Monel in Subsea Applications

From some of my other lines of research, it would appear that the problem may lie in the Cathodic Protection systems causing hydrogen embrittlement. But the same research also showed that Inconel 718, X-750, and Duplex Stainlesses are also subject to attack. That would seem to limit my bolting material choices quite heavily. So what's left? Inconel 725? Haynes C-276? I could use some ideas...

C-276:

"Versatile corrosion-resistant alloy, usually does not need to be solution heat treated after welding and has vastly improved fabricability, excellent resistance to localized corrosion and to both oxidizing and reducing media. Resists formation of grain-boundary precipitates and is therefore suitable for chemical process applications in the as-welded condition. C-276 has excellent resistance to strong oxidizers like ferric and cupric chlorides, hot contaminated media (organic and inorganic), chlorine, formic and acetic acids, acetic anhydride, seawater, brine solutions, sulfur compounds, wet chlorine gas, hypochlorite and chlorine dioxide."

I've used it on ferric and cupric chloride etchers with good luck.

Zeron 100, a super Duplex Stainless steel and is in wide use in these areas...........it would also be worth checking the other Super Alloys, e.g Iconel, Nimonic, etc

I know duplexes are verboten, so Zeron 100, Foroni 918, SAF2507 and SAF2205 are all off the table. 718 and X-750 are both questionable too. I'm leaning towards 725. The data shows that in some cases it exceeds 625 in corrosion resistance and immunity to hydrogen embrittlement, The customer is just going to have to eat the cost. It is their own requirements driving this after all. Reality is what it is.

Just curious, but is it because of the copper component that duplexes are forbidden?

it is a hydrogen embrittlement issue. generally speaking, the duplexes and the monels don't lose strength, but they lose toughness. Most of these assemblies have a lot of sacrificial anodes mounted on them and that impressed current from the anodes creates a lot of hydrogen ions.

If you are intending to use for bolting, I suggext a charpy or Izod impact test at slightly lower temperatures than the lowest that you will ever see. If yo can get 100 ft.-lbs, then you have a good bolting material. If you can't get that, then you will have the potential to have stress corrosion cracking. The hydrogen from seawater corrosion will supply enough hydrogen to cause the embrittlement.