Seamless pipe vs Welded pipe: Which one is more likely to rust?

Generally speaking, welded pipes are more likely to rust than seamless pipes, which is mainly attributed to the differences in their production processes and internal structures.

Seamless steel pipes are produced by processes such as cold drawing or cold rolling, and no welding is required during the entire process, so their internal structure and surface are relatively dense and smooth. This dense structure makes it difficult for water molecules, oxygen molecules, etc. to penetrate, thereby reducing the formation of internal rust. In addition, seamless steel pipes also have fewer structural defects during the production process, such as burrs, slag, pores, etc. These defects are usually the gathering places of corrosive substances such as water molecules and oxygen molecules, which contribute to the formation of rust.

In contrast, the production process of welded pipes requires welding processes, so some structural defects may be left inside the pipe and at the weld. These defects will not only become the gathering places of corrosive substances, but also may cause changes in the organization and properties of the material due to the heat-affected zone generated during the welding process, thereby reducing its corrosion resistance. In addition, oxidation and rust problems are also prone to occur at the weld, which accelerates its rusting speed.

However, it is worth noting that the corrosion resistance of seamless steel pipes and welded pipes is also affected by many other factors, such as material, use environment, surface treatment process, etc. Therefore, in practical applications, it is necessary to select the appropriate type of steel pipe according to specific use conditions and requirements.

1. When the material is the same (such as carbon steel)

Comparison items:

a. Weld seam influence
Seamless pipe: no weld seam, uniform overall
Welded pipe: There may be component segregation and residual stress in the weld seam area, increasing the risk of local corrosion

b. Inner wall oxidation
Seamless pipe: The inner wall is smoother after cold drawing/hot rolling and is not easy to accumulate dirt
Welded pipe: The inner wall of the welded pipe may have welding slag and burrs, which are easy to hide dirt and accelerate corrosion

c. Anti-corrosion treatment
Seamless pipe: The surface treatment (such as galvanizing, painting) is more uniform
Welded pipe: The coating at the weld may be uneven, and the anti-corrosion effect is slightly worse

Conclusion:
If the material is the same (such as Q235 carbon steel) and there is no anti-corrosion treatment, the welded pipe may be more prone to rust than the seamless pipe due to weld defects.
If galvanized, plastic coated or other anti-corrosion treatments are applied, the anti-rust performance of the two is similar.

2. The influence of different materials (stainless steel vs. carbon steel)

Stainless steel pipes (whether seamless or welded): corrosion resistance is much better than carbon steel, and the risk of rust is extremely low (unless in a chloride ion environment).
Carbon steel pipes (whether seamless or welded): easy to rust, need to rely on anti-corrosion coatings (such as galvanizing, epoxy resin).

Example:
304 stainless steel welded pipe is more rust-resistant than 20# carbon steel seamless pipe.
Galvanized welded pipe may be more rust-resistant than bare carbon steel seamless pipe.

Selection suggestions:

Ordinary environment (such as building structure):
If the budget is limited, choose galvanized welded pipe (high cost performance, good rust prevention).
If high reliability is required, choose seamless carbon steel pipe + anti-corrosion coating.

Corrosive environment (such as chemical industry, ocean):
Preferably use stainless steel seamless pipe (such as 316L) or duplex steel welded pipe.

High temperature and high pressure scenarios (such as boiler tubes):
Seamless pipes are more reliable (no weld weaknesses), but they need to be matched with high temperature resistant materials (such as alloy steel).

In general, although seamless steel pipes have more advantages than welded pipes in terms of corrosion resistance, both may rust under certain conditions (such as highly corrosive environments). Therefore, corresponding protective measures need to be taken during use to extend their service life.

Read more: ERW pipe vs Seamless pipe cost or China seamless steel pipe manufacturer