Galvanized seamless pipe vs. Galvanized welded pipe

What is galvanized steel pipe?

Galvanized steel pipe (GI Pipe) is an anti-corrosion steel pipe with a layer of zinc (Zn) on the surface of ordinary steel pipe (carbon steel or low alloy steel). The zinc layer can effectively isolate air and moisture, prevent the steel pipe from rusting, and extend the service life. Galvanized steel pipes are divided into galvanized seamless steel pipes and galvanized welded steel pipes according to the base pipe type.

Galvanized seamless pipe: The steel pipe is made of heated perforated, hot rolled or cold drawn steel billets and then galvanized. It has no welds, is uniform as a whole, has strong pressure bearing capacity, and is suitable for high pressure and high stress environments. The seamless pipe process is complex and the cost is high. The diameter is mostly small-diameter thick-walled pipe.

Galvanized welded pipe: The steel plate is curled and welded into a pipe (ERW, SAW, etc.) and then galvanized. The pipe body has a weld and has a low pressure bearing strength, which is suitable for medium and low pressure scenarios. Welded pipes have high production efficiency and low cost, and are often used to manufacture large-diameter thin-walled pipes.

Galvanized seamless pipe vs. Galvanized welded pipe: How to choose?

The choice of galvanized seamless pipe or galvanized welded pipe (ERW) should be based on the specific application scenario, performance requirements and budget. The details are as follows:

(1) Different application scenarios

Preferential selection of seamless pipe:
High-pressure system: such as hydraulic oil pipe, boiler pipe, high-temperature steam pipe.
High mechanical stress: automobile transmission shaft, aerospace parts.
Precision instruments: occasions requiring uniform wall thickness and no defects.

Preferential selection of welded pipe:
Low-pressure fluid transportation: building water supply and drainage, HVAC (galvanized welded pipe is sufficient).
Structural support: scaffolding, guardrail, cable casing (low cost, easy to process).
Large diameter requirements: such as ventilation ducts (welded pipes can produce larger diameters).

(2) Corrosion resistance

The galvanizing process of both is the same (hot-dip galvanizing or electro-galvanizing). The difference in corrosion resistance mainly depends on the galvanizing quality and galvanizing layer thickness, and has nothing to do with seamless/welding.
Note: If the weld seam of the welded pipe is not properly treated, such as the zinc layer is not evenly covered, it may become a corrosion weakness.

(3) Budget and construction period

Seamless pipes are cost-effective: suitable for projects with sufficient budget and performance priority.
Welded pipes are cost-effective: the advantages are obvious when purchasing in large quantities, and the delivery cycle is shorter.

Notes:
Inspection requirements: Seamless pipes need to pay attention to internal defects (such as ultrasonic testing), and welded pipes need to check the quality of the weld seam (such as X-ray flaw detection).

Other classification methods:

Galvanized steel pipes can also be divided into hot-dip galvanized steel pipes and electro-galvanized steel pipes (cold-dip galvanized steel pipes) according to the galvanizing process. Among them, hot-dip galvanized steel pipes are more widely used, while electro-galvanized steel pipes are only used in specific scenarios and are relatively limited.

Hot-dip galvanized steel pipes: mainstream choice, strong corrosion resistance, suitable for outdoor, humid or high-demand environments.
Electro-galvanized steel pipes: limited to low-corrosion, low-cost scenarios, gradually replaced by hot-dip galvanizing.

If there are specific application scenarios (such as chemical industry and marine environment), the thickness of the zinc layer or special coating (such as epoxy resin + galvanized composite pipe) needs to be further considered.

Common misunderstandings clarified:

Misunderstanding 1: "Seamless pipes must be more corrosion-resistant than welded pipes"
Wrong. Corrosion resistance depends on the galvanizing process and the thickness of the zinc layer, and has nothing to do with whether it is welded.

Misunderstanding 2: "Welded pipes have low strength and cannot withstand pressure"

Not entirely correct. High-quality welded pipes (such as ERW high-frequency welded pipes) can achieve higher pressure-bearing capacity through process optimization.

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