Steel pipe surface treatment method for pipeline anti-corrosion operation

with the continuous development of my country's economy and the country's vigorous development of the energy industry, long-distance oil and gas pipelines are an important way to ensure energy. it is the premise of whether the anti-corrosion layer and the steel pipe can be firmly combined. it has been verified by research institutions that the life of the anti-corrosion layer depends on factors such as the type of coating, coating quality, and construction environment. the impact of the surface treatment of the steel pipe on the life of the anti-corrosion layer accounts for about 50%. the requirements of the surface of the steel pipe are constantly explored and summarized, and the surface treatment method of the steel pipe is continuously improved. anti-corrosion steel pipe become more and more popular in the market.

1. cleaning

solvents and emulsions are used to clean the steel surface to remove oil, grease, dust, lubricants, and similar organic substances, but they cannot remove rust, oxide scale, welding flux, etc. on the steel surface, so it is only used as an auxiliary means in anti-corrosion production.

2. tool rust removal

mainly use tools such as wire brushes to grind the surface of the steel, which can remove loose or raised oxide scale, 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 steel surface has adhered to a firm scale of iron oxide, the rust removal effect of the tool is not ideal, and the anchor pattern depth required for anti-corrosion construction cannot be achieved.

3. pickling

generally, two methods of chemical and electrolysis are used for pickling treatment. pipeline anti-corrosion only adopts chemical pickling, which can remove oxide scale, rust, and old coating. sometimes it can be used for reprocessing after sandblasting and rust removal. although chemical cleaning can make the surface achieve a certain degree of cleanliness and roughness, its anchor pattern is shallow and it is easy to cause pollution to the environment.

4. spray (throwing) rust removal

spraying (throwing) rust removal is to drive the spraying (throwing) blades to rotate at high speed through a high-power motor, so that abrasives such as steel sand, steel shot, iron wire segments, minerals, etc. are sprayed (throwing) on the surface of the steel pipe under the action of centrifugal force. , not only can completely remove rust, oxides, and dirt, but also the steel pipe can achieve the required uniform roughness under the action of the violent impact and friction of the abrasive.

after spraying (throwing) rust removal, it can not only expand the physical adsorption on the surface of the pipe but also enhance the mechanical adhesion between the anti-corrosion layer and the surface of the pipe. therefore, spraying (throwing) rust removal is an ideal rust removal method for pipeline anticorrosion. generally speaking, shot blasting (sand) derusting is mainly used for the inner surface treatment of pipes, and shot blasting (sand) derusting is mainly used for the outer surface treatment of pipes. several problems should be paid attention to when using spray (throwing) rust removal.

4.1 derusting level

for the construction process of epoxy, vinyl, phenolic, and other anti-corrosion coatings commonly used in steel pipes, the surface of steel pipes is generally required to reach a near-white level (sa2.5). the practice has proved that with this rust removal grade, almost all oxide scale, rust, and other dirt can be removed, and the depth of the anchor pattern reaches 40~100μm, which fully meets the adhesion requirements of the anti-corrosion layer and the steel pipe. the rusting process can reach the technical condition of near white level (sa2.5) with low operating cost and stable and reliable quality.

4.2 spray (throwing) abrasive

to achieve the ideal rust removal effect, the abrasive should be selected according to the hardness of the steel pipe surface, the degree of original rust, the required surface roughness, the type of coating, etc. for single-layer epoxy, two-layer, or three-layer polyethylene coating, using the mixed abrasive of steel grit and steel shot is easier to achieve the ideal rust removal effect. steel shot has the effect of strengthening the steel surface, while steel grit has the effect of etching the steel surface. the mixed abrasive of steel grit and steel shot (usually the hardness of steel shot is 40-50 hrc, and the hardness of steel grit is 50-60 hrc can be used on various steel surfaces, even on steel surfaces with c and d rust, the rust removal effect is also very good.

4.3 abrasive particle size and ratio

to obtain better uniform cleanliness and roughness distribution, the particle size and ratio design of abrasive is very important. if the roughness is too large, the anti-corrosion layer will become thinner at the peak of the anchor pattern; at the same time, because the anchor pattern is too deep, the anti-corrosion layer will easily form bubbles during the anti-corrosion process, which will seriously affect 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 serious internal pitting corrosion, the cleaning effect cannot only be achieved by the high-strength impact of large-particle abrasives, but also by small-particle abrasives to remove corrosion products. at the same time, a reasonable ratio design can not only slow down the abrasive wear on pipes and nozzles (blade), and 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 grit is 0.4-1.0 mm, of which 0.5-1.0 mm is the main component. the sand-to-pill ratio is generally 5 to 8. it should be noted that in practice, the ideal ratio of steel grit to steel shot in the abrasive is difficult to achieve because the hard and brittle steel grit has a higher breakage rate than steel shot. for this reason, the mixed abrasive should be continuously sampled and tested in operation. according to the particle size distribution, new abrasives should be added to the rust remover, and the amount of steel grit should be the main part of the new abrasives added.

4.4 derusting 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 and the kinetic energy e1 of the single particle abrasive applied to the steel pipe by the abrasive per unit time. in the formula: m - the amount of abrasive spraying (throwing); v - the running speed of the abrasive; m1 - the quality of the single particle abrasive. m. the size of the abrasive is related to the abrasive crushing rate, and the size of 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, but due to the crushing of the abrasive, m1 changes, so generally, the abrasive with a lower loss rate should be selected, which is conducive to improving the cleaning speed and longevity. life of leaves.

4.5 cleaning and preheating

before the spraying (throwing) treatment, use the cleaning method to remove the grease and fouling on the surface of the steel pipe, and use the heating furnace to preheat the pipe body to 40-60 °c to keep the surface of the steel pipe in a dry state. during the spraying (throwing) treatment, since the surface of the steel pipe does not contain grease and other dirt, the effect of rust removal can be enhanced. the surface of the steel pipe is cleaner.