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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Locating Disbonded Coatings with EMAT Technology— Part 2 RICHARD NORSWORTHY, Polyguard Products, Inc., Ennis, Texas JÖRG GRILLENBERGER, MATTHIAS JURGK, AND CARSTEN HEINKS, Rosen Technology and Research Center, Lingen (Ems), Germany Disbonded coatings that shield pipeline cathodic protection are a leading cause of external corrosion and stress corrosion cracking. Electromagnetic acoustic transducer (EMAT) technology can locate disbonded coating. Part 1 described EMAT technology (October 2012 MP). Part 2 discusses corrosion under disbonded coating. 44 MATERIALS PERFORMANCE November 2012 W hen inspecting pipelines, not only is it important to find locations of disbonded coat- ing, but it is equally important to determine if corrosion or stress corro- sion cracking (SCC) is occurring under disbonded coating. Electromagnetic acoustic transducer (EMAT) technology can perform both of these functions, giving the pipeline operator valuable in- formation about the pipeline integrity. Stress Corrosion Cracking 7VM WN \PM ÅZ[\ UIRWZ QVKQLMV\[ ZM- corded of external SCC on a natural gas pipeline in the United States happened on March 4, 1965. Numerous failures have been reported since then. Most SCC has been reported on natural gas pipe- lines, but there have been reports of SCC on liquid pipelines. The four components for electrochemical corrosion to occur on metals must be present for SCC to occur. Since three of these are inherent in all metals susceptible to SCC, the electrolyte at the pipe surface (under the disbonded coating) and the magnitude of tensile stress become the critical factors. High pH SCC is intergranular with little evidence of general corrosion; near- neutral pH SCC is transgranular with corrosion on the cracked faces. High pH SCC occurs under the disbonded coating when the concentrated carbonate- bicarbonate solution has a pH range of 9.5 to 11.5 and a potential range of –0.60 to –0.75 V at room temperature. Near- neutral SCC occurs in dilute bicarbonate solutions with a pH range from 5.5 to 7.5 and can occur when there is inadequate cathodic protection (CP) being provided in combination with a shielding and disbonded coating, highly resistant soils, and suitable temperature.1 Cathodic Protection and SCC For CP to be effective it must move the potential of the steel out of the crack- NACE International, Vol. 51, No. 11