Body in ductile iron with aspecial 4till5 layer coating
Exotic materials are verry nice but far to expensive fore these applications!
a verry good and widly accepted disc construction is ductile iron with hardrubber coating (+/-6mm)
when the butterfly valve is full rubberlined the shaft is not in contact with the fluid 17-4 or aisi431 is sufficient
If the valve disc is uncoated it should be made from a seawater compatible material.
Do not use stainless of any grade as it is very suspect to chloride pitting corrosion.(this is not true of super-duplex)
LG2 bronze is acceptable but a much better option would be a cupro-nickel alloy like nickel aluminium bronze or monel 400.If you have lots of money in the budget you could look at incolloy 825
The same applies to the body but as it is rubberlined it shouldnt be in contact with the seawater and couuld be externally epoxy coated.

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