Patterns of localized corrosion in stainless steel

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I. Knife-like stress corrosion of stainless steel 1. Phenomenon and identification: Severe corrosion occurs in a very narrow area at the junction between the weld and the base metal of Cr-Ni austenitic stainless steels containing Ti and Nb, while the base metal and the weld itself suffer only mild corrosion or no corrosion at all. Under a metallographic microscope, the characteristics of intergranular corrosion in the sensitized state can be observed. Studies have shown that Ti-containing Cr-Ni stainless steels can suffer from knife-edge corrosion in both oxidative and reducing media. 2. Mechanism: When steel mills produce Cr-Ni austenitic stainless steels containing Ti(Nb) (such as 1Cr18Ni9Ti, 0Cr18Ni9Ti, 0Cr18Ni11Ti, 0Cr18Ni11Nb, etc.), after processes such as smelting, casting, forging, and rolling to shape the material, it is generally subjected to a solution treatment at temperatures of 920–1150°C followed by rapid cooling before being shipped out. At this point, most of the Ti (or Nb) in the steel should exist in the form of TiC(NbC). However, after welding, in a narrow region adjacent to the weld where the temperature is high (≥1150°C), TiC(NbC) decomposes, and the carbon in the steel dissolves into the austenitic matrix. During the subsequent cooling process, when this high-temperature region passes through 450–850°C, i.e., the sensitization temperature range, a large amount of chromium-rich M23C6 (Cr23C6) precipitates along the grain boundaries. This leads to chromium depletion at the grain boundaries, and under the influence of the medium, knife-line corrosion occurs. Therefore, knife-edge corrosion is a type of intergranular corrosion that occurs on the weld fusion line in Cr-Ni austenitic stainless steels containing Ti(Nb); it results from the decomposition of TiC(NbC) in the steel, the dissolution of Ti and C, and subsequent precipitation of chromium-rich M23C6, thereby creating chromium-deficient regions. Essentially, it is no different from intergranular corrosion in the sensitized state. 3. Material selection: Fundamentally, knife-edge corrosion is still caused by the relatively high carbon content often present in Cr-Ni austenitic stainless steels containing Ti(Nb). Therefore, when selecting materials, low-carbon (0.04–0.06%) and ultra-low-carbon (≤0.03%) Cr-Ni austenitic stainless steels should be considered first as substitutes for Ti(Nb)-containing stainless steels ； When it is necessary to use Cr-Ni austenitic stainless steel containing Ti(Nb), the carbon content in the steel must also be kept within the allowable range and as low as possible. II. Pitting corrosion of stainless steel 1. Phenomenon and identification: Pitting corrosion is the formation of small holes or rust spots of certain depth on the surface of stainless steel. Pitting is often difficult to detect because it is covered by rust layers, corrosion products, etc. Under a metallographic microscope, pitting exhibits various cross-sectional morphologies. Pitting generally occurs in specific corrosive media, especially in those containing Clˉ (including Brˉ, Iˉ). Common media that cause pitting corrosion in stainless steel include: the atmosphere, aqueous media and water vapor, seawater, bleach, various organic and inorganic chlorides, etc. Pitting can occur at room temperature, and becomes more likely to occur and more severe as the temperature of the corrosive medium rises. Pitting corrosion can not only cause perforation and damage to equipment, pipelines, etc., but also often induces intergranular corrosion, stress corrosion, and fatigue corrosion. Although pitting corrosion of stainless steel accounts for only about 20% of the corrosion-related damage in industries such as chemicals and petroleum, nearly 80% of the damage to stainless steel used in atmospheric environments is caused by pitting and rust spots. 2. Mechanism: It is generally believed that pitting in stainless steel occurs at defects on the metal surface such as non-metallic inclusions, precipitated phases, grain boundaries, and exposed dislocations; due to the vulnerability of the passivation film, its ability to repair itself is poor under the action of certain corrosive media, leading to damage. The occurrence of pitting involves two stages: nucleation and propagation. 