CHINA National Petroleum Corporation has introduced the enterprise standard of double-layer epoxy coating for this purpose. That is: Q/CNPC 38-2002 "Technical Specifications for Buried Steel Pipeline Double Fusion Bonded Epoxy Powder External Coating". The so-called double-layer fusion-bonded epoxy powder outer coating is a composite coating structure, which is formed by one-time spraying of an anti-corrosion epoxy powder bottom layer and an anti-mechanical damage epoxy powder surface layer. Double-layer fusion-bonded epoxy powder coating was invented to solve the disadvantages of single-layer epoxy coating that are prone to mechanical damage in general construction environments and the defects of not being resistant to humidity and heat in use in actual use. The double-layer epoxy coating has been used since 1992, and the raw material of the powder is provided by DuPont of the United States. It is mainly used for the anti-corrosion layer of the high-temperature pipeline and the river crossing section of the pipeline.
At present, 3M and DuPont in the United States can provide different varieties of double epoxy powder coating system coatings for different soil environments, including different varieties with high wear resistance and high toughness. There are also epoxy powder manufacturers in my country that can produce two kinds of epoxy powder supporting products, such as the Yanmei joint venture powder production enterprise in Langfang. The thickness of the double-layer epoxy powder coating specified in the Q/CNPC 38-2002 standard is shown in the table below.
| level regulation | Low layer epoxy powder coating thickness/μm | Surface layer epoxy powder coating thickness/μm | Total thickness of double layer coating/μm |
| Ordinary | >250 | >370 | ≥620 |
| Enhanced level | >300 | >500 | ≥800 |
The external coating acceptance index of the double-layer epoxy powder coating specified in the Q/CNPC 38-2002 standard is shown in the table below.
| experimental project | Quality Index | experiment method |
| 24h or 48h cathodic stripping | ≤6mm | SY/T 0315-97 Appendix C |
| Porosity of bottom section | Level 1~Level 4 | SY/T 0315-97 Appendix E |
| Bottom bonding surface porosity | Level 1~Level 4 | |
| 24h or 48h adhesion | Level 1~Level 2 | SY/T 0315-97 Appendix H |
| 0°C~30°C, 2°bending resistance (common-grade coating thickness, test temperature selected according to engineering requirements) | no crack | Appendix D |
| 0°C~30°C, 1.5°bending resistance (reinforced coating thickness, test temperature selected according to engineering requirements) | no crack | |
| 10J impact resistance (ordinary coating thickness) | no pinholes | Appendix E |
| 15J impact resistance (reinforced coating thickness) | no pinholes | |
| 30kg scratch resistance (common grade coating thickness) | Scratch depth ≤ 350μm without leakage | Appendix F |
| 50kg scratch resistance (enhanced coating thickness) | Scratch depth ≤ 500μm without leakage |
Obviously, the thickness regulations and quality acceptance items of the double-layer epoxy powder coating indicate that the coating type selection mainly serves the pipeline crossing construction technology. There are no corresponding coating quality inspection and acceptance standards for other pipeline anti-corrosion coatings used for crossing.
