What issues need our attention regarding the quality control of plastic-coated steel pipes

First, how to inspect plastic-coated steel pipes?

Plastic-coated steel pipes, also known as plastic-coated pipes, steel-plastic composite pipes, or plastic-coated composite steel pipes, are steel-plastic composite pipes with a steel pipe base. A layer of plastic anti-corrosion coating is fused to the inner surface of the steel pipe (base pipe) or to both the inner and outer surfaces through spraying, rolling, dipping, or suction processes.

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

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

3. Pinhole test of plastic-coated steel pipes: The pipe section specimen is approximately 1000mm long. Use an electric spark leak detector to inspect the coating of the steel pipe under a specified test voltage. For a coating thickness not exceeding 0.4mm, the test voltage is 1500V; for a coating thickness greater than 0.4mm, the test voltage is 2000V. Check for the generation of electric sparks.

4. Adhesion Test of Plastic-Coated Steel Pipe: The adhesion test shall be conducted according to section 7.4.2 of CJ/T 120-2008, and the test results shall comply with the requirements of section 5.6.

5. Bending Test of Plastic-Coated Steel Pipe: Plastic-coated steel pipes with DN≤50mm shall undergo a bending test. The length of the pipe section specimen shall be (1200±100) mm. The bending shall be performed at a temperature of (20±5) ℃, with a radius of curvature of 8 times the nominal diameter of the steel pipe and a bending angle of 30°, on a pipe bending machine or mold. No filler material shall be present inside the pipe during the bending test, and the weld seam shall be located on the side of the main bending surface. After the test, the specimen shall be cut open from the middle of the bending arc to inspect the inner coating. The test results shall comply with the requirements of section 5.7.

6. Flattening Test of Plastic-Coated Steel Pipe: Plastic-coated steel pipes with DN>50 mm shall undergo a flattening test. The length of the pipe section specimen shall be (50±10) mm. Under an environment with a temperature of (20±5) ℃, place the specimen between two flat plates and gradually compress it on a pressure testing machine until the distance between the two plates is four-fifths of the specimen's outer diameter. When flattened, the weld seam of the plastic-coated steel pipe should be perpendicular to the direction of load application. After the test, inspect the inner coating; the test results should meet the requirements of 5.8.

7. Impact Test of Plastic-Coated Steel Pipe: Cut a specimen approximately 100 mm long from any position on the plastic-coated steel pipe. Under an environment with a temperature of (20±5) ℃, during the test, the weld seam 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. Plastic-coated steel pipes not only possess the advantages of high strength, easy connection, and resistance to water flow impact found in steel pipes, but also overcome the disadvantages of steel pipes, such as susceptibility to corrosion, pollution, and scaling when exposed to water, as well as the low strength and poor fire resistance of plastic pipes. Their design life can reach 50 years.

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

1. Weld defects in plastic-coated steel pipes: Severe weld defects are often repaired by manual mechanical grinding, resulting in uneven surfaces and affecting aesthetics. Acid pickling and passivation only on the weld also leads to uneven surfaces.

2. Difficulty in removing scratches from plastic-coated steel pipes: Even overall acid pickling and passivation cannot remove various scratches generated during processing. Furthermore, it cannot remove carbon steel, spatter, and other impurities adhering to the surface of the plastic-coated steel pipe due to scratches and welding spatter, leading to chemical or electrochemical corrosion and rusting in the presence of corrosive media.

3. Uneven grinding, polishing, and passivation of plastic-coated steel pipes: After manual grinding and polishing, acid pickling and passivation treatment are difficult to achieve a uniform finish for large workpieces, resulting in an unsatisfactory surface. Furthermore, labor and material costs are high.

4. Limited pickling capability of plastic-coated steel pipes: 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, impacts, dragging, and hammering cause severe scratches, increasing the difficulty of surface treatment and being a major cause of post-treatment corrosion.

6. Equipment factors affecting plastic-coated steel pipes: 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 affecting plastic-coated steel pipes: Impacts and scratches caused during the procurement and storage of raw materials, particularly during hoisting and transportation, are also significant contributing factors to corrosion.