What are the common types of corrosion for carbon steel pipes?
Electrochemical corrosion (such as rust in humid environments), chemical corrosion (such as acid erosion) and local corrosion (such as pitting and crevice corrosion) are the main types. Soil corrosion may occur in buried pipelines, and chloride ion corrosion is susceptible to seawater environment. High temperature oxidation (such as boiler tubes) and stress corrosion cracking (SCC) should also be vigilant. Sulfide stress corrosion (SSCC) in H₂S environment is the key prevention and control target of the oil industry.
How to protect carbon steel pipes through coating?
Epoxy resin coating is suitable for indoor or buried pipelines and provides chemical isolation. The galvanized layer protects the substrate through sacrificial anodes and is suitable for atmospheric environment. 3PE anti-corrosion layer (fused epoxy + adhesive + polyethylene) is the gold standard for buried pipelines, which is acid and alkali resistant and has high mechanical strength. Sandblasting is required to achieve Sa2.5 cleanliness before painting, and the coating thickness is usually 200-400μm. Regular detection of coating damage (such as spark leak detection) is the key to maintenance.
What is the principle of cathodic protection technology?
Cathodic protection is divided into sacrificial anode method (such as installing zinc blocks) and forced current method (external DC power supply). The former protects steel pipes through electron transfer of active metals (Mg/Al/Zn), which is suitable for small facilities; the latter applies negative potential to the pipeline through a rectifier, which is suitable for long-distance pipelines. When designing, it is necessary to calculate the protection current density (such as 10mA/m²) and set the monitoring potential of the detection pile (-0.85V vs CSE standard).
How to deal with the inner wall corrosion of carbon steel pipes?
Internal corrosion protection can be achieved by lining plastic (such as PP/PE lining), internal epoxy ceramic lining or cement mortar lining. The corrosion rate of water pipelines can be reduced by adding corrosion inhibitors (such as phosphates). Regular cleaning (PIG cleaning) to remove sediments can avoid local corrosion. For acidic media (such as CO₂), the choice of sulfur-resistant materials (such as L245NS) or chrome plating of the inner wall is a solution.
How to prevent high-temperature oxidation corrosion?
Boilers or thermal pipelines can be treated with aluminization to form an Al₂O₃ protective film. Choosing chromium-molybdenum steel (such as 15CrMoG) can increase the upper limit of oxidation resistance temperature (to 600°C). Spraying ceramic coating on the surface or covering with insulation materials are also effective means. Controlling the oxygen content of flue gas (such as boiler combustion adjustment) and regular descaling can indirectly slow down oxidation.
