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Generally, stainless steel exhibits good corrosion resistance because its surface is covered with a Cr2O3 passivation film that is 1–5 nm thick. For metals, Cl- has a small radius and strong penetration ability, allowing it to penetrate the corrosion product film on stainless steel and adsorb onto the metal surface. When stainless steel is exposed to an environment containing active anions such as Cl-, these corrosive Cl- ions combine with Fe2+ in the stainless steel to form FeCl2. This highly acidic salt can create electrochemical cells on a small scale, thereby dissolving the passivation layer. Pitting nuclei begin to form on the metal surface; as Cl- continues to penetrate through the corrosion product layer and combine with Fe2+, the concentration of FeCl2 increases, and more pitting nuclei appear, leading to localized corrosion such as pitting. In acidic oil and gas environments, in addition to Cl-, a large amount of acidic gases such as H2S, CO2, and SO2 are also dissolved in the solution. These acidic gases act in synergy with Cl- to further promote the occurrence and progression of pitting in stainless steel. Liu Liewei and others believe that when H2S gas dissolves in water, the HS- ions produced undergo a reaction with the passivation layer on the surface of stainless steel, resulting in the formation of a sulfuride film on that surface. This causes the passivation layer to dissolve gradually until it is completely destroyed.