Specifications And Uses Of GI Pipes
Galvanized iron (GI) pipes are mild steel tubes coated with zinc, used mainly for water supply, plumbing, irrigation, borewells, fire protection and light structural work. This guide gives the classes, dimensions, standards and limits you need to choose the right pipe, including where GI is not the best choice.
The specifications below follow IS 1239 (Part 1) for the tube and IS 4736 for the zinc coating. Dimensions are nominal, so always confirm against your project specification and the current standard.
What is a GI Pipe?
A GI pipe is a mild steel pipe coated with zinc by hot-dip galvanizing. The zinc acts as a barrier against moisture, and it also corrodes in preference to the steel if the surface is scratched.
Despite the name, GI pipes are made of low-carbon mild steel, not iron. Three terms are often confused:
- GI pipe: mild steel pipe with a hot-dip zinc coating on the inside and outside.
- Black pipe: the same type of pipe without zinc, left with its dark mill finish and usually oiled or painted.
- MS pipe: a general term for mild steel pipe. It covers both black and galvanized pipe, so check whether a quote means plain or galvanized.
GI pipes carry water, air and other non-corrosive fluids, and they also serve as structural members in fencing, scaffolding and frames.
How are GI Pipes Manufactured?
GI pipes are made by forming and welding mild steel strips into tubes, testing them, and then dipping them in molten zinc.
- A low-carbon mild steel strip is selected and checked.
- The strip is roll-formed into a round section.
- The edges are joined by high-frequency electric resistance welding (ERW).
- The tube is sized, straightened and cut to length.
- Each tube is pressure-tested for strength and leakage.
- Tubes are cleaned (degreased, pickled and fluxed) and dipped in molten zinc, which coats the inside and outside surfaces.
- Coating mass, uniformity, adhesion and dimensions are inspected before dispatch.
Testing before galvanizing means leaks and weld defects are found before any zinc is applied. The coating checks that follow then confirm the finished tube.
Classes of GI Pipes
IS 1239 (Part 1) groups tubes into three classes by wall thickness, and thicker walls give more pressure capacity and corrosion margin.
| Class | Colour band | Typical uses |
|---|---|---|
| Light (Class A) | Yellow | Domestic plumbing, low-pressure internal water lines, small irrigation |
| Medium (Class B) | Blue | Commercial plumbing, municipal and rural water supply, irrigation, general engineering |
| Heavy (Class C) | Red | Borewells, industrial water lines, fire protection, higher-pressure and buried lines |
For buried lines, choose medium or heavy class and add external protection such as wrapping or coating.
Technical Specifications of GI Pipes
The tables below give the nominal dimensions of light and medium class tubes under IS 1239 (Part 1). Mass is the nominal mass of a plain-end black tube, so the galvanized tube weighs slightly more because of the zinc.
Light Class (Class A)
| NB (mm) | NB (inch) | Outside diameter (mm) | Wall thickness (mm) | Mass (kg/m) |
|---|---|---|---|---|
| 15 | 1/2 | 21.0 to 21.4 | 2.0 | 0.947 |
| 20 | 3/4 | 26.4 to 26.9 | 2.3 | 1.380 |
| 25 | 1 | 33.2 to 33.8 | 2.6 | 1.980 |
| 32 | 1 1/4 | 41.9 to 42.5 | 2.6 | 2.540 |
| 40 | 1 1/2 | 47.8 to 48.4 | 2.9 | 3.230 |
| 50 | 2 | 59.6 to 60.2 | 2.9 | 4.080 |
| 65 | 2 1/2 | 75.2 to 76.0 | 3.2 | 5.710 |
| 80 | 3 | 87.9 to 88.7 | 3.2 | 6.720 |
| 100 | 4 | 113.0 to 113.9 | 3.6 | 9.750 |
Medium Class (Class B)
| NB (mm) | NB (inch) | Outside diameter (mm) | Wall thickness (mm) | Mass (kg/m) |
|---|---|---|---|---|
| 15 | 1/2 | 21.0 to 21.8 | 2.6 | 1.210 |
| 20 | 3/4 | 26.5 to 27.3 | 2.6 | 1.560 |
| 25 | 1 | 33.3 to 34.2 | 3.2 | 2.410 |
| 32 | 1 1/4 | 42.0 to 42.9 | 3.2 | 3.100 |
| 40 | 1 1/2 | 47.9 to 48.8 | 3.2 | 3.560 |
| 50 | 2 | 59.7 to 60.8 | 3.6 | 5.030 |
| 65 | 2 1/2 | 75.3 to 76.6 | 3.6 | 6.420 |
| 80 | 3 | 88.0 to 89.5 | 4.0 | 8.360 |
| 100 | 4 | 113.1 to 115.0 | 4.5 | 12.200 |
| 125 | 5 | 138.5 to 140.8 | 5.0 | 16.600 |
| 150 | 6 | 163.9 to 166.5 | 5.0 | 19.800 |
Heavy Class (Class C)
Heavy class tubes have thicker walls than medium class at every size.