3. Nucleation of pitting: In the presence of Cl˻ in the solution, at areas on the metal surface where manganese sulfide inclusions are present, it is difficult to achieve passivation; as a result of further attempts at passivation, preferential dissolution occurs, leading to the formation of small pits. The dissolution of sulfides produces H+ (or H2S), which activates the fresh surface of stainless steel and prevents the repassivation of small pits, thus avoiding the formation of pitting sources. III. Crevice Corrosion of Stainless Steel 1. Phenomenon and Identification: If there are metallic and non-metallic inclusions on the surface of stainless steel, such as metal particles, sand grains, dust, dirt, marine organisms, or due to structural reasons such as riveting, bolted connections, gaskets (rings), expansion joints between pipes and tube sheets, and contact with non-metals, crevices can form. Under the action of corrosive media, corrosion occurs within the gap, which is known as crevice corrosion. Crevice corrosion generally takes on a certain shape depending on the shape of the crack. In mild cases, it can be ordinary (general) corrosion within the gaps; in severe cases, it often appears as patches of pitting or ulceration. Research shows that almost all corrosive media can cause crevice corrosion in stainless steel, with no specific medium preferred. However, crevice corrosion in environments containing Clˉ is the most common ； Crevice corrosion has certain requirements regarding the size of the crack: it is necessary to make it difficult for substances to migrate between the solution inside and outside the crack, while still allowing the solution to enter the crack. The width of cracks in stainless steel that lead to crevice corrosion is generally in the range of 0.025 to 0.1 mm. 2. Mechanism: Crevice corrosion can be divided into pitting-type crevice corrosion and activation-type crevice corrosion. The former is crevice corrosion that originates from pitting, mainly caused by the oxidative degradation of the passivation film within the crack ； The formation mechanism of the latter is briefly described as follows: Due to the presence of gaps, it is difficult for the components in the solution within those gaps to migrate. For example, oxygen in the corrosion solution that can passivate stainless steel and enter the gaps can only do so through diffusion, resulting in a slow process. To maintain the passive state of stainless steel, oxygen in the gaps is rapidly depleted and not replenished in time, causing the passivation film on the surface of the stainless steel to begin undergoing reductive dissolution. As a result of this dissolution, the metal salts formed as corrosion products gradually concentrate, and due to hydrolysis, the pH value of the solution in the gaps drops sharply. When the pH value drops to the depassivation pH of stainless steel in the solution (that is, the critical pH at which the solution can no longer maintain the passive state of stainless steel), the passive film on the surface of the stainless steel within the gap suffers from reductive degradation, resulting in intergranular corrosion. 3. Material selection: Crevice corrosion in stainless steel is mainly caused by the acidification of the solution within the crevices and oxygen deficiency, which leads to the destruction of the surface passivation film. Therefore, improving the stability and passivation as well as repassivation ability of the stainless steel passive film is also an important measure for enhancing the resistance of stainless steel to crevice corrosion. Therefore, some of the measures for selecting materials resistant to pitting are also applicable to the selection of materials resistant to crevice corrosion. IV. Stress Corrosion of Stainless Steel 1. Phenomenon and Identification: Stress corrosion of stainless steel is a type of failure that occurs under the combined action of static tensile stress and specific working media. It is the most common and most harmful type of localized corrosion damage in stainless steel. Experience in analyzing engineering accidents shows that in stainless steel equipment and components, including those that have not been used, sudden leaks or damage often occur due to stress corrosion, especially when no obvious plastic deformation is observed at the site of the leak or damage. The main criteria for identifying stress corrosion are the characteristics of the cracks and the morphology of the fracture surfaces: (1) Crack characteristics: The macroscopic cracks resulting from stress corrosion all originate from the surface of the stainless steel, and their distribution is distinctly localized ； The direction of the crack is closely related to the stress applied, especially to the residual stress ； The cracks often appear as crackling patterns similar to those in air-dried wood, and no plastic deformation is observed near the cracks ； Except for the cracked areas, corrosion is mild in other parts, which often retain a metallic luster. The microstructure of stress corrosion cracks is mostly transgranular, but intergranular and a mixed transgranular+intergranular type are also common ； The cracks are narrow but extend deep; their depth is often several orders of magnitude greater than their width ； Cracks have both main branches and secondary branches; typical cracks resemble tree trunks and branches after the leaves have fallen, with sharp tips. (2) Fracture morphology: The macroscopic fractures of stress corrosion are mostly characterized by brittle fracture. The microscopic morphology of the fracture surfaces shows that transgranular fractures are mostly quasi-cleavage fractures; patterns such as rivers, fans, fishbones, and feathers are commonly observed ； Along the grain pattern, it usually takes on the appearance of rock sugar lumps. 