What is polyoxysulfuric acid corrosion of austenitic stainless steel equipment? ? ? ?

I’ve heard of it for the first time too; I’ll go ask the person who created the question.

Even under the corrosion effect of poly-sulfuric acid, during the operation of hydrogenation units, the high-temperature (H2+H2S) medium reacts with steel to form FeS. When the equipment is stopped for maintenance and opened, the FeS on its inner surface comes into contact with air (O2) and water, resulting in the formation of polyoxosulfates. The reaction equations are as follows: 3FeS + 5O2 → Fe2O3·FeO + 3SO2; SO2 + H2O → H2SO3; H2SO3 + 1/2O2 → H2SO4; FeS + H2SO3 → mH2SxO6 + nFe2+; FeS + H2SO4 → FeSO4 + H2S; H2SO3 + H2S → mH2SxO6 + nS. Polyoxosulfates (H2SXO6) can cause corrosion cracking in austenitic stainless steel. Even at ambient temperature, cracking occurs very rapidly, especially in areas with residual tensile stress and in regions where intergranular sulfides may be present (such as the heat-affected zone near the weld). Although stabilized austenitic stainless steels (TP321, TP347, etc.) have some resistance to polyoxosulfate stress corrosion, preventive measures must still be taken. The best approach is to keep the surface of austenitic steel (including surfacing layers, composite layers, and linings, etc.) dry and free from contact with air; this is usually achieved by using an inert gas under slight positive pressure, but this is often not possible. Therefore, the equipment should be cleaned with an alkaline solution before being turned on to neutralize acidic substances such as polyoxysulfuric acid.

Mechanism of polythiic acid stress corrosion 1. Formation of polythiic acid In the petroleum refining and chemical industry, the media contain varying amounts of H2S and reactive sulfur. Due to their reactive chemical properties, they can directly react with the iron in the metal surfaces of equipment at high temperatures in an anhydrous environment to form FeS. The reaction process is as follows: H2S + Fe → FeS. This FeS forms a dense layer on the surface of the equipment; in a sense, it provides some protection for the equipment by preventing further corrosion of its surface by other substances. However, when the device is shut down, cooled down, and its components are opened, the air contains large amounts of O2 and moisture, which react with FeS on the surface of the equipment to produce large quantities of polythiosulfates (H2SxO6, where x=3,4,5…). The reaction is as follows: FeS + O2 + 2H2O → Fe2O3 + H2SxO6. The formation of polythiosulfates creates an environment conducive to stress corrosion. Yet not all alloy steels are susceptible to stress corrosion caused by polythiosulfates, as stress corrosion operates according to distinct corrosion mechanisms compared to ordinary corrosion. 2. Mechanism of stress corrosion cracking in polyoxysulfuric acid (1) Influence of concentration factors: The effect of acid concentration on grain-boundary stress corrosion cracking in sensitized stainless steel (650°C/4h) in polyoxysulfuric acid solutions. As the concentration of polydithionic acid increases, the pH value of the solution decreases, and the breaking time shortens accordingly. (2) Influence of material factors ① Influence of heat treatment: The solutioning temperature and degree of sensitization have a significant impact on SCC in 321 stainless steel with insufficient Ti content in polyoxysulfuric acid. In such materials with a low Ti/C ratio and C that is not fully stabilized, the higher the solution treatment temperature and the greater the sensitization, the more prone they are to intergranular cracking. This phenomenon, which results from the decomposition of TiC at high temperatures followed by sensitization treatment, causes a large amount of C to precipitate as Cr23C6 at the grain boundaries, forming continuous Cr-deficient regions that increase the intergranular sensitivity. Furthermore, if carbides nucleate at the grain boundaries in advance, sensitization will occur even at lower temperatures (231–300°C). ②Welding heat-affected zone: Stress generated by welding remains in the welding heat-affected zone. Its presence creates favorable conditions for stress corrosion cracking; cracking occurs preferentially in these areas, and stress is released during the cracking process. ③Effect of hardness: Generally, the higher the hardness of steel, the greater its sensitivity to SSCC. When the hardness (HRC) of steel is less than 20, its sensitivity to SSCC is very low ； And when the HRC of steel is greater than 30, the sensitivity of the steel to SSCC increases. To this end, the API recommends that for engineering components in contact with H2S, the HRC of their material should be kept below 22. ④Effect on steel microstructure: Stainless steels with an austenitic microstructure are prone to sensitization due to the precipitation of carbides at their grain boundaries. However, steels with a ferritic-austenitic duplex stainless steel structure exhibit excellent SCC resistance in polyoxysulfuric acid. The ferrite content is taken as a threshold of 10%; steels with a ferrite content exceeding 10% do not suffer from SCC. Furthermore, even when subjected to sensitization treatment, SCC hardly occurs in duplex stainless steel. (3) Stress factors Stress factors include two types: internal and external. Internal stress refers to the residual stresses inherent in a material due to its manufacturing and welding processes; the magnitude of these stresses directly influences whether stress corrosion will occur ； External stress refers to the stress generated during the installation and alignment of the workpiece, as well as other external stresses acting on the workpiece; its impact on SCC is less than that of internal stress. 3. The formation process of stress corrosion cracking caused by polyoxysulfates: During shutdown periods, polyoxysulfate deposits form on the inner surfaces of the equipment, creating an corrosive environment that leads to corrosion of the inner walls of the equipment. Grain boundaries are sensitive areas where impurity segregation and carbide precipitation lead to pitting. When the area surrounding a grain is corroded, the grain or the precipitated phases will fall off one by one, resulting in pitting that gradually expands into pits visible to the naked eye. These erosion holes themselves cause stress concentration, leading to the formation of cracks and serving as sources of fracture. Furthermore, the discontinuities resulting from the processing of the equipment’s inner walls, along with the right-angled connections of the fittings rather than smooth transitions, inevitably increase stress concentration. This causes the corrosion pits in these areas to become primary sources of failure; under the influence of stress, cracks expand radially and circumferentially, penetrating the entire inner wall. Therefore, pitting caused by poly-sulfuric acid is the main cause of cracking. This is also confirmed by the grain-edge corrosion morphology of the intergranular corrosion fractures and pits, as well as the metallographic structure of the grain-edge corrosion near the pits, since it corresponds to the characteristics of intergranular corrosion caused by polyoxysulfuric acid.

