Materials Performance

OCT 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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32 OCTOBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 10 Applying Cathodic Prevention Cathodic prevention CD is approxi- mately one order of magnitude lower than the typical requirement for CP. This, in part, is because the steel/concrete potentials required for cathodic prevention are less negative than those required for CP. Further- more, passive steel is more easily polarized. For this project, the CD was assumed to be 2 mA/m 2 for the steel. Since the surface area of the steel in each foundation is 2 m 2 , the required current is 4 mA. The necessary w e i g ht of th e a n o d e m at er i al , w hi ch includes utilization and efficiency factors, was calculated using Faraday 's law, Equa- tion (1): W = (ARC * CR * L) / (E * U) (1) where ARC is the average required current (0.004 A), CR is the consumption rate of the anode (11.2 kg/y for zinc), L is the designed lifetime (20 years), E is efficiency (0.9), and U is the utilization factor (0.85). The calcu- lated weight of zinc is 1,200 g, which is pro- TABLE 1. RESULTS OF CORROSION ASSESSMENT TESTS ON A TYPICAL ELECTRIC TRANSMISSION TOWER FOUNDATION (QESHM-34-LEG B) Parameter Measurement Age 14 years Distance from sea 6 km Height above the sea level 802 m Concrete cover depth 56 mm Rebar diameter 16 mm Alkalinity pH 10.9 Chloride ion concentration 0.06% Concrete compressive strength 28 MPa Soil resistivity 5 Ω-m Corrosion potential –410 mV (CSE) Corrosion CD 20.3 µA/cm 2 Concrete electrical resistivity 168 Ω-m FIGURE 3 Galvanostatic pulse measurements determine corrosion potential, corrosion rate, and concrete resistivity. FIGURE 4 Cathodic prevention is applied to the selected foundations (a) by discrete zinc anodes and (b) zinc sheet anodes. based on an artificial neural network. This software classified the examined tower foundations into one of four corrosion risk categories (low, medium, high, and ver y high). Th e results showed ~60% of th e selected foundations were placed in either t h e h i g h o r v e r y h i g h c o r r o si o n r i s k groups. In addition to using sacrificial anode CP systems with the foundations' patch repairs, th e own er of th e high-v oltage power lines and towers decided that CP syst ems w ould al so b e used for n ew ly installed foundations, which was the first time this was done in Iran. This type of CP, called cathodic prevention, applies to new structures, which are expected to become contaminated by chlorides during their service life, as well as in-service structures with chloride ions that have not reached the steel and depassivation has not yet occurred. The distinction in these terms relates to the historical practice of apply- ing CP primarily as part of the repair/ retrofit strategy after corrosion has been initiated. Cathodic prevention is a proac- tive approach. CATHODIC PROTECTION

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