Other Specification Points
| Parameter | Requirement |
|---|---|
| Material and process | Mild steel, ERW welded |
| Length | Standard random length of 4 to 7 m; 6 m is the most common supplied length |
| Ends | Plain end, or threaded to BSP (IS 554) with sockets |
| Zinc coating | Hot-dip galvanized to IS 4736, minimum zinc mass 360 g/m² |
| Hydraulic test | 5 MPa (about 50 kgf/cm²) |
| Wall thickness tolerance | Minus 8% for light class; minus 10% for medium and heavy class |
| Mass tolerance (single tube) | Light class minus 8% to plus 10%; medium and heavy class plus or minus 10% |
| Identification | Colour band: yellow (light), blue (medium), red (heavy) |
Standards Followed by GI Pipes
In India, GI pipes are specified to IS 1239 (Part 1) for the tube and IS 4736 for the zinc coating.
- IS 1239 (Part 1): mild steel tubes for water, gas, air and steam. It sets classes, dimensions, tolerances and tests. Part 2 covers the matching fittings and sockets.
- IS 4736: hot-dip zinc coatings on mild steel tubes, covering coating mass, uniformity and adhesion.
- IS 554: pipe threads where pressure-tight joints are made on the thread.
- BS EN 10255 (formerly BS 1387): the British and European equivalent, with closely aligned light and medium series dimensions.
- ASTM A53: the US standard for black and hot-dipped zinc-coated pipe, specified by schedule rather than by light, medium and heavy class.
When buying, ask for the ISI mark and test certificates that cover zinc coating mass, hydrostatic testing and dimensions.
Common Uses of GI Pipes
GI pipes are used wherever a strong, weather-resistant steel pipe is needed for cold water and non-corrosive services.
- Water supply: residential, commercial and rural or municipal distribution lines, usually in medium class.
- Agricultural irrigation: field distribution, tube wells and pump connections.
- Plumbing: cold water lines in homes and commercial buildings, usually in light or medium class.
- Borewells: casing and rising mains, usually in heavy class.
- Fire protection: sprinkler and hydrant lines.
- Industrial services: water, compressed air and other non-corrosive fluids.
- Structural work: fencing, scaffolding, solar mounting structures and shed frames.
Key Benefits of GI Pipes
GI pipes combine strength, corrosion protection and easy installation at a moderate cost.
- Corrosion protection: the zinc coating covers the inside and outside and protects the steel even where the surface is scratched.
- Strength: the steel wall handles loads, impact and outdoor exposure better than most plastic pipes.
- Service life: in dry, non-aggressive conditions a well-coated line can last for decades. Aggressive or soft water, acidic or saline soil, and hot water can shorten that considerably. Coating mass, pipe class, water chemistry, temperature and installation quality all decide the outcome.
- Easy installation: standard lengths, threaded joints and widely available fittings.
- Low upkeep outdoors: no repainting is needed in many outdoor settings, unlike black pipe.
- Wide range of use: one material covers water, irrigation, fire lines and structural work.
Limitations: When GI is Not the Right Choice
GI pipe performs poorly in hot water, corrosive service and some long-term drinking water uses, so check these limits before specifying it.
- Hot water: the zinc layer protects less well as water temperature rises (above roughly 60 °C is commonly cited). CPVC, PPR or stainless steel suit hot water lines better.
- Drinking water over the long term: the inside surface can corrode and zinc can flake, causing rusty water, scale build-up and reduced flow. Many new domestic plumbing jobs now use CPVC, PPR or stainless steel. If you use GI for potable water, choose medium or heavy class, follow local requirements and test water quality.
- Corrosive fluids and environments: acids, strong alkalis, saline conditions and aggressive soils can attack zinc quickly. Choose another material or add linings and external protection.
- Threads and cut ends: threading and cutting remove zinc. Seal threaded joints and treat exposed steel with zinc-rich paint.
- Welding: heating galvanized steel releases zinc fumes. Weld in ventilated areas or with fume extraction and respiratory protection, grind back the coating at the weld, then recoat.
