I put a little galvanized steel roof on the garage a built in 1990. Nothing special just regular ripped galvanized sheet with no paint. I took the building down last summer 2017,and I even kept the roof panels they were in such good shape. About all I can say about it was there was some rust beginning, nothing pitted just the beginning’s. I could’ve thrown a coat of paint on it and put it all up again.
For fasteners I had Your typical drill driver driven sealed self tapping roof screws. Those were startingTo show some rust but they weren’t in any way compromise to including the seals. I’ll listen New York state where the weather is lousy it rains constantly and snows and it is damp Much of the time.
Now my house has a 30 year shingles installed in 1995. I’ve had the typical where you’d expect and they’re still OK but I did get tired of putting shingles back on to get popped off by the winds we have here.