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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Original Message:
Sent: 09-13-2022 06:57 AM
From: David Breakey
Subject: SCC in Alloy 600 in caustic service
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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