With the development of economy, the advantages of buried pipelines are more and more recognized by the public. Therefore, the problem caused by buried pipeline leakage is also taken seriously. Most buried pipelines are laid in topsoil, and different environments in the soil cause different corrosion of pipelines.
Metal corrosion is divided into chemical corrosion and electrochemical corrosion, of which electrochemical corrosion occupies the majority. It is a kind of chemical reaction, must have metal electrode, electrolyte as reaction environment and conductive loop. The corrosion of buried pipeline is mainly caused by electrochemical reaction. There are many types of corrosion, such as galvanic corrosion, microbial corrosion, concentration corrosion and so on.
Galvanic corrosion is the corrosion caused by different potential in different parts due to the structure of the metal itself. Microbial corrosion is essentially bacterial corrosion, which is also a kind of electrochemical corrosion, but its medium changes the physical and chemical properties of the interface between its contact materials due to the propagation and metabolism of corrosive microorganisms.
Based on the principle of corrosion, the most effective anticorrosion method of buried pipeline is the combination of anticorrosive layer and cathodic protection. The anticorrosive layer has different structure and thickness according to different pipe diameter, soil environment and engineering anticorrosive requirements and construction conditions.
The anticorrosive layer is an insulating material applied to the surface of the pipe, which separates the pipe from the electrolyte in many different potential areas and eliminates the corrosive cell pathway to prevent corrosion. It is the first line of defense for corrosion control and is responsible for 99% of the corrosion protection tasks. However, as an organic material, it will age over time. Sunlight will accelerate the aging, and leakage points will appear after impact.
Cathodic protection is an anti-corrosion method to protect buried pipelines according to the galvanic cell principle. It can be divided into two types: sacrificial anode cathodic protection and applied current cathodic protection. Sacrificial anode cathodic protection uses a metal material that is more reactive than steel, such as magnesium or aluminum, as an anode, which is buried in the ground and connected to the pipe at a certain distance to form an anode in the soil.
The applied current uses an external power source and an auxiliary anode to force the current to flow from the surrounding medium to the protected structure through the auxiliary anode, so as to eliminate corrosion. Therefore, it is also called forced current cathodic protection.