Hot-Dip Galvanizing Thickness Compared: GB/T 13912 vs ASTM A123 vs EN ISO 1461
2026/10/20
Hot-Dip Galvanizing Thickness: Comparing GB/T 13912, ASTM A123 and EN ISO 1461
Three standards dominate hot-dip galvanizing specifications in steel structure procurement: China's GB/T 13912, the ASTM family in North America and EN ISO 1461 in Europe. They do not share the same logic, and their figures are not interchangeable — a mismatch that regularly turns into an acceptance dispute at delivery.
1. GB/T 13912 (China)
Covers hot-dip galvanized coatings on iron and steel products, including some castings, with total alloying elements not exceeding 2%. Coating thickness is graded by substrate thickness:
| Steel thickness | Local minimum | Average minimum |
|---|---|---|
| t ≥ 6 mm | 70 μm | 85 μm |
| 3 mm ≤ t < 6 mm | 55 μm | 70 μm |
| 1.5 mm ≤ t < 3 mm | 45 μm | 55 μm |
| t < 1.5 mm | By agreement | |
Local thickness is the lowest value at any measuring point — protection against premature corrosion at the thinnest spot. Weight conversions use zinc density 7.14 g/cm3, so 1 μm ≈ 7.14 g/m2: 85 μm ≈ 606 g/m2, 55 μm ≈ 395 g/m2, 45 μm ≈ 320 g/m2. Measurement is mainly magnetic (GB/T 4956), with gravimetric as the referee method; the gauge reads from steel substrate to outer surface, including the zinc-iron alloy layer.
2. ASTM (North America)
There is no single US figure — requirements split by product type. Structural steel and fabrications follow ASTM A123/A123M, graded by material category and steel thickness:
| Steel thickness | Typical minimum average |
|---|---|
| ≥ 6.4 mm (≥ 1/4 in) | Grade 100, about 100 μm (705 g/m2) |
| 4.8 – < 6.4 mm | about 75 μm (530 g/m2) |
| 3.2 – < 4.8 mm | about 75 μm (530 g/m2) |
| 1.6 – < 3.2 mm | about 65 μm (460 g/m2) |
| < 1.6 mm (< 1/16 in) | about 45 μm (320 g/m2) |
Fasteners galvanized then centrifuged follow ASTM F2329 (about 50 μm average above 3/8 in, 43 μm at or below) or A153 (Class C about 53 μm, Class D about 43 μm); electroplated bolts at roughly 5–8 μm belong to a different, weaker system. Continuous galvanized coil follows ASTM A653, ordered by coating designation in total double-sided zinc mass: G30/Z90 ≈ 90 g/m2, G60/Z180 ≈ 180 g/m2, G90/Z275 ≈ 275 g/m2, G235/Z700 ≈ 700 g/m2.
3. EN ISO 1461 (Europe)
Also graded by substrate thickness, but with two requirement sets. Non-centrifuged parts — normal structural members — mirror GB/T 13912: 85 μm average / 70 μm local at t > 6 mm, 70/55 μm at 3–6 mm, 55/45 μm at 1.5–3 mm and 45/35 μm below 1.5 mm. Centrifuged small parts such as bolts, nuts and washers drop to 55/45 μm above 6 mm and 35/25 μm below 3 mm. Continuous coil sits outside this standard (EN 10346).
4. Where the Standards Actually Diverge
| Item | GB/T 13912 | ASTM | EN ISO 1461 |
|---|---|---|---|
| Scope | Galvanized steel products | A123 / A153 / F2329 / A653 by product | Fabricated steel parts (not coil) |
| Grading basis | Substrate thickness | Material category and thickness | Substrate thickness |
| t ≥ 6 mm average | ≥ 85 μm | A123 about 100 μm | ≥ 85 μm |
| Coil designation | No equivalent | A653 G/Z codes | EN 10346 Z codes |
| Edge distance for readings | No single rule | ≥ 13 mm from edge | ≥ 10 mm from edge |
| Limits | Minimums specified | Generally minimums only | Minimums only; thicker is better |
5. Selection and Acceptance Notes
- Match the standard to the product. A123 for structural steel, F2329 or A153 for fasteners, A653 for coil — they cannot substitute for one another.
- Separate single-side from double-side. A653 designations such as G90/Z275 are double-sided totals, roughly 19–21 μm per face — not comparable with 85 μm single-side on a heavy section.
- Write it into the contract. Standard, thickness, test method and edge distance, all three, to avoid two parties measuring to different rules.
- Thickness is not the only variable. High-silicon reactive steel accelerates coating growth, high-strength bolts above about 33 HRC carry hydrogen embrittlement risk, and sealed cavities need vent and drain holes before dipping.
One correction worth making in any technical discussion: coating requirements rise with steel thickness, because thicker sections heat slowly and spend longer in the zinc bath, growing a fuller alloy layer. "Thicker steel, thinner coating" contradicts every one of these three standards.
