I'm not sure from the photos but it seems that there is one pit through adjacent to a weld, and one internal corroded area adjacent to a flange, which I am guessing may have been welded. The third I can't tell where it is.
Any alloy will have limits of its resistance. Perhaps these failed adjacent to welds because of the change in the percentage of Aus to Fer phases from the welding heat? If you're looking for repair solutions, perhaps a welding procedure needs to be qualified for replacement sections that won't change the phases so much, or perhaps surface condition post-weld was inferior from shielding loss.
Could an overlay in vulnerable areas be applied of a more resistant grade? The edges of the overlay would of course then possibly be vulnerable, this would need to be checked. Adjust the welding procedure to minimize heat input and base metal phase percentage change. Finally, when stainlesses can't hack it, go superalloy, or use plastic pipe or plastic lined or fiberglass. These are often available in standardized lengths and flanges that could be swapped into existing systems.
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Paul Tibbals
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Original Message:
Sent: 08-26-2020 09:53
From: Waleed Khalifa
Subject: Failures of Duplex Stainless Steel, Solutions Out of the Box
Duplex stainless steel pipe and flange conforming to ASTM A790 2507 and ASTM A182 F51, failed after two years of service. It was a multiphase flow of natural gas, condensate and water. The conditions were severe: salinity was 12% and temperature was 57 to 75 C. The ferrite % in the weld root was low: 30-25% in some locations. The failures are seen in the attachments. Both damages occur at the 6 o'clock position. Installing dehydration units, as a remedy, seems impractical at this stage. I would appreciate if you can share some or your ideas and experience for solutions, solutions out of the box.
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Waleed Khalifa
Principal and CEO
Arabic Consultancy Center for Engineering Materials, Inspection
Maadi, Cairo
01098163293
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