I would not recommend the use of 316 ss in this application. As a minimum you would need 316L ss and even in this situation you are looking at pitting and serious crevice corrosion problems especially with a ball valve application in sea water.
I would upgrade the valve material to an Alloy 20. This material can be welded to the Duplex 2205 using a nickel-base filler metal. The interpass temperature must be less than 300 deg F during welding.
I assume that this is not on the seawater side of the RO, since 2205 will not survive for long at those chloride levels either. 316 will fail quickly from crevice corrosion.
I wouldn't bother with 904L. It is expensive (high Ni) and not very corrosion resistant. Go to a 6%Mo alloy.
If you do need to weld an autenitic stainless to a duplex you will want to use a high alloy austenitic filler so that the weld is austenitic. If the weld is 'sort of' duplex you can get some very poor mechanical properties.
If I were welding 316L to 2205 I would use a 6% Mo filler.
If I were welding a 6%Mo stainless to 2205 I would use a 'C' type Ni-Cr-Mo alloy (C-276 or similar).
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Corrosion never sleeps, but it can be managed.
2205 can be easily welded to 316 SS. Whether 316 SS is appropriate for the valve is a differnet story as stated by metengr and EdStainless. We have had many cases where the two materials have been specified by Owners and Engineering firms to be welded together and have used E/ERNiCrMo-3 or E/ER309LMo for the filler metal using the GTAW and SMAW processes.
I would not use the 309 filler. This will leave the welds less corrosion resistant that either of the alloys being joined.
The NiCrMo-3 (625) fill is a good idea Stan. It is versitile, strong and corrosion resistant.
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Corrosion never sleeps, but it can be managed.
Brackish water can vary in chloride content based on tide and location. I would still use 316L SS to minimize the effects of sensitization along the weld HAZ. Maintain the interpass temperature as mentioned above.
Brackish water can have stuff in it that makes sea water look as pure as driven snow, so to speak. Find out exactly what is in your water and post back.
I would be concerned with stagnation in the legs of the socket made as you have indicated. If sufficient stagnation occurs adjacent to the weld, its life expectancy may be very, very short.
Make sure the weld geometry doesn't leave a crevice on the inside. I would rather see a butt weld.
A 6%Mo austenitic stainless filler would be a good option. Your filler must have higher Mo than either of the materials that you are joining in order to get decent weld metal corrosion resistance.
And as always, clean welds are good welds. Good shielding and post-weld acid cleaning will go a long way.
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Corrosion never sleeps, but it can be managed.
