09545 Interference of an Impressed CP System with an Adjacent Sacrificial CP System and its Mitigation

Tuesday, March 24, 2009: 10:20 AM
C302 (Georgia World Congress Center)
Ezeddin R. Busba , Great Man-Made River Authority, Benghazi, Libya
Fawzi S. AL-Madani , Great Man-Made River Authority, Benghazi, Libya
David Scantlebury , The University of Manchester, Manchester, United Kingdom
Hussain S. Boshalla , Great Man-Made River Authority, Benghazi, Libya
An existing impressed CP system of steel tanks was found to be interfering with newly installed zinc anodes of an adjacent Pre-stressed Concrete Cylinder Pipeline. The interference prevented the zinc anodes from providing protection current. The two protected structures are electrically bonded. Field measurements showed that some zinc anodes were providing current. Other anodes acted as if receiving current from the pipes. The latter was thought to be a cause of accelerated pipe corrosion (reversed circuit). The tank bottom plates were also found to have lost protection.  In this paper, analysis of field data revealed that all of the zinc anodes in the vicinity of the tanks were picking up stray current. A mitigation measure was proposed to remove the existing resistive bond between the two structures and install a few zinc anodes at the discharge area. An experimental approach was undertaken to determine the effectiveness of this method by simulating the problem on a lab scale. Experimental measurements showed that installing zinc anodes at the discharge area successfully prevented the current discharge from the pipe. It might therefore be possible to operate the two CP systems separately.