I'd look at the nickel alloys, Inconel 625 and Hastelloy C22 would be a good start. They should be orders of magnitude more corrosion resistant than stainless s

I'd look at the nickel alloys, Inconel 625 and Hastelloy C22 would be a good start. They should be orders of magnitude more corrosion resistant than stainless steel and still have respectable strength. Of course they are more expensive and harder to fabricate.