51315-5453-Effect of Over and Under Protection CP System on Buried Duplex Stainless Steel in Soil Environment

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Product Number: 51315-5453-SG

ISBN: 5453 2015 CP

Author: Talal Al Nabhani

Publication Date: 2015

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Effect of Over and Under Protection CP system on Buried Duplex Stainless Steel in Soil EnvironmentTalal Al Nabhani Nasser Al Behalni Barry Johnson Petroleum Development Oman (PDO) PO. Box81 Postal Code 100 Muscat Sultanate of Oman E mail: Talal.Nabhani@Pdo.co.omAbstractThis paper investigates the effect of over /under protection of impressed current cathodic protection (ICCP) system that applied on Dupplex Stainless Steel (DSS) buried in soil environment.Stepped constant load testing (SCLT) at different ICCP potentials and temperature has been carried out in 30 % (saturated) NaCl solution in a programmed slow strain rate test rig from Cormet. In addition constant potential testing has been carried out in two different Sabkha muds from Oman.The SCLT found that girth welded material specimens with a machined V-notch similar to a weld undercut withstand a load equal to the actual yield strength (AYS) of the material at a CP potential of down to -1250 mV(Cu/CuSO4) at both 40 and 90 C in a 30 % NaCl solution. Further from the hydrogen analysis it is found that 62-70 ppm of hydrogen has entered the tested material after 20 days of exposure at the lowest CP potential of -1250 mV(Cu/CuSO4) at 40C.From the polarization testing it is found that pitting will not occur at room temperature under normal circumstances but may take place at 95 C. the corrosion potential is however lower than the pitting potential indicating that pitting corrosion will not initiate. Since the potential different between them is approximately 200 mV(Cu/CuSO4) and the tests are short time tests the recommended polarization of the pipe to an ICCP potential is less than -629 mV(Cu/CuSO4) to avoid local corrosion. Finally the constant potential polarization testing shows that when the mud dries up the polrisation current decreases to near zero. Hence in order to ensure that the ICCP system functions and will protect the pipeline from corrosion the backfill mud surrounding the pipe line and the ICCP anodes must be wet.Keywords; Impressed current cathodic protection ICCP DSS Soil Polarisation