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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49 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 tial, macro-cell corrosion current (I macro ), and electrical concrete resistance (R). It can also measure temperature at the sensor location. The second corrosion sensor is an experimental prototype. It is a multi-func- tional, single probe type that can measure corrosion potential; instantaneous corro- sion rate (CR), determined by the linear polarization resistance method; electrical concrete resistance; and temperature. The third sensor is a manganese dioxide refer- ence electrode (MnO 2 RE) that can measure the corrosion potential of the sensors and the reinforcing steel of the structure. 2-3 Unfortunately, it was discovered during the first data collection that none of the sen- sors provided accurate temperature read- ings. The cause of the inaccurate tempera- ture readings could not be determined. Sensor Installation and Data Collection The sensor locations were strategically chosen to compare corrosion activities on the sea-facing and lake-facing sides along four exposure zones at different elevations (atm o sp h er i c , sp l a sh , ti d al , a n d sub - m erged). Sp ecif ically, 13 sensors were installed on the sea side and the remaining six were installed on the lake side. To evalu- ate the corrosion state at different eleva- tions, four sensors were installed in the atmospheric zone, six in the splash zone, eight in the tidal zone, and one in the sub- merged zone. Prior to concrete casting, sensor lead wires were fed to six monitor- ing stations installed on the top of the structure. Periodic data collection was made sys- tematically at each monitoring station using an electrochemical testing instrument. Data Analysis This section presents selective data analysis results based on 12 sets of moni- toring data. As the volume of data is not sufficient yet, only limited data analysis was possible. The data trends are exempli- fied by the monitoring data collected in the tidal zone, which was determined to be the location where reinforcing steel was most susceptible to corrosion. FIGURE 1 Aerial view of the tidal power plant in operation. FIGURE 2 Schematic of sensor installation: elevation view (a) and plan view (b). (a) (b) Corrosion Potential Figure 3(a) summarizes overall mean corrosion potential of six groups: three steel types (the multi-probe corrosion sen- sor, single-probe corrosion sensor, and structural reinforcing steel) and two struc- ture orientations (sea side vs. lake side). The potential data were divided into four exposure zones (submerged, tidal, splash, and atmospheric). Figure 3(b) shows how mean corrosion potentials of six groups in the tidal zone varied during three years of monitoring. The measurement sequence on the x-axis corresponds to quarterly data collections such that sequence numbers 1, 5, and 9 correspond to three consecutive winter seasons and sequence numbers 3, 7, and 11 to the following summer seasons, respectively. According to the sea side mean corro- sion potential data, structural reinforcing steel in the submerged and tidal zones exhibited values around –1,100 mV vs. MnO 2 . These are likely due to the ICCP sys- tem installed for the turbine structure. The corrosion sensors in the same zones also e x h i b i t e d q u i t e n e g a t i v e p o t e n t i a l s between –600 and –900 mV vs. MnO 2 . The splash and atmospheric zones exhibited more positive potentials between –200 and –300 mV vs. MnO 2 . However, the structural steel in the atmospheric zone exhibited a questionable potential of –520 mV vs. MnO 2 . Th e c orro sion sens ors and th e

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