2. Common media: The most common media causing stress corrosion in various types of stainless steels are air and industrial water containing Cl⁻ and oxygen, seawater, etc. Since Cr-Ni austenitic stainless steels are used the most, they are also associated with the highest number of stress corrosion incidents. 3. Mechanism: Due to the numerous factors influencing stress corrosion and the complexity of the process, to date, there is no unified understanding of stress corrosion in stainless steels. Regarding stress corrosion in high-strength stainless steels, such as martensitic and martensitically precipitated hardening stainless steels, many believe that hydrogen embrittlement plays a dominant role. However, some believe that in neutral aqueous solutions, it is anodic dissolution rather than hydrogen embrittlement that plays a dominant role in the stress corrosion of 13%Cr martensitic stainless steel. For Cr-Ni austenitic stainless steels, many researchers have also proposed that hydrogen embrittlement is the main mechanism underlying their stress corrosion. The main reason is that in boiling chlorides of Mg, Li, Ca, etc., in high-temperature water and steam, and in a room-temperature H2SO4+NaCl mixture, the plasticity of steel is significantly reduced due to hydrogen adsorption ； Under corrosion potential and cathodic polarization, hydrogen evolution is possible ； Under stress, austenite deformation can locally generate martensite; meanwhile, an increase in hydrogen content in the steel can promote this martensitic transformation ； After fracture, observation of the fracture surface indicated that it was a case of hydrogen embrittlement fracture. However, for widely used Cr-Ni austenitic stainless steels, stress corrosion caused by anodic dissolution at the crack tip is currently tended to be explained using a slip-dissolution-fracture model. Under the influence of a medium, a protective film (passivation film) that provides corrosion resistance exists on the surface of Cr-Ni austenitic stainless steel. Under tensile stress, dislocations move along the slip planes toward the metal surface, creating slip steps on the surface; this leads to local rupture of the surface film and exposure of the bare metal without any protective layer. Microcells are formed between membrane-covered and membrane-free metals. Under the influence of the medium, the bare metal acting as the anode undergoes anodic dissolution. At this point, the function of the protective film is not only to provide a cathode for the corrosion process, but also to concentrate the anodic dissolution in a localized area. Obviously, once the protective film is broken, if the exposed bare metal remains in an activated corrosion state, corrosion will inevitably spread laterally as well. As a result, the radius of curvature at the crack tip increases, the degree of stress concentration decreases, which in turn slows down the rate at which the crack propagates deeper until it eventually stops. However, it can be seen both in the laboratory and in the analysis of stress corrosion engineering accidents that the tips of stress corrosion cracks in stainless steel are extremely fine. Therefore, it is generally believed that while the bare metal is corroding, there must also be a process that prevents corrosion from spreading laterally, so that cracks can propagate longitudinally. This process is the repassivation of stainless steel. Therefore, the slip-dissolution-fracture model includes at least the formation of a surface film ； Under stress, metal slips, causing the surface film to break ； Four processes: anodic dissolution of bare metal and repassivation of bare metal. The repeated occurrence of these processes leads to stress corrosion cracking in stainless steel. Regarding intergranular stress corrosion in Cr-Ni austenitic stainless steels, the current main views are that under stress, selective dissolution occurs in the chromium-deficient regions at the grain boundaries of the stainless steel ； Preferred dissolution in stainless steel caused by the segregation of impurities along grain boundaries under stress ； Under stress, phenomena such as the dissolution of grain boundary precipitates in stainless steel occur. 