April 11, 2023 | Technical Note
These three coatings get treated as successive generations, one better than the last. They are not. Each has a clear boundary, and choosing the wrong one costs more than choosing the expensive one.
| Property | Hot-dip zinc (GI) | Galvalume (55% Al) | Zn-Al-Mg (ZAM) |
|---|---|---|---|
| Coating | Zinc | 55% Al / 43.4% Zn / 1.6% Si | Zinc with 1–11% Al, 1–3% Mg |
| Corrosion | Baseline | Roughly 2–3× GI | Usually better than GL, especially at edges |
| Heat resistance | Degrades above about 200 °C | Continuous service to 315 °C | Moderate |
| Cut-edge protection | Zinc sacrifice | Zinc at the edge, alumina sealing the face | Magnesium drives self-healing — strongest |
| Weldability | Best | Poor; needs dedicated consumables | Moderate |
| Relative cost | Lowest | Middle | Highest |
Welded structural parts → zinc. Brackets, hinge mounts, anything spot-welded. The aluminium in galvalume ruins weld quality and the trade is not worth it.
Near heat, heat resistance required → galvalume. Compressor bays, water heaters, oven surrounds. 200 °C is the wall for zinc; galvalume holds to 315 °C.
Many cut edges, damp environment → Zn-Al-Mg. Densely perforated parts and surfaces that condense moisture benefit genuinely from edge self-healing — but it is the most expensive option, so do not specify it where nobody looks and nothing is loaded.
1. Treating galvalume as a better zinc. Welding and pre-treatment differ completely. Swap the material without changing the process and you create more problems than you solve.
2. Reading salt-spray hours alone. Salt spray is an accelerated test and does not map linearly to service life. Condensation, wet-dry cycling and the proportion of cut edge all reshuffle the ranking.
3. Ignoring paint adhesion. Galvalume has a dense surface and will shed paint in sheets if pre-treatment is wrong; zinc is more forgiving. This is the one most often missed when switching.
Describe four things — where the part sits, whether it is welded, how many cut edges it has, and whether it sees condensation — and the coating usually picks itself. If you cannot answer all four, run a sample comparison before going to production.
