Materials Performance

DEC 2016

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.

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33 NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS PERFORMANCE DECEMBER 2016 m Q Q E U Mag CP Mag = × × (2) w h ere Q Mag (A-y/kg ) i s th e th eoretical capacity of anode material, E is the current efficiency, and U is the utilization factor, as listed in Table 4. In general, CP systems with distributed anodes provide lower anode-bed resistance and better protection coverage for founda- tions with irregular geometries; but the cost for excavation and installation is a lim- iting factor. The objective of the proposed CP design method is to compare different anode arrangements to find an optimum design in terms of cost and performance. Numerical Analysis and CP Design Optimization A f i n i t e e l e m e n t s o lv e r, C OM S O L MULTI PHYSICS † (Version 5.2), was used to solve the governing electrochemical equa- tions. In this example, CP simulations were developed for a foundation with a single leg stub. Two brace angles of the structure are also partially buried (Figure 2). In thi s exampl e, th e found ation i s uncoated and buried in a neutral soil with a resistivity of 5,000 Ω-cm. From the 3-D geometry model, the buried surface area is calculated as 70 ft 2 (6.5 m 2 ). Current requirement tests in neutral soils indicate that 37 mA would be required for CP of the buried members. Alterna- tively, the information in Table 2 can be used to estimate the required current. Aft er using Equations (1) and (2), th e minimum mass of magnesium anodes for 20 years of CP in neutral soil can be calcu- lated as 15.3 lb (7 kg). Cylindrical 5, 9, and 17 lb (2.3, 4, and 7.7 kg) magnesium anodes were considered for CP modeling. Accord- ingly, anode beds with one 17-lb anode, two 9-lb anodes, or three 5-lb anodes were selected to investigate different CP design scenarios. In Figure 2, simulation results for differ- ent CP system designs are shown. Four dif- ferent anode bed designs with horizontal anodes are presented in each row. Results in the top row correspond to neutral soil with soil resistivity of 5,000 Ω-cm. To illus- † Trade name. FIGURE 2 IR-free potential distributions (V) on buried surfaces of the grillage foundation are shown for different anode arrangements. The top and bottom rows correspond to soil resistivities of 5,000 and 2,000 Ω-cm, respectively. White cylinders around the foundation represent the anodes. trate the effects of soil resistivity on CP per- formance, simulation results at a slightly acidic soil with a resistivity of 2,000 Ω-cm are presented in the bottom row. To pro- vide a fair comparison between these cases, the anode size is the same. The required CP current obviously increases as the soil cor- rosivity increases, which in turn increases the required mass for anodes for a certain CP system life. Distribution of polarized potentials on buried sur faces of th e foundation was investigated to assess the performance of each anode bed design. According to the NA C E st a n d a rd , 3 a m i n i m u m su r f a c e potential of –0.850 V vs. CSE is required for CP of steel (Table 1). In Figure 2, dark-red areas are protected portions of the founda- tion, while orange, yellow, green, and blue areas, in that order, represent surfaces with decreasing levels of protection. The results show that anode beds provide bet- ter protective current distribution in soils with lower resistivity and that highly dis- tributed anode beds provide more uniform coverage. Only a few anode bed designs are dis- cussed here; but the design tool allows investigation of various designs, and its high-resolution results provide the basis for sound decisions. Conclusions These simulations confirm that areas w ith ge om etri c feature s (c orn ers and edges) located in the vicinity of anodes receive the maximum protective current w hi l e f l at sur faces, par ticul arly w h en shielded, are least polarized/protected. As a result of geometrical complexities, mul- tiple anodes for CP of the grillage founda- tion are required. Furthermore, in soils with high resistivities, it is necessar y to consider a greater number of anodes bur- ied close to the structure ( < ~ 2 ft [0.6 m] away) to achieve a good level of protection. For large grillage foundations, horizon- tally buried anodes are preferred to protect the horizontal members of the grillage, while vertically buried anodes are recom- mended for protection of vertical ( leg ) components. Nonetheless, it is always rec- ommended to provide full CP to critical load-bearing members of the foundation— usually the legs—thus, a combination of vertical and horizontal anodes might be required. For galvanized structures, the equilib- rium potential of the structures gradually shifts toward electropositive values as the zinc layer is consumed and corrosion pro- gresses into the steel substrate. Accord- ingly, the design of CP systems for two Galvanic Cathodic Protection for Power Transmission Tower Grillage Foundations

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