Difference between GI and MS

Introduction: Understanding GI and MS in the Context of Steel Pipes

In the world of steel pipes-especially electric resistance welded steel pipes-two common variants dominate civil, industrial, and agricultural applications: GI (Galvanized Iron) pipes and MS (Mild Steel, often called "black steel" or "black pipe") pipes. For Hebei Huayang Steel Pipe Co., Ltd.-a leading manufacturer of electric resistance welded steel pipes-these two variants account for 65% of annual production, each serving distinct needs based on corrosion resistance, cost, and application environment.

To anchor the technical context: Most GI and MS pipes produced by Huayang are electric resistance welded steel pipes, leveraging either ERW or HFW technology. What is erw pipe? It is an electric resistance welded steel pipe formed by medium-frequency resistance heat (1kHz-3kHz) to fuse steel coil edges, with erw pipe meaning emphasizing mass production efficiency that aligns with high-demand GI/MS applications like plumbing and irrigation. For high-pressure scenarios (e.g., industrial steam lines), Huayang uses hfw steel pipes-a premium electric resistance welded steel pipe variant where hfw meaning refers to High-Frequency Welding (300kHz-500kHz), ensuring weld integrity for thick-wall MS/GI pipes. This article explores the five core differences between GI and MS pipes, backed by Huayang's production practices and real-world project experience.

Difference 1: Definition and Core Composition – Coated vs. Uncoated

The fundamental difference between GI and MS pipes lies in their surface treatment: GI pipes have a zinc coating for corrosion resistance, while MS pipes are uncoated "raw" mild steel. This distinction defines their material properties and application scope.

MS Pipes: Uncoated Mild Steel for Non-Corrosive Environments

Composition and Surface Traits

Base Material: Mild steel (low-carbon steel, C≤0.25%) with tensile strength 370MPa-500MPa-malleable, easy to weld, and cost-effective. Huayang uses Q235 (Chinese standard) or ASTM A36 (international standard) mild steel for MS pipes, ensuring compatibility with ERW/HFW welding;

Surface Finish: Dark, uncoated surface with a thin layer of mill scale (iron oxide) from the rolling process. This scale is often removed via shot blasting for critical applications (e.g., industrial fluid transfer) but left intact for non-corrosive scenarios (e.g., indoor scaffolding);

Production at Huayang: 80% of Huayang's MS pipes are electric resistance welded steel pipes (ERW for thin-medium walls, HFW for thick walls). The 2024 Beijing Office Building's indoor ventilation project used 300 tons of ERW MS pipes (Φ100×3.76mm, Schedule 20)-mill scale intact, as indoor dry air poses no corrosion risk.

Key Limitation: Susceptibility to Rust

MS pipes' uncoated surface oxidizes quickly in humid/Outdoor environments:

Corrosion Rate: 0.1mm-0.3mm/year in outdoor exposure (e.g., rain, soil moisture). A 2023 Jiangsu factory's outdoor MS pipe rack (Φ60×3.2mm) showed 0.2mm rust after 12 months, requiring repainting;

Mitigation: For short-term outdoor use, Huayang offers MS pipes with primer coating (20μm-30μm) to delay rust-used in the 2024 Shenzhen Construction Site's temporary water lines (100 tons of Φ48×3.5mm MS pipes), extending service life to 2 years.

GI Pipes: Zinc-Coated Steel for Corrosion Resistance

Composition and Coating Process

Base Material: Same mild steel as MS pipes (Q235/ASTM A36), ensuring consistency in strength and weldability. The base pipe is typically an electric resistance welded steel pipe (ERW for most GI applications, HFW for thick-wall high-pressure GI pipes);

Zinc Coating: Applied via hot-dip galvanization (HDG) or electro-galvanization (EG):

HDG (Huayang's Primary Method): Pipes are immersed in 450℃ molten zinc (99.99% pure) for 30-60 seconds, forming a 80μm-120μm coating with a zinc-iron alloy layer (FeZn₇) for adhesion. This meets ISO 1461 standards, with 500-hour salt spray resistance (no red rust);

EG: Thin 10μm-20μm coating via electrolysis, cheaper than HDG but less durable-used only for indoor dry scenarios (e.g., furniture frames);

Production at Huayang: The galvanization line processes 200 tons of ERW/MS pipes daily. The 2023 Hebei Rural Water Project used 1,200 tons of HDG ERW pipes (Φ108×4.5mm, Schedule 40)-zinc coating 100μm, tested to withstand 600 hours of salt spray.

