90/10 and Monel 400 would work fine together.
Yes the Monel is more noble but not by a huge amount.
316 or 2205 will not survive very long in seawater.
Superduplex is asking for issues.

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P.E. Metallurgy, consulting work welcomed

I have experienced the 90/10 cuni to be highly susceptible to galvanic corrosion from 316 SS. Our installation has 316 SS studs in blind holes CuNi flanges to through holes in steel flanges with SS nuts. This works because we have anodes on the carbon steel structure. However, there are isolated SS bolts attaching the CuNi exchanger to the hull and if the CuNi exchanger is isolated as well we will see corrosion rates exceeding 1/16 inch per year on the CuNi in the vicinity of the SS bolt.

These dissimilar couplings can work provided your anodes never wear out. Once the anodes are gone the system will fail rapidly.

@ EdStainless

How about Incinel, SB-446 UNS NO6625 Gr.1 for sea water service and againt the gavonized corrosion with SB-283 C70620?

Thanks,

TBE, I think that some other mechanism is at work in your case. It is generally accepted that 90/10 is more noble in seawater than 316.

So going to 625 hardware would be even worse.

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P.E. Metallurgy, consulting work welcomed

May I ask what condition would cause SS to be active vs passive?

Anything that will breakdown the passive film; low pH, high temperature, mechanical damage (scouring), applied current from a stray source (making the SS the anode).
Once corrosion starts the SS is no longer passive.

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P.E. Metallurgy, consulting work welcomed

My big assumption is that if there are less noble materials nearby the SS is going to favor its passive state in the galvanic cell. In salt water the SS is going to be passive which makes it quite a bit more noble than the CuNi.

Once corrosion starts the SS is no longer passive is certainly true when that applies to corrosion of the SS. Copper alloys near SS are anodic and prevent that corrosion of the SS from starting.

That is correct.
A potential difference of 0.25V is plenty to drive accelerated corrosion in warm water, it will take a bit more in colder water.

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P.E. Metallurgy, consulting work welcomed

Can we just clarify the location of the bolts: internal in the seawater; external in the atmosphere?

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.

@EdStainless, thank you. It is a good guideline for me to understand what materials may be possible in the gavanized corrosion condition from the table.
@SJones, the whole floating head soak in the sea water.