September 14, 2021 | Knowledge Sharing
Coating weight is the first line buyers cut from a quotation, and the first one that comes back as rust spots a year later. It means the total zinc mass on both faces in g/m²: Z100 is 100 g/m² combined, roughly 7 μm per side.
Zinc protects steel sacrificially, so in theory more zinc means longer life. But it costs three things: price rises with the zinc market, the coating cracks and powders when you bend a heavy gauge, and zinc vapor causes porosity and spatter during welding. The goal is not the thickest coating — it is the coating that matches the service environment of that part.
| Application | Suggested coating | Note |
|---|---|---|
| Indoor panels (fridge side, washer shell) | Z60 – Z120 | Sufficient protection, best value |
| High humidity or condensation (freezer liner, water heater) | Z120 – Z180 | Pushes back the first rust point |
| Outdoor or coastal (AC outdoor unit) | Z180+, or switch to galvalume | Zinc is consumed quickly in salt spray |
| Welded structural parts | Z60 – Z80 | Heavier coating makes welding harder |
1. Cut edges and folds rust first. The face has paint; the cut edge has only zinc. More cuts and deeper folds mean you should step up a grade.
2. Condensation is more common than salt spray. Most appliance failures are not salt corrosion but long-term condensation. On freezer liners and water heater jackets, humidity matters more than salt.
3. Coating and paint must match. Thin zinc with thick paint means the edge fails first; heavy zinc with cheap paint means the paint fails first. Price them together, not separately.
Ask for a coating mass test report with each shipment, or take a sample and check it by the strip-and-weigh method on arrival. Put GB/T 2518 or EN 10346 in the contract and agree in advance what happens if a re-test fails. That clause is worth more than two points of price.
