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There are mainly three types: abrasion or erosion, which refers to the mechanical wear on a metal surface caused by the migration or adsorption of material on that surface as a result of the action of corrosive solid particles. Underground corrosion occurs when insoluble salts in water tend to deposit in the areas with defects on the surface of the material, forming initial crystal nuclei. Various insoluble salts and suspended particles in the water gather around these crystal nuclei, gradually expanding to form layers of dirt. Oxygen concentration difference cells are likely to form in these dirt layers: Anodic reaction: Fe → Fe2+ + 2e; Cathodic reaction: O2 + 2H2O + 4e → 4OH-, thereby accelerating the corrosion of the substrate. Microorganisms and attached organisms cause corrosion of metal substrates. Marine microorganisms adhere to steel substrates by adsorption or by secreting mucus that solidifies; they obtain energy from oxidation reactions, and the products of these reactions are acidic substances that can even reduce the pH value of the surrounding environment to below 0.6. Over time, this can cause corrosion of the steel, and even perforation. However, in high-temperature operating environments, the corrosion behavior of materials is highly complex; they may be subjected to single or combined forms of corrosion from gaseous, liquid, or solid phases, representing a mixed-type corrosion resulting from the coexistence of multiple corrosion mechanisms.