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Classification of cracking in carbon steel and low-alloy steel: 1. Corrosive cracking of steel in alkaline solutions 2. Amine-induced corrosive cracking 3. Nitrate-induced corrosive cracking 4. Stress corrosion cracking of carbon steel caused by ammonia solution 5. H2S-induced stress corrosion cracking 6. Corrosion of steel by salt solutions 7. Carbonate-induced corrosive cracking I. Corrosive cracking of steel in alkaline solutions Stress corrosion cracking that occurs in steel in alkaline solutions is commonly referred to as alkali embrittlement. In hot, concentrated alkaline solutions, the protective film on the steel surface is destroyed, resulting in a higher corrosion rate. Oxygenation of alkaline solutions, the presence of CO2 or chlorides, and elevated temperatures all accelerate the corrosion of carbon steel. Although the OH– concentration in ammonia water is much lower than that in caustic alkali solutions, various types of steel can be widely used for storing and handling anhydrous ammonia or ammonia water solutions. Sodium salts are added during the water treatment process, causing the water to contain NaOH. At the gaps and the ends of the expanded tubes, the evaporation of water leads to the concentration of alkali, resulting in a fairly high concentration of alkaline solution. Combined with stress concentration, this area of metal is subjected to very high local stress. The high temperature of the alkaline solution also promotes stress corrosion cracking in steel. Alkali embrittlement is related not only to the concentration of the alkali but also to the temperature of the alkali solution. Alkali embrittlement cracking is mainly intergranular in nature, but there are also mixed types. The corrosion potential lies in the activation-passivation transition potential range, where conditions are created for the activation of grain boundaries while the grain bodies remain passivated, thereby leading to intergranular corrosion failure. Corrosion of carbon steel in alkalis: In NaOH solution, carbon steel is quite stable because an insoluble iron hydroxide protective film forms on the metal surface. In alkali solutions with a concentration of over 46%, iron dissolves in the form of ferrates, thereby losing its stability. As the temperature rises, the membrane is more susceptible to dissolution, and the corrosion rate increases. In hot alkaline solutions, carbon steel suffers from corrosion and cracking, a phenomenon known as \"alkaline embrittlement\". The minimum temperature for caustic embrittlement is 50–60°C, and the minimum concentration is 5%. The risk is highest at an NaOH concentration of around 30%, and caustic embrittlement is most likely to occur at high temperatures near the boiling point. Alkali washing of oils requires caustic soda, and the stress corrosion cracking that occurs in carbon steel equipment in NaOH solutions is commonly referred to as \"alkali embrittlement\". The factors affecting stress corrosion cracking in carbon steel in NaOH solution are concentration, temperature, and residual stress. Therefore, it is specified that carbon steel equipment storing NaOH at various concentrations shall undergo stress-relief heat treatment after welding when the temperature is higher than the values shown in the table below. Concentration %: 5, 10, 15, 20, 30, 40, 50, 60, 70; Temperature °C: 85, 76, 70, 65, 54, 48, 43, 40, 38. When the operating temperature of all carbon steel amine liquid pipelines is ≥90°C, post-weld heat treatment is required, with the hardness of the welds and heat-affected zones to be ≤200 HB. After stress-relief heat treatment, the welds in the lean amine liquid pipelines exhibit significantly improved resistance to hydrogen embrittlement and stress corrosion. Methods to prevent steel from suffering from alkali embrittlement: Adding aluminum, titanium (0.2–0.7%), niobium, vanadium, chromium, and rare earth metals (less than 0.2%) to low-carbon steel can reduce or even eliminate the steel’s susceptibility to alkali embrittlement. l Holding low-carbon steel at temperatures above 500°C during heat treatment can significantly reduce its susceptibility to caustic embrittlement in boiling 34% NaOH solution. Lower the operating temperature, eliminate areas where the alkaline solution becomes concentrated, and reduce the stress on the metal: this can be achieved by using appropriate water treatment techniques and suitable corrosion inhibitors. II. Amine corrosion cracking damage. Amine corrosion cracking