I think you're probably going to be looking at nickel alloys. The most commonly available one that might work for your service is C276.
If there is a fair amount of air exposure or other oxidizer, then C-22 will probably work better. I've seen C-22 come out of a 3 day 6% FeCl3 test with mirror finish intact.
If 1% is an occasional maximum, perhaps one of the 6% Mo stainless steels will work (Al6XN, for example). But even 1% FeCl3 has a low enough pH that it will probably challenge this subfamily. I'd really want to see some test exposures before committing to this choice.
You have an application for which there is no shortage of data. ASTM G150 is the test for critical pitting temperature and it uses 10%FeCl3 at increasing temperatures until it reaches a tempature at which pitting occurs. For 316L that temperature ranges from 0C to 20C depending on surface condition, heat chemistry and other variables.
The temperature increases 2C for each 1% chromium and 6.6C for each 1% molybdenum. 0.1% nitrogen gives you 3.2C. Subtract for sulfur and surface abrasion..more complicated. Welding takes 10 to 15C off the rsistance. In effect you can build your own alloy.
Go for a good 10C over the intended use temperature. That says at least 2205, better 2507 or a 6%moly austenitic.
If your "structural" application will permit polymer lining, consider it. Ferric chloride is used as an etchant- it's nasty stuff. If not, you're into a high alloy or titanium per the previous poster's recommendations. If it's not a heat transfer surface and the geometry is amenable to lining, there's little sense in choosing a corrosion resistant material if you can choose one which is corrosion-immune.