4. Material selection: Research and practice have shown that the stress corrosion resistance of any stainless steel or alloy is conditional. There are no, and it is impossible to have, stainless steels and alloys that are resistant to stress corrosion under all conditions. Therefore, a reasonable selection must be made based on the equipment, the operating conditions of the components, and the characteristics of the stress-corrosion-resistant stainless steel itself. At the same time, during the processes of equipment and component processing, shaping, manufacturing, and use, it is also necessary to take into account the performance characteristics of the stainless steel selected; only by making appropriate choices and using it correctly can satisfactory results be achieved. Numerous statistics on stress corrosion accidents involving stainless steel both domestically and internationally show that accidents caused by chlorides account for over 80%. Therefore, it becomes even more important to select materials appropriately in order to address chloride stress corrosion in conventional Cr-Ni stainless steels. V. Fatigue corrosion of stainless steel 1. Phenomenon and identification: The failure of stainless steel caused by the combined action of a medium and alternating stress is known as corrosion fatigue. Since stainless steel is often used in corrosive environments, the failure of such steel under alternating stress is mostly due to corrosion fatigue. Compared to ordinary mechanical fatigue, corrosion fatigue in stainless steel is characterized by obvious corrosion and pitting on its surface. Corrosion fatigue can involve either a single crack or multiple cracks existing simultaneously, which is related to the fact that corrosion fatigue in stainless steel can initiate and propagate at one point or at multiple points. In stainless steel, corrosion fatigue cracks typically propagate tangentially or longitudinally at the macroscopic level, and often exhibit a serrated or stepped pattern ； Microscopically, cracks generally have no branches and their tips are blunt. Apart from corrosion and cracks, the most important characteristic of corrosion fatigue in stainless steel is the presence on the fracture surface of various features typical of mechanical fatigue. For example, the macrofracture surface is relatively smooth, appearing porcelain-like or shell-like, with fatigue arcs, fatigue steps, fatigue sources, etc ； Microfractures show fatigue striations, etc. Stainless steel can experience corrosion fatigue in any corrosive medium, with no preference for a particular medium. To verify whether it is corrosion fatigue, it can also be determined by checking whether failure still occurs after enhancing the steel’s strength and corrosion resistance or by removing the corrosive agents. If, due to the increased strength of the steel, fatigue fracture in stainless steel disappears or its service life is extended, it can be concluded that the original fracture was caused by mechanical fatigue ； If the corrosion resistance of the steel is improved or the effect of corrosive agents is eliminated, and as a result the fatigue fracture of stainless steel disappears or its service life is extended, it can be concluded that the original fracture was caused by corrosion fatigue. Stress corrosion can be accurately distinguished from corrosion fatigue based on fracture characteristics. 2. Mechanism At present, the mechanisms of corrosion fatigue in stainless steel mainly include the following models. (1) Pitting stress concentration model: It is believed that the stress concentration at the bottom of the pitting pits in stainless steel is the main cause of crack nucleation ； (2) Deformed metal preferential dissolution model: It assumes that the deformed metal acts as the anode, while the undamaged metal acts as the cathode, resulting in the preferential dissolution of the deformed portion ； (3) Surface film rupture model: It is believed that under alternating stress, metal slip zones penetrate the surface film, creating steps without a protective film, which puts them in an activated state and leads to dissolution, thereby initiating crack nucleation. Corrosion fatigue is formed by the repeated action of sliding and dissolution ； (4) Adsorption model: It is believed that the active substances in the corrosive medium adsorb onto the metal surface, reducing the surface energy and altering the mechanical properties of the material; this in turn facilitates the formation of slip zones on the stainless steel surface as well as the propagation of cracks. 3. Material selection: Choosing stainless steels with better corrosion resistance, as well as duplex stainless steels with a multiphase structure, are the main measures to address corrosion fatigue in stainless steels. Since corrosion fatigue in stainless steel usually originates from pitting, various stainless steels with good pitting resistance can be selected to prevent corrosion fatigue. For example, martensitic stainless steels with high levels of Cr and Mo, Cr-Ni austenitic stainless steels, and ferritic stainless steels, etc. Since some duplex stainless steels not only have high levels of Cr and Mo but also often contain N, they exhibit good pitting resistance. Moreover, due to their multiphase structure, this not only significantly enhances the corrosion fatigue strength of the steel but also makes it more difficult for fatigue cracks to propagate compared to steels with a