- Mixed metals: connecting GI to copper in a wet system can cause galvanic corrosion. Use dielectric unions or isolating fittings, and avoid placing copper upstream of GI.
GI Pipe vs MS Pipe vs Black Pipe
GI pipe is MS pipe with a zinc coating, and black pipe is the same steel without it. A quote that says "MS pipe" usually means plain black pipe, so confirm the finish before comparing prices.
| Feature | GI pipe | Black pipe (plain MS) |
|---|---|---|
| Base material | Mild steel, ERW | Mild steel, ERW |
| Surface | Zinc coating inside and outside | Uncoated, dark mill finish, often oiled or painted |
| Corrosion resistance | High in normal water and weather exposure | Low; rusts quickly in moisture unless painted or coated |
| Initial cost | Higher | Lower |
| Upkeep | Little for most outdoor use | Needs painting or coating and regular checks |
| Welding | Needs coating removal and fume control | Easier to weld |
| Best for | Water supply, irrigation, outdoor lines, fencing | Dry indoor lines, welded fabrication, work that will be painted or coated afterward |
How GI Compares with HDPE, PVC/CPVC and Stainless Steel
GI pipe wins on rigidity and outdoor durability, while plastics and stainless steel win on internal corrosion and hot water.
| Material | Best for | Strengths | Limits |
|---|---|---|---|
| GI pipe | Cold water mains and distribution, irrigation, fire lines, structural use | Strong and rigid, handles outdoor exposure and mechanical loads, threaded joints | Internal corrosion over time, heavier, poor for hot water |
| HDPE | Buried water mains, irrigation, cable ducts | Flexible, corrosion free, long continuous lengths, fused joints | Lower stiffness, needs support above ground, needs UV protection |
| PVC / CPVC | Domestic plumbing and drainage (PVC); hot and cold plumbing (CPVC) | Light, corrosion free, easy solvent-cement joining | Lower impact strength, UV degradation unless protected, PVC unsuitable for hot water |
| Stainless steel | Potable water, food and hygiene-critical lines, corrosive settings | Excellent corrosion resistance and long life | Highest cost, needs skilled installation |
The right choice depends on temperature, pressure, fluid, burial or exposure, and budget. Utkarsh India also supplies HDPE and PVC pipes, so we can help match the material to the duty.
Choosing the Right GI Pipe Manufacturer
A good supplier can show proof of conformity, not just a product list. Look for:
- BIS licence and ISI marking against IS 1239 (Part 1)
- Test certificates for zinc coating mass, hydrostatic pressure and dimensions
- A uniform, adherent coating with no bare patches at ends and threads
- A full range of classes, sizes and end types, including cut-to-length and threaded options
- Technical support for class selection and installation
- A track record on projects similar to yours
Utkarsh India manufactures GI pipes to IS 1239 for water, irrigation, fire protection and structural use. To discuss your requirement, download the product brochure or request a quote.
Conclusion
GI pipes remain a dependable choice for cold water, irrigation, fire protection and structural work when the class, coating and installation match the duty. Choose medium or heavy class for buried and high-demand lines, avoid GI for hot water and aggressive fluids, and buy from a manufacturer that can show IS 1239 and IS 4736 test results.
Get Insights On: 5 Features That Make GI Pipes Durable
Frequently Asked Questions
1. What is a GI pipe used for?
GI pipes are used for cold water supply, plumbing, irrigation, borewells, fire protection, industrial water and air lines, and structural work.
2. How long do GI pipes last?
Life depends on coating quality, water chemistry, temperature and installation. In dry, non-aggressive conditions they can last for decades, but aggressive water, soil or hot water can shorten that sharply.
3. What is the difference between GI, MS and black pipe?
GI pipe is mild steel with a zinc coating. Black pipe is mild steel without zinc, and "MS pipe" is a general term that can mean either, so confirm the finish. GI costs more upfront but needs less upkeep outdoors; black pipe is cheaper and easier to weld but must be painted or coated.
4. Can GI pipes be used for drinking water?
Yes, hot-dip galvanized pipe to IS 1239 is used for potable water, but the inside can corrode over time and affect water quality and flow. Choose medium or heavy class, follow local requirements and consider CPVC, PPR or stainless steel for new domestic plumbing.
5. Can GI pipes carry hot water?
They are not recommended for hot water, because the zinc layer protects less well as temperature rises. Use CPVC, PPR or stainless steel instead.
6. Which class of GI pipe should I choose?
Use light class for low-pressure internal plumbing, medium class for general water supply and irrigation, and heavy class for borewells, fire lines, buried pipes and higher pressures.