are there similar metal structures near where you are ?
I'd talk to companies that do coatings and paint for corrosion protection ... see what they recommend.
I expect your choices will be ...
1) an expensive premium steel that probably won't corrode (CRES),
2) a mid-range product (treated steel) that will have a life ... ie it'll corrode but slowly and maybe not significantly if the protection is reapplied periodically), or
3) a cheap product that may last the life of your project (whatever that is).
"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.