STEEL COATING & CORROSION SCIENCE
Galvanized Metal Roofing: G90 Zinc vs. Galvalume Steel
Galvanized metal roofing is carbon steel coated with a sacrificial layer of molten zinc (ASTM A653) to prevent rust and oxidation. While traditional G90 galvanized steel has protected roofs for over a century, modern aluminum-zinc Galvalume (ASTM A792) has become the dominant standard for painted residential standing seam systems.
Coating Comparison at a Glance
- Galvanized Steel (ASTM A653 – G90): 100% Zinc coating ($0.90text{ oz/sq ft}$). Excellent sacrificial edge protection; highly compatible with animal confinement/agricultural barns.
- Galvalume Steel (ASTM A792 – AZ50/AZ55): 55% Aluminum, 43.4% Zinc, 1.6% Silicon alloy. Provides 2 to 4 times greater corrosion resistance in general residential applications.
How Zinc Galvanization Protects Steel
Zinc provides galvanic barrier protection. When the metal is cut or scratched, zinc corrodes sacrificially before the underlying carbon steel can oxidize, forming a protective zinc carbonate patina that seals the exposed cut edge.
Galvanized vs. Galvalume vs. Aluminum
| Substrate Metal | Coating Chemistry | Expected Lifespan | Coastal Saltwater Resistance |
|---|---|---|---|
| G90 Galvanized Steel | 100% Hot-Dip Zinc | 25 – 40 Years | Moderate (zinc depletes rapidly in sea spray) |
| Galvalume Steel | 55% Aluminum / 45% Zinc | 40 – 60+ Years | Good (Avoid within 1,500 ft of breaking surf) |
| Solid Aluminum | Solid Alloy (No steel core) | 50+ Years | highly resistant to marine salt-spray rust (Best for coastal) |
Related Metal Guides
Explore Aluminum Roofing, Metal Roofing Panels, or calculate project pricing with our Metal Roof Cost Calculator.