[Coated #1] The Armor War: GI (Galvanized) vs. GL (Galvalume)
In our previous series, we mastered the art of Stainless Steel. Now, let's talk about the steel that is born to fight rust: Coated Steel.
The market is dominated by two giants: GI (Galvanized) and GL (Galvalume).
Most buyers think: "GL is newer and more expensive, so it must be better."
This blind faith causes buildings to collapse. Today, we choose the right armor.
1. The Chemistry: Sacrifice vs. Barrier
To understand the difference, you must understand how they protect the steel base.
| Type | Composition | Defense Style |
| GI (Galvanized) | 100% Zinc | "Sacrificial" The Zinc dies first to save the steel. Even if scratched, the Zinc "flows" to cover the wound (Galvanic Action). |
| GL (Galvalume) | 55% Aluminum + 43.4% Zinc | "Barrier" The Aluminum creates a strong wall. It resists rust better on the flat surface, but it cannot "heal" scratches well. |
2. The Trap: The Pig Farm Disaster
Here is a classic mistake.
A farmer builds a pig barn. He buys GL roofing because he heard "GL lasts 3 times longer than GI."
Two years later, the roof is full of holes. Why?
🚨 GL's Weakness: Ammonia & Alkali
Aluminum (the main part of GL) is very weak against Alkaline substances.
- Animal Manure (Ammonia): Highly alkaline. It eats GL alive.
- Wet Concrete: Alkaline. If you pour concrete over GL decking, it corrodes instantly.
In these environments, the old-school GI (Zinc) performs much better.
3. The Edge Problem: Cut Edges
When you cut a steel sheet, the inner steel is exposed at the edge.
- GI: The Zinc nearby reacts and forms a protective film over the cut edge. (Self-healing).
- GL: The Aluminum barrier stops the Zinc from moving. The cut edge rusts quickly.
💡 Pro Tip for Traders: The "Density" Secret
Why do roll-forming factories love GL? Because of Density.
- Zinc Density: 7.14 g/cm³
- Aluminum Density: 2.70 g/cm³
Since GL contains 55% Aluminum, the coating is lighter.
If you buy 1 ton of GI and 1 ton of GL (same thickness), the GL coil is longer.
(More Length = More Roofing Sheets = More Profit).
Final Thoughts: Right Tool for the Job
GL is not an "upgrade" of GI. It is a different tool.
✅ Use GL (Galvalume) if:
- You are building near the beach (Salt resistance).
- You need heat reflection (Cool roofing).
- You want general long-life roofing.
✅ Use GI (Galvanized) if:
- You are building a farm (Animal manure).
- The steel touches wet concrete.
- You need to cut the edges frequently.
Don't follow the trend. Follow the chemistry.
📉 Technical Source:
1. GalvInfo Center, "Galvalume vs. Galvanized: Performance Guide".
2. BlueScope Steel, "Technical Bulletin: Usage in Animal Shelters".
Next Post
👉 [Coated #2] The Invisible Scam: Zinc Coating Weight (Z-Designation)