Steel pipe surface treatment methods for pipeline anti-corrosion operations

With the continuous development of my country's economy, the country is vigorously developing the energy industry. Long-distance oil and gas pipelines are an important way to ensure energy security. During the anti-corrosion construction process of oil (gas) pipelines, the surface treatment of steel pipes is a key factor in determining the anti-corrosion service life of the pipeline. It is the prerequisite for whether the anti-corrosion layer and the steel pipe can be firmly combined. It has been verified by research institutions that, in addition to factors such as coating type, coating quality, and construction environment, the surface treatment of steel pipes accounts for about 50% of the impact on the life of the anti-corrosion layer. Therefore, the anti-corrosion layer specifications should be strictly followed. The requirements for the surface of steel pipes are constantly explored and summarized, and the surface treatment methods of steel pipes are constantly improved.

1. Cleaning: Use solvents and emulsions to clean the steel surface to remove oil, grease, dust, lubricants, and similar organic matter. However, it cannot remove rust, oxide scale, soldering flux, etc. on the steel surface, so it is only used in anti-corrosion production. as an auxiliary means.

2. Tool rust removal: Mainly use tools such as wire brushes to polish the steel surface to remove loose or lifted oxide scales, rust, welding slag, etc. The rust removal of hand tools can reach the Sa2 level, and the rust removal of power tools can reach the Sa3 level. If the iron oxide scale is firmly attached to the surface of the steel, the rust removal effect of the tool will not be ideal and the anchor pattern depth required for anti-corrosion construction will not be achieved.

3. Pickling: Generally, chemical and electrolytic methods are used for pickling treatment. Only chemical pickling is used for pipeline anti-corrosion, which can remove scale, rust, and old coatings. Sometimes it can be used as a reprocessing after sandblasting and rust removal. Although chemical cleaning can achieve a certain degree of cleanliness and roughness on the surface, its anchor lines are shallow and it can easily cause environmental pollution.

4. Spray (throw) rust removal: Spray (throw) rust removal uses a high-power motor to drive the spray (throw) blade to rotate at high speed, so that steel sand, steel shots, wire segments, minerals, and other abrasives are removed under the action of centrifugal force. Spraying (throwing) the surface of the steel pipe can not only completely remove rust, oxides, and dirt, but also the steel pipe can achieve the required uniform roughness under the action of violent impact and friction of abrasives. After spraying (throwing) rust removal, it can not only expand the physical adsorption on the pipe surface but also enhance the mechanical adhesion between the anti-corrosion layer and the pipe surface. Therefore, spraying (throwing) rust removal is an ideal rust removal method for pipeline anti-corrosion. Generally speaking, shot blasting (sand) rust removal is mainly used for the inner surface treatment of pipes, and shot blasting (sand) rust removal is mainly used for the outer surface treatment of pipes. Several issues should be paid attention to when using spray (throwing) to remove rust.

4.1 Rust removal level: For the construction technology of epoxy, vinyl, phenolic, and other anti-corrosion coatings commonly used for steel pipes, the surface of the steel pipe is generally required to reach the nearly white level (Sa2.5). The practice has proved that using this level of rust removal can remove almost all oxide scale, rust, and other dirt. The depth of the anchor pattern reaches 40~100μm, which fully meets the adhesion requirements between the anti-corrosion layer and the steel pipe. The rust process can achieve near-white level (Sa2.5) technical conditions with lower operating costs and stable and reliable quality.

4.2 Spray (throw) abrasive: To achieve the ideal rust removal effect, the abrasive should be selected according to the hardness of the steel pipe surface, the original rust degree, the required surface roughness, the coating type, etc. For single-layer epoxy, two-layer epoxy, etc. One or three layers of polyethylene coating, using a mixed abrasive of steel sand and steel shot to more easily achieve the ideal rust removal effect. Steel shot has the function of strengthening the steel surface, while steel grit has the function of etching the steel surface. Mixed abrasives of steel grit and steel shot (usually the hardness of steel shot is 40 to 50 HRC, and the hardness of steel grit is 50 to 60 HRC) can be used on various steel surfaces, even on grade C and D rusted steel surfaces. , the rust removal effect is also very good.

4.3 Particle size and ratio of abrasives: To obtain better uniform cleanliness and roughness distribution, the particle size and ratio design of abrasives are very important. Too much roughness will easily cause the anti-corrosion layer to become thinner at the peaks of the anchor lines; at the same time, because the anchor lines are too deep, bubbles will easily form in the anti-corrosion layer during the anti-corrosion process, seriously affecting the performance of the anti-corrosion layer. If the roughness is too small, the adhesion and impact strength of the anti-corrosion layer will decrease. For severe internal pitting corrosion, we cannot rely solely on high-intensity impact with large-grain abrasives. We must also rely on small particles to grind away the corrosion products to achieve the cleaning effect. At the same time, a reasonable ratio design can not only slow down the wear of the abrasives on the pipes and nozzles (blade) but the utilization rate of abrasives can also be greatly improved. Usually, the particle size of steel shot is 0.8~1.3 mm, and the particle size of steel sand is 0.4~1.0 mm, of which 0.5~1.0 mm is the main component. The sand shot ratio is generally 5 to 8.
It should be noted that in actual operation, the ideal ratio of steel grit and steel shot in the abrasive is difficult to achieve because the hard and brittle steel grit has a higher breakage rate than the steel shot. For this reason, the mixed abrasives should be continuously sampled and tested during operation, and new abrasives should be added to the rust remover according to the particle size distribution. Moreover, among the new abrasives added, steel grit should account for the majority.

4.4 Rust removal speed: The rust removal speed of the steel pipe depends on the type of abrasive and the displacement of the abrasive, that is, the total kinetic energy E applied to the steel pipe by the abrasive per unit time and the kinetic energy E1 of the single particle abrasive. In the formula: m - the amount of abrasive spraying (throwing); V - the running speed of abrasive; m1 - the mass of single-grain abrasive.
m. The size is related to the abrasive crushing rate, and the crushing rate directly affects the cost of surface treatment operations and the cost of rust removal equipment. When the equipment is fixed, m is a constant and y is a constant, so E is also a constant. However, due to the crushing of the abrasive, m1 changes. Therefore, abrasives with a lower loss rate should generally be selected, which will help improve the cleaning speed and long-term maintenance.

4.5 Cleaning and preheating: Before spraying (throwing) treatment, use cleaning methods to remove grease and scale on the surface of the steel pipe, and use a heating furnace to preheat the pipe body to 40-60°C to keep the surface of the steel pipe dry. During spraying (throwing) treatment, since the surface of the steel pipe does not contain grease and other dirt, the rust removal effect can be enhanced. The dry steel pipe surface is also conducive to the separation of steel shot, steel sand, rust, and oxide scale, making the rust removed The surface of the steel pipe is cleaner.