single-phase structure. Therefore, the use of duplex stainless steels is an important approach to addressing corrosion fatigue failure in stainless steels. VI. Intergranular corrosion of chromium-nickel austenitic stainless steels in the sensitized state 1. Phenomenon and identification: Intergranular corrosion in the sensitized state occurs in the heat-affected zones of welded components, or in those parts that have been heated to 450–850°C; under the action of certain media, this leads to leakage or damage in these areas ； Equipment, components, etc., that cause sensitized intergranular corrosion show almost no change in size or shape, with no plastic deformation at all ； Apart from the corroded areas, there are no signs of corrosion elsewhere, and it still retains a distinct metallic luster ； Local sampling tests showed that the strength and plasticity of the corroded areas had been severely reduced; cracks appeared during cold bending, and in severe cases brittle fracture and grain detachment occurred, with no metallic sound heard when the fragments hit the ground. Under a metallographic microscope and a scanning electron microscope, it is evident that the grain boundaries of the steel widen due to corrosion, often taking on a network-like appearance; in severe cases, grain detachment also occurs. 2. Mechanism: The common intergranular corrosion in sensitized states can be satisfactorily explained by the low-chromium theory. Cr-Ni austenitic stainless steels are usually in a solution-treated state before use or at the time of delivery from the smelter. The stainless steel is heated to a high temperature (around 1000–1150°C, depending on the steel grade), held at that temperature for a while, and then rapidly cooled (usually by water cooling). At this point, when the carbon content in Cr-Ni austenitic stainless steel is above 0.02–0.03% (depending on the Ni content in the steel), carbon is in a supersaturated state within the steel. Subsequently, during the processing of stainless steel and its equipment, as well as the manufacturing and use of components, if heating at a sensitization temperature of 450–850°C is required (for example, during welding or when using the material within this temperature range), the supersaturated carbon in the steel will diffuse toward the grain boundaries, where it precipitates and forms carbon compounds with the chromium present there. In commonly used Cr-Ni austenitic stainless steels, this carbide is generally Cr23C6. Since this type of carbide contains a high amount of Cr, the precipitation of chromium carbides along the grain boundaries leads to a decrease in the Cr concentration in the steel matrix surrounding these carbides, resulting in so-called \"chromium-depleted zones\". When chromium carbides precipitate along the grain boundaries in a network pattern, the chromium-deficient areas also take on a network structure. Stainless steel is corrosion-resistant because, under the action of certain media, it contains enough chromium to enable the steel to become passivated in that medium. In the chromium-deficient areas, the insufficient amount of chromium reduces the passivation ability, or even eliminates it, while the austenite grains themselves still possess sufficient passivation (corrosion resistance) capacity. Therefore, under the action of corrosive media, the chromium-deficient areas that form a network-like structure near the grain boundaries dissolve preferentially, leading to intergranular corrosion. 3. Common media: There are many common media that can cause intergranular corrosion in Cr-Ni austenitic stainless steels. The table below lists only some of them for reference: Common media that cause intergranular corrosion in Cr-Ni austenitic stainless steels – Nitric acid, Nitric acid + Hydrochloric acid, Nitric acid + Hydrofluoric acid, Nitric acid + Acetic acid, Nitric acid + Chlorides, Fluorides, Nitric acid + Nitrates, Phosphoric acid, Phosphoric acid + Nitric acid, Phosphoric acid + Sulfuric acid, Formic acid, Lactic acid, Urea ammonium solution, Sulfuric acid, Sulfuric acid + Nitric acid, Sulfuric acid + Methanol, Sulfuric acid + Ferrous sulfate, Sulfuric acid + Sulfamic acid, Sulfuric acid + Copper sulfate, Copper sulfate, Ferric sulfate + Hydrofluoric acid, Hydrofluoric acid, Ferric chloride, Human body fluids. 