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99356 PREVENTION OF CREVICE CORROSION IN DUPLEX SS FLANGES USING CARBON STEEL BOLTS WITH CATHODIC PROTECTION

Product Number: 51300-99356-SG
ISBN: 99356 1999 CP
Author: W. H. Thomason, Randall G. Ivie, John A. Marlow
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Achieving reliable long-term performance of high strength bolts for flange connections in subsea service is a critical issue for the offshore industry. Viable bolting materials with high strength that are not susceptible to embrittlement or galvanic corrosion when the flanges are made of stainless steels are limited. A laboratory study was performed to determine the viability of using B7 carbon steel stud bolts and 316 stainless steel (SS) seal rings in a Duplex SS flange for subsea service The laboratory test system used full size commercial flanges, bolts and seal rings to simulate electrochemicalconditions that will occur in crevices associated with carbon steel bolts in a Duplex SS flange and with the use of a 316 stainless steel seal ring in a Duplex SS flange. The flange systems were instrumented to enable monitoring of current densities and potentials at precise locations within the crevices throughout the tests as test parameters were changed. Test parameters included cathodic protection level, temperature, and sealing the outer flange gap. Cathodic protection was provided by remote aluminum sacrificial anodes to achieve potentials typical for a subsea manifold. Both electrochemical data and examination of the components at the end of the 164 day exposure indicated that sufficient cathodic protection occurred in the crevices to provide long term corrosion control to all of the components involved. The capability to use B7 bolts rather than high alloy bolts enables a significant project savings.

Achieving reliable long-term performance of high strength bolts for flange connections in subsea service is a critical issue for the offshore industry. Viable bolting materials with high strength that are not susceptible to embrittlement or galvanic corrosion when the flanges are made of stainless steels are limited. A laboratory study was performed to determine the viability of using B7 carbon steel stud bolts and 316 stainless steel (SS) seal rings in a Duplex SS flange for subsea service The laboratory test system used full size commercial flanges, bolts and seal rings to simulate electrochemicalconditions that will occur in crevices associated with carbon steel bolts in a Duplex SS flange and with the use of a 316 stainless steel seal ring in a Duplex SS flange. The flange systems were instrumented to enable monitoring of current densities and potentials at precise locations within the crevices throughout the tests as test parameters were changed. Test parameters included cathodic protection level, temperature, and sealing the outer flange gap. Cathodic protection was provided by remote aluminum sacrificial anodes to achieve potentials typical for a subsea manifold. Both electrochemical data and examination of the components at the end of the 164 day exposure indicated that sufficient cathodic protection occurred in the crevices to provide long term corrosion control to all of the components involved. The capability to use B7 bolts rather than high alloy bolts enables a significant project savings.

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