Materials Performance

NOV 2018

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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18 NOVEMBER 2018 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Continued f rom page 17 Concrete deterioration at upper section of the cooling tower for Unit #3 in 2010, including removed concrete from "de-spalling" efforts. (assumed to be 0.035% acid soluble chlo- ride by weight sample) at both the interior and exterior surfaces of the shell, and up to three and six times higher than the threshold concentration at the depth of exterior and interior reinforcement, respectively. Additionally, visible deterioration was found in 30 of the 32 X-columns. The lower half of the X-columns (below the f ill) exhibited a greater amount of deteri- oration, which primarily consisted of lon- gitudinal cracking at the corners that was associated with corrosion of embedded steel reinforcement. By 2010, the amount of spalled con- crete on the shell's exterior had increased, but the most dramatic change was a sub- stantial increase in the extent of delami- nations at the interior face of the shell, which signif ied an increase in the rate that corrosion-related damage was occurring. This high grow th rate of con- crete delamination was considered a sig- nif icant risk to the structure's integrity because the structural stability and strength of the tower shell rely on the con- crete wall thickness being intact so the reinforcing steel is capable of resisting in- plane tensile stresses as well as out-of- place bending stresses. Because the delaminations in the upper section of the tower shell had resulted in widespread interior and exte- rior deterioration of the concrete, the owner decided to perform an engi- neered demolition and rebuild of the upper one-third of the tower shell. Below the throat, concrete deterio- ration was less severe and struc- tural concrete repairs were selected, which included localized partial-depth con- crete repairs at both the interior and exterior of the shell with some full-wall thickness repairs. Since the X-columns had suff i- cient capacity in their observed state, they were not structurally repaired. Corrosion mitigation of the reinforc- ing steel was specif ied for the entire shell and supporting X-columns to protect against future deterioration and main- tain the tower's structural integrity. A hybrid cathodic protection (CP) system, comprised of both galvanic CP and impressed current CP (ICCP), was installed on this tower. For the shell, ICCP was selected because the voltage levels at different locations on the large shell could be controlled over the course of the CP system's service life. Since the X-columns are exposed to a wet environment and a wide range of service temperatures (from –20 to 50 °C), galvanic CP was chosen to protect them because its components have the ability to withstand the harsh environment. The ICCP system is comprised of 48 individual zones, which are evenly dis- tributed around the tower shell in six ver- tical stacks of eight zones. Each zone has four embedded reference electrodes for a total of 192 reference electrodes in the entire shell to monitor the polarization of the steel reinforcement. The lower two zones—the heavily reinforced, thickened concrete base of the shell—have discrete titanium suboxide ceramic tube anodes. The next four zones use mixed metal oxide (MMO) ribbon mesh anodes grouted into slots cut into the exterior surface to protect the repaired height of the existing shell, and the top two zones use MMO ribbon mesh anodes placed between the reinforcing layers in the reconstructed portion of the shell above the throat. In total, over 6 mi (9.7 km) of ribbon mesh anodes and over 4,300 dis- crete ceramic tubular anodes were installed on the project. At each vertical stack, a rectifier cabi- net that contains eight independent trans- former/rectifier direct current (DC) power supplies—one for each zone—and a remote monitoring unit (RMU) that moni- tors and logs the reference electrode data along with the DC voltage and current of each individual circuit is located on the canopy walkway. The RMU can be con- trolled by web-based software to measure voltage, current, and potentials, as well as perform polarization decay testing. The galvanic CP system on the X-columns uses galvanic jackets that contain alkali-activated zinc anodes installed with cement mortar inside stay- in-place f iber-reinforced polymer forms. These galvanic anodes essentially operate as a low-voltage battery that supplies pro- tective current to the columns' reinforc- ing steel. These jackets do not require any maintenance or monitoring to provide long-term corrosion protection; however, four of the 32 X-columns were instru- mented with manganese dioxide (MnO 2 ) reference electrodes and wired for moni- toring so representative data can be obtained to assess the performance of the galvanic CP system. For these columns, manual monitoring boxes were mounted at the base of the tower shell so they would be easily accessible from the can- opy walkway. The system was energized in March 2017. Adjustments were made after two weeks, three months, and six months. Further testing, adjustments, and inspec- tions will occur every six months. More information on the CP systems can be found in CORROSION 2018 paper no. 11005, "Evaluation and Repair of Natural Draft Cooling Towers," by M.B. Gries, et al. —K.R. Larsen

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