4. Material selection: For a long time, people have used Cr-Ni austenitic stainless steels containing stabilizing elements such as Ti and Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Ti, 1Cr18Ni12Mo2Ti, 1Cr18Ni12Mo3Ti, 1Cr18Ni11Nb, 0Cr18Ni11Nb, etc., to prevent intergranular corrosion in the sensitized state, and satisfactory results have been achieved. The role of Ti and Nb is mainly to form stable carbides such as TiC and NbC with the supersaturated carbon in the steel, thereby preventing or reducing the formation of chromium carbide Cr23C6. However, stainless steels containing stabilizing elements such as Ti and Nb, especially those containing Ti, have many disadvantages. In today’s world where stainless steel smelting technologies are constantly evolving. Some of these drawbacks have severely hindered technological progress in stainless steel smelting and production, causing unnecessary losses and hazards in its use. For example, the addition of Ti increases the viscosity of the steel and reduces its fluidity, posing difficulties for the continuous casting process of stainless steel ； The addition of Ti deteriorates the surface quality of steel ingots and billets; it not only **increases the amount of grinding required in metallurgical plants but also significantly reduces the yield of steel, thereby raising the cost of stainless steel** ； The addition of Ti reduces the purity of the steel due to the formation of non-metallic inclusions such as TiN; this not only deteriorates the polishing properties of the steel but also lowers its corrosion resistance, as these inclusions often serve as sources of pitting ； In stainless steels containing Ti, under the influence of certain media, \"knife-like corrosion\" tends to occur along the weld seam, which likewise leads to corrosive damage in welded structural components. Due to the aforementioned drawbacks of Ti-containing stainless steels, in Japan, which has the highest production volume of stainless steel, the production of Ti-containing 18-8Cr-Ni stainless steels accounts for only 1–2% of the total production of Cr-Ni stainless steels; in China, however, it still makes up over 90% of the Cr-Ni stainless steel production. This reflects both the irrationalities in China’s stainless steel production and the use of various steel grades, as well as the backwardness in the structure of steel grades in China’s stainless steel production and utilization. It is recommended to use ultra-low carbon Cr-Ni austenitic stainless steel. Since the strength of ultra-low carbon Cr-Ni austenitic stainless steels is lower than that of those stabilized with Ti and Nb, when strength is insufficient, ultra-low carbon Cr-Ni austenitic stainless steels with controlled nitrogen content or nitrogen alloying can be used. These steels not only possess high strength but also exhibit better resistance to intergranular corrosion and pitting compared to stainless steels containing Ti and Nb. It is recommended that Cr-Ni austenitic stainless steels containing Ti and Nb be used only in situations where low-carbon or ultra-low-carbon stainless steels cannot serve as alternatives, such as for use as heat-resistant steels or in applications involving polyoxysulfuric acid. VII. Intergranular corrosion in the unsensitized state (solution-treated state) of chromium-nickel austenitic stainless steels 1. Intergranular corrosion in the unsensitized state of chromium-nickel austenitic stainless steels was discovered only in 1949; although some research has been conducted, to date no satisfactory theoretical or practical explanations or solutions have been found. 2. Phenomenon and identification: Intergranular corrosion in the unsensitized state (solution-treated state) refers to the intergranular corrosion that occurs in Cr-Ni austenitic stainless steels when they are heated to high temperatures (1000–1150°C), held at those temperatures, and then rapidly cooled. No further sensitization treatment (such as welding or heating at sensitization temperatures of 450–850°C) is required; this type of intergranular corrosion appears in certain corrosive media as well. Cr-Ni austenitic stainless steels that suffer from intergranular corrosion in the non-sensitized state include both ordinary stainless steels, as well as ultra-low carbon stainless steels resistant to intergranular corrosion in the sensitized state and stainless steels containing stabilizing elements such as Ti and Nb. Intergranular corrosion in the unsensitized state mainly occurs in HNO3 containing Cr6+. Except for 65% HNO3, it occurs most easily in concentrated HNO3, especially in fuming nitric acid. Furthermore, under the conditions of urea production by the carbon dioxide stripping method in China, intergranular corrosion in the un-sensitized state of urea-grade and non-urea-grade 00Cr17Ni14Mo2 and 00Cr25Ni22Mo2N, as well as the Fe-Ni-based corrosion-resistant alloy 00Cr20Ni35Mo2Cu3Nb (Carpenter 20cd-3), was observed in the high-temperature, high-pressure urea ammonium solution, at the liquid-phase-gas-phase interface, and in the gas phase. Intergranular corrosion in the unsensitized state generally occurs in the base metal far from the weld. Its identification is essentially the same as that of intergranular corrosion in the sensitized state. However, observations under a metallographic microscope and a scanning electron microscope revealed that the intergranular corrosion pattern of the un-sensitized Cr-Ni austenitic stainless steel in urea production plants differed significantly from the intergranular corrosion pattern in the sensitized state mentioned earlier. It is mainly characterized by wide intergranular corrosion cracks that often extend only shallowly and are frequently accompanied by grain detachment, but no precipitates are observed at the grain boundaries. 