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High-temperature sulfur corrosion and naphthenic acid corrosion will promote each other, leading to increased corrosion. When the two coexist, naphthenic acid will dissolve the FeS protective film generated by sulfur corrosion, causing sulfur corrosion to continue, thus causing more serious composite corrosion. Detailed explanation of mutual relations: Protective film destruction mechanism FeS protective film will be generated in the early stage of high-temperature sulfur corrosion, which can slow down further corrosion. However, naphthenic acid has a strong cleaning effect and can react with FeS to form oil-soluble iron naphthenate, destroying the protective film and continuously exposing the metal to corrosive media. Synergistic corrosion effect: Iron naphthenate acts as a surfactant to enhance the solubility of corrosion products and promote the development of corrosion in depth. After losing the protection of the FeS film, hydrogen sulfide (H₂S) can continue to react with the metal, forming a vicious cycle of "acceleration of corrosion - dissolution of the protective film - acceleration again". When the two corrosion morphological characteristics coexist, the corrosion surface presents smooth grooves (characteristics of naphthenic acid) and scaly products (characteristics of sulfur corrosion) coexisting, and the corrosion rate is much higher than that of a single corrosion type. The influencing factors are superposition of temperature, flow rate, acid value and sulfur content. Synergistic corrosion is the most serious in the range of 220–400°C, especially in the 340–400°C range.