Pipeline anti-corrosion encompasses measures aimed at retarding or preventing the degradation of pipelines caused by the chemical and electrochemical effects of internal and external media, or the metabolic activities of microorganisms. The primary purpose of pipeline anti-corrosion is to impede corrosion resulting from chemical or electrochemical reactions within pipelines.
Two predominant anti-corrosion methods are commonly employed for petroleum steel casing pipes: coating anti-corrosion and electrical protection.
Coating Anti-corrosion:
Coating anti-corrosion involves the uniform and dense application of paint on the surface of rust-treated metal pipes to isolate them from various corrosive media.
This method serves as one of the fundamental approaches to pipeline anti-corrosion. With the increasing demands on coating performance in challenging environments such as polar regions and oceans since the 1970s, as well as the transportation of oil products at elevated temperatures, pipeline anti-corrosion coatings are progressively incorporating composite materials or composite structures.
Types and Usage Conditions of Pipe Coating Materials:
Inner Wall Anti-corrosion Coating: Applied as a thin film on the inner wall to prevent corrosion, reduce friction resistance, and enhance output. Common coatings include amine-cured epoxy resin and polyamide epoxy resin, with a thickness ranging from 0.038 to 0.2 mm. Surface treatment of the inner wall is essential to ensure a firm bond with the pipe wall.
Anti-corrosion and Thermal Insulation Coating: Utilized on medium- and small-diameter thermal pipelines transporting crude oil or fuel oil to diminish heat dissipation. It involves adding a composite layer of insulation and anti-corrosion to the exterior of the pipeline. Common insulation material is rigid polyurethane foam, with a suitable temperature range of -185 to 95℃. A high-density polyethylene layer is applied outside the insulation layer to enhance strength and prevent groundwater penetration.
Electrical Protection:
Electrical protection involves altering the electrode potential of metal relative to the surrounding medium to shield it from corrosion.
In the context of long-distance pipelines, electrical protection primarily refers to cathodic protection and methods for preventing electrical corrosion.
1. Cathodic Protection
- Impressed Current Method:
In the impressed current method, a DC power supply is utilized, with the negative electrode connected to the protected pipeline and the positive electrode linked to the anode ground bed.
Upon connecting the circuit, the pipeline undergoes cathodic polarization. Complete cathodic protection is achieved when the potential of the pipeline to the ground reaches the minimum protection potential.
- Sacrificial Anode Method:
The sacrificial anode method involves connecting a metal with a more negative potential than the protected metal electrode to the protected metal, forming a primary battery in the electrolyte.
Metals with a relatively negative potential (e.g., magnesium, zinc, aluminum, and their alloys) serve as sacrificial anodes, gradually corroding as they output current. The protected pipeline metal becomes a cathode, preventing corrosion. This method is commonly used for underground pipeline protection.
Determining factors include the current generated by the anodes, the number of anodes, and the length of protection. Sacrificial anodes offer advantages such as low investment, easy management, no external power supply requirement, and effectiveness in preventing interference and corrosion.
2. Electric Corrosion Prevention
Minimizing Leakage Current:
Implement measures on facilities associated with stray current sources to reduce leakage current to a minimum.
Optimizing Pipeline Layout:
Endeavor to avoid stray current areas during pipeline installation. Improve the quality of insulation and anti-corrosion layers in disturbed pipe sections using methods such as shielding and installing insulating flanges.
Drainage Protection:
Provide drainage protection for interference pipelines to discharge stray current back to the power grid that generates leakage current, eliminating corrosion.
Drainage methods include direct drainage, polar drainage, and forced drainage, each tailored to different applications and performance requirements.
