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Help CenterSteel Pipe & Tube & Hot-dip Galvanized Steel Pipe，Q235B、Q215、Q355B
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1. Basic Definition & Production Process
Manufactured based on black welded or seamless steel pipes, hot-dip galvanizing refers to immersing steel pipes into molten zinc liquid at 440~460℃. Metallurgical reaction occurs between steel and zinc to form a composite coating consisting of zinc-iron alloy layer and outer pure zinc layer, fully covering both inner and outer walls.
Complete production flow:Degreasing & oil removal → Pickling & rust removal → Water washing → Flux dipping → Drying → Hot-dip galvanizing (1~5 min) → Air knife zinc residue blowing → Water/air cooling → Passivation for white rust prevention → Finished product inspection
Key Coating Standards (GB/T 3091-2025)
Total zinc coating weight ≥300g/㎡; heavy anti-corrosion type can reach over 500g/㎡
Coating thickness: 50~100μm, strong adhesion, not easy to peel off in large areas after collision
Full galvanization protection on pipe inner & outer walls, cutting ends and weld seams
2. Material Grades & Executive Standards
Base material grades: Q235B (main grade), Q215, Q355B (thick-walled for high pressure)
National Standard: GB/T 3091-2025 Welded Steel Tubes for Low-pressure Fluid Service
Standard fixed length: 6 meters per piece; customized lengths of 3m, 9m, 12m available
Hydrostatic test pressure: DN15~DN150: 3MPa; DN175 and above: 5MPa
3. Common Specification Table
| Nominal Diameter DN | Inch Size | Outer Diameter mm | Standard Wall Thickness mm | Reinforced Wall Thickness mm | Typical Application |
|---|---|---|---|---|---|
| DN15 | 1/2" | 21.3 | 2.75 | 3.25 | Domestic water branch pipes, small sprinkler pipes |
| DN20 | 3/4" | 26.8 | 2.75 | 3.50 | HVAC pipes, indoor branch pipes |
| DN25 | 1" | 33.7 | 3.25 | 4.00 | Fire sprinkler branch pipes |
| DN40 | 1.5" | 48.3 | 3.50 | 4.25 | Indoor water supply main pipes |
| DN50 | 2" | 60.3 | 3.50 | 4.50 | Fire standpipes, gas pipelines |
| DN80 | 3" | 88.9 | 4.00 | 4.75 | Outdoor water supply trunk pipes |
| DN100 | 4" | 114.3 | 4.00 | 5.00 | Fire main ring pipes, municipal gas pipelines |
| DN150 | 6" | 168.3 | 4.50 | 5.50 | Large municipal water supply, industrial fluid pipelines |
4. Main Advantages & Disadvantages
Advantages
Excellent anti-corrosion performance: Zinc acts as sacrificial anode, providing automatic rust protection even where coating is damaged; service life 10~15 years in dry outdoor environment, 5~8 years underground.
Firm coating: Metallurgical bonding, no large-area peeling unlike electro-galvanized pipes, full protection for inner and outer walls.
Wide construction compatibility: Threaded, grooved, flanged and welded connections available, complete matching pipe fittings.
Cost-effective: Much lower cost than stainless steel with low maintenance cost, mandatory pipe for fire protection and municipal projects.
Weather and acid-alkali resistance: Suitable for open-air, buried, humid workshop and saline-alkali environments.
Disadvantages
Matte silver-grey surface, less smooth and bright than electro-galvanized pipes.
High energy consumption during high-temperature galvanizing, higher unit price than electro-galvanized pipes.
Zinc coating will be damaged after welding, secondary anti-corrosion treatment required for welding joints.
5. Main Application Fields
Fire protection engineering (mandatory): Fire hydrant pipes, sprinkler pipelines, pump house main pipes (hot-dip galvanized pipes specified for fire acceptance).
Fluid transportation: Domestic potable water, medium-low pressure gas, air, steam, agricultural irrigation pipes.
Municipal infrastructure: Road guardrails, street lamp poles, bridge supports, buried water supply & drainage pipe networks.
Steel structure & scaffolding: Construction scaffolding, factory columns, protective railings, iron art works.
Power & communication: Cable protection sleeves, outdoor mechanical & electrical supports.
Mining & chemical industry: Mine support pipes, cooling heat exchange pipes for common media.
6. Core Comparison: Hot-dip Galvanized VS Cold Galvanized (Electro-galvanized) Steel Pipe
| Comparison Item | Hot-dip Galvanized Steel Pipe | Cold Galvanized (Electro-galvanized) Pipe |
|---|---|---|
| Coating Thickness | 50~100μm, full coverage on inner & outer walls | 5~20μm, extremely thin coating inside pipe |
| Bonding Mode | High-temperature metallurgical fusion, no delamination | Electrostatic adsorption, easy peeling off |
| Anti-corrosion Service Life | Over 10 years outdoors, usable underground | 2~3 years indoor, rust within 1 year outdoors |
| Policy Regulation | Mandatory for fire protection, gas and water supply pipelines | Forbidden for water supply and gas pipelines by regulations |
| Appearance | Matte silver grey with obvious zinc spangles | Mirror bright and smooth surface |
7. Purchasing & Acceptance Inspection Points
Surface inspection: Complete zinc coating, no missing galvanization, black spots, massive white rust or peeling.
Weighing standard: Weight of galvanized pipe = black pipe weight × galvanizing weight gain coefficient (coefficient closer to 1 for thicker walls).
Performance test: Qualified coating after 5 times copper sulfate immersion test without reddening.
Welding reminder: Polish welding positions and brush zinc-rich primer for secondary rust prevention.
Storage requirement: Keep rainproof and moisture-proof, avoid long-term ponding causing white rust.
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