Materials Performance

MAR 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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35 MATERIALS PERFORMANCE: VOL. 57, NO. 3 MARCH 2018 In unusual circumstances, such as in high-chloride environments, the passivating properties of the highly alkaline cement may be com- promised. Then, supplemental pro- tection may be necessary. This is usually in the form of barrier coat- ings (i.e., coal tar epoxy). In some rare cases, CP is provided. 6 Potent ial criteria a nd c urrent densit y requirements for buried PCCP are generally not available. Current densit y desig n require- ments of 1 mA/ft 2 to 2 mA/ft 2 (10.8 mA/m 2 to 21.6 mA/m 2 ) have been used. The most accepted criterion of –850 mV copper-copper sulfate ref- erence electrode (CSE) often is used on bare or organically coated steel. 7 A potential of –500 mV CSE was re- ported as a criterion to protect un- corroded steel in an alkaline envi- ronment in t he presence of high levels of chloride ions, with –710 mV CSE enough to prevent f urt her corrosion, once initiated. 8 A 100 mV depolarization shift is another cri- terion used. 6-7,9 Polarization shifts of only 20 mV have been found to ef- fectively protect corroding steel in mortar. 10 Theoretical Considerations HE of Prestressing Wire Many corrosion engineers and technicians did not recognize that the prestressing wire in PCCP, as a high tensile steel, was susceptible to hydrogen embrittlement (HE). Some pipelines were cathodically protected as if they were oil and gas pipelines. Unfortunately, HE of the prestressing wire from excessive CP appeared to have caused iso- lated ruptures. 3 The most negative polarization (instant current-off) potential on one line was –1,265 mV CSE , with rupture adjacent to an anode bed 12 months after CP acti- vation. On the other, it was –1,330 mV with rupture occurring 18 years after activation. This pipeline was in an area of frequent light n ing Cathodic Protection Criteria for Prestressed Concrete Pipe—An Update FIGURE 1 Components of installed PCCP. FIGURE 2 Tensile strength of 6-gage prestressing wire from manufacturers A and B stressed at 60% of its tensile strength and subjected to CP in a saturated Ca(OH) 2 solution.

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