Pyrosulfuric acid (PTA) and sulfurous acid are major factors to consider in petroleum processing, especially in units such as hydrocracking, diesel hydrogenation, gasoline hydrogenation, and naphtha hydrodesulfurization. These chelates are formed when equipment containing sulfur impurities is exposed to air and moisture while shut down. In this acidic environment, some sensitive materials (austenitic materials with a carbon content of less than 10%) suffer from rapid intergranular corrosion and cracking after sensitization (prolonged exposure at 370–815°C) or welding treatments similar to sensitization, as well as when austenitic stainless steels with low carbon content and stabilized by titanium or niobium are exposed for long periods to areas with high sensitization levels. 　　The medium environment that leads to stress corrosion cracking caused by polyoxysulfuric acid must have a certain level of acidity for this phenomenon to occur; generally, for 18-8 stainless steels, this can happen when the pH value is less than or equal to 5. Since most of the austenitic steels used in the production process of hydrogenation units operate at temperatures near the sensitization threshold, great care must be taken to prevent stress corrosion cracking caused by polythionic acids. 　　The characteristic of polyoxysulfate stress corrosion cracking is the brittle cracking that occurs in a certain metal (steel) under the combined action of tensile stress and a specific corrosive environment. Austenitic stainless steels are relatively sensitive to sulfide stress corrosion cracking. Stress corrosion cracking caused by polythiosulfuric acid (H2SxO6, x=3–6) also falls under the category of sulfide stress corrosion cracking, and it is generally intergranular cracking. During the shutdown of refining units, when the system temperature and pressure drop, water vapor is condensed; or when the equipment is opened for maintenance, its interior and pipelines come into contact with moist air. When iron/chromium sulfides react with hydrogen and oxygen, sulfurous acid and polythionic acid are produced, thereby causing corrosion. In the hydrogenation units of the petrochemical industry, sulfide stress corrosion cracking in austenitic stainless steel equipment is a common occurrence.

I’ve learned it; thanks for sharing

Thank you to the people above; I’ll reply to see what’s hidden.

Take a look at the hidden content and learn* it

1111111111111111111