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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsGeneral Corrosi...General Corrosi...Sea water line flange corrosionSea water line flange corrosion
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7/8/2010 9:00 AM
 
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We are getting crevice corrosion in the Carbon steel cement lined pipe flange gasket seating area.Other end of the flange is connected to DSS (Duplex stainess less) strainer without internal coating. CNAF (1.5 mm thick)gasket is used for CS Pipe flange and DSS strainer flange connection. When we opened the pipe,flange gasket was found cut at one end and sticking to the CS flange. Underground line is having impressed CP system and no insulating joint provided between the underground and above ground portion. The above problem is encountered within 3 months of operation at wherever the CS Pipe and DSS strainer is connected(Valve pit location) . The line opearting pressure is 400 psig. I would like to know the reason for crevice corrosion and i assumed the following scenario , 1)Chance of galvanic corrosion between DSS flange and CS flange. What is the probable period of getting galvanic corrosion? 2)Improper seating of gasket at the interface location 3)Gasket thickness was too low 4)Non-application of sealant coating at the interface location (cement lining to CS flange gasket area) 5)CP Problem You expert opinion in this regard is highly appreciated. regards,
 
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8/30/2010 9:00 AM
 
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The likelihood of galvanic corrosion between Duplex and Carbon Steel in sewater service is high, have a look at the galvanic series of metals in seawater. In order to avoid galvanic corrosion between these two material, in my opinion, is necessary to foreseen something more than a simple gasket like an isolating spool. This is taken from a NACE Conference paper: "When uncoated carbon steel is the anodic: part in a galvanic coupling to stainless steel or copper nickel the attack seems to be dominantly uniform in nature. Depending on the diameter ratios of the coupled pipe sections the maximum corrosion rate at the coupling is calculated to be in the range from a few tenths of a mm/year to everal mm/year. For all reasonable diameter ratios, however, the use of a spool piece of a length 1-2 meters reduces the galvanic corrosion to an acceptable level." 96496 HOW TO PREVENT GALVANIC CORROSION IN SEAWATER PIPING SYSTEMS http://www.corrosionist.com/Galvanic_Corrosion.htm hope this help Vitt
 
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