Galvanized Carbon Steel Pipe, Galvanized Iron (GI) Pipe

What is Galvanized carbon steel pipe?

Galvanized carbon steel pipe (GI Pipe) is an anti-corrosion steel pipe with a layer of zinc coated on the surface of ordinary carbon steel pipe (seamless and welded pipe), which is achieved by hot-dip galvanizing or electro-galvanizing process. The zinc layer combines with the steel pipe matrix to form a physical barrier and electrochemical protection, which can effectively isolate the steel pipe from contact with air and water, prevent rust, significantly improve corrosion resistance, and extend service life.

Galvanized steel pipes are widely used. In addition to being used as pipeline pipes for general low-pressure fluids such as water, gas, and oil, they are also used as oil well pipes and oil pipelines in the petroleum industry, especially offshore oil fields, oil heaters, condenser coolers, coal distillation and washing oil exchangers for chemical coking equipment, and pier pipe piles and support frames for mine tunnels.

Features of galvanized carbon steel pipes:

1. Double anti-corrosion
Physical isolation (zinc layer blocks air/water) + electrochemical protection (zinc preferentially corrodes and protects iron).

2. Economical and practical
The cost is only 1/3~1/5 of that of stainless steel pipes, and the anti-corrosion life can reach 20~50 years (normal environment).

3. Convenient processing
Can be welded, cut, and bent, but high-temperature welding requires zinc supplementation (such as cold-dip galvanizing spray).

4. Surface characteristics
Silver-gray zinc layer, smooth and beautiful, suitable for exposed installation.

5. Limitations
The zinc layer is not resistant to strong acid (pH<6), strong alkali (pH>12) or high-salt environment (such as seawater).

Production process types of galvanized carbon steel pipes:

The galvanizing process is mainly divided into three categories: hot-dip galvanizing, electro-galvanizing and cold-dip galvanizing. Each process has significant differences in anti-corrosion performance, cost and application scenarios.

1. Hot-dip galvanized pipe

Hot-dip galvanizing is an anti-corrosion coating technology that forms a zinc-iron alloy layer on the surface of steel through a high-temperature galvanizing process. It is widely used in construction, transportation, energy and other fields.

Hot-dip galvanizing is to pickle the steel pipe first. In order to remove the iron oxide on the surface of the steel pipe, after pickling, it is cleaned in an ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to the hot-dip galvanizing tank. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion and long service life.

Hot-dip galvanizing process flow:
1) Surface treatment
Degreasing (alkali washing) → pickling (hydrochloric acid to remove oxide scale) → plating (ammonium zinc chloride solution to prevent secondary oxidation).

2) High-temperature zinc immersion
The pretreated steel parts are immersed in molten zinc liquid (445~465℃), and iron-zinc reaction occurs to form a zinc-iron alloy layer.

3) Cooling and forming
After taking out, air cooling or water cooling is performed to form a dense coating on the surface.

2. Electrogalvanized pipe (power is required)

Electrogalvanizing is an anti-corrosion technology that uses an electrolytic process at room temperature to deposit a zinc layer on the surface of the steel pipe by zinc ions in the electrolyte. It is widely used in precision parts, electronic components and other fields.

Electrogalvanizing process flow:
1) Pretreatment
Degreasing (alkaline cleaning) → pickling (removing oxides) → activation (improving adhesion).

2) Electroplating process
The steel pipe is immersed in an electrolyte containing zinc salt (such as ZnSO₄) as the cathode. After passing direct current, zinc ions (Zn²⁺) are reduced and deposited on the surface.

3) Post-treatment
Passivation (chromate treatment) → drying (enhancing corrosion resistance).

3. Cold galvanized pipe (no power required)

Cold galvanizing is an anti-corrosion technology that forms a zinc protective layer on the metal surface through a room temperature coating process. Compared with traditional hot-dip galvanizing and electrogalvanizing, it has the characteristics of flexible construction and low cost.

Cold galvanizing process flow:
Surface treatment:
Sandblasting/grinding (Sa2.5 cleanliness) → degreasing (solvent cleaning).

Zinc layer coating:
Use zinc-rich paint (containing zinc powder ≥96%) by spraying, brushing or dipping.

Curing method:
Self-curing at room temperature (1~24 hours) or low-temperature baking (accelerated curing at 60~80℃).

Application areas of galvanized carbon steel pipes:

Galvanized carbon steel pipes are widely used in the following fields due to their anti-corrosion performance, economy, and easy processing.

1. Construction and municipal engineering
Hot-dip galvanized steel pipes are widely used in building water and electricity installation, fire protection pipes and municipal water supply and drainage systems.
For example, the laying of fire protection lines in large commercial complexes must use galvanized steel pipes to ensure circuit stability. Data from urban water supply network reconstruction projects show that the service life of underground pipe networks using hot-dip galvanized steel pipes can reach 30 years.

2. Industry and machinery manufacturing
The petrochemical industry often uses thickened galvanized seamless pipes to transport steam, compressed air and other media. In sulfur-containing environments, its cost-effectiveness is higher than that of stainless steel pipes;
Automobile production line equipment frames and automated machine tool guardrails mostly use galvanized round pipes. Electrogalvanized pipes are particularly suitable for precision industrial equipment because of their smooth surface (roughness ≤1.6μm). ‌

Cable protection tube: Electro-galvanized thin-walled tube is used for indoor electrical wiring, and its fire resistance is better than plastic tube.
Ventilation duct: Galvanized spiral duct (thickness 0.5~1.2mm) is resistant to mild corrosion and easy to install.

3. Agriculture and water conservancy projects
Galvanized steel pipes are used to build arch sheds for greenhouse planting, with a wind pressure resistance of 0.5kN/m² and a service life that is 5-8 years longer than ordinary black pipes;
N80 grade galvanized steel pipes are commonly used in water conservancy irrigation systems, and their zinc layer adhesion is ≥275g/m², which can withstand pH6-12 environmental corrosion. ‌

4. Automobile manufacturing
Body panels (doors, hoods) are made of electro-galvanized steel plates, and the surface roughness is ≤5μm to meet the requirements of mirror coating;
The hot-dip galvanized bracket of the suspension system can withstand 500,000 fatigue tests.

5. Traffic facilities
Road guardrails & street lamp poles: hot-dip galvanizing + spray-coated double-layer protection, excellent weather resistance.
For example, highway corrugated beam guardrails.

Restrictions on the use of galvanized steel pipes:

1) Restrictions on hot-dip galvanized steel pipes

High-temperature steam pipes (＞200℃):
Reason: The zinc layer reacts with the iron matrix at high temperature to form a brittle zinc-iron alloy, resulting in peeling (the melting point of zinc is 419℃ and the boiling point is 907℃). The long-term working temperature of the zinc layer is ≤200℃.

Alternative solution:
Seamless steel pipe (20#/15CrMo) + high-temperature resistant coating (such as aluminum silicon coating).

Strong acid/strong alkali medium environment:
Reason: pH <4 (such as hydrochloric acid, sulfuric acid): The zinc layer dissolves quickly (corrosion rate >1mm/year); pH >12 (such as sodium hydroxide): Zinc undergoes "alkali embrittlement" cracking. Galvanized pipes are prohibited from being used for acid and alkali transportation.

Seawater or high salt fog environment:
Reason: Chloride ions (Cl⁻) penetrate the zinc layer to cause pitting corrosion, and the service life is only 3~5 years (up to 50 years in fresh water environment). Offshore platforms use duplex stainless steel or nickel-based alloys.

Direct contact with media in the food/pharmaceutical industry:
Reason: Zinc ion (Zn²⁺) migration may contaminate the product.

2) Restrictions on electrogalvanized steel pipes

Building water supply system:
Electrogalvanized steel pipes are prohibited from being used in drinking water systems (only hot-dip galvanized steel pipes are allowed).
Reason: The electrogalvanized layer is thin (5~30μm) and is prone to corrosion and peeling, resulting in excessive zinc ions. Due to insufficient anti-corrosion ability and water pollution risks, it is prohibited from being used in drinking water and gas systems.

Gas pipelines:
Electrogalvanized pipes are prohibited for buried gas pipelines (PE pipes or 3PE anti-corrosion steel pipes are required).

Allowed use scenarios:
Electrical threading pipes: such as JDG electrogalvanized steel pipes, used for wiring in dry environments.
Furniture/decoration: non-load-bearing structural parts (such as display racks, lamp brackets).

3) Restrictions on cold-dip galvanized steel pipes

Permanent building structures:
Cold-dip galvanizing (zinc-rich coating) is not allowed to be used as the anti-corrosion layer of the main structure (such as beams and columns of steel structure factories) due to its insufficient durability (5-10 years).
Due to the poor durability of cold-dip galvanized steel pipes, it is prohibited to be used as the anti-corrosion layer of permanent structures. The main load-bearing components must use hot-dip galvanizing or heavy anti-corrosion coatings.

Water pipelines:
The cold-dip galvanized coating has poor adhesion and is easy to fall off after long-term immersion. It is prohibited to be used in drinking water systems.

Permitted use scenarios:
Temporary facilities: temporary enclosures and scaffolding on construction sites (regular repainting is required).
Maintenance projects: local repair of hot-dip galvanized layers after damage (such as bridge bolts).

Hot-dip galvanizing vs. electro-galvanizing: How to choose?

Choose hot-dip galvanizing:
Require long-term outdoor corrosion protection (such as high-voltage transmission towers).
Understand mechanical impact or wear (such as mining equipment).

Choose electro-galvanizing:
Precision small parts (such as bolts, electronic components).

Short-term anti-corrosion + aesthetics (such as furniture accessories).

Read more: Galvanized ERW pipe or Guide for Selecting the Thickness of Galvanized Steel Pipe