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The main types of corrosion present in hydrocracking units include: hydrogen damage (including high-temperature hydrogen corrosion, hydrogen embrittlement, and hydrogen-induced delamination), high-temperature H2+H2S corrosion, polyoxysulfuric acid corrosion, temper embrittlement of Cr-Mo steel, high-temperature S corrosion, H2S+H2O corrosion in low-temperature areas, and H2S+NH3+H2O corrosion. 1. Hydrogen damage 【Definition】The degradation of the properties of metal materials caused by the diffusion of hydrogen atoms into the metal matrix or their reaction with the metal is referred to as hydrogen damage. 【Location】The main locations where hydrogen damage occurs are the equipment and pipelines in reaction systems exposed to high-temperature, high-pressure hydrogen environments. 【Classification】Hydrogen damage can mainly be divided into: hydrogen embrittlement, high-temperature hydrogen corrosion, and hydrogen-induced delamination. (1) Hydrogen embrittlement [Definition] When steel is used in hydrogen-rich environments, hydrogen diffuses into the crystal lattice in atomic form, and aggregates around grain boundaries or non-metallic inclusions in molecular form. 【Feature】A physical process that is reversible is known as a primary embrittlement phenomenon. There is no significant change in the tensile strength or hardness of the material, but its notched strength or toughness decreases at room temperature, and cracks may sometimes occur. Materials affected by hydrogen embrittlement can have their ductility and toughness restored after dehydrogenation, provided no cracks occur. (2) High-temperature hydrogen corrosion [Definition] Hydrogen, under high temperature (T>220°C) and high pressure, undergoes a chemical reaction with impurities in the alloy (such as carbides F3C or dissolved carbon C) or alloy additives (such as Si), resulting in the formation of high-pressure gases. This leads to decarburization and grain boundary cracks in the steel. The tensile strength, ductility, and toughness of materials subjected to high-temperature hydrogen corrosion are significantly reduced. 【Characteristics】High-temperature hydrogen corrosion is completely different from hydrogen embrittlement; it is a chemical reaction process that is irreversible, and is known as permanent embrittlement. There are mainly two forms of high-temperature hydrogen corrosion: one is surface decarburization ； The first is internal decarbonization. Surface decarburization can occur when steel comes into contact with hydrogen. Surface decarburization does not cause cracks, but the strength and hardness of the material decrease slightly, while the elongation increases. The main reactions that occur are: Fe3C + 2H2 → CH4 + 3Fe. This reaction generally starts at the surface of the steel and gradually moves inward. The methane gas produced does not easily escape; it accumulates around grain boundaries or impurities, creating local pressures that can exceed several thousand atmospheres. As a result, not only does the surface and inner layers of the steel become decarburized and brittle, but severe bubbling and cracking also occur. The carbon dissolved in steel also reacts with the hydrogen present in the steel: C+4H→CH4 ； Si + 4H → SiH4. A characteristic of high-temperature hydrogen corrosion is the presence of a latency period; depending on the material and environmental conditions, this period can range from a few hours for shorter durations to several years for longer ones. After being subjected to high-temperature hydrogen corrosion, steel exhibits only minor changes in surface dimensions; the main effects are a decrease in the material’s yield strength and impact toughness, which leads to cracking of the material. There is a combination of temperature and pressure, along with a threshold value, for steel to suffer from hydrogen corrosion; once this limit is exceeded, hydrogen corrosion occurs. 【Main factors affecting high-temperature hydrogen corrosion】 ■The influence of temperature, pressure, and exposure time. The higher the temperature or pressure, the earlier the onset of high-temperature corrosion. ■The influence of alloying elements and impurity elements. ■Effect of heat treatment. The resistance of steel to hydrogen corrosion is also closely related to its microstructure. ■Effect of stress. The greater the stress value, the stronger the corrosion. (3) Hydrogen-induced spalling [Definition] Hydrogen diffuses into steel under high temperature and pressure. During equipment maintenance or cooling, when the temperature drops below 150°C, the hydrogen cannot be released in time; as a result, hydrogen remains trapped within the steel. Under certain conditions, this leads to cracking between the surfacing layer and the base material. (4) Protective measures against hydrogen damage ■ Strictly control the rate of cooling and pressure reduction; it must not be too high. Cooling rate: 20–25°C/h; pressure reduction rate: 1.0–1.5 MPa/h. (It can effectively prevent hydrogen embrittlement and hydrogen-induced cracking.) ■Control the content of impurity elements in the material, and perform post-weld heat treatment. ■Select hydrogen-resistant materials based on the latest version of the Nelson curve. It is strictly prohibited to allow the equipment to exceed its temperature or pressure limits. 