Corrosion factors and protective measures of buried metal pipelines

Pipeline system is an indispensable part of industrial production and civil facilities, they play a vital role in the field of pipeline transportation. With the rapid development of China's petrochemical industry and the acceleration of urban infrastructure construction, pipeline construction has also shown a rapid growth trend. Since most pipelines are buried underground, the corrosion effect of soil on pipelines and its protective measures have always been the key factors determining the reliability and service life of pipelines.

When metal pipes are in contact with the surrounding medium, they may suffer damage due to chemical or electrochemical reactions, which is called metal corrosion. Soil corrosion of the metal occurs when the metal material is subjected to chemical and electrochemical action in the surrounding soil medium. Corrosion of buried metal pipes usually occurs in aqueous environment and is essentially an electrochemical process. With the change of environment and soil properties, soil corrosion of metal pipelines is becoming more and more serious.

The harmfulness of pipeline corrosion cannot be ignored. With the continuous development and production of oil and gas fields, the national investment in pipeline transportation and storage continues to increase, and the underground pipeline network continues to expand, pipeline corrosion will cause huge economic losses. Soil is heterogeneous and heterogeneous, containing many micropores that can penetrate water and gases, so different soils have different corrosivity. The stability of soil makes soil corrosion different from other electrochemical corrosion processes. In soil, oxygen is transferred through the pores of the soil, and its rate of transfer depends on the structure and moisture of the soil. In soil, there may be not only micro-cell corrosion related to microstructure heterogeneity, but also macro-cell corrosion due to macroscopic differences in soil media. The types of macrocell corrosion include the macrocell corrosion formed by long-distance oil pipelines passing through different soils, and the macrocell corrosion caused by the difference in the material of the pipes buried in the soil. The different buried depth of the pipeline leads to the difference in the density and oxygen content of the soil in the upper and lower parts of the pipeline, resulting in different electrode potentials in the upper and lower parts of the pipeline, forming macro cell corrosion.

The properties of soil have a great influence on corrosion. Factors such as soil porosity, water content, resistivity, pH, and salt content all affect its corrosiveness. The greater the porosity, the easier the penetration of oxygen and water, and the more serious the corrosion of the pipeline. The increase of water content will accelerate the corrosion rate, but when the water content exceeds a certain value, the corrosion rate will decrease due to the obstruction of oxygen diffusion. The lower the soil resistivity, the higher the corrosion rate. Most of the soil in China is neutral, the pH value is between 6 and 8, and the decrease of pH value will lead to the increase of corrosion rate. As the salt content in the soil increases, so does the electrical conductivity, which increases the corrosiveness of the soil. However, when the soil contains CaCO3, the corrosion rate will decrease with the increase of CaCO3 content.

The influence of stray current cannot be ignored. Stray currents generated by trams, electrified railways, transmission and distribution systems and electrolysis devices can flow into the ground and water environment and may cause interference corrosion when buried pipelines or metal structures are present in the environment. The degree of stray current corrosion is much more severe than that of ordinary soil corrosion.

Microbes in the soil can also affect the corrosion process. Sulfate-reducing bacteria are a type of anaerobic bacteria that live in soil and participate in electrode reactions that convert soluble sulfate into hydrogen sulfide, thus accelerating corrosion.

Temperature also has a significant effect on the corrosion rate. In general, every 20℃ increase in temperature, the corrosion rate will be doubled.

In order to protect metal pipes from soil corrosion, the following measures can be taken:

Improve the nature of the metal: choose different materials according to different uses to form corrosion resistant alloys, or add alloying elements to the metal to improve its corrosion resistance, such as adding nickel to steel to make stainless steel to enhance corrosion resistance.

Forming a protective layer: Covering the metal surface with various protective layers, such as non-metallic protective layer and metal protective layer, effectively isolating corrosive media.

Improve the corrosion environment: reduce and prevent metal corrosion by reducing the concentration of corrosive media, removing oxygen in the media, and controlling the ambient temperature and humidity. Adding corrosion inhibitor to the corrosive medium is also an effective anticorrosive means.

Electrochemical protection law: According to the principles of electrochemistry, measures are taken on the metal equipment to make it become the cathode in the corrosion battery, so as to prevent or reduce metal corrosion.