What are the common processing problems and quality inspection methods for plastic-coated steel pipes

First, common problems in the production and processing of plastic-coated steel pipes:

1. Weld defects: Severe weld defects are often remedied by manual mechanical grinding, resulting in uneven surfaces and affecting aesthetics. Pickling and passivation only on the weld also leads to uneven surfaces.

2. Difficulty in removing scratches: Overall, pickling and passivation cannot remove various scratches generated during processing, nor can they remove carbon steel, spatter, and other impurities adhering to the surface of the plastic-coated steel pipe due to scratches or welding spatter. This leads to chemical or electrochemical corrosion and rusting in the presence of corrosive media.

3. Uneven grinding, polishing, and passivation: After manual grinding and polishing, pickling and passivation are difficult to achieve uniform treatment results for large workpieces, resulting in an unsatisfactory surface. Furthermore, labor and material costs are high.

4. Limited pickling capacity: Pickling and passivation paste is not a panacea; it is difficult to remove black oxide scale produced by plasma cutting and flame cutting.

5. Severe scratches caused by human factors: During hoisting, transportation, and structural processing, scratches caused by bumps, dragging, hammering, and other human factors are relatively severe, increasing the difficulty of surface treatment and being a major cause of post-treatment corrosion.

6. Equipment factors: Scratches and creases caused during the bending and folding of profiles and plates are also a major cause of post-treatment corrosion.

7. Other factors: Bumps and scratches caused during the procurement and storage of raw materials for plastic-coated steel pipes due to hoisting and transportation are also relatively severe, contributing to corrosion.

Second, how to inspect the quality of plastic-coated steel pipes?

1. Visual inspection: Inspect the appearance quality of the plastic-coated steel pipes.

2. Thickness measurement: Take two cross-sections of different lengths from both ends of the plastic-coated steel pipe, and use an electromagnetic thickness gauge to measure the coating thickness at any four orthogonal points on the circumference of each cross-section.

3. Pinhole Test: The steel pipe section specimen length is approximately 1000 mm. An electric spark leak detector is used to check the steel pipe coating under the specified test voltage. For coating thickness not exceeding 0.4 mm, the test voltage is 1500 V; for coating thickness exceeding 0.4 mm, the test voltage is 2000 V. Check for the generation of electric sparks.

4. Adhesion Test: The adhesion test is conducted according to 7.4.2 of CJ/T 120-2008. The test results should comply with the provisions of 5.6.

5. Bending Test: Coated steel pipes with DN ≤ 50 mm undergo a bending test. The pipe section specimen length is (1200±100) mm. In an environment with a temperature of (20±5) ℃, the bending radius is 8 times the nominal diameter of the steel pipe, and the bending angle is 30°, performed on a pipe bending machine or mold. No filler material is present inside the pipe during the bending test, and the weld is located on the side of the main bending surface. After the test, the specimen is cut open from the middle of the curved arc to inspect the inner coating. The test results should meet the requirements of 5.7.

6. Flattening Test: Coated steel pipes with DN>50 mm are subjected to a flattening test. The pipe section specimen length is (50±10) mm. At a temperature of (20±5) ℃, the specimen is placed between two flat plates and gradually compressed on a pressure testing machine until the distance between the two plates is four-fifths of the specimen's outer diameter. During flattening, the weld of the coated steel pipe is perpendicular to the direction of load application. After the test, the inner coating is inspected, and the test results should meet the requirements of 5.8.

7. Impact Test: A specimen approximately 100 mm long is cut from any position on the coated steel pipe. During the test at a temperature of (20±5) ℃, the weld should be in the opposite direction to the impact surface. The test results should meet the requirements of 5.9.

Plastic-coated steel pipes have excellent corrosion resistance and relatively low frictional resistance. Epoxy resin coated steel pipes are suitable for conveying media such as water supply and drainage, seawater, warm water, oil, and gas. Polyvinyl chloride-coated steel pipes are suitable for conveying media such as drainage, seawater, oil, and gas. Coated steel pipes not only possess the advantages of steel pipes, such as high strength, easy connection, and resistance to water flow impact, but also overcome the disadvantages of steel pipes, such as easy corrosion, pollution, and scaling when exposed to water, and the disadvantages of plastic pipes, such as low strength and poor fire resistance. Their design life can reach 50 years.