Chromium (10.5%+ in stainless round steel) forms a passive oxide layer (Cr₂O₃) that prevents further oxidation, granting corrosion resistance. In grades like 304 (18-20% Cr), chromium works with nickel (8-10%) to enhance resistance to acids, salts, and atmospheric moisture. Higher chromium (e.g., 25% in 310 stainless) improves resistance to high-temperature oxidation, making it suitable for furnace components. Without chromium, carbon steel round bars corrode rapidly in humid or chemical environments, requiring coatings like galvanization for protection.