Cases of sensitization-intergranular corrosion in stainless steel

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I. Case 1: Overview of the cracking of the stainless steel composite layer in the heat exchanger tube box: During maintenance of a certain facility, severe corrosion was found on the inner wall of one of the heat exchanger tube boxes. The basic parameters of the tube train are as follows: Tube train pressure: designed at 2.68 MPa, operating pressure ≤ 2.34 MPa ； Gang temperature: Designed at 186°C, operating ≤166°C℃ ； Governing medium: moderate-temperature gas (CH4: 2.3%, CO2: 13%, H2: 80%, CO: 1.1%, N2: 2.5%) ； Material of tube bank and tube box: 16MnR+304 ； The post-weld heat treatment temperature for the tube box is: 620°C. Figure: Causes of failure due to cracking in the stainless steel composite layer – Improper heat treatment led to sensitization of the stainless steel, resulting in intergranular corrosion in a CO2 (carbonic acid) environment. II. Concept 1: Description of damage and mechanisms of damage a) Sensitization: Ordinary 300-series stainless steels (such as 304 and 316) have a relatively high carbon content and are in an unstable state, meaning they do not contain stabilizing elements such as titanium or niobium. At room temperature, the solubility of carbon in austenite is very low, at around 0.02%–0.03%, which is far lower than the actual carbon content in these stainless steels. As a result, the supersaturated carbon remains dissolved in the austenite. When the temperature exceeds 425°C and remains within that range for a certain period of time, the supersaturated carbon begins to diffuse toward the grain boundaries of austenite, where it combines with chromium to form chromium carbide compounds, such as tricarbide of chromium (Cr23C6). The diffusion rate of chromium within the grains is slower than that along the grain boundaries; the chromium inside has insufficient time to diffuse to the grain boundaries. The chromium required for the formation of chromium carbide in the intergranular regions comes mainly from the area near the grain boundaries, resulting in a significant reduction in the chromium content in that vicinity. When the chromium mass fraction at the grain boundaries drops below 12%, so-called \"chromium-deficient regions\" are formed. There are differences in electrochemical properties between these chromium-deficient regions and the grains themselves, which results in the creation of an activation-passivation cell with a large potential difference between the chromium-deficient regions (anode) and the matrix in its passivated state (cathode). The small anode in the chromium-deficient region and the large cathode of the matrix form a corrosion cell; under the action of the corrosive medium, the chromium-deficient region is rapidly corroded. The grain boundaries are damaged first, the bonding strength between the grains is significantly reduced, the mechanical properties deteriorate, and the mechanical strength **decreases**, although deformation is not noticeable. The precipitation of such carbides at the grain boundaries is generally referred to as sensitization. In austenitic stainless steels containing stabilizing elements, repeated heating and cooling cycles in the welded joint area cause carbon atoms, which were originally dissolved in titanium carbide (TiC) or niobium carbide (NbC), to precipitate out in narrow areas. These carbon atoms then combine with chromium to form chromium carbide compounds such as tricarbide twenty-three chromium (Cr23C6), thereby creating chromium-deficient regions that reduce the material’s corrosion resistance. b) Intergranular corrosion: After metal materials become sensitized, the grain boundaries suffer preferential corrosion in a corrosive medium due to their lower resistance to corrosion ； It is a localized failure process in which, in materials that are not sensitized or in which sensitization has not occurred, corrosion occurs preferentially at the grain boundaries or near them in specific corrosive media, resulting in a loss of cohesion between the grains. Sensitized austenitic stainless steel is highly susceptible to intergranular corrosion. 