I'll add my two cents here. My shop is in the carport, closed in and insulated with some vapor barrier. Some of the walls are concrete as is the floor. These are not insulated; yet. As it was a car port the perimeter drain tile (hollow concrete pipe sections) were not around the carport. The house was built in 1967.
We bought it in 1997 and a few years ago started seeing water coming up through the basement floor drain. Because we're on a hill the underground water level would come up against the basement wall and not flow through the now plugged up perimeter drain which BTW, was connected to the floor drain in the basement by that same wall. Water ran like a river through the shop diagonally from mid January to the end of February. The odd thing is with just a 1500W fan heater on the floor near the lathe and mill I had no rust problems.
Long story short, we had new plastic perimeter drain done, ($20K) which included now the outside of the car port. The next winter instead of water the shop was dry. The water level in the floor drain remained below the concrete level instead of 6" above. (I had a plastic pipe inserted and calked).
And that winter although I ran the same fan heater everything started rusting. Steel that had been rust free for 20 years now was in awful shape. The Gecko Stepper Driver that had been functional for more than 10 years developed fuzz on the power connections, shorted and destroyed itself. However it was now inside the CNC electrical cabinet rather than hanging on the wall so more likely to attract condensation.
I bought a PA refurbished dehumidifier and also ran the fan heater and was able to stop the rusting the next winter. This year I plan on insulating the concrete walls in the shop and I'm considering different options for sealing the floor to prevent moisture from coming in that way.
And I have a small Raspberry PiZeroW connected to a DHT-22 RH/Temperature sensor broadcasting a web page.
The trick has been to keep the metal about 5 degrees warmer than the dew point. Don't care what the RH is since it's derived from the dew point and the temperature (hence 'relative')
And in the last few days I've been running the metal bandsaw with coolant which has often splashed onto the floor so the dehumidifier is fill the bucket more often but nothing is rusting.
IMHO, the key thing is to keep the items that can rust at a temperature above the dew point to prevent condensation from forming on them.
