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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50 NOVEMBER 2018 W W W.MATERIALSPERFORMANCE.COM CM CORROSION MANAGEMENT structural steel in the lake side tidal zone exhibited potentials near –200 mV vs. MnO 2 , at least 600 mV more positive com- pared to the sea side counterpart. During the 11th data collection, the corrosion potential of structural steel was measured on the sea-facing exterior con- crete surface to verify potential data col- lected using the embedded MnO 2 RE. The selected test area extended from EL +1.2 m to EL +3.9 m encompassed the upper sec- tion of the tidal zone, the entire splash zone, and bottom section of the atmo- spheric zone. The potential measurements were made on a 300- by 300-mm grid sys- tem using a portable copper/copper sulfate (Cu/CuS O 4 ) reference electrode (C SE). Upon completing the potential measure- ments, concrete powder samples for acid- soluble chloride analysis were taken at two depths (10 to 20 mm and 60 to 70 mm from the surface) at four elevations (EL 0, +2.0 m, +5.1 m, and +7.5 m). Figure 4 shows the mean potential data and chloride concen- tration profiles. In general, the external mean corrosion potential data showed the same trend as those measured with the embedded MnO 2 shown in Figure 2(a) (i.e., more positive potentials at higher elevations). The atmo- spheric zone's negative mean potential of –520 mV vs. MnO 2 must have been caused by a malfunctioning MnO 2 RE. Several other MnO 2 REs in the atmospheric zone also exhibited erroneous potential readings during the last several rounds of data col- lection, apparently because of progressive deterioration. The chloride analysis profiles verified that the shallow cover samples contained higher chloride concentrations than the d e e p er on e s. Thi s trend i s more pro- nounced in the tidal zone at EL 0 m and the lower section of the splash zone at EL +2.0 m. Although sufficient chloride ions have not yet reached the concrete interior (60 to 70 mm), the 10 to 20 mm concrete in the tidal zone already contained 400 ppm of chloride. More negative corrosion potentials at lower elevations, despite insufficient chlo- ride ions to initiate corrosion at the rebar depth, suggest that exposure to seawater lacking dissolved oxygen, rather than cor- ro sion , wa s re sp on si bl e. Th erefore, it should be emphasized that the corrosion criteria specified in ASTM C876 4 should not apply to reinforced concrete structures exposed to varying times of wetness. The reinforcing steel in the atmospheric zone exhibit ed relatively positive corrosion potentials and negligible chloride concen- tration, indicating that corrosion here is very unlikely. FIGURE 3 (a) Overall mean corrosion potentials classified by steel type and exposure condition and (b) variation of mean corrosion potentials with time in the tidal zone. (a) (b) FIGURE 4 Mean corrosion potential and total chloride concentrations as a function of elevation on a section of generator house concrete exterior wall.

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