Discuss the causes of corrosion in petrochemical pipelines
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Corrosion refers to the chemical and electrochemical reactions between the surface of a metal material and the environmental medium, which lead to the degradation and destruction of the material. The common corrosion of oil pipelines is caused by corrosive substances present in crude oil, such as CO2, H2S, C1–, a small amount of dissolved oxygen, and bacteria; these substances react directly with the metal, leading to chemical corrosion. Chemical corrosion is not very harmful, while the main cause of bulges on the surface of steel pipes, leading to perforations, is electrochemical corrosion. When a metal undergoes an electrochemical reaction, the area with a lower electrode potential tends to lose electrons and becomes the anode ； The area with a higher electrode potential receives electrons and becomes the cathode. In the presence of O2 and H2O, Fe(OH)2 transforms into hydrated iron oxide, thereby causing corrosion.
The main reasons for corrosion in petrochemical pipelines are as follows: 1. Acidic gases in the crude oil medium: Crude oil contains acidic gases such as CO₂ and H₂S; these substances create an acidic environment when in contact with water, thereby accelerating the electrochemical corrosion of the pipelines. 2. Presence of chloride ions (Cl⁻): Crude oil contains a certain amount of chloride ions, which can cause pitting or localized corrosion of metals, leading to the formation of pits and holes. 3. Dissolved oxygen: A small amount of dissolved oxygen is present in the pipes, forming an oxygen concentration cell with the steel, which exacerbates local electrochemical corrosion on the pipe surface. 4. Presence of water: Water is often present during the transportation of crude oil; its presence provides an environment for electrolytic reactions, thereby promoting electrochemical corrosion. 5. Microbial action: The metabolic activities of bacteria such as sulfate-reducing bacteria produce corrosive metabolites that accelerate pipe corrosion and damage. 6. Flow erosion: Under conditions of high flow rates over extended periods, as well as mechanical impact of the medium on the pipe walls, the protective coating is damaged or weak areas become exposed, thereby accelerating corrosion in those areas. These factors work together to accelerate the corrosion and damage of petrochemical pipelines, severely affecting their safe operation. .