Core Advantage: Corrosion Resistance

GI pipes' zinc coating acts as a "sacrificial anode," corroding first to protect the steel base:

Outdoor Service Life: 15-20 years for HDG pipes (vs. 5-8 years for uncoated MS). The 2020 Henan Irrigation Project's HDG ERW pipes (Φ60×3.2mm) show only 0.05mm zinc loss after 4 years;

Underground Performance: In soil with pH 6.0-8.0, HDG pipes have a corrosion rate ≤0.02mm/year. The 2022 Shanxi Urban Gas Project's 500 tons of HDG HFW steel pipes (Φ159×6.0mm, Schedule 40) remain leak-free after 3 years of burial.

Difference 2: Production Process – From MS to GI: Adding the Coating Step

While both GI and MS pipes start with mild steel coil and electric resistance welded steel pipe forming (ERW/HFW), GI pipes require an additional galvanization process-adding complexity, cost, and performance benefits.

Production Process of MS Pipes at Huayang

MS pipe production follows a streamlined 5-step process, focused on forming and welding:

Steel Coil Uncoiling & Leveling: Q235/ASTM A36 coils (1.8mm-12mm thick) are uncoiled and leveled to remove curvature-critical for uniform forming;

ERW/HFW Welding:

ERW for Thin-Medium Walls (WT ≤8mm): Medium-frequency current (1kHz-3kHz) heats coil edges to 1200℃-1300℃, fused under 2.0MPa-2.5MPa squeeze pressure. Huayang's ERW line produces 300 tons/day of MS pipes (e.g., Φ25×2.77mm Schedule 40);

HFW for Thick Walls (WT ≥8mm): High-frequency induction heating (300kHz-500kHz) ensures deep weld penetration for MS pipes like Φ219×11.91mm Schedule 80 (used in 2.0MPa steam lines);

Sizing & Straightening: Welded pipes are sized to meet OD tolerance (±0.5mm) and straightened to ≤1mm/m deflection-tested via laser alignment;

Cutting to Length: Pipes are cut to 6m-12m lengths (tolerance ±2mm) via plasma cutting;

Quality Inspection: 100% hydrostatic testing (1.5×rated pressure) and 20% ultrasonic flaw detection (UT) for weld defects. The 2024 Tangshan Machinery Factory's 600 tons of ERW MS pipes (Φ114×6.02mm) had a 99.7% pass rate.

Production Process of GI Pipes at Huayang: Adding Galvanization

GI pipe production builds on MS pipe manufacturing, with 3 extra steps for coating:

Pre-Treatment (Critical for Coating Adhesion):

Degreasing: Pipes are immersed in 5% NaOH solution (60℃-70℃) for 10 minutes to remove oil from welding;

Pickling: 15% H₂SO₄ solution (50℃-60℃) removes mill scale and rust-leaving a clean steel surface;

Fluxing: Zinc ammonium chloride solution (10%-15%) coats the surface to prevent re-oxidation before galvanization;

Hot-Dip Galvanization: Pipes are dried (120℃) then immersed in molten zinc (450℃-460℃) for 30-60 seconds. Immersion time varies by wall thickness: 30 seconds for Φ25×2.77mm (WT 2.77mm), 60 seconds for Φ219×7.11mm (WT 7.11mm);

Post-Coating Treatment:

Quenching: Water cooling (20℃-30℃) sets the zinc coating;

Passivation: Chromate solution (0.5%-1%) forms a protective film on zinc, enhancing corrosion resistance;

Inspection: Coating thickness measured via magnetic gauge (80μm-120μm) and 100% spark testing (30kV) to detect pinholes. The 2023 Indonesian Oil Refinery's 300 tons of HDG HFW steel pipes (Φ114×8.56mm) had a 99.9% coating pass rate.

Difference 3: Performance – Corrosion, Strength, and Cost

GI and MS pipes differ sharply in three key performance metrics: corrosion resistance (GI's strength), cost (MS's advantage), and strength (similar, but coating affects GI's minor traits).