2 Damage morphology a) When sensitization occurs, there are generally no significant changes in size or shape, and no plastic deformation takes place ； When intergranular corrosion occurs, if the grain detachment is not obvious, it is difficult to detect the damage by visual inspection. b) Corrosion traces are only visible in the sensitized areas; if the sensitized zone is narrow, such as those formed during welding, narrow corrosion grooves or cracks generally appear. c) The sensitized area may still retain a bright metallic luster, but it loses all plasticity; it is prone to cracking during cold bending, and in severe cases, brittle fracture and the shedding of metal grains occur, with no sound of metal impact even when it hits the ground. If intergranular corrosion has occurred, obvious grain detachment can sometimes take place, causing the metal surface to lose its luster, and significant thinning may occur in certain areas. d) Under a metallographic microscope or scanning electron microscope, it can be observed that the grain boundaries become significantly wider and often take on a network-like appearance; in severe cases, obvious grain detachment can be seen. e) When sensitization and intergranular corrosion occur in the welded joints of austenitic stainless steels containing stabilizing elements, a distinct \"knife-like corrosion\" (or blade-like corrosion) can be observed in the joint area. f) The sensitized material is prone to intergranular corrosion under the action of corrosive media, and it also often leads to stress corrosion cracking along grain boundaries in areas with high tensile stress. 3 Affected materials include 300 series stainless steels (without stabilizing elements such as titanium or niobium, or containing such elements but not stabilized), and similar situations sometimes occur with nickel-based alloys and aluminum alloys as well. 4 Main influencing factors a) Carbon content: The higher the carbon content, the greater the sensitivity to sensitization, the stronger the tendency for intergranular carbide precipitation, and the more likely intergranular corrosion will occur ； b) Alloy composition: The addition of elements such as titanium and niobium, which can form stable carbides (TiC or NbC), along with stabilization treatment, can reduce sensitivity to sensitization and intergranular corrosion ； c) Heat treatment: Heating to a high temperature for solution treatment, followed by rapid cooling (such as water cooling) to form a single austenite phase, can prevent sensitization. However, on-site construction generally cannot meet the requirements for solution treatment; therefore, this method is usually used only in manufacturing facilities ； d) Process conditions: In sections where 300-series stainless steel is used, reducing the operating temperature to below 425°C can prevent sensitization. 5 Devices or equipment prone to problems a) Catalytic cracking units used at high temperatures – 300 series stainless steel equipment ； b) 300-series stainless steel shift gas pipes for gasification units ； c) Superheated steam furnace tubes and furnace tubes for liquid pyrolysis feedstock made of 300 series stainless steel in the convection section of pyrolyzers in ethylene cracking units, where the operating temperature in the cracking and quenching systems is above 400°C ； d) Stainless steel lining of the urea synthesis tower in the urea plant ； e) Other equipment and piping made or installed by welding methods and not subjected to solution heat treatment, made of 300 series stainless steel, which are relatively sensitive if the material is not of a low-carbon grade ； f) Equipment or linings made of 300-series or 400-series stainless steels that suffer from σ-phase embrittlement are prone to intergranular corrosion (i.e., intergranular corrosion occurs even without sensitization). 6 Main preventive measures a) Using austenitic stainless steels with low carbon content can effectively reduce the occurrence of sensitization, such as the ultra-low carbon austenitic stainless steel series ； b) Add certain alloying elements, such as titanium and niobium, to form stable carbides ； c) Solution heat treatment is generally applied only to equipment and pipelines that are under fabrication in the factory; it is not recommended for use at the construction site ； d) Adjust the ratio of austenite-forming elements to ferrite-forming elements in the steel to obtain an austenite + ferrite dual-phase structure, which is less prone to grain boundary sensitization ； e) For ferritic stainless steels prone to intergranular corrosion, annealing is carried out at 700°C–800°C. 7 Detection or monitoring methods a) Sensitization generally cannot be observed directly; however, if the sensitized material corrodes or cracks under the influence of the medium, it may be observable directly ； b) When intergranular corrosion occurs in unsensitized materials, the grains separate noticeably; visual inspection reveals an uneven surface, and in some areas the loss of grains results in distinct corrosion zones. Otherwise, it is generally difficult to detect this through visual inspection ； c) Metallographic analysis or scanning electron microscopy observation ； d) Install corrosion coupons at areas prone to intergranular corrosion to regularly measure the amount of metal loss ； e) Penetrant testing.