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Failures of Duplex Stainless Steel, Solutions Out of the Box

  • 1.  Failures of Duplex Stainless Steel, Solutions Out of the Box

    Posted 08-26-2020 10:27
    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.

    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo

  • 2.  RE: Failures of Duplex Stainless Steel, Solutions Out of the Box

    Posted 08-27-2020 12:18
    Yes, if the condensed water sits stagnant and gets hot (75C), that appears to be too much for superduplex. Seems to me that one solution is to ensure all condensed water drains to one particular area, and either put in a drain plug there and drain it as often as needed, or put in a dead leg that can be drained, and perhaps easily replaced if draining is not possible or cannot be easily done.

    Gary Coates
    Manager, Technical
    Nickel Institute
    Mississauga ON

  • 3.  RE: Failures of Duplex Stainless Steel, Solutions Out of the Box

    Houston Admin
    Posted 08-28-2020 11:07
    Just coming at this from the welding side, since your root had low ferrite and it appears that this is where ​you corroded through (although there was certainly plenty of corrosion in other areas as well) - it may be worthwhile to take a closer look at your welding procedure. Getting a good ferrite/austenite balance in as-welded super duplex can be tricky, and is influenced by several factors, namely shielding gas, filler metal composition, and cooling rate after welding. Therefore:

    • For shielded welding processes like TIG, it's common to add 2% or so nitrogen to stabilize austenite in the weld. However, if nitrogen was too high (or if shielding was poor and you were getting excess nitrogen from the air), then this may have contributed to the high austenite content. If this was welded out with SMAW (or even TIG root and SMAW hot pass), then that might have exacerbated the nitrogen pickup.
    • Check the FN of your filler as well as its nominal composition. Some fillers are more over-alloyed than others. If you're using a filler with a high Ni(eq), that will predispose you to get more austenite. You'll also want to adhere to the manufacturer's recommended interpass temp for that grade filler.

    Ultimately the issue is the corrosivity of the environment but it may still be helpful to mitigate the susceptibility of the weld.

    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX

  • 4.  RE: Failures of Duplex Stainless Steel, Solutions Out of the Box

    Posted 08-30-2020 12:27
    Pitting away from the weld says it's not a welding issue. Looks like this grade of duplex is not up to the job.

    Walter Sperko
    Sperko Engineering Services Incorporated
    Greensboro NC
    (336) 674-0600

  • 5.  RE: Failures of Duplex Stainless Steel, Solutions Out of the Box

    Posted 08-28-2020 09:22
    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.

    Paul Tibbals

  • 6.  RE: Failures of Duplex Stainless Steel, Solutions Out of the Box

    Posted 08-30-2020 12:27
    Over 50 degrees C with 12% brine is too much for any duplex stainless steel to handle in an aerated solution. But, it is likely that your environment is nearly oxygen-free. So, Paul Tibbals' suggestion about alternative materials is good, but might not be necessary.
    I cannot determine from your pictures where the gasket is on the flanged joint, so cannot say whether corrosion initiated under the gasket. If it did, facing the flange with a more resistant alloy (N06022, N10276, etc.) may solve your problem. (Corrosion initiation at gasketed joints is a common problem. Eliminating such joints is another remedy.)
    The corrosion inside the pipe appears to be attacking the material layer-by-layer, which is typical for corrosion (including, but not limited to, crevice corrosion) in wrought duplex stainless steel.
    Usually, welds in duplex stainless steel are plagued by excessive ferrite. Low ferrite is unusual. I suspect that a nickel-enriched filler was used and that it was a multi-pass weld. The heat of subsequent passes caused the root pass to become more austenitic. This is seldom a problem and is unlikely to be the cause of the failure. 

    John Grubb