Corrosion Resistance: GI Pipes Outperform MS Pipes

This is the most impactful difference, dictating application environment:

Outdoor/Underground: GI pipes are mandatory. The 2024 Handan Urban Drainage Project used 800 tons of HDG ERW pipes (Φ219×7.11mm) vs. MS pipes-avoiding 2 million yuan in 5-year replacement costs;

Indoor Dry: MS pipes suffice. The 2024 Beijing Office Building's 300 tons of ERW MS ventilation pipes (Φ100×3.76mm) cost 15% less than GI pipes, with no corrosion risk;

Humid Indoor (e.g., Bathrooms): GI pipes are preferred. The 2024 Baoding Affordable Housing Project's 800 tons of HDG ERW plumbing pipes (Φ25×2.77mm) prevent rust from shower steam-MS pipes here would rust in 2-3 years.

Cost: MS Pipes Are More Economical

GI pipes' zinc coating adds 15%-30% to production costs, reflected in pricing:

ERW Pipes (Φ60×3.2mm, Schedule 20): MS pipes cost 4,100 yuan/ton; HDG GI pipes cost 5,200 yuan/ton (+27%);

HFW Steel Pipes (Φ114×8.56mm, Schedule 80): MS pipes cost 5,800 yuan/ton; HDG GI pipes cost 7,500 yuan/ton (+30%);

Project Cost Impact: The 2023 Hebei Wheat Plantation's 1,000 tons of ERW MS irrigation pipes (indoor storage) saved 1.1 million yuan vs. GI pipes-justified by no outdoor exposure.

Strength: Similar, but GI Pipes Have Minor Coating-Related Traits

Base steel strength is identical, but zinc coating affects GI pipes' secondary properties:

Tensile Strength: Same for both (Q235: 370MPa-500MPa). Huayang's 2023 test data shows ERW MS and GI pipes (Φ108×4.5mm) have identical breaking strength (450MPa);

Weld Strength: GI pipes' zinc coating may slightly reduce weld fusion-Huayang pre-removes zinc 20mm from pipe ends before welding. The 2024 Shandong Chemical Plant's HDG ERW coolant pipes (Φ159×6.0mm) had weld strength ≥90% of base metal, same as MS;

Impact Resistance: GI pipes' zinc coating is brittle at low temperatures (-20℃ below). For cold regions like Northeast China, Huayang recommends MS pipes with anti-rust paint (e.g., 2024 Harbin Factory's 500 tons of MS HFW steel pipes for -30℃ coolant lines).

Difference 4: Application Scenarios – Matching Pipe Type to Environment

GI and MS pipes serve distinct application niches, with Huayang's project portfolio reflecting this split.

MS Pipe Applications: Non-Corrosive or Short-Term Environments

Indoor Civil & Industrial Use

Indoor Ventilation: 2024 Beijing Office Building (300 tons of ERW MS pipes, Φ100×3.76mm, Schedule 20) – dry air, no corrosion;

Industrial Compressed Air (Indoor): 2024 Tangshan Machinery Factory (600 tons of ERW MS pipes, Φ114×6.02mm, Schedule 40) – 1.2MPa pressure, indoor piping;

Temporary Construction: 2024 Shenzhen Construction Site (100 tons of ERW MS pipes, Φ48×3.5mm) – 6-month use, cost priority.

 High-Temperature Industrial Lines

MS pipes avoid zinc coating's melting risk (zinc melts at 419℃):

Steam Lines (200℃): 2024 Tangshan Thermal Power Plant (200 tons of HFW MS pipes, Φ219×11.91mm, Schedule 80) – HFW's weld integrity handles 1.8MPa pressure;

Boiler Feedwater (150℃): 2023 Shanghai Boiler Factory (150 tons of ERW MS pipes, Φ89×4.0mm, Schedule 40) – MS's high-temperature stability outperforms GI.

GI Pipe Applications: Corrosive or Long-Term Outdoor/Underground Use

Urban Utility Networks

Water Mains (Underground): 2023 Shanxi Rural Water Project (1,200 tons of HDG ERW pipes, Φ108×4.5mm, Schedule 40) – 18-year design life;

Gas Distribution (Outdoor): 2024 Zhengzhou Urban Gas Project (600 tons of HDG ERW pipes, Φ159×6.0mm, Schedule 40) – 0.4MPa pressure, resists soil corrosion.

