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1. What is metal corrosion? Metal corrosion is a phenomenon in which metal undergoes chemical, electrochemical, or physical reactions with its surrounding environment, resulting in the formation of metal compounds and thus damage to the metal. 2. According to their mechanism of action, into which categories can the common types of corrosion in metal pipes be divided? The common types of corrosion in metal pipes can be divided into chemical corrosion and electrochemical corrosion based on their mechanisms of action. 3. What are the common anti-corrosion measures? Common anti-corrosion measures include coating, lining, electrochemical protection, and corrosion inhibitors. 4. What is chemical corrosion? Chemical corrosion refers to the degradation of a metal surface caused by a pure chemical reaction between the metal surface and a non-electrolyte. Chemical corrosion can be further divided into gas corrosion and corrosion in non-electrolyte solutions. 5. What is electrochemical corrosion? Electrochemical corrosion refers to the phenomenon of damage to metals caused by electrochemical reactions with electrolytes. 6. How does crevice corrosion occur? Many metal components are connected using screws, rivets, welding, and other methods. Narrow gaps can form at defects in these connections or weld joints; the width of such gaps (usually between 0.025 and 0.1 mm) is sufficient for electrolyte solutions to enter. This allows the metal inside the gap to form a short-circuited galvanic cell with the metal outside the gap, leading to severe corrosion within the gap. This type of localized corrosion is known as crevice corrosion. 7. What is pitting corrosion? Pitting corrosion refers to a type of corrosion in which the degradation is concentrated in localized areas on the metal surface and penetrates deep into the metal as pores. 8. What are the characteristics of pitting and corrosion pits respectively? Pitting: the diameter of the pit is smaller than its depth ； Pit erosion: The diameter of the pit is greater than its depth. 9. What is stress corrosion? How many types can stress corrosion be classified into based on its corrosion mechanism? The material failure process resulting from the combined effect of strain and corrosion caused by residual or applied external stresses is known as stress corrosion. Stress corrosion can be classified according to its corrosion mechanism into: (1) anodic dissolution and (2) hydrogen-induced cracking. 10. What is the definition of corrosion fatigue? The degradation of a material caused by the combined effect of metal in a corrosive environment and alternating stresses is known as corrosion fatigue. 11. How does oxygen concentration cell corrosion occur? The most common corrosion phenomenon in underground pipelines is the oxygen concentration cell. Due to the varying oxygen concentrations in different parts of the pipeline, the natural potential (unbalanced potential) of the pipeline in the oxygen-poor areas is low; these areas act as the anode of a corrosion galvanic cell. The rate of anodic dissolution in these areas is significantly higher than that on the other surfaces, resulting in corrosion. When pipes pass through the junctions of soils with different properties, the clay-rich sections become oxygen-poor and are prone to corrosion; corrosion is most severe at the junctions between the two types of soil or near the end of buried pipes where they emerge from the ground. For oil storage tanks, the oxygen concentration gradient is mainly manifested by poor contact between the tank bottom plate and the sand base, as well as differences in permeability between the perimeter and the center of the tank. The lower oxygen concentration at the center leads to corrosion there as an anode. 12. What is bacterial corrosion? How was it created? Bacterial corrosion occurs when metal corrodes in soil containing sulfates; the hydrogen from the cathodic reaction reduces the sulfates to sulfides, and sulfate-reducing bacteria use the energy from this reaction to reproduce, thereby accelerating the corrosion of the metal. In some oxygen-poor soils that contain sulfates, sulfate-reducing bacteria multiply. During their metabolic processes, they require hydrogen or certain reducing agents to convert sulfates into sulfides, using the energy from this reaction to grow. SO42- + 8H+ → S2- + 4H2O. Due to the presence of sulfates and other H+, the cathodic reaction in the corrosion process of metals in soil results in the generation of atomic hydrogen. In the soil, it adheres to metal surfaces; if it cannot escape as bubbles continuously, cathodic polarization occurs, which significantly slows down the corrosion process. However, the presence of bacteria in sulfuric acid provides a way for atomic hydrogen to be utilized; it reduces SO42- to S2-, which then reacts with Fe2+ to form black FeS deposits. Bacterial reproduction is most favorable when the soil pH is between 5 and 9 and the temperature is between 25 and 30°C. 13. How does galvanic corrosion occur? When two metals or alloys with different electrode potentials come into contact with each other and are in an electrolyte solution, the metal with the more negative electrode potential corrodes continuously, while the metal with the more positive electrode potential is protected; this type of corrosion is known as galvanic corrosion.