Discussing high-temperature sulfur corrosion in petrochemical refining engineering

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Let’s discuss high-temperature sulfur corrosion in petrochemical refining engineering. High-temperature sulfur corrosion generally refers to sulfur corrosion at temperatures of ≥240°C, and it is characterized by uniform corrosion occurring on the surface of steel. When the equipment starts operating, corrosion occurs relatively quickly; however, as the operating time prolongs, the corrosion rate gradually slows down. High-temperature sulfur corrosion is a type of chemical corrosion, in which the medium reacts directly with the metal: Fe + HCl (gas) → FeCl2 + H2; Fe + H2O (vapor) → iron oxide + H2; Fe + H2S → iron sulfide + H2; R-COOH + Fe → R-COOFe; Fe + O2 (air) → iron oxide; Fe + S → FeS. When the sulfur content in crude oil is below 0.5%, its corrosive effect is weak, while it becomes more severe when the sulfur content is between 0.5% and 1.0% on a weight basis. However, there are also crude oils with a sulfur content of less than 0.5% that possess strong corrosivity due to their high content of active sulfur. At around 260°C, the sulfides in petroleum begin to decompose, becoming corrosive to carbon steel; at 345–400°C, the corrosivity is very strong. At 480°C, the decomposition is nearly complete and corrosion begins to decline. The corrosiveness of some sulfides follows this order: disulfides > alkyl sulfides > hydrogen sulfide > thiols > elemental sulfur and thiophene. Mechanism of high-temperature sulfur corrosion: Sulfur in crude oil includes elemental sulfur, hydrogen sulfide, thiol, thioether, disulfides, thiophene compounds, and more complex sulfides. Before 100°C, the main components in crude oil and its fractions are thiol and thioether ； In the 100–150°C fraction, in addition to the aforementioned sulfides, there are also alkyl thiophenes and a small amount of disulfides ； Above 150–250°C, it is mainly dibenzothiophene and benzothiophene, with cyclic sulfides being predominant ； The 200–400°C fraction mainly contains polyaromatic sulfides, such as complex sulfides like benzothiophene, dibenzothiophene, and benzothiophenes. As the boiling point of petroleum fractions increases, the structures of sulfur-containing compounds become more complex and stable. Around 130–160°C, thioethers and disulfides begin to decompose: R-CH2-CH2-S-CH2-CH2-R → R-CH2-CH2-SH + R-CH=CH2. As the temperature rises, the decomposition accelerates: CH3-CH2-S-CH3 → H2S + 2H2 + C2H4; CH3-CH2-CH2-CH2-SH → H2S + 2H2 + C2H4; R-CH2-CH2-S-S-CH2-CH2-R → R-CH2-CH2-S-CH2-CH2-R + S; R-CH2-CH2-S-S-CH2-CH2-R → R-CH2-CH2-SH + S + R-CH=CH2. The decomposition of other sulfides begins at 250°C; for example: R-C-CH2-CH2-S-S-CH2-CH2-R → S + H2S + 2H2. The decomposition of R-C-CH2-CH2-S-S-CH2-CH2-R into H2S occurs most rapidly at 343–371°C, while the decomposition slows down at temperatures above 427°C, with complete decomposition occurring around 480°C. Thiophene does not decompose even at 450°C. As the temperature rises, the reactions between hydrogen sulfide, mercaptans, elemental sulfur generated by decomposition and metals become more intense; at 240–500°C, high-sulfur corrosion is particularly severe. High-temperature sulfur corrosion generally occurs in areas exposed to sulfur-containing oils at temperatures above 230°C, and the corrosion intensifies as the temperature rises. Methanethiol is the main corrosive agent.