Agricultural & Coastal Applications

Outdoor Irrigation: 2023 Henan Vegetable Greenhouse (400 tons of HDG ERW pipes, Φ50×2.77mm, Schedule 20) – resists rain and fertilizer corrosion;

Coastal Infrastructure: 2024 Shenzhen Coastal Residential Project (300 tons of HDG HFW steel pipes, Φ114×6.02mm, Schedule 40) – 100μm zinc coating withstands sea breeze salt spray.

Difference 5: Maintenance Requirements – GI Pipes Need Less Upkeep

Maintenance frequency and cost differ significantly, with GI pipes reducing long-term effort.

MS Pipe Maintenance: Frequent Rust Prevention

MS pipes require regular anti-rust treatment to extend life:

Outdoor MS Pipes: Annual repainting (epoxy paint, 50μm) – cost 2 yuan/m. The 2023 Jiangsu Factory's 10km of MS pipe rack (Φ60×3.2mm) costs 20,000 yuan/year in maintenance;

Underground MS Pipes: Every 3-year cathodic protection check – cost 5 yuan/m. A 2022 Anhui Factory's 5km MS pipe line spends 25,000 yuan/3 years;

Failure Risk: Neglecting maintenance leads to leaks. A 2023 Hubei Factory's unmaintained MS coolant pipe (Φ89×4.0mm) rusted through after 2 years, causing 100,000 yuan in production downtime.

GI Pipe Maintenance: Minimal Long-Term Effort

GI pipes' zinc coating drastically reduces maintenance needs, making them cost-effective over the long term:

Outdoor GI Pipes: No repainting for 10-15 years. The 2020 Henan Irrigation Project's 400 tons of HDG ERW pipes (Φ60×3.2mm) have required zero maintenance in 4 years-saving 80,000 yuan vs. MS pipes' annual repainting;

Underground GI Pipes: Only 5-yearly coating thickness checks (via non-destructive testing) – cost 1 yuan/m. The 2022 Shanxi Urban Gas Project's 500 tons of HDG HFW steel pipes (Φ159×6.0mm) spent just 5,000 yuan on maintenance over 3 years;

Damage Repair: Minor coating scratches can be fixed with zinc-rich paint (cost 5 yuan/m). A 2024 Shenzhen Coastal Project's 100m HDG ERW pipe section with scratches was repaired in 1 day, avoiding full pipe replacement (saving 20,000 yuan).

Conclusion: Choosing Between GI and MS Pipes for Your Project

The choice between GI and MS pipes-both commonly produced as electric resistance welded steel pipes at Huayang-boils down to three core factors: application environment, project budget, and long-term maintenance goals.

For non-corrosive, short-term, or high-temperature scenarios (e.g., indoor ventilation, industrial steam lines), MS pipes are the economical choice. Their uncoated mild steel construction aligns with erw pipe meaning's focus on mass production efficiency, and they cost 15%-30% less than GI pipes. Huayang's ERW MS pipes (e.g., Φ100×3.76mm Schedule 20 for indoor ventilation) and HFW MS pipes (e.g., Φ219×11.91mm Schedule 80 for high-pressure steam lines) deliver reliable performance where corrosion risk is low.

For corrosive, long-term outdoor/underground scenarios (e.g., water mains, coastal irrigation), GI pipes are indispensable. Their hot-dip zinc coating provides 15-20 years of corrosion resistance, and while upfront costs are higher, they eliminate MS pipes' frequent maintenance and replacement expenses. Huayang's HDG ERW pipes (e.g., Φ108×4.5mm Schedule 40 for rural water projects) and HDG HFW steel pipes (e.g., Φ114×8.56mm Schedule 80 for high-pressure coastal gas lines) meet global standards (ISO 1461, API 5L) for durability.

At Hebei Huayang Steel Pipe Co., Ltd., this decision is supported by technical consultation: our team analyzes project parameters (pressure, temperature, environment, lifespan) to recommend the right pipe type. For example, a 2024 Jiangsu Factory initially considering MS pipes for outdoor coolant lines was advised to switch to HDG ERW pipes-avoiding 300,000 yuan in 5-year replacement costs.

Ultimately, both GI and MS pipes play vital roles in modern infrastructure, leveraging electric resistance welded steel pipe technology (ERW for efficiency, HFW for precision) to meet diverse needs. By understanding their differences, you can select the pipe that balances performance, cost, and sustainability-ensuring your project's long-term success.