Surface treatment methods of high-frequency welded steel pipes

With the continuous advancement of China’s economy, the nation has vigorously promoted the development of the energy industry. Long-distance API oil and gas pipelines play a crucial role in ensuring energy security. In the anti-corrosion construction process of oil and gas pipelines, the surface treatment of high-frequency welded steel pipes is one of the key factors affecting the service life of the anti-corrosion coating. It is the fundamental condition for achieving strong adhesion between the coating and the steel pipe.

Super Steel Manufacturing Co.,Ltd is professional pipe flange manufacturer, for more details, please contact:sales@super-steels.com

According to research, the lifespan of the anti-corrosion layer is influenced by factors such as coating type, construction quality, and environmental conditions. Among these, surface treatment of the steel pipe accounts for approximately 50% of the impact. Therefore, it is essential to strictly adhere to the technical specifications for the surface of thick-walled steel pipes and to continually explore and refine surface treatment methods to improve coating performance and durability.

1. Cleaning

Solvents and emulsions are used to remove oil, grease, dust, and other organic contaminants from the steel surface. However, this method cannot remove rust, scale, or welding slag and is generally used only as a supplementary process in anti-corrosion treatment.

2. Mechanical Rust Removal

Tools such as wire brushes are used to clean the steel surface, removing loose scale, rust, and welding residues. Manual cleaning can reach a Sa2 level, while powered tools can achieve Sa3. However, this method is less effective against firmly attached scale and cannot provide the required anchor profile depth for coating adhesion.

3. Pickling

Chemical pickling (commonly used over electrolytic methods in anti-corrosion processes) can remove oxide scale, rust, and old coatings. It is sometimes used as a secondary treatment after abrasive blasting. While it achieves a certain level of cleanliness and roughness, it produces a relatively shallow anchor profile and poses environmental risks.

4. Abrasive (Blasting) Rust Removal

Abrasive blasting is considered the most effective method for rust removal. A high-power motor drives a rotating blade to propel steel grit, steel shot, wire segments, or mineral abrasives onto the pipe surface via centrifugal force. This process not only removes rust, oxides, and dirt but also creates a uniform surface roughness, significantly enhancing the physical and mechanical adhesion between the steel pipe and the anti-corrosion coating.

4.1 Rust Removal Grade

For commonly used coatings such as epoxy, polyethylene, and phenolic resins, the steel surface should reach a near-white metal finish (Sa2.5). This level effectively removes all rust and contaminants and produces an anchor profile depth of 40–100μm, which meets adhesion requirements. Abrasive blasting can achieve this level efficiently, with lower costs and consistent quality.

4.2 Abrasive Selection

The choice of abrasive should consider the steel surface hardness, the degree of corrosion, required roughness, and the type of coating. For single-layer epoxy, two-layer, or three-layer polyethylene coatings, a mix of steel grit and steel shot is ideal. Steel shot strengthens the surface, while steel grit provides etching. A mix (with steel shot hardness of 40–50 HRC and steel grit at 50–60 HRC) is effective even on heavily rusted surfaces.

4.3 Abrasive Particle Size and Ratio

Proper abrasive size and distribution are essential for achieving uniform cleanliness and roughness. Excessive roughness can thin the coating at anchor peaks and lead to bubbles; insufficient roughness reduces adhesion and impact resistance. For deep pitting, large particles are not enough—smaller particles must also be used to polish off corrosion. A balanced ratio reduces wear on the pipe and equipment while maximizing abrasive efficiency. Typically, steel shot particles range from 0.8–1.3 mm, steel grit from 0.4–1.0 mm, with 0.5–1.0 mm as the main component. The optimal grit-to-shot ratio is around 5–8. However, due to the higher breakage rate of grit, continuous sampling and adjustment are necessary during operation.

4.4 Rust Removal Speed

The speed of rust removal depends on the abrasive type and volume. Abrasives with lower wear rates are preferred to improve cleaning speed and extend the life of the blades.

4.5 Cleaning and Preheating

Before blasting, the steel pipe should be cleaned to remove oil and scale, then preheated to 40–60°C. A dry surface enhances the blasting effect, improves abrasive separation from rust and scale, and results in a cleaner surface post-treatment.

In practice, we place great importance on surface treatment and strictly control the parameters during the rust removal process. This ensures that the peel strength of the anti-corrosion coating on the steel pipe far exceeds standard requirements. By refining the process without additional equipment investment, we have significantly improved both quality and efficiency while reducing production costs.