1. Each material has different failure mechanisms. For SS and Ni alloys pitting and crevice corrosion are important, especially with biofouling. A steel would have HE and impact concerns.
2.Has anyone presented support data for a 50 year design? Sacrificial anodes are not lasting 50 years.
3. Yes you can CP SS alloys.Current draw is lower since the corrosion rates are lower, but in crevices you can still get attack.
4. I would suggest looking to older literature.
The only alloys that I would look are Ni based, either Monel 400 or C276.
The problem with bolts is that you don't really have a corrosion allowance. Once you get any crevice corrosion the flanges loose clamping. So it isn't like a bulk stucture.

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P.E. Metallurgy

Just to go outside of the box, have you considered silicon bronze? It's much less expensive than any of the materials listed, resists corrosion in sea water and is not subject to crevice corrosion. High strength shouldn't be a requirement if you're using plastic piping. Silicon bronze fasteners are an off-the-shelf option as well.

Start with reviewing EEMUA 194 for guidance on materials and corrosion control. Then work through the degradation mechanisms listed in API RP 571, eliminating those deemed as not credible for the various materials options.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

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

Good thought TBE, though NiAlBronze might even be a better option for this.

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P.E. Metallurgy

Thank you all very much! Some queries on your replies:
1. Sand backfill will enhance biofouling?
2. HDG will face HE due to the CP? Is it a viable option? It is already installed in a nearby contruction with huge Aluminium alloy sacrificial anodes...
3. Is there a possibility CP sacrificial anodes to revert and sacrify the cathode (flanged connection) under conditions described, i.e. backfill & subsea? Especially for the 316LSS. What will be the material of the anodes? Aluminium?
4. HDPE bidders have quoted for HDG with CP or 316LSS with CP or 1.4410. CP will be maintained to last 50 years. But I need the opinion of a specialist in metals, not in plastics!
5. Service will be chlorinated seawater, should it be considered for the material selection?
6. What are the grades for NiAl bronze flange/bolt/nut? There shall be all the same grade?
A search has shown no pitting/crevice/galling/biofouling. Maybe this will be my solution since no other metals will be near for galvanic corrosion issues.
I will also review EEMUA and API when I archive them, super references, thanks!

Read, and digest, the documents that you will be "archiving" (hopefully not filching illicit copies), and also check DNVGL-RP-B401. Incidentally, it's the steel substrate, not the hot dip galvanising, that could have a susceptiblity to hydrogen embrittlement.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

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

Ed, TBE, do you know of an ASTM spec for silicon bronze or NiAlBronze fasteners?

Huub

I believe that would be ASTM F468 for the silicon bronze though it's normally referred to as 651 bronze. The aluminum bronze would be B150.

The NAB is alloy 630 (C63000) and some derivatives.
The three digit numbers are usually the old CDA (Copper Development Association) designations. You can go to the CDA web site and look up a lot of technical info.

By being buried you assure an anaerobic environment, so the bugs will be different. And alloys like SS which require oxygen for passive film development will have more problems.
I cannot picture anodes or HDG holding up for 50 years, 20 yes.

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P.E. Metallurgy

I appreciate your help, even conveyed by "non-digestive" "incidental" comments.

EdStainless I hoped from a reply from you. Thank you so much!

My comment was incident free and complied with all health and safety regulations.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

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

The inner service will be chlorinated sea waste but that's irrelevant. Your flange is buried subsea.

Really not sure how you plan to add CP to a flange??

Forget CP for a 50 yr life. Replacing it is a nightmare.

Just use Super Duplex or Aluminium Bronze.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

NAB will suffer from corrosion being backfilled with sand? Do we have to take some precautions in the quality of the sand?

LittleInch, thank you very much for your reply! I know nothing about cathodic protection and how you can add sacrificial anodes on a flange. The HDPE bidders have quoted so!, this is why I need an expert approach corrosion point of view. Mr. EdStainless said above that crevice corrosion should be considered for super duplex. If I go with Nickel Alu Bronze, it will be affected from the fact that it will be backfilled with sand?

Ed will know a lot more than me so maybe, but I think you need to look at the service and determine the stresses and what happens if the bolt became loose or even "failed". For a seawater intake line this will be lower pressure than the sea to make it flow, unless you have a subsea pump??

~Or is there a way to get rid of the flange completely? PE fuses really well and unless this flange is being connected subsea?? then there are alternatives,

But use or find a good metalurgist. 50 years in seawater is a loooong time for any material, even something as realtively bullet proof as Super Duplex.

The only thing you find intact from wrecks are the bronze fittings....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Also, find and use a certificated CP consultant.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

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

Dear all, just to inform you that I have contacted copper.org for the suitability of copper alloys and I will share any feedback. Furthermore I have just finish reading Corrosion Performance of Metals for the Marine Environment EFC 63: A Basic Guide, where a lot of queries found a reply.

It's amazing what you find when you read up.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

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

We may go with 1.4410 to avoid cathodic protection. If anyone has any kind objection, please do so. Thank you all for your help!

The reason that we mentioned Cu based alloys is that they have a long term track record in seawater. The main reason is that the Cu in them slowly dissolves and is toxic to marine life, this serves to prevent/minimize fouling.
I don't know that 2507 would not work, it certainly should, but I don't know of any long term sub-sea data.

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P.E. Metallurgy, consulting work welcomed

The biggest concern with copper alloys in salt water is sulfide corrosion. If the back fill has a high concentration of microorganisms then this could be an issues. Otherwise, copper alloys are very reliable.

The concerns about Copper alloys are:
1. if they can be fabricated in DN2300 required for the flange backing ring.
2. What will be the material grade for the three mating items, flange, bolts, nuts. Can we have NAB flange with silicon bronze bolts and what shall be the material for nuts. Will any galvanic corrosion present?
3. if they will face high corrosion rate being immersed and buried under sand backfill.
On the other hand, same applies for the Super duplex as well! I am really in trouble...

TugboatEng, I saw your reply after my submittal. Thank you both indeed, so helpful posts! Can I request for the backfill to be clean? Or the bacteria will be self born?

Well you never said it 2.3m diameter pipe!!

I think you might be over thinking this a little bit. Super Duplex in this sort of service is about as bullet proof as you're going to get and doesn't need CP.

What is this for - A power plant water channel or something else?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

You are correct, I have not mentioned the NPS. It is an HDPE Seawater Intake pipeline.

Well at least you grasp the challenge.
Location is a big factor here. If this is deep and away from populated areas you have a very good chance.
If it is shallow and near lots of people (more pollution) then the risk is higher.
Have you looked at non-metallic options? Could you weld the tube or use FRP flanges?

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P.E. Metallurgy, consulting work welcomed

Locations will be 2 km from coast and 10 meters depth. So I guess there will be a pollution.
Welding the long length HDPE pipes is an option that I have suggested.
Non metallic backing rings according to ISO 9624 have been suggested as well, but we have a concern regarding ease of installation at site, bolt torquing requirements, gaskets, bolts materials etc.