ASK MTI - PUBLIC DISCUSSION

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  • 1.  SCC in Alloy 600 in caustic service

    Posted 09-13-2022 06:58 AM
    Hello,

    We design and build systems for the treatment of caustic contaminated with sulphur compounds (e.g. sodium sulphide and mercaptans), and our reactor operates in the 300-330 C range. A typical application has a caustic concentration around 7%wt, and a chloride level below 200 ppm.

    We normally use Alloy 600 in this service, and this selection is backed up by MTI documents such as the "Technical Awareness Bulletin No. 13 | Caustic Corrosion Cracking" (MTI, 2008) and the "Materials Selector for Hazardous Chemicals. MS-6: Ammonia and Caustic Soda" (Davies, MTI, 2004), which note that caustic stress corrosion cracking (SCC) is not an issue at caustic concentrations below 70%wt and that stress relief is generally used to counter this risk. Note that Alloy 600 is preferred over nickel 201 because of the presence of the sulphur compounds.

    Though we have not yet experienced issues, we've become aware that there is a large body of experience from the nuclear industry demonstrating that Alloy 600 suffers from SCC in both pure water and 10%wt caustic at around 300-315 C. This seems to be a main reason that the nuclear industry is moving to Alloy 690. We're having difficulty reconciling this data with the MTI recommendations for relatively low-concentration caustic applications. Can anyone shed some light on the risk of SCC in this caustic application and why the experience from the nuclear industry should or should not apply in this case? Are there specific mechanisms at play in the nuclear applications that do not apply to general caustic service?

    Kind regards,

    David Breakey

    SCFI Ltd.
    Cork, Ireland

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    David Breakey
    SCFI Ltd
    Bishopstown
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  • 2.  RE: SCC in Alloy 600 in caustic service

    Posted 09-14-2022 08:59 AM

    Hi David, I have worked in the nuclear industry previously and I do not know right offhand why the publication states that SCC is not an issue in Alloy 600 for caustic concentrations below 70 weight % for the range of temperatures you cite.  Early in the commercial nuclear industry phosphate-based chemistries were used on the secondary side of steam generators.  This led to the generation of caustic conditions in the heat transfer crevices and cracking in mill annealed Alloy 600 steam generator tubing.  As a result, tests were performed to understand cracking in Alloy 600 tubing under caustic conditions.  The tests were generally performed in 10 weight % NaOH solutions using U-bend specimens manufactured from Alloy 600 tubing and  SCC readily occurred in the U-bend specimens.  Thermally treated Alloy 690, however, provides much better performance in caustic environments and is one reason the industry is moving towards Alloy 690.  Attached are three seminal papers from Roger Staehle summarizing the nuclear industry experience with Alloy 600 and Alloy 690, as well as the testing done to compare the two alloys in a variety of environments, including 10 weight % NaOH with and without contaminants (e.g., lead, cooper) added to the test solution. 



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    Edward Richey
    Linde
    Tonawanda NY
    (716) 879-2805
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  • 3.  RE: SCC in Alloy 600 in caustic service

    Posted 09-14-2022 10:52 AM
    Hi David,

    Note that the MS-6 does state that "Alloy 600 (N06600) is used worldwide as a steam generator tube material in pressurized light water reactor nuclear plants where caustic SCC is also encountered. In this case, the environment is typically 10% NaOH at around 300°C (572°F)."

    Alloy 600 actually also cracks by SCC in the primary water of PWRs, which is high purity water with some additions. This has been known for a long time. In my experience, many materials become sensitive to SCC in high temperature (pressurized) water. I am aware of SCC failures of low alloy CrMo steels, 300 series steels, and Ni-alloys, mostly in power applications.

    Initiation of proto-cracks in 600 seems to take place through GB oxidation. When the cracks become long enough, they will form a specific crack fluid composition with a potential difference between the inside of the crack and outside, driving the SCC cracking. To prevent SCC initiation, it seems alloys with better/more oxide formers (690 contains almost twice as much Chromium) are better at preventing oxidation. That is as far as my knowledge goes, I'm afraid.

    Hope this helps, Regards.

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    Jan-Willem Rensman
    Fluor
    Hoofddorp Netherlands
    31 23 5432164
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  • 4.  RE: SCC in Alloy 600 in caustic service

    Posted 09-15-2022 09:45 AM
    Hello David
    I was involved in the alloy 690 development and thermal treatment optimization for 600 and 690.  A big difference in nuclear and general environments is that the nuclear environment is strictly deaerated, down to the ppb range.  The 10% NaOH, 300-315C environment was used as an accelerated test to show the effects of alloy composition and heat treatment in a much shorter time than in long term deaerated high purity water tests.  In laboratory testing the oxygen content was often <50 ppb oxygen.

    Jim Crum
    Nickel Institute

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    Jim Crum
    Nickel Institute
    Ona WV
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  • 5.  RE: SCC in Alloy 600 in caustic service

    Posted 09-26-2022 12:04 PM

    Hello all,

    Thank you for your responses. In particular, the three-part paper by Staehle and Gorman is comprehensive and was very illuminating. It seems that there a few main differences between the nuclear applications and general caustic service: among them are electrochemical potential, deaerated vs. aerated environments, and the fact that the caustic environment in steam generators is formed in crevices where heat transfer occurs.

     

    That being said, it is also clear that in laboratory environments in deaerated caustic--even without crevices--SCC readily occurs at high stress levels with certain applied potentials.

     

    I think in our case, it is a matter of reviewing these factors more closely, and I believe that our application sits in a good position with regard to applied stress and no mechanism for further caustic concentration increase. Our environment also has a high dissolved oxygen level. We will have to give these details some more thought, but the discussion has certainly moved us in the right direction, so thank you again.

     

    Kind regards,

     

    David Breakey

    SCFI Ltd.



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    David Breakey
    SCFI Ltd
    Bishopstown
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