I live on the other coast, about 1 1/2 miles from the Pacific and do a lot of my riding even closer to the coast and corrosion is an issue.

You shouldn't have much of a problem with rust on a painted steel frame, but any unpainted steel can be an issue. For example, some parts on my pedals are steel and corrode. I use corrosion cleaner (Rust-Oleum, for example) on them frequently. Generally, keep the steel parts clean and dry and treat any rust that does occur and your bike with last a long time.

Could you put a dehumidifier in the shed or consider an aluminum framed bike?