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32 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 once the bare surface area is calculated/ approximated. Anodic and cathodic Tafel equations were used to model electrochemical pro- cesses at the surfaces of sacrificial anodes a n d st e el str u c tu re s (c ath o d e s) . Thi s required laboratory voltammetry tests to evaluate the relevant kinetic parameters (i .e., exchange CD and Tafel slopes for anode and cathode materials at a sample soil collected at the tower footing ). 8 The kinetic parameters can vary with factors such as soil resistivity, pH level, oxygen concentration, metal ion concentration, surface area of electrodes, temperature, chloride contamination, and organic mat- ter content. Examples of values for kinetic parame- ters are listed in Table 3. The listed coeffi- cients are provided for demonstration pur- poses—considerably different values may be obtained in different soil environments. System Selection and Primary Calculations In contrast to pipelines, which are con- ti nu o u s str u c tu re s w ith l a rge su r fa c e areas, foundations of T&D; towers are sep- arate structures with relatively small sur- face areas. Accordingly, it is preferred to install individual galvanic CP systems for each tower and implement the same for each tower within a group that share com- mon characteristics. Magnesium and zinc anodes are gener- ally recommended for soil application ; nonetheless, use of zinc anodes is suggested only for low soil resistivity conditions. H i g h - p o t e n t i a l m a g n e s i u m a l l o y anodes (Type M1, per ASTM B843 9 ) were selected for the example (Table 4 7 ). The required capacity of the CP system (Q CP ) can be calculated from Equation (1): Q CP = I CP × Life (1) where I CP (A) is the required protection cur- rent obt ain ed from on-sit e t e stin g or approximated from Table 2. A minimum life of 20 years is considered for the CP system. Once the capacity of the CP system is deter- mined, the minimum mass of the anode (m Mag ) for the system can be calculated from Equation (2): TABLE 2. TYPICAL CURRENT REQUIREMENTS FOR CP OF BARE STEEL Environment Current Density mA/m 2 mA/ft 2 Neutral soil 4.3 to 16.1 0.4 to 1.5 Highly acidic soil 32.3 to 161.4 3 to 15 Heated soil 32.3 to 269 3 to 25 Wet concrete 32.3 to 161.4 3 to 15 TABLE 3. SAMPLE KINETIC PARAMETER VALUES USED IN ELECTROCHEMICAL SIMULATION Anodic Tafel slope +50 mV vs. CSE Cathodic Tafel slope –160 mV vs. CSE Anodic exchange CD 100 mA/m 2 Cathodic exchange CD 1 mA/m 2 Anodic equilibrium potential –1,750 mV vs. CSE (Table 4) Cathodic equilibrium potential –600 mV vs. CSE (Table 1) TABLE 4. SPECIFICATION OF HIGH-POTENTIAL MAGNESIUM ANODES (M1 TYPE) Nominal potential –1,750 mV vs. CSE (in packaged backfill) Current efficiency (E) 50% Utilization factor (U) 85% Theoretical capacity (Q Mag ) 0.251 (A-y/kg) or 0.114 (A-y/lb) Theoretical consumption rate 3.98 (kg/A-y) or 8.76 (lb/A-y) • Soil resistivity test • S oi l-to-str ucture electro ch emical potential test • CP current requirement test • Voltammetry tests While the first three tests must be per- formed on-site, voltammetry tests require laboratory facilities. Soil resistivity measurements can be performed either on-site or in a laboratory per ASTM G187 4 and AASHTO T288 5 stan- dards. It is recommended, however, to use the Wenner four-pin method, ASTM G57, 6 to perform in situ soil resistivity tests, which allows identification of soil layers, if any. Since distribution of protection cur- rent in soil environments strongly depends on the soil resistivity, the presence of soil horizons with different resistivity values can make a considerable difference in the performance of a CP system. Based on field experience, the electro- chemical potential of a directly buried structure, measured with respect to a refer- ence electrode (e.g., copper/copper sulfate [Cu/CuSO 4 ] electrode [CSE]), indicates the corrosion condition, as listed in Table 1. 7 Note that this potential and the corrosion rate will change during different seasons mainly due to variations of soil tempera- ture and moisture content. The required current to cathodically protect a grillage foundation can be mea- sured on-site by the current interruption technique, which uses temporary anodes and a portable direct current power sup- ply. Based on soil resistivity and bare sur- face area at the foundation, the required current may vary from a few mA to a few hundred mA. Where CP current require- ment tests are not possible, the required current can be estimated from Table 2 7 CATHODIC PROTECTION