is generally observed in amine treatment units that use alkylamine aqueous solutions to remove acidic gases such as H2S or CO2 from various gases or liquid hydrocarbon fluids. Cracks mainly occur intergranularly, appearing as a network of fine cracks filled with corrosion products. Amine corrosion cracking is most common in ethanolamine (MEA) and diisopropanolamine (DIPA) plants, followed by diethanolamine (DEA) plants. In the MEA solution, the crack sensitivity is high within a concentration range of 15–35%. Cracking generally occurs in lean solutions that are strongly alkaline and contain very low concentrations of acidic gases; it is unlikely to occur in rich solutions containing high concentrations of acidic gases. At high temperatures, the susceptibility to cracking is generally high; at low temperatures, it occurs in equipment and pipelines that are in normal operation but equipped with heating or steam injection. III. Nitrate corrosion cracking: The phenomenon of cracking or fracturing of low-carbon steel in factories that produce nitrogen fertilizers and nitrates in the presence of concentrated nitrates is known as \"nitrate embrittlement\", which can be abbreviated as \"NC embrittlement\". Nitrate is an oxidizing agent, and the overall reaction that takes place is as follows: 10Fe + 6NO3- + 3H2O → 5Fe2O3 + 6OH- + 3N2. The tendency for nitration cracking increases in the following order: NaNO3, KNO3, LiNO3, Ca(NO3)2, and NH4NO3; the decrease in pH that occurs during the hydrolysis of these nitrates leads to an increased tendency for nitration cracking. IV. Stress corrosion cracking of carbon steel in ammonia solution Carbon steel is commonly used to manufacture liquid ammonia storage tanks, ammonia solution storage vessels, and transportation containers. Impurities in ammonia have a significant impact on steel cracking: air (oxygen) dissolved in ammonia promotes cracking, while water acts as a corrosion inhibitor. For ammonia storage containers, performing stress-relief treatment after manufacturing can minimize the risk of stress corrosion cracking in the steel. V. H2S stress corrosion cracking: The presence of H2S in oil and gas causes corrosion of steel, which often leads to fracture accidents. It is known as sulfide stress corrosion cracking. The most sensitive temperature for rupture is 20–50°C. The microstructure of steel after welding has a significant impact on sulfide stress corrosion cracking. The martensitic structure is the most sensitive. To eliminate the adverse effects of the martensitic structure, post-weld heat treatment is very important; in addition to high-temperature tempering, long-duration low-temperature tempering or secondary tempering can also be employed. Its carbon content can also be reduced. The complexity of cracking caused by wet hydrogen sulfide environments: The four forms of cracking induced by wet hydrogen sulfide environments each have their own characteristics, but under actual wet hydrogen sulfide conditions, multiple forms often coexist. In actual failure analysis, among the four aforementioned types of cracking, the more dangerous forms are SSCC and SOHIC. Because they extend along the wall thickness direction of the pressure vessels and pipes, they directly reduce their load-bearing capacity. VI. Corrosion of steel by salt solutions: The ionization of salts increases the conductivity of water, thereby promoting pitting corrosion. Ammonium salts and salts of trivalent elements (such as Cr3+, Fe3+) promote corrosion the most. Solutions of NH4Cl, NaCl, Na2SO4, and KNO3 form soluble compounds with iron, accelerating corrosion ； AlCl3 and MgCl2 can hydrolyze to produce free acids, which corrode steel. When the positive charge of the cation in salt is stronger than that of iron, ions such as Cu2+ can undergo a displacement reaction with iron, causing corrosion of steel. Oxidizing salts such as KSCrO4 and KMnO4 cause steel to become passivated. VII. Corrosion and cracking due to carbonates: During the pyrolysis of coal, stress corrosion occurs due to the presence of carbonates containing trace or small amounts of H2S and CN—. This stress corrosion caused by carbonates is, like alkaline embrittlement and nitric acid embrittlement, of the anodic dissolution type. In an aqueous solution of (NH4)2CO3, the following equilibrium reactions occur: 2(NH4)2CO3 → NH4HCO3 + NH2CO2NH4 + NH3 + H2O; 2 NH4HCO3 → NH2CO2NH4 + 2H2O + CO2. The resulting ammonium carbamate (NH2CO2NH4) serves as an accelerator in stress corrosion tests involving liquid ammonia.