How about a little heat and insulation for the winter months to keep the moisture out? I have a metal garage that sweats in the winter causing rust on my tools and bikes.
My bro put up a plastic shed for his Triumph. Small, but he likes it.
Metal sheds are notorious for sweating, especially if they sit in direct sunlight. Sealing the concrete will help, but ventalation is key whether passive (simple vents) or power (like a solar attic vent).