What Z275 Actually Means: Zinc Coating on Steel, Explained

Every spec sheet for galvanized steel struts, channels, and sheet carries a code like Z275, Z600, or Z90. Most buyers treat it as a label. It is not. The number tells you exactly how much zinc sits on the steel, and that number decides how long your material survives outdoors. Here is how to read it.

The number is grams per square meter, total

Z275 means 275 grams of zinc per square meter of steel, double-sided. That is the standard coating class for structural galvanized steel under EN 10346 / GB/T 2518. A coil labeled Z275 carries, on average, 275 g/m2 across both faces combined, roughly 137 g/m2 per side, or about 20 microns of zinc per face.

The scale runs from Z60 up to Z600 and beyond. Thin Z90 sheet goes into indoor appliances. Z275 is the workhorse for construction. Z600 sits on coastal fences and solar mounting systems that need double the life.

How the coating is actually applied

Galvanized sheet is not dipped piece by piece. Steel strip runs continuously through a molten zinc bath at around 450 °C, then air knives wipe off the excess to hit the target thickness. The result is a bonded zinc layer that metallurgically fuses with the steel, not a paint, not a plating you can flake off with a thumbnail.

For finished parts like bolts or brackets that get dipped after machining, the reference standards are different: ISO 10684 for hot-dip galvanized fasteners (50 µm average on sizes over M8) and ISO 4042 for zinc electroplating (3 to 20 µm). Same metal, different application logic. Do not compare a Z275 coil spec to a bolt plating spec, because they answer different questions.

What Z275 buys you in corrosion life

Zinc sacrifices itself to protect steel , and that is the point. Corrosion rates depend on the environment, but as rough field figures for zinc:

• Rural inland: 0.5 to 1 µm per year → Z275 lasts several decades
• Urban/industrial: 1 to 3 µm per year → decades down to about 15 years
• Coastal with salt spray: 3 to 6 µm per year → plan for Z400 or Z600, or paint over HDG

So when a strut channel supplier claims Z275 and you need 20+ years in an industrial zone, the honest answer is that it meets spec, so budget for it. If a supplier quotes you "Z275 equivalent" at a suspiciously low price, the coating is usually where they cut.

The number is a minimum average — verify it

Here is what most buyers never check. Z275 is a coating class, guaranteed as an average across the coil lot. Individual spots can run lower. The only way to know what you actually received is a third-party test: weigh the coating, measure the steel, compare against the standard.

We recently had our slotted strut channel, specified Z275, tested by SGS under ISO 1460. Results: average coating 295 g/m², 7% above the Z275 requirement, with individual samples up to 339 g/m². Base steel came back at 371 MPa tensile, 31% elongation, and wall thickness measured 2.499 mm against a 2.5 mm nominal.

We are not sharing this to boast. It is what a buyer should demand from any supplier: the spec sheet says Z275, the test report says what actually shipped. When the two match or beat the number, you can build your quote on real data.

Quick reference: coating classes

The takeaway

Z275 is not a marketing label. It is a measurable promise. Read the class, match it to your environment, and ask the supplier for the coating test report, not just the catalog page. If you are sourcing strut channels or galvanized components and want the numbers behind the spec, send us your requirement list, we will work out a quotation on those lines within 24 hours.