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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40 MARCH 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C mental barrier protection, such as an organic coating, to obtain a min- imum 100 mV shift and a maximum pola r i zat ion potent ia l of –1,0 0 0 mV CSE while realizing that the ac- tual current densit y required for other type pipelines will differ. Test Setup This project consisted of install- ing an impressed current CP system on a 48-in. (1.22-m) diameter by 240- ft (73-m) long PCCP line and mea- suring polarization and depolariza- tion potentials and current of the pipeline during 1 year of system ac- tivation. The pipeline consisted of 10 24-ft (7.3-m) long PCCP sections. Each pipe was manufactured with two 1-in. wide shorting straps that were 180 degrees apart to reduce the electrical attenuation along the pre- stressing wire in each pipe section. The prestressing wire was made electrically continuous to the steel cylinder at each end of the pipe. The steel joints were specially manufac- tured with oversized bells, epoxy coated, and installed wit h over- sized gaskets to ensure electrical d i scont i nu it y bet we en adjacent pipe sections. Only the joint bonds provided electrical continuity. Two of the pipe sections were coated with a 0.026-in. (660-μm) thick supplemental coal tar epoxy coat i ng a nd t wo of t he sect ions were wrapped in a 0.008-in. (200 μm) thick polyethylene (PE) film. Pinholes were present in both coat- ing systems. Six sections had no ad- ditional supplemental protection. The pipe sections were installed with 6 ft (1.8 m) of cover in an arid environment. Mortared night caps were provided at each end with two access manholes. The native soil at the site was a sandy gravel ranging from 100,000 Ω-cm to 200,000 Ω-cm dr y a nd 16,000 Ω-c m to 30,500 Ω-cm saturated, and a pH range from 7.8 to 8.2. The pipeline was backfilled with sand (13.400 Ω-cm to 63.200 Ω-cm). Provisions were made to allow electrical connection or disconnection between adjacent pipe sections to simulate bonded and unbonded pipelines. A 4-in. (10-cm) diameter by 45- in. (110-cm) long steel pipe buried 20 ft (6.1 m) perpendicular to the last pipe was used as the anode, to simulate CP systems where anodes are installed close to the pipeline. Gypsum was placed around t he anode. A variable power supply was used. A baseline potential sur- vey was taken prior to activating t he CP system. Potent ia ls were measured approximately every 5 ft (1.5 m) along the centerline of the pipeline. Current-on and polariza- tion (instant current-off) potentials and current were recorded. Results Current Density to Achieve 100 mV Shift and –1, 0 0 0 mV CSE . The baseline potentials of the six un- coated pipe sections were –0 (CSE) (Figure 7). Since previous work 13 showed that 25 μA/ft 2 (270 μA/m 2 ) produced a polarization shift of 180 mV during 3 months of CP, the cur- rent density on the six sections of uncoated PCCP was adjusted to 12 μA/ft 2 (130 μA/m 2 ) based on the mortar coating surface area. The polarization (instant current-off) potent ials at 3 week s of CP are shown. The polarization shift was –120 mV. The depolarization shift after 3 weeks of polarization was 100 mV in 4 h and 120 mV after 1 week. In addition, the average current density required to achieve polar- ization potentials not < –1,000 mV CSE on the pipe nearest the anode was 100 μA/ft 2 (1,080 μA/m 2 ). Figure 7 gives the polarization potentials of the pipeline. This indicates that the design current density for PCCP is roughly 100 μA/ft 2 . Wit hin each 24-ft long PCCP section with short- ing straps, the most negative poten- tial is in the center of each pipe sec- tion, not at the joint. Cur rent Flow to Prestressing Wire and Cylinder. The prestress- ing wire surface area was 65% of t he cylinder surface area. It was found that 47% to 49% of the cur- rent flowed onto the prestressing w i r e, a n d t h e b a l a n c e t o t h e cylinder. Discussion When serious PCCP pipe corro- sion exists, CP can be installed. Typically, corrosion does not affect the entire prestressing wire surface. Much of the wire is still passive and not corroding. Requiring the pipe- line to be cathodically protected to –850 mV CSE , as typically required for steel pipelines, is unnecessary and uneconomical since only the corroding areas need to be pro - tected. Even though –850 mV is typ- ically used for bur ied a nd sub- merged pipelines in the oil and gas industry, NACE RP0169 allows a 100 mV polarization or depolariza- tion shift. This 100 mV shift also is used as a criterion in RP0290 to pro- tect the steel in atmospherically ex- posed reinforced concrete struc- tures. This shift needs to occur at the corroding, anodic sites of the pipeline. Polarization shifts of only 20 mV have been found to effec- t ively protect corrodi ng steel i n mortar. 7 In most cases, t he pipe joints have a bell and spigot configuration with a rubber gasket. This requires that the joints be bonded for CP to be effective. In the tests performed, embr it t lement d id not occ u r at –1,000 mV CSE but signs of embrittle- ment were evident at –1,100 mV. CP Design Concept. When ap- plying CP to a PCCP line, the polar- i zat ion potent ia l shou ld not be more negative than –1,000 mV CSE to prevent HE of t he prest ressi ng wire. This criterion also applies to prest ressi ng wire i n carbonated mortar and to corroding wire where the pH around the wire is acidic. If a few pipe sections are corrod- ing, only those sections need to be protected, not the entire pipeline. This may require only one anode at C A T H O D I C P R O T E C T I O N

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