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Help CenterSteel Pipe & Tube & ERW Galvanized Pipe，Q195、Q215B、Q235A、Q235B、Q275B、SPHC、Q355B、Q355C、27SiMn
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1. Definition & Core Function
Galvanized steel pipe refers to ordinary welded or seamless steel pipe coated with a layer of zinc on the surface. Based on the sacrificial anode principle, zinc isolates air and moisture to deliver excellent rust and corrosion resistance. It is widely used for low-pressure fluid transportation, fire protection and steel structure construction.
Base material grades: Q235B, Q215BNational Standard: GB/T 3091-2015 Welded Steel Pipes for Low-Pressure Fluid Delivery
2. Two Main Categories (Key Distinction for Engineering Projects)
2.1 Hot-dip Galvanized Steel Pipe (Qualified Mainstream for Engineering, Applicable for Water Supply & Fire Protection)
Full name: Hot-dip Galvanized Steel Pipe, standard qualified pipe for construction projects
Production Process: Steel pipe pickling to remove oxide scale → flux coating & drying → dipping into molten zinc liquid at 450℃. Metallurgical reaction occurs between zinc and iron to form a composite coating consisting of iron-zinc alloy layer and pure zinc layer.
Zinc Coating Thickness: 50–100μm, coating weight 300–600g/㎡, uniform coating on inner and outer walls
Appearance: Natural spangled silver-gray matte surface with slight roughness
Advantages: Strong coating adhesion; resistant to large-area zinc peeling during cutting, threading and welding; suitable for outdoor and buried installation; service life of 15–30 years under normal environment
Applications: Fire sprinkler systems, municipal water supply, gas pipeline, heating system, steel structure, greenhouses, scaffolding
2.2 Cold Galvanized Steel Pipe (Electro-galvanized Pipe, Banned by Housing & Urban-Rural Development Ministry for Water Supply, Gas & Fire Protection)
Production Process: Electrolytic plating at normal temperature; zinc adheres to pipe surface physically without alloy layer
Zinc Coating Thickness: Only 5–20μm; extremely thin or even no coating on inner wall
Defects: Coating easily peels off; severe rust occurs within 1–3 years in humid environment, causing pipe blockage and equipment corrosion
Permitted Usage: Indoor dry temporary supports, cable conduits; strictly prohibited for pressure-bearing fluid pipelines such as water delivery, gas and fire protection
Comparison of Hot-dip & Cold Galvanized Pipe
| Comparison Item | Hot-dip Galvanized Pipe | Cold Galvanized Pipe |
|---|---|---|
| Coating Bonding | Metallurgical bonding, hard to peel | Physical adsorption, easy to fall off |
| Zinc Coating Thickness | 50–100μm | 5–20μm |
| Anti-rust Service Life | Over 15 years outdoors | 1–3 years in humid environment |
| Engineering Approval | General use for fire protection & water supply | Forbidden for pressure-bearing fluid pipelines |
| Surface Texture | Matte with zinc spangles | Super bright and smooth |
3. Specification System (National Standard Low-pressure Fluid Pipe)
Two marking methods: DN nominal diameter (widely used in engineering), outer diameter × wall thickness (for production & foreign trade marking)Classified into ordinary pipe and reinforced pipe (higher pressure resistance, mostly adopted for fire protection)
Common Specification Table
| Nominal Diameter DN | Inch Size | Outer Diameter (mm) | Ordinary Wall Thickness (mm) | Reinforced Wall Thickness (mm) |
|---|---|---|---|---|
| DN15 | 1/2" | 21.3 | 2.5 | 2.8 |
| DN20 | 3/4" | 26.9 | 2.5 | 2.8 |
| DN25 | 1" | 33.7 | 2.8 | 3.2 |
| DN32 | 1 1/4" | 42.4 | 2.8 | 3.2 |
| DN40 | 1 1/2" | 48.3 | 3.0 | 3.5 |
| DN50 | 2" | 60.3 | 3.2 | 3.8 |
| DN80 | 3" | 88.9 | 3.5 | 4.0 |
| DN100 | 4" | 114.3 | 4.0 | 4.5 |
| DN150 | 6" | 168.3 | 4.5 | 5.0 |
Pipe End Types: Plain end, threaded end, grooved end (mainstream for fire protection)
4. Main Applications
Fire Engineering: Automatic sprinkler and fire hydrant pipelines (mandatory hot-dip reinforced galvanized pipe)
Water Supply, Drainage & HVAC: Municipal tap water, community heating, low-pressure natural gas transportation
Steel Structure Fabrication: Greenhouse frames, scaffolding, guardrails, brackets
Industrial Fluid Transportation: Low-pressure air, oil delivery
Lining Composite Pipe Base: PE/EP lined galvanized pipe for chemical industry and water purification (to avoid secondary pollution)
5. Advantages & Disadvantages
Advantages
Far superior rust resistance compared with black steel pipe, long service life
High mechanical strength, pressure and impact resistance, convenient installation and maintenance
Good machinability for welding, threading and bending
Lower cost than stainless steel pipe, high cost performance
Disadvantages
Tiny zinc ion precipitation exists during long-term tap water delivery; PPR pipe is widely adopted for household drinking water at present
Zinc coating is corroded by strong acid and alkali; lining treatment is required for heavy corrosive working conditions
Low-quality cold galvanized pipe rusts easily; process identification is required during procurement
6. Key Identification Points for Procurement
Visual Inspection: Qualified hot-dip galvanized pipe has uniform zinc spangles without large-area missing coating and black spots; cold galvanized pipe surface is overly bright
Coating Test: Pass 5 times copper sulfate test without red discoloration for national standard qualified products
Document Check: Request material certificate and galvanized coating inspection report to confirm compliance with GB/T 3091
Scenario Matching: Only select hot-dip reinforced galvanized pipe for fire protection and water supply, reject cold galvanized pipe
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