Prepainted GI Steel Coils

Continuous Hot-Dip Metallic Coating
The foundational corrosion resistance of pre-painted steel coils begins with the continuous hot-dip metallic coating process, where steel strip passes through molten zinc (galvanized/GI) or zinc-aluminum alloy (Galvalume®/AZ) baths at 450-465°C. For GI coatings, the bath contains >99% zinc with minor aluminum additions (0.15-0.25%) to control spangle formation, achieving coating weights of 60-275 g/m² (Z60-Z275). Galvalume® baths contain 55% aluminum, 43.4% zinc, and 1.6% silicon, forming a layered microstructure: aluminum-rich outer layer for barrier protection and zinc-rich intermetallic layer for sacrificial protection. Coating thickness is precisely controlled within ±3g/m² by air knives blowing nitrogen at 10-50 kPa pressure, stripping excess metal while optimizing adhesion. The silicon content prevents brittle Fe-Al intermetallic formation, enabling post-coating forming without cracking.

This metallurgical foundation dictates ultimate durability. GI coatings provide superior sacrificial protection – zinc corrodes preferentially to protect steel scratches at a rate of ~1µm/year in rural atmospheres. Galvalume® offers 2-4x longer life in marine/industrial environments due to aluminum’s barrier properties and zinc’s self-healing capability at cut edges. Coating microstructure analysis reveals Galvalume®’s characteristic dendritic structure with 80% Al-rich phase and 20% Zn-Al eutectic, verified by SEM-EDS. Salt spray testing (ASTM B117) shows Galvalume® (AZ150) withstands 1,500+ hours to red rust versus 240-500 hours for GI (Z275). The coating’s bendability is proven by T-Bend tests (ASTM D4145) achieving 0T-2T ratings, while impact resistance (ASTM D2794) exceeds 100 in-lbs without cracking. This metallic substrate enables pre-painted coils to achieve 30-50 year service lives in architectural applications.