How are corrosion‑resistant spiral steel pipes for drainage protected against corrosion?

With the continued development of China’s economy, the state has been vigorously promoting the energy sector. Long-distance oil and gas pipelines are a crucial means of ensuring energy security. During the anti-corrosion construction of oil (and gas) pipelines, the treatment of corrosion‑resistant steel pipes is one of the key factors determining the service life of the pipeline’s protective coating; it is also the prerequisite for achieving a strong bond between the coating and the pipe. Therefore, it is essential to strictly adhere to the specifications governing the surface preparation of steel pipes, continuously explore and refine best practices, and constantly improve the methods used to treat corrosion‑resistant steel pipes. Today, Nanning Canghai Steel Pipe Co., Ltd. will introduce the anti-corrosion treatment procedures for spiral‑welded steel pipes used in drainage applications.

　　I. Anti-Corrosion Process for Water Service Steel Pipes

　　1. Cleaning: Solvents and emulsions are used to clean the steel surface, removing oil, grease, dust, lubricants, and similar organic contaminants. However, they cannot eliminate rust, scale, welding flux, or other surface imperfections; therefore, in corrosion‑prevention processes, they serve only as an auxiliary step.

　　2. Manual rust removal primarily involves using tools such as wire brushes to grind the steel surface, thereby removing loose or flaking scale, rust, weld spatter, and other contaminants.

　　The manual surface treatment of spiral steel pipes can achieve a Sa2 grade, while power‑tool blast cleaning can reach Sa3. However, if the steel surface is covered with tightly adherent iron oxide scale, tool‑based rust removal will be ineffective and fail to attain the required anchor profile depth for anti‑corrosion application.

　　3. Pickling is typically performed using either chemical or electrolytic methods; for pipeline corrosion protection, only chemical pickling is employed. It can remove scale, rust, and old coatings, and is sometimes used as a post‑treatment following sandblasting to remove rust.

　　Although chemical cleaning can achieve a certain level of surface cleanliness and roughness, the resulting anchor profile is shallow and it is prone to environmental pollution.

　　4. Blast (or shot) cleaning: Blast (or shot) cleaning involves using a high‑power motor to drive blast wheels at high speed, which propel abrasive materials such as steel grit, steel shot, wire segments, and mineral abrasives onto the pipe surface under centrifugal force. This process not only thoroughly removes rust, oxides, and contaminants but also imparts the desired uniform surface roughness through the intense impact and friction of the abrasive media. Following blast (or shot) cleaning, the pipe’s surface exhibits enhanced physical adsorption, while the mechanical adhesion between the anti‑corrosion coating and the pipe surface is significantly improved. Consequently, blast (or shot) cleaning represents an ideal method for preparing spiral‑welded and other types of corrosion‑resistant steel pipes prior to coating.

　　II. Properties of Spiral-Welded Steel Pipes for Water Applications

　　If the gap is too small, the proximity effect increases, leading to excessive heat input and burn‑through of the weld; alternatively, after extrusion or roll‑forming, deep pits may develop, degrading the weld’s surface quality. Once the two edges of the tube blank have been heated to the welding temperature, they are subjected to the compressive action of the squeeze rolls, causing the metal grains at the interface to interpenetrate and recrystallize, ultimately forming a strong weld. If the squeezing force is insufficient, fewer shared crystals are formed, reducing the weld metal’s strength and increasing the likelihood of cracking under load; conversely, excessive squeezing can cause the material to enter a molten state.

　　Using spiral welded steel pipe, the strip steel is fed into the welded pipe mill. As it passes through multiple sets of rolling rolls, the strip gradually curls up to form a circular tube blank with an open seam. The penetration of the squeeze rolls is adjusted to maintain the weld gap within 1–3 mm and ensure that both ends of the weld are flush.

　　If the gap is too large, the proximity effect is reduced, resulting in insufficient eddy‑current heating and poor intergranular bonding in the weld, which can lead to lack of fusion or cracking.

　　If the gap is too small, the proximity effect intensifies, leading to excessive heat input and burn‑through of the weld; alternatively, after extrusion or roll‑forming, the weld may develop deep depressions, compromising its surface quality.

　　After the two edges of the tube billet are heated to the welding temperature, they are pressed together by the squeeze rolls, causing the metal grains at the interface to interpenetrate and recrystallize, ultimately forming a strong weld.

　　If the extrusion pressure is too low, fewer intergranular crystals form, resulting in reduced weld metal strength and susceptibility to cracking under load. Conversely, excessive extrusion pressure can force molten metal out of the weld joint, not only diminishing weld strength but also generating substantial internal and external burrs and potentially leading to defects such as incomplete fusion.