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13 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 SEE CORRPRO. Successfully loading deep anode wells is an art and will depend a lot on the geolog y and drilling method. A good, experienced anode well driller is key. A: Refer to NACE SP0572, "Design, Installation, Operation, and Maintenance of Impressed Current Deep Anode Beds." Pouring is not the best way to fill in the active area. A water/coke mix is advisable but not always economical if your anode bed is not very deep. Aluminum Anodes Q: Could you tell me in what resis- tivities aluminum anodes work eff iciently? Is there any standard stipulat- ing the water resistivity values? For example, NACE standards state that zinc with g y psum and bentonite backf ill are used in soils having relatively low resis- tivity (<2,000 Ω-cm). A: Normally, aluminum anodes are used only in seawater, which has a resistivity of ~25 Ω-cm. Even then they have a small amount of alloying to prevent passivation. A luminum anodes generally require chloride ions in the electroly te to function properly. As the quantity of chloride ions decreases below normal seawater concentrations (3.5%, or 35,000 ppm), the current capacity of the anode decreases, and the anode potential becomes more noble. Refer to the NACE Corrosion Engineer's Reference Book , ird Edition, in the chapter "Design Criteria for Offshore Cathodic Protection Systems." A: A luminum alloy anodes require the presence of chloride ions to prevent passivation. Obviously, an environment with a sufficient amount of chlorides to operate the anodes will have a low resistivity, but resistivity is not the key factor. We could have a medium with low resistivity, but if there are no chlorides, the anodes will not work. According to S.N. Smith, et al. (MP 17, 3 [1978], p. 32), the minimum required chloride concentration in waters is ~1,800 to 2,000 ppm. C.F. Schrieber and R.N. Murray (MP 27, 7 [1988], p. 70) found that A l-Zn-In-Si anodes in brackish waters >12% seawater strength exhibit current capacities (A-h/kg) equal to those observed in full- strength seawater. At saline strength <12%, severe capacity scatter is observed. Anode potentials show acceptable values 9–1.07 to –1.10 V vs. silver/silver chloride (Ag/AgCl) through 33% strength seawater. At ~20% seawater strength and less, noticeable potential variance will occur. Continued f rom page 11 NACE International offers more than 400 books covering every aspect of corrosion control in all industries. For more information, visit the NACE Store at www.nace.org.