As-hot-rolled A36 H-beams have a surface of mill scale over bare steel. Mill scale is brittle, can interfere with weld quality and paint adhesion, and provides only limited corrosion protection.
Surface Preparation: For long-term corrosion protection, the mill scale must be removed. The most common method is abrasive blasting to a "near-white" or "commercial" blast finish (per SSPC-SP 10 or SP 6), which creates a clean, slightly rough surface for paint adhesion.
Corrosion Protection Systems:
Paint/Coatings: The most common method. A multi-coat system (primer, intermediate, and topcoat) is applied after blasting. Epoxy and polyurethane systems are widely used for durability.
Hot-Dip Galvanizing: Immersing the fabricated beam in molten zinc provides superior, long-term corrosion resistance through barrier and cathodic protection. It is common for exposed outdoor structures.
Weathering Steel: It is critical to note that A36 is not a weathering steel. For the characteristic rust-like patina and enhanced atmospheric corrosion resistance, a different material like ASTM A588 must be specified. Uncoated A36 will corrode uniformly and continuously if left exposed to the elements.