Materials Performance

NOV 2014

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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36 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 CATHODIC PROTECTION away from the anodes. The estimated rebar surface area in the test zone is >5 m 2 . The rebar half-cell potentials before CP applica- tion ranged from –220 to –400 mV vs. a cop- per/copper sulfate (Cu/CuSO 4 ) electrode (CSE). Results of Field Trials Bridge in Denmark The rectifier was set to operate at an output of 340 mA (10 mA/m 2 ) at 4.8 V in the constant current mode. During initial test- ing (commissioning ), polarization decay was measured using six portable reference electrodes located on the top portion of the bridge deck's bottom slab (inside the box girder). Aft er a on e- and three-month period of operation, eight portable refer- ence electrodes were used for testing. The results are presented in Table 1. All refer- ence electrode locations satisfied the 100 mV potential decay criterion over a 4 h period. It should be noted that the design CD of this system was 20 mA/m 2 , indicating highly uniform CP current distribution dur- ing this test. TABLE 1. CP SYSTEM TEST RESULTS—DANISH BRIDGE Depolarization Amount (mV) at 4 h Date Voltage CD (mA/m 2 ) RE 1 RE 2 RE 3 RE 4 RE 5 RE 6 RE 7 RE 8 Commissioned December 23, 2012 4.8 10 95 174 214 164 220 146 — — January 2013 3.94 10 271 370 165 259 242 311 239 311 March 2013 3.49 10 159 292 133 192 163 244 151 242 Note: RE = reference electrode. TABLE 2. DEPOLARIZATION TEST RESULTS—AUSTRALIAN BRIDGE Depolarization Amount (mV) Commissioned April 2013 RE1 RE2 RE3 RE4 Two weeks after commissioning (200 mm tape spacing): 16 mA/m 2 109 187 181 114 Three months after commissioning (400 mm tape spacing): 16 mA/m 2 88 130 166 109 Note: RE = reference electrode. Bridge in Australia The rectifier was set to operate at an output of 80 mA (16 mA/m 2 ) at 2.1 V in the constant current mode. Table 2 summarizes the polarization decay test results. The first measurements were obtained after a two- week period. The spacing of five MMO tita- nium tape anodes was 200 mm, and all ref- erence electrode locations satisfied the 100 mV polarization decay criterion over a 24 h period. After this test, the anode spacing was changed to 400 mm in order to evaluate the current distribution from two tape anodes, and the second polarization decay test was conducted six weeks later. The rec- tifier current output was 60 mA, and the operating voltage was 2.3 V (an increase of ~10% from the previous setting). Three of the four reference electrodes satisfied the 100 mV polarization decay cri- terion. At reference electrode R1 (Figure 6[a]), a 24 h potential decay of 88 mV was measured during continuing depolariza- tion. There fore, it is speculated that full protection would be achieved given a lon- ger decay period. As part of the performance testing, adhesion of the semi-conductive layer was tested at four locations, approximately four months after installation, using pull-off tests. These tests, carried out by an inde- pendent consulting firm, concluded that the first mode of disbondment is within the semi-conductive layer, occurring at ~0.5 MPa. This indicates that the bond strength of the semi-conductive layer to the con- crete substrate is >0.5 MPa. Conclusions • T h e MMO t i t a n i u m t a p e a n o d e system covered with a thin (5 mm) semi-conductive layer demonstrated outstanding performance. • The computer modeling indicated that the system can provide more uniform CP current distribution than slotted ribbon mesh anode systems. • The new system is simple and cost- e f f e c t iv e t o i n st a l l b e c a u s e t h e cutting or drilling of concrete is not required. • Field trials have indicated that the system is highly cost-effective, with the installation costs significantly lower than conventional CP systems such as slotted or concrete overlay type. • Field trials have also indicated that this system successfully operated on a dry concrete structure that was not directly exposed to occasional wet conditions. • Due to the simple configuration of the system, complex CP engineering and testing during the installation are not required. Acknowledgments The authors acknowledge Danish Road and COWI in Denmark and NSW Roads &

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