Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/879896
35 NACE INTERNATIONAL: VOL. 56, NO. 10 MATERIALS PERFORMANCE OCTOBER 2017 niques to fight external corrosion and to preserve the value of buried assets; how- ever, a standardized and defensible ap- proach does not exist. The level of corrosion control imple- m ented varies significantly within th e water industry. Large utilities and water purveyors generally maintain large, perma- nent dedicated staffs of corrosion control personnel . Medium-sized utilities may have one or two responsible individuals whose duties include corrosion control. Small utilities often have few corrosion control resources, and leak repair may be their major effort for corrosion control. The CP techniques discussed next may be applied by both large and small water utili- ties, even those with limited resources. Direct Application for Existing Water Infrastructure The effective methods of implementing C P on exi sting wat er utilities initially installed without CP has been discussed in various publications including those by Kleiner & Rajani. 3-4 Such methods include: • Hotspot CP, which is the practice of opportunistically installing a protec- tive sacrificial anode at the location of a pipe repair. • Retrofit CP, which refers to the prac- ti c e of sy st emati cal ly prot e ctin g existing pipes with galvanic protec- tion. If the pipe is electrically discon- tinu ou s, th en an an o d e i s of t en attached to each pipe segment rather than the tedious process of locating and bonding all pipe joints. Measuring the Benefits of a Cathodic Protection Program The increase in the rate of breaks as structures age has been modeled as an exponential function. 5 Therefore, the cost of repairs exponentially increases if no pro- visions other than spot repairs are consid- ered. The initial cost of replacement is sig- nificantly high and may force utilities to endure increasing numbers of repairs as opposed to pipe replacement. Based on case studies and historical data, the continuous implication of the hot-spot technique tends to stabilize the number of breaks at little cost; in another w ord s, hot-spot programs pre vent th e exponential increase in the projected rate of breaks for aging structures. Anode retro- fit programs have a higher initial cost but are expected to reduce the number of pipe breaks and leaks (as opposed to stabilizing the number of breaks). In many cases, doc- umenting the number of breaks before and after implication provides a means of mea- suring the outcome of the CP programs. Case Studies T h e f o l l o w i n g c a s e s t u d i e s w e r e obtained from available literature sources or as part of the WRF4618 utility surveys and workshops. East Bay Municipal Utility District Long-Term Cathodic Protection Performance Study This case study 6 describes one of the earliest implementations of CP for water pipes. It should be noted that during the time of this investigation, no advances were made in the efficiency of anode materials, rectifiers, and other necessary tools for CP. Even though maintaining the CP system with the basic tools available was difficult, this early study still concluded that imple- menting CP will result in significant cost savings. The East Bay Municipal Utility District (EBMUD) aqueduct from the Pardee Reser- voir on the Mokelumne River to San Pablo Creek (in Northern California) was com- pleted in 1928. It extended ~93 miles (150 km), of which ~81 miles (130 km) consisted of steel pipe. Approximately 71 miles (114 km) of this pipe is buried in a wide variety of soil types (rocks to peat lands). The bur- ied portion of the pipe outside the river crossing has a bituminous dip coating and spirally wrapped felt covering. The pipe in the river has a concrete jacket. After only three years in service, the soil movement/ stress caused some damage and cracks in the pipe covering, which led to corrosion of the pipeline. The corrosion had not pro- gressed to a point that caused immediate danger to the pipe. To prevent further pro- gression of corrosion, CP (which was suc- cessfully used for nearby oil and gas lines) was implemented. Two to three 20-ft (6-m) long (2.5-in [64-mm] diameter) heavy steel pipes were used as anodes and were placed vertically at 25-ft (8-m) spacing in a line parallel to the aqueduct. Impressed current of up to 50 A was applied at individual loca- tions using motor -gen erators. Various other types of current sources such as a copper oxide rectifier, vacuum tube recti- fier, and selenium-type rectifier were used, each having their disadvantages for spe- cific circumstances. After six months of operation, it was found that one of the anodes was con- sumed. The anodes at some of these loca- tions were replaced with carbon rods (80-in [2-m] long with 2-in [51-mm] diameter) embedded in coke backfill. After two years of operation, the carbon rods showed sig- nificant failures. At that time, other materi- als were tested to be used as anodes but no sufficient progress was made. To test the performance of CP, the EBMUD visually inspected each rectifier, measured poten- tials, and embedded 3 by 6-in (152-mm) coupons for weight-loss tests. The coupons were buried in pairs at springline depth and ~2 ft (0.6 m) from the pipe. While the performance of early anodes in this study varied significantly, based on the surround- ing soil in each location, the cost analysis for prolonging the life of the structure indi- cated that the application of CP was justi- fied for the Mokelumne Aqueduct. The assumption was made that the average use- ful life of 35 years without protection and 70 years with protection could be achieved. An annual cost of $85 per mile per year and a saving of $1,300 per mile per year were estimated in the study. The study concluded that: • The time may come when CP will not be regarded as some sort of last ref- uge to be sought after others have failed. It is the only form of protec- tion that controls the elements of corrosion and may eventually be a competitive rival of various coatings. There is much yet to be learned con- cerning its application and ample