Not all galvanized pipe is equal. The difference between hot-dip galvanized (HDG) and pre-galvanized (also called continuous galvanized or electro-galvanized) pipe comes down to zinc coating thickness, bond structure, and long-term corrosion performance. Here is what you need to know.
Hot-dip galvanizing immerses fabricated steel pipe into a bath of molten zinc at approximately 450°C. A metallurgical reaction forms three iron-zinc intermetallic alloy layers beneath a pure zinc outer layer. The total coating is typically 45–85 microns on pipe of standard wall thickness, with thicker coatings of 85–140 microns possible on heavier sections per ISO 1461.
Pre-galvanized pipe is manufactured from steel coil that was zinc-coated in a continuous hot-dip process before the pipe was formed and welded. The coil passes through a zinc bath at high speed, producing a thinner, more uniform coating of 10–25 microns (equivalent to zinc mass of 100–275 g/m², commonly referred to as Z100–Z275).
| Standard | Coating Type | Minimum Local Thickness | Typical Total | |----------|-------------|------------------------|---------------| | ISO 1461 | Hot-dip (after fabrication) | 45 μm (pipe 3–6 mm wall) | 55–85 μm | | EN 10240 | Hot-dip galvanized tube | 40 μm (wall < 6 mm) | 50–80 μm | | ASTM A123 | Hot-dip general | 65 μm (≥ 6 mm steel) | 70–100 μm | | EN 10346 | Pre-galv (Z275) | — | 19 μm per side (275 g/m²) | | ASTM A653 | Pre-galv (G90) | — | 20 μm per side |
The practical takeaway: hot-dip galvanized pipe carries 2–4× more zinc than pre-galvanized pipe.
Zinc protects steel through barrier protection and galvanic (sacrificial) protection. Thicker zinc means longer life. In a C3 (moderate) corrosivity environment per ISO 12944, 85 microns of HDG zinc provides roughly 40–60 years to first maintenance. A 20-micron pre-galvanized coating under the same conditions lasts 12–18 years.
The zinc-iron alloy layers in HDG are harder than the steel substrate (250 DPN vs 159 DPN for steel), providing superior abrasion resistance during handling, transport, and installation. Pre-galvanized pipe's pure zinc outer layer is soft (70 DPN) and easily scratched during threading or coupler tightening.
Pre-galvanized pipe has an uncoated weld seam where the zinc was burned off during ERW welding. The factory applies a zinc-rich paint touch-up, but corrosion commonly initiates at the weld line before anywhere else. Hot-dip galvanized pipe is fully coated after welding—there is no bare weld seam.
For buried or continuously wet applications, this difference alone makes hot-dip galvanizing the standard choice. The zinc-rich paint on a pre-galvanized weld seam fails cathodic protection tests within 2–5 years in soil exposure.
Pre-galvanized pipe works for:
Hot-dip galvanized pipe is required for:
TXD Steel Group manufactures hot-dip galvanized steel pipe from 21 mm to 1,200 mm OD with batch test certificates for zinc coating weight and uniformity. Contact admin@txdmetal.com or WhatsApp +86 15216408075 for specification details.
