Why Galvanized Layer Thickness Is Quoted as Coating Mass
The zinc layer on a galvanized sheet is normally specified by coating mass in grams per square metre rather than by thickness in micrometres. The reason is practical: coating mass is what the production line controls and what the gravimetric test measures directly, and it can be converted to an equivalent thickness with a fixed factor. Because zinc has a density of about 7.14 g/cm³, a coating of 1 g/m² equals approximately 0.14 µm of zinc. A designation such as Z275 therefore means 275 g/m² total on both sides, or about 137 g/m² per side, which converts to roughly 19 µm per side. The table below converts the common coating classes.
Coating Mass to Thickness Conversion (Continuous Hot-Dip Sheet)
| Designation | Total both sides (g/m²) | Per side (g/m²) | Approx. per-side thickness |
|---|---|---|---|
| Z100 | 100 | 50 | ≈ 7 µm |
| Z140 | 140 | 70 | ≈ 10 µm |
| Z180 | 180 | 90 | ≈ 13 µm |
| Z225 | 225 | 112 | ≈ 16 µm |
| Z275 | 275 | 137 | ≈ 19 – 20 µm |
These values describe continuously galvanized sheet and strip. They are not the same as the coating applied to fabricated steelwork in a batch galvanizing bath, which is much thicker and is measured differently.
Typical Thickness by Process
| Process | Product | Typical coating | Typical thickness per side |
|---|---|---|---|
| Electro-galvanizing | Electro-galvanized sheet and strip | ZE25 – ZE50 (2.5 – 5 g/m² per side range classes) | ≈ 2.5 – 10 µm |
| Continuous hot-dip galvanizing | Hot-dip galvanized sheet (GI) | Z100 – Z275 total both sides | ≈ 7 – 20 µm |
| Continuous hot-dip galvannealing | Zinc-iron alloy coated sheet (GA / +ZF) | ZF classes matched to Z60 – Z180 | ≈ 4 – 13 µm |
| Batch hot-dip galvanizing | Fabricated components after galvanizing | Coating thickness as a minimum test value | ≈ 45 – 85 µm, depending on steel thickness |
The last row is the one most often confused with the first three. A batch-galvanized structural member routinely carries 45 µm or more of zinc on a face, which is several times the thickness found on a continuously galvanized sheet. Conversely, electro-galvanized sheet is deliberately thin, with a very smooth surface suited to painting, and should never be compared with a hot-dip coating on a like-for-like basis.
Relevant Standards
| Standard | Scope |
|---|---|
| EN 10346 | Continuously hot-dip coated steel flat products; defines Z, ZA and ZF coating designations |
| GB/T 2518 | Continuously hot-dip zinc-coated steel sheet and strip; the Chinese equivalent designation system |
| ASTM A653 | Steel sheet, zinc-coated by the hot-dip process; coating class expressed in g/m² or oz/ft² |
| EN 10152 / GB/T 15675 | Electrolytically zinc-coated steel flat products |
| ISO 1461 | Hot-dip galvanized coatings on fabricated iron and steel articles; minimum coating thickness by steel thickness |
| ISO 1463 / ISO 2178 | Measurement of coating thickness by microscopical and magnetic methods |
| ISO 1460 / ASTM A90 | Gravimetric (weigh-strip) determination of coating mass |
How the Coating Thickness Is Measured
Magnetic method (ISO 2178). A magnetic gauge measures the lift-off distance over the non-magnetic zinc layer. It is fast and non-destructive, and is the normal shop-floor check, but needs calibration on the actual substrate.
Gravimetric method (ISO 1460). A measured specimen is stripped of zinc in an inhibited acid solution and reweighed; the mass loss is converted to coating mass. This is the reference method used for acceptance testing.
Microscopical method (ISO 1463). A polished cross-section is examined under a microscope and the layer thickness measured directly. It is the most accurate way to see coating uniformity and any alloy layer.
X-ray fluorescence. A fast, non-destructive check of coating mass on a moving line, widely used for process control.
Practical Selection Guidance
Coating thickness should be chosen from the corrosivity category of the final installation and from the expected service life, not from habit. Indoor appliance parts are usually adequate with a light to medium coating, outdoor roofing and cladding normally start at Z120 to Z180, and coastal, chemical or heavily polluted sites justify Z275 or a thicker batch-galvanized coating. Cut edges, drilled holes and fixing points are the first places to corrode, so the coating mass should be selected with those details in mind rather than from the average value on a flat panel. Sheet gauge and coating thickness are independent choices: the base metal thickness controls the structural strength, while the zinc layer controls how long the part lasts before the first red rust.
FAQ
Q: What is the standard thickness of the galvanized layer on galvanized sheet?
For continuously hot-dip galvanized sheet the usual range is about 7 µm to 20 µm per side, corresponding to coating classes from Z100 to Z275 measured as total mass on both sides.
Q: How many micrometres is Z275?
Z275 means 275 g/m² total on both sides, or about 137 g/m² per side. Using the 0.14 µm per g/m² factor, that is approximately 19 µm to 20 µm of zinc on each face.
Q: Why is hot-dip galvanizing thicker than electro-galvanizing?
The hot-dip process forms a metallurgically bonded zinc and zinc-iron layer while the strip leaves a molten bath, whereas electro-galvanizing deposits a thin, very smooth layer by electrolysis. This is why electro-galvanized sheet paints so well and hot-dip sheet lasts so much longer outdoors.
Q: Can I order galvanized sheet thinner than 0.35 mm?
Yes. Galvanized sheet and strip are produced well below 0.35 mm for light-gauge applications, and 0.35 mm should be treated as a common mill minimum for heavier building grades rather than an industry-wide lower limit.
Q: How is the zinc coating thickness verified on a mill certificate?
The certificate normally states the coating class, such as Z120, together with the test method used, typically the gravimetric weigh-strip method or a magnetic gauge check. Both sides of the strip are reported.
Q: Does a thicker zinc layer always mean a longer life?
Up to a point, yes, because service life in a given environment scales with the zinc mass. The limit is formability: very heavy coatings can crack or flake during severe forming, which is why prepainted and deep-drawn products generally use moderate coatings.