Intergranular corrosion of stainless steel materials

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I. Rationale for the issue Technical standards generally stipulate that \"containers made of austenitic stainless steel, when used in environments where intergranular corrosion may occur, should undergo solution treatment or stabilization treatment after welding.\" There is a valid reason for such requirements. However, even if designers specify in the technical requirements for the drawings that the manufacturer carry out post-weld heat treatment on stainless steel containers (such as heat exchangers), it is often difficult to meet the ideal requirements set by the designers due to the challenges associated with controlling the actual heat treatment parameters and other unforeseen difficulties. In practice, the vast majority of stainless steel equipment in use is operated in its post-weld state. This leads us to wonder: intergranular corrosion is the most common form of corrosion in austenitic stainless steels, so what is the mechanism behind its occurrence? In what medium environments can intergranular corrosion occur? What are the main methods for preventing and controlling intergranular corrosion? Do containers made of austenitic stainless steel require heat treatment after welding in environments where intergranular corrosion may occur? This article reviews relevant standards, specifications, and monographs, and presents personal views based on practical production experience. II. Mechanism of intergranular corrosion Intergranular corrosion is a common type of localized corrosion; it occurs along the boundaries between the grains of a metal or alloy, or in the areas adjacent to those boundaries. Since grain corrosion is minimal in such cases, this type of corrosion is referred to as intergranular corrosion. It weakens the bonds between the grains. Severe intergranular corrosion can cause the metal to lose its strength and ductility, leading to fracture under normal loads. The modern theories of intergranular corrosion mainly include the chromium-deficiency theory and the theory of selective dissolution of grain boundary impurities. 2.1 The chromium-deficiency theory: Intergranular corrosion in commonly used austenitic stainless steels in oxidizing or slightly oxidizing media is mostly caused by improper heating during processing or use. The so-called improper heating refers to the situation where steel is heated or cooled slowly through the temperature range of 450–850 °C, which makes the steel susceptible to intergranular corrosion. Therefore, this temperature is a dangerous temperature for the use of austenitic stainless steel. Stainless steel is subjected to solution treatment upon leaving the factory. Solution treatment involves heating the steel to 1050–1150 °C and then quenching it, with the aim of obtaining a homogeneous solid solution. Austenitic steel contains a small amount of carbon, and the solubility of carbon in austenite decreases as the temperature drops. For example, in 0Cr18Ni9Ti, the solubility of carbon is about 0.2% at 1100 °C, and about 0.02% at 500–700 °C. Therefore, in steel that has undergone solution treatment, carbon is supersaturated. When steel is heated or cooled through 450–850 °C, carbon can form (Fe, Cr)23C6, which precipitates from austenite and distributes at the grain boundaries. The chromium content in (Fe, Cr)23C6 is much higher than that in the austenitic matrix; the precipitation of this compound consumes a large amount of chromium near the grain boundaries. Since chromium cannot be replenished in time from within the grains through diffusion – due to the slow diffusion rate of chromium – the chromium content near the grain boundaries falls below the level required for passivation (i.e., 12% Cr), resulting in chromium-deficient regions. This disrupts the passivated state, causing the potential near the grain boundaries to drop, while the grains themselves remain in a passivated state with a higher potential. The grains and grain boundaries thus form a active-passive microelectrochemical cell, with a large cathode area relative to a small anode area, which leads to corrosion in the grain boundary region. 2.2 Theory of selective dissolution of grain boundary impurities In practical production, it has also been observed that austenitic stainless steels can suffer from intergranular corrosion in strongly oxidizing media such as concentrated nitric acid; however, the nature of this corrosion differs from that in oxidizing or weakly oxidizing media. It usually occurs in steel that has undergone solution treatment; it generally does not occur in steel that has been passivated. When the solute contains 100 ppm of phosphorus as an impurity, or 1000–2000 ppm of silicon as an impurity, they will precipitate at the grain boundaries. These impurities dissolve under the action of strongly oxidizing media, leading to intergranular corrosion. When steel is sensitized, either carbon can combine with phosphorus to form (MP)23C6, or the preferential accumulation of carbon limits the diffusion of phosphorus toward the grain boundaries; in either case, this reduces or eliminates the accumulation of impurities at the grain boundaries, thereby reducing or eliminating the steel’s susceptibility to intergranular corrosion. The two theories mentioned above for explaining the mechanism of intergranular corrosion are each applicable to certain microstructural states of alloys and certain media; they do not exclude each other but rather complement one another. In production practice, most cases of intergranular corrosion of stainless steel occur in weakly oxidizing or oxidizing media; therefore, the chromium-depletion theory can be used to explain the vast majority of corrosion instances. III. Medium environments that cause intergranular corrosion There are mainly two types of media that cause intergranular corrosion in commonly used austenitic stainless steels. One category consists of oxidizing or weakly oxidizing media, while the other category includes strongly oxidizing media such as concentrated nitric acid. The first type is the most common; below are the common medium environments that cause intergranular corrosion in austenitic stainless steels. 3.1 Common media that cause intergranular corrosion in austenitic stainless steel
In the “Corrosion Data Charts” compiled by G. A. Nelson, the common media that can cause intergranular corrosion in austenitic stainless steel are listed as follows: acetic acid, acetic acid + salicylic acid, *ao acid an, sulfuric acid an, chromic acid, copper sulfate, fatty acids, formic acid, ferric sulfate, hydrofluoric acid + ferric sulfate, lactic acid, nitric acid, nitric acid + hydrochloric acid, oxalic acid, phosphoric acid, seawater, salt spray, sodium bisulfate, sodium hypochlorite, sulfur dioxide (wet), sulfuric acid, sulfuric acid + copper sulfate, sulfuric acid + ferrous sulfate, sulfuric acid + methanol, sulfuric acid + nitric acid, sulfurous acid, phthalic acid, sodium hydroxide + sodium sulfide. 3.2 Intergranular corrosion tendency test: When austenitic stainless steels are used in environments that may cause intergranular corrosion, an intergranular corrosion tendency test shall be conducted in accordance with GB4334.1–GB4334 \"Test methods for intergranular corrosion of stainless steels\". The selection of test methods for assessing the intergranular corrosion tendency of austenitic stainless steels, as well as the acceptance criteria, shall comply with the following provisions: (1) Austenitic stainless steels and stainless steels designed for use in concentrated nitric acid, when used in nitric acid at a temperature of 60 °C or higher and a concentration of 5 % or higher, shall be tested in accordance with GB4334.3 \"Test Method for Corrosion of Stainless Steels in 65% Nitric Acid\". The average corrosion rate over five cycles, or the corrosion rate over three cycles, shall not exceed 0.6 g/m2·h (or equivalent to 0.6 mm/year). The sample state can be in service or sensitized state. (2) Chromium-nickel austenitic stainless steels (such as 0Cr18Ni10Ti, 0Cr18Ni9, 00Cr19Ni10 and similar grades): General requirement: In accordance with GB4334.5 \"Test methods for corrosion of stainless steels in sulfuric acid—copper sulfate solution\", no intergranular corrosion cracks shall be present on the surface of the specimen after the bending test. Higher requirements: In accordance with GB4334.2 \"Test methods for corrosion of stainless steel by sulfuric acid and ferric sulfate\", the average corrosion rate shall not exceed 1.1 g/m2·h. (3) Molybdenum-containing austenitic stainless steels (such as 0Cr18Ni12Mo2Ti, 00Cr17Ni14Mo2, and similar grades): General requirement: In accordance with GB4334.5 \"Test methods for corrosion of stainless steels in sulfuric acid–copper sulfate solution\", no intergranular corrosion cracks shall be present on the surface of