Which type of metal would be least suitable for outdoor applications without protective coatings?

Cold rolled steel is the least suitable metal for outdoor applications without protective coatings because it is susceptible to rust and corrosion when exposed to moisture and environmental elements. Unlike galvanized steel, which has a protective zinc coating to prevent rust, or stainless steel, which contains chromium that enhances corrosion resistance, cold rolled steel does not have inherent protective characteristics.

Aluminum is lightweight and resistant to corrosion due to its natural oxide layer, making it a better option for outdoor use without additional coatings. Galvanized steel provides a protective barrier against corrosion thanks to its galvanization process. Stainless steel offers excellent resistance to oxidation and corrosion, making it well-suited for outdoor conditions. Therefore, the lack of protective measures in cold rolled steel makes it the least appropriate choice for outdoor environments.