What elements make weathering I-beams corrosion-resistant?

Weathering I-beams (e.g., ASTM A588) contain copper (0.20-0.35%), chromium (0.40-0.65%), nickel (0.25-0.50%), and phosphorus (0.04-0.08%). These elements form a dense, adherent oxide layer ("patina") that slows corrosion by 7-10 times compared to carbon steel. Copper initiates patina formation, chromium stabilizes the layer, and nickel enhances its durability. Phosphorus improves strength but is limited to avoid cold brittleness. This composition eliminates the need for painting in outdoor applications like bridges and transmission towers, reducing lifecycle costs by 30-40% over 50 years.