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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31 NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS PERFORMANCE DECEMBER 2016 members, which are extensions of a tower leg. Common grillage foundation con - figurations that connect the tower leg to the grillage plate include pyramid sup- por ts, a single tubular m emb er, and a single leg stub. The main advantages of grillage foun- dations are their low cost and ease of installation. They are entirely shop-fabri- cated and typically can be purchased with the tower. Furthermore, grillage founda- tions require minimal installation time and allow immediate tower assembly. They usu- ally do not need a concrete pour, so con- crete-related transporting and curing time is not required. 2 The major drawback of grillage founda- tions is the necessity of relatively deep excavations for their installation. Some- times, due to variations in soil environ- ments along the power line route, tower foundations need to be enlarged by pouring a concrete base around the grillage if actual soil conditions are not as good as those assumed in the original design. In addition, large grillages are dif ficult to set and require accurate adjustments for tower installation. 2 Cathodic Protection Model The primary goal of the proposed CP design tool is to determine detailed distri- bution of potential and current density (CD) on the surfaces of grillage founda- tions. Such information allows examina- tion and optimization of the anode bed design so the structure can be sufficiently polarized in accordance with the NACE International criteria for CP. 3 Three-Dimensional Geometry Model Since CP is a geometry-related matter, inclusion of more details in the geometry leads to a more accurate design. Three- dimensional (3-D) geometr y models for grillage foundations are used in CP model- ing. Such detailed geometry models enable accurate calculation of the total surface area and al low pre ci se predictions of under- protected and overprotected areas. The surface area for grillage founda- FIGURE 1 A sample of a computational soil domain for electrochemical simulation of a CP system for a grillage foundation with two vertical anodes. TABLE 1. STRUCTURE VS. POLARIZED SOIL POTENTIAL AND CORROSION CONDITION FOR STEEL Potential (vs. CSE) Corrosion Condition More electropositive than –500 mV Severe corrosion –500 to –600 mV Corrosion –600 to –700 mV Mild corrosion –700 to –850 mV Slow corrosion –850 to –1,100 mV Cathodically protected More electronegative than –1,100 mV Overprotected tions varies from ~60 ft 2 (~6 m 2 ) to ~260 ft 2 (~24 m 2 ) depending on the size and design of the grillage plate. A soil hemisphere with a radius of a few feet is considered the main soil domain for electrochemical simulations. Another soil domain, which surrounds the main soil domain , accounts for the ef fects of the infinite soil environment. In Figure 1, a s ampl e c omput ation al s oi l dom ain i s depicted. Input Data: Field Survey and Lab Tests For the sake of modeling accuracy, cer- tain tests must be performed to character- ize the soil environment and collect rele- vant electrochemical data. These include:

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