3. Mechanism: Studies have shown that the application of the solute (impurity) polarization theory can satisfactorily explain the causes of intergranular corrosion in the solid solution state (non-sensitized state). In nitric acid media containing Cr6+, the highly pure Cr-Ni stainless steels Cr14Ni14 and 1Cr18Ni11Ti were used to study the effect of elements such as C, P, Si, and B on intergranular corrosion in the unsensitized state. However, whether the intergranular corrosion in the unsensitized state is caused by a simple electrochemical corrosion process resulting from chemical concentration gradients at and within the grain boundaries due to the segregation of impurity elements such as P, Si, and B, or by a decrease in the corrosion resistance of the grain boundaries due to this segregation, or by other factors, remains to be further investigated. 4. Material selection: Theoretically, developing high-purity Cr-Ni austenitic stainless steels with P≤0.01%, Si≤0.10%, and B≤0.008% is the most fundamental approach to addressing intergranular corrosion in the non-sensitized state. Currently, to address intergranular corrosion in the non-sensitized state in applications involving nitric acid, stainless steels with high silicon content (Si~4%), such as 0Cr18Ni11Si4AlTi, 00Cr20Ni24Si4Ti, 00Cr14Ni14Si4, and 00Cr17Ni15Si4Nb, are primarily used. To address intergranular corrosion in the non-sensitized state of Cr-Ni austenitic stainless steel in the four high-pressure equipment used in urea production via the carbon dioxide stripping method – namely, the urea synthesis tower, high-pressure condenser, high-pressure washer, and carbon dioxide stripping tower – urea-grade grades 00Cr17Ni14Mo2 and 00Cr25Ni22Mo2N, which have extensive proven experience in practical use, still need to be selected. However, it is necessary to control the levels of C, P, and Si in the steel as much as possible, especially keeping the P level as low as possible. VIII. Intergranular Corrosion of Ferritic Stainless Steels 1. Phenomenon and Identification: The intergranular corrosion of ferritic stainless steels differs from that of the aforementioned Cr-Ni austenitic steels; it generally occurs after heating at temperatures above 900–950°C (or after welding), and cannot be avoided even under rapid cooling conditions such as in water ； Short-term heating at 750–850°C can reduce, or even eliminate, the intergranular corrosion susceptibility of ferritic stainless steels ； Intergranular corrosion in ferritic stainless steels occurs in the area immediately adjacent to the weld fusion line, rather than in the heat-affected zone of Cr-Ni austenitic stainless steels. Apart from the differences in location, the identification of intergranular corrosion in ferritic stainless steels is essentially the same as that of intergranular corrosion in the sensitized state of Cr-Ni austenitic stainless steels. Intergranular corrosion in ferritic stainless steels occurs not only in highly corrosive media but also in less aggressive environments, such as tap water. 2. Mechanism: Numerous studies have shown that the low-chromium theory can also satisfactorily explain the intergranular corrosion phenomenon in ferritic stainless steels. When high-chromium ferritic stainless steel is heated above 900–950°C, C and N dissolve in the matrix of the steel. Since the diffusion rate of Cr in ferrite is about 100 times that in austenite, C and N not only diffuse rapidly in ferrite (at 600°C, the diffusion rate of C in ferrite is about 600 times that in austenite), but also have a low solubility (in ferritic steel containing 26% Cr, the solubility of C at 1093°C is 0.04%, while at 927°C it is only 0.004%; at even lower temperatures, it drops below 0.004%) ； The solubility of N is 0.023% at temperatures above 927°C, whereas it is only 0.006% at 593°C). Therefore, after high-temperature heating, during the subsequent cooling process, even rapid cooling often fails to prevent the precipitation of chromium-rich carbon and nitrogen compounds along the grain boundaries and the formation of chromium-poor regions. Treatment at 750–870°C can reduce and eliminate the intergranular corrosion tendency of ferritic stainless steels. However, in the range of 500–700°C, the diffusion rate of chromium in steel decreases, making it impossible to eliminate the chromium-deficient areas in a short period of time. Therefore, high-temperature heating is carried out first; during cooling, the ferritic stainless steel passes through the 500–700°C temperature range. Due to the presence of chromium-deficient areas at the grain boundaries, intergranular corrosion occurs under the action of corrosive agents. Studies have shown that in ferritic stainless steels containing 20% Cr, the Cr content in the chromium-deficient regions can