2. High-temperature H2 + H2S corrosion 【Definition】High-temperature H2 + H2S corrosion refers to the corrosive degradation of metals that occurs in an environment of high temperature (204°C; some sources mention 300–420°C), along with H2 and H2S. 【Corrosion sites】High-temperature H2+H2S corrosion mainly occurs on the equipment in the reaction system after hydrogen mixing, such as hydrogenation reactors, high-pressure heat exchangers for reaction effluents, reactor tubes, thermal cracking units, and the corresponding process pipelines. The corrosion mode is the chemical corrosion of steel by H2S, manifesting as uniform corrosion, hydrogen embrittlement, and hydrogen corrosion. 【Corrosion factors】During the hydrogenation process, hydrogen is also a factor that causes equipment corrosion. It can not only directly corrode metals but also acts as a catalyst for high-temperature H2S. The influencing factors include: ■Concentration: When the H2S concentration is below 1% (by volume), the corrosion rate increases as the concentration rises, increasing sharply; once the concentration exceeds 1% (by volume), the corrosion rate remains essentially constant. ■Temperature: Between 315–480°C, as the temperature increases, the corrosion rate increases accordingly; moreover, for every 50°C increase in temperature, the corrosion rate roughly doubles. ■Time: The corrosion rate decreases over time; generally, it is at its highest within 5,000 hours of operation of the device. In the subsequent period, the corrosion rate decreases by 2-10 times. ■Pressure: In high-temperature H2S+H2 corrosion, pressure has no effect on the corrosion rate, whereas in pure high-temperature hydrogen, pressure has a significant impact on corrosion. 【Protection measures】High-temperature H2S + H2 causes uniform corrosion. The material's corrosion rate must be estimated strictly in accordance with the Couper curve, and materials should be selected and designed appropriately. Generally, carbon steel can be used at temperatures below 250°C ； For temperatures above 250°C, chromomolybdenum steel (in the presence of only H2) and/or austenitic stainless steel (resistant to H2+H2S corrosion) are used. 3. Polysulfuric acid corrosion 【Affected areas】Austenitic stainless steel equipment (such as the surfacing layer of reactors, furnace tubes, austenitic stainless steel air coolers, heat exchangers, etc.). 【Protection measures】Use ultra-low carbon or stable austenitic stainless steel ； In manufacturing, efforts should be made to eliminate or reduce residual stresses caused by cold working and welding, and care should be taken to design the structure in such a way that stress concentrations do not occur or are minimized as much as possible ； Protected by nitrogen, with the equipment temperature maintained at around 150°C, for neutralization cleaning. 4. Tempering brittleness of Cr-Mo steel 【Definition】The phenomenon in which the fracture toughness of a material deteriorates when it is maintained at temperatures between 325–575°C for an extended period of time, or when it is cooled slowly from these temperatures. This is due to the segregation of trace impurity elements and alloying elements in the steel toward the original austenite grain boundaries, which reduces the cohesion at those boundaries. Once a material becomes tempered and brittle, its transformation temperature shifts toward higher temperatures. 【Corrosion sites】The areas where Cr-Mo steel is used are mainly found in equipment made of 2.25Cr-1Mo and 3Cr-1Mo materials (such as reactors, high-pressure heat exchangers, thermal crackers, etc.). [Main protective measures] ■ Strictly control the values of the temper brittleness coefficients (J-coefficient and X-coefficient). J = (Si + Mn) × (P + Sn) × 10⁴ ； (X)=(10P+5Sb+4Sn+As)×10^-2). ■Through step cooling tests, the temper embrittlement degree vTr54 +3△vTr54 is controlled to be ≤ 0℃. ■Adopt a hot-state startup/shutdown scheme (first raise the temperature then the pressure; first lower the pressure then the temperature). 5. High-temperature sulfur corrosion 【Definition】In hydrocracking units, high-temperature sulfur corrosion is a phenomenon in which sulfides present in the feedstock (mainly H2S and elemental S) react with metals at temperatures above 240–260°C, resulting in damage to the metallic properties. 【Corrosion site】It causes uniform corrosion, primarily occurring in the high-temperature areas of the feed oil system and the distillation system before hydrogen mixing (at the bottom of the towers, at the elbows, tees, and reducers of the inlet and outlet pipelines of the reboilers). In the design, the corrosion rate of materials can be estimated using the McConomy curve. 【Corrosion mechanism】 H2S+Fe→FeS（