GI (Galvanised Iron) rods and pipes are mild steel products coated with a thin protective layer of zinc through the hot-dip galvanising process. This zinc coating dramatically improves corrosion resistance, making GI products ideal for outdoor, plumbing, structural, and marine applications.
GI pipes in India are governed by IS 1239 — Steel Tubes, Tubulars and Other Wrought Steel Fittings, while the galvanizing process follows IS 4736 / IS 4759.
Galvanised iron is mild steel that has been coated with a zinc layer (typically 60–120 g/m²) by hot-dip galvanising or electroplating. The zinc layer protects the steel from rust through both barrier action and sacrificial (cathodic) protection.
Typical zinc coating thickness: 50–80 microns (350–560 g/m²) for structural GI; 30–40 microns for thin sections.
| Class | Wall Thickness | Identification Colour | Application |
|---|---|---|---|
| Light (Class A) | Thinnest | Yellow strip | Low pressure water supply, conduits |
| Medium (Class B) | Medium | Blue strip | Domestic water, sewage |
| Heavy (Class C) | Thickest | Red strip | High-pressure water, gas, fire fighting |
| Nominal Bore (NB) | OD (mm) | Wall Thk - Light (mm) | Wall Thk - Medium (mm) | Wall Thk - Heavy (mm) |
|---|---|---|---|---|
| 15 mm (1/2") | 21.3 | 2.0 | 2.6 | 3.2 |
| 20 mm (3/4") | 26.9 | 2.3 | 2.6 | 3.2 |
| 25 mm (1") | 33.7 | 2.6 | 3.2 | 4.0 |
| 32 mm (1-1/4") | 42.4 | 2.6 | 3.2 | 4.0 |
| 40 mm (1-1/2") | 48.3 | 2.9 | 3.2 | 4.0 |
| 50 mm (2") | 60.3 | 2.9 | 3.6 | 4.5 |
| 65 mm (2-1/2") | 76.1 | 3.2 | 3.6 | 4.5 |
| 80 mm (3") | 88.9 | 3.2 | 4.0 | 4.8 |
| 100 mm (4") | 114.3 | 3.6 | 4.5 | 5.4 |
| 150 mm (6") | 165.1 | 4.5 | 4.85 | 5.4 |
Standard length: 6 metres (with one end threaded with socket).
| Diameter (mm) | Length | Common Use |
|---|---|---|
| 6 mm | 6 m | Light fencing, ties |
| 8 mm | 6 m | Light grilles, brackets |
| 10 mm | 6 m | Heavy grilles, gates |
| 12 mm | 6 m | Anchor bolts, structural ties |
| 16 mm | 6 m | Anchor rods, foundation |
| 20 mm | 6 m | Heavy anchor bolts, support rods |
| 25 mm | 6 m | Heavy duty applications |
| Fitting | Use |
|---|---|
| Elbow (90°, 45°) | Change pipe direction |
| Tee | Branch off a main line |
| Cross | 4-way connection |
| Reducer / Bushing | Connect different pipe sizes |
| Socket / Coupling | Join two pipes of same size |
| Union | Allow disassembly |
| Nipple | Short pipe with threads on both ends |
| Cap / Plug | Close pipe end |
| Gate Valve / Globe Valve | Flow control |
| Check Valve / Foot Valve | One-way flow / pump suction |
| Flange | Bolt-on joint for large pipes |
| Property | Value |
|---|---|
| Tensile Strength (Min.) | 320 MPa |
| Yield Strength (Min.) | 195 MPa |
| Elongation (Min.) | 20% |
| Bend Test | Pass at 6D mandrel |
| Hydrostatic Test Pressure (Light) | 5 MPa (50 bar) |
| Hydrostatic Test Pressure (Heavy) | 7 MPa (70 bar) |
| Zinc Coating Mass | 350 – 500 g/m² (depends on class) |
Most common for GI pipes. BSP (British Standard Pipe) threads. Use Teflon tape or hemp+paste for sealing.
For large diameters or non-removable connections. Be careful as welding damages the galvanising near the weld zone.
For large diameter pipes (100 mm+). Bolt-on with gaskets.
Less common for GI; more for stainless or CPVC.
| Parameter | GI | CPVC | Copper |
|---|---|---|---|
| Cost | Moderate | Low to Moderate | High |
| Service Life | 25 – 40 years | 40 – 60 years | 50+ years |
| Corrosion Resistance | Good | Excellent (no corrosion) | Excellent |
| Joining | Threaded | Solvent cement | Solder / press fit |
| Hot Water | Limited | Yes (up to 93°C) | Excellent |
| Weight | Heavy | Light | Moderate |
| Fire Resistance | Excellent | Limited | Excellent |
| Use | Old buildings, conduits | Modern residential | Premium, commercial |
| Item | Size | Price per metre / per kg (Rs) |
|---|---|---|
| GI Pipe 15 mm Light | 1/2" | 120 – 160 per 6m length |
| GI Pipe 20 mm Medium | 3/4" | 180 – 240 per 6m length |
| GI Pipe 25 mm Medium | 1" | 260 – 350 per 6m length |
| GI Pipe 50 mm Heavy | 2" | 650 – 850 per 6m length |
| GI Pipe 100 mm Heavy | 4" | 1400 – 1800 per 6m length |
| GI Rods (general) | various | 90 – 120 per kg |
| Standard | Description |
|---|---|
| IS 1239 (Part 1) | Steel tubes for plumbing |
| IS 1239 (Part 2) | Wrought steel fittings |
| IS 4759 | Hot-dip zinc coating on structural steel |
| IS 4736 | Hot-dip zinc coating on wire products |
| IS 2629 | Recommended practice for hot-dip galvanising |
| IS 2633 | Methods for testing uniformity of zinc coating |
| IS 6911 | Stainless steel sheet (related) |
GI rods and pipes have served Indian construction for decades. While modern alternatives like CPVC and stainless steel have replaced GI for many plumbing applications, GI remains the preferred choice for structural pipework, fire-fighting lines, fencing, gates, anchor bolts, and high-strength outdoor applications.
Choose the correct class (Light/Medium/Heavy) based on the working pressure, conform to IS 1239, verify zinc coating thickness, and use proper threading and Teflon sealing for leak-proof joints.