability for use in reinforced concrete PERFORMANCE AND DURABILITY OF SYSTEM COMPONENTS structures and failed within 5 years, which was reported by several respondents. Missouri reported failure of conductive Performance and durability of the anode materials is crucial polymer systems at 0 to 5 years when carbon fibers were to the overall success of the cathodic protection systems. All used as the primary anode. However, the conductive poly- anodes have some performance and durability limitations; mer anode systems using platinum as the primary anode have for example: lasted between 21 and 25 years. Missouri now only uses plat- inum as the primary anode in conjunction with conductive Conductive polymer material used as secondary anodes polymer anodes. is susceptible to acid attack, Conductive paint can weather within 5 to 10 years Failure of the conductive coke asphalt system was reported depending on the exposure conditions, by four agencies, Missouri (11 to 15 years), North Carolina Mixed metal oxide anodes when operated above and Virginia (0 to 5 years), and Ontario (6 to 10 years). The 10 mA/ft2 of anode surface area can generate chlo- failure in Virginia was judged to have resulted from acid rine, which can result in acid attack of the concrete, generation at the anode. In addition, coke asphalt systems Zinc anodes can passivate in certain environments, are susceptible to damage during replacement of the riding Ceramic anodes can have low contact resistance and surface (some operational systems have been lost during if the gasses are not vented properly can result in acid replacement of the asphalt riding surface). This is an inher- attack, ent flaw in the design of this type of system and it is not used Adhesive in the zinc foil anodes can dissolve and loss any more. of bond can occur if water infiltrates, Coke breeze systems can suffer from high resistance Failure of a zinc foil with adhesive system was reported by due to loss of coke around anodes and corrosion of wire five agencies; three had experienced failure in the 0- to 5-year connectors, and time frame and two in the 6- to 10-year time frame. Florida, Arc sprayed zinc and aluminumzincindium alloy may Illinois, and Missouri reported failure of the adhesive and loss experience bond problems, etc. of bond. High rate of anode consumption at the edges was

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39 TABLE 25 TABLE 27 LENGTH OF OPERATION OF FREQUENCY OF CATHODIC CATHODIC PROTECTION SYSTEMS PROTECTION COMPONENT MAINTENANCE No. of Length of Operation Responses Frequency No. Less than 1 year 2 Once a week 0 1 to 5 years 3 Once a month 1 5 to 15 years 15 Once a quarter 0 Greater than 15 years 3 Once every six months 2 Note: Table based on results of Question 43 Once a year 7 of the survey. Once every two years 2 Once every five years 4 Note: Table based on results of Question reported by Oregon. Missouri indicated that in year 1 the bond 49 of the survey. failure was noted, and by year 4 the anode was consumed. This anode is no longer available. The durability of the rectifier is dependent on several Florida reported failure of the localized (hockey puck) zinc factors. As discussed earlier, lightning strikes on a rectifier anodes in 0 to 5 years, as the anodes did not provide adequate are a significant problem and 10 agencies noted that to be protection in their application. one of the most important problems. The rectifying ele- ment, the control cards, and the remote monitoring units Most agencies indicated that on average cathodic protection were identified by the agencies to be the components most systems were operational for 5 to 15 years. Three agencies, susceptible to failure in the rectifier. Their responses to New Brunswick, Missouri, and Washington State, indicated this question are tabulated in Table 28. In the interview, that their systems lasted more than 15 years. A summary of the Missouri indicated that certain rectifiers have performed responses to this question is provided in Table 25. It may be better than others and have been operational for the entire noted that with a few exceptions, not many agencies have service life of the system. systems old enough to have been operational for more than 15 years. Vandalism is also an issue that must be considered during design. Placing system components out of reach of vandals is The rectifier on an impressed current system can often be essential. At least one agency has reported the failure of a the weakest link and requires the most maintenance. Several system as a result of vandalism. In one instance, it was found agencies that reported failure of various kinds of systems that homeless people living under a bridge had cut the con- indicted that the failure had occurred as a result of the fail- duit and wires for the cathodic protection system in an effort ure of the rectifier. It was selected by the greatest number of to obtain power for their heaters and television sets. respondents to require the most maintenance (Table 26). Cables, wiring, and conduits were also identified by the same North Carolina installed five different types of impressed number of respondents. However, the rectifier is easy to repair cathodic protection systems, the mixed metal oxide mesh, con- or replace and is not a fatal flaw. The cable, wiring, and con- ductive paint, conductive polymer, conductive coke breeze, duit failure, where they are embedded in concrete, could turn and aluminumzincindium alloy, on five different bents and out to be a fatal flaw. A maintenance frequency of once a year all of them failed within one year. The mode of failure for was experienced by the most number of respondents as the each of the five systems was listed as "whole system failed." summary in Table 27 indicates. In the rectifier experience section they indicated that they had problems with the rectifier and the control cards. They also commented that these systems were not considered "tough TABLE 26 CATHODIC PROTECTION COMPONENTS THAT REQUIRE THE MOST MAINTENANCE TABLE 28 Component No . RECTIFIER COMPONENTS MOST SUSCEPTIBLE Rectifier 6 TO FAILURE Remote Monitoring Unit 3 Component No . Anode 1 Rectifying Element Failure 8 Cable, Wiring, and Conduit 6 Control Card Failure 7 Reference Cells 1 Lighting Strikes 10 Current Probes 0 Remote Monitoring Unit Failure 6 Concrete Overlay or Backfill Material Used to 1 Other 6 Encapsulate the Anode Note: Table based on results of Question 52 of the Note: Table based on results of Question 48 of the survey. survey.