Black Iron Pipe

What is black iron pipe?

Black iron pipe refers to untreated welded steel pipe. It typically refers to straight-seam electric resistance welded (ERW) carbon steel pipes that have not undergone galvanizing and are used for transporting general low-pressure fluids such as water, gas, air, and oil.

Black iron pipe, also known as ordinary welded steel pipe, is a welded steel pipe formed by rolling and welding steel plates or strips. It is mainly used for transporting low-pressure fluids or gases. It is made of carbon steel and has not undergone rust-proofing treatment, hence its black surface. Black iron pipes are usually round, but can also be other shapes, such as square or rectangular. It is widely used in construction, plumbing, fire protection, and low-pressure industrial fields.

Difference between black steel pipe and black iron pipe: 

Black steel pipe includes all ungalvanized steel pipes (such as seamless structural pipes, boiler tubes, etc.), while black iron pipe specifically refers to welded steel pipes used for transporting low-pressure fluids.

Characteristics and Advantages of Black Iron Pipes:

1. Material and Performance

Typically made of common low-carbon steel (such as ASTM A53 GR.B or Q235), with moderate strength and excellent plasticity, toughness, and weldability.

Mechanical strength (tensile and compressive strength) is higher than most plastic pipes but lower than alloy steel pipes.

2. Surface and Corrosion Protection

The surface has a black oxide scale, providing some short-term rust prevention, but long-term exposure to humid environments will continue to cause rusting.

It lacks long-term corrosion resistance. Therefore, secondary anti-corrosion treatment (such as applying anti-rust paint, asphalt anti-corrosion layer, epoxy resin coating, etc.) is necessary before burying or exposing to the elements.

3. Manufacturing Characteristics

Mature manufacturing process: High-frequency resistance welding (ERW) production lines are highly efficient, automated, and produce large volumes, thus reducing manufacturing costs. However, as it is a welded pipe, there is a longitudinal weld seam, which is a structural weakness.

The pipe wall thickness is relatively uniform, with a complete range of sizes and lengths available for customization.

4. Economic Efficiency

Inexpensive, with mature manufacturing processes and low raw material costs, black iron pipes offer excellent value for money as a general-purpose pipeline. Considering the pipe itself, connectors, and installation labor, the price is far lower than stainless steel, copper, and alloy pipes. Black iron pipe systems are typically the lowest-cost metal piping solution.

Disadvantages of Black Iron Pipes:

1. Highly Susceptible to Corrosion and Rust

The black oxide scale on the surface is very thin, making it highly susceptible to corrosion and rust once damaged or exposed to prolonged humid environments.

2. Limited Pressure Bearing Capacity

As a welded pipe, its longitudinal weld seam is a weak point in terms of strength. Its yield strength and compressive strength are lower than seamless and alloy pipes. Therefore, it is only suitable for medium- and low-pressure systems (such as fire water, low-pressure gas, and heating) and cannot be used in high-pressure, high-temperature critical conditions.

3. Poor Hygiene

The rough inner wall is prone to rust and easily breeds bacteria, forming a biofilm. This is the main reason why it has been completely phased out in domestic water supply pipelines.

4. Relatively Inconvenient Installation

Traditional connection methods use threaded connections, requiring on-site threading, which is labor-intensive, inefficient, and demands high-quality sealing materials (hemp fiber, PTFE tape). The significant weight also increases the difficulty of handling and installation.

5. Rough Appearance

The untreated black surface is unsightly when installed exposed and clashes with modern architectural styles.

Applications of Black Iron Pipes:

Due to their economy and sufficient pressure resistance, black iron pipes have been widely used in the following areas, both in the past and present:

1. Building Water Supply and Fire Protection Systems
Fire Sprinkler Systems: Still the main type of pipe used for indoor fire sprinkler systems due to their low cost and ability to meet the pressure requirements of fire protection systems.

Water Supply and Drainage: Widely used in water supply and drainage systems in residential, commercial buildings, and industrial facilities. While previously used extensively for domestic water supply, they have been largely replaced by galvanized pipes, PPR pipes, and stainless steel pipes due to their susceptibility to rust and water pollution.

2. Low-Pressure Fluid Transportation
Gas and Natural Gas Transportation: Initially used for low-pressure gas pipelines, they are now mostly replaced by PE pipes and higher-standard steel pipes.
Heating pipes: Used for hot water transportation in heating systems.
Compressed air and ventilation ducts.

3. Protection and Building Structures
Temporary structural supports and scaffolding (but formal scaffolding now mostly uses specialized high-frequency welded pipes or seamless pipes).
Electric wire and cable protection pipes (conduit pipes).
Agricultural irrigation, factory drainage, and other applications where corrosion resistance requirements are not high.

How to Select and Maintain Black Iron Pipes?

When selecting suitable black iron pipes, factors such as size, thickness, and material need to be considered to meet specific application needs. For applications requiring frequent transportation of liquids or gases, black iron pipes with internal anti-corrosion coatings can be selected to reduce the risk of rust. The key to maintaining black iron pipes is regular inspection and cleaning, timely treatment of rust and corrosion problems, and implementation of necessary protective measures. At the same time, avoiding prolonged exposure to humid environments can extend the service life of black iron pipes.

Conclusion: 

Black iron pipe is the industry term for welded steel pipes used for low-pressure fluid transportation. It is economical, practical, technologically mature, and has good weldability, but external anti-corrosion treatment is necessary. Its core manufacturing process is resistance welding (ERW), and the most common materials are Q235B or ASTM A53 Gr. B. Although it has been replaced in the domestic water supply of new buildings, it is still one of the main pipe materials in fire protection systems, low-pressure industrial pipelines and other fields.

Read more: Black Iron Pipe vs. Black Steel Pipe or MS and GI Pipe Difference