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Alloy 625 - Norsok M-001

Alloy 625 - Norsok M-001

Alloy 625 - Norsok M-001

Norsok M-001 (Ed 5, Section 5.11.3) specifies that nickel grades 625, 59 & 686 are to be used in structural applications but that 625 can only be used at ambient temperatures. Table 10 goes on further to state that 30degC is the maximum seawater operating temperature.
Elsewhere, the same Norosk standard seems to imply that's it's ok for any subsea temperature in 'unstressed' conditions, i.e. as weld overlays etc.
I searched the Norsok standards (and various other places) and didn't find any reason or explanation for this limitation but I did find a 1987 MIT report (AD-A190252) which seemed to suggest that 625 experiences hydrogen embrittlement "...when subjected to potentials that produce hydrogen evolution..." but the report wasn't clear on what those specific conditions were in terms of environment, temperature and stress.

I'm just wondering if any of you subsea guru's out there can enlighten me on a referencable source that explains the reason for the 30degC operating limitation?

Thanks in advance.

RE: Alloy 625 - Norsok M-001

Could be a concern about crevice corrosion as per F J Martin, et al, Corrosion, Vol 59, (6), 2003, pp498 - 504, J S Lee, et al, Materials Performance, September 2006, pp36 - 40.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.

RE: Alloy 625 - Norsok M-001

I don't know a lot about subsea-HISC outside of steels but typically it is driven by:
Exposure to hydrogen, from CP system
Loading, residual (eg from welding) or external to material (eg pressure, mass, etc)
Yield/Proof value

I'm not sure about the 30°C maximum value, it seems a bit odd but it might due to Hydrogen production occurring at a greater rate.

RE: Alloy 625 - Norsok M-001

For the ser, service of Alloy 625 in high temperature:
1st, the mechanical of Alloy 625 in high temperature is quite stable up to 130 degree.
2nd, the corrosion resistance is also very good as following:

Alloy 625 has withstood many corrosive environments. In alkaline, salt water, fresh water, neutral salts, and in the air, almost no attack occurs. The nickel and chromium provide resistance to oxidizing environments. Nickel and molybdenum provide for resistance to nonoxidizing atmospheres. Pitting and crevice corrosion are prevented by molybdenum. Niobium stabilizes the alloy against sensitization during welding. Chloride stress-corrosion cracking resistance is excellent. The alloy resists scaling and oxidation at high temperatures.

From my experience, Alloy 625 is very popular alloy in subsea equipment and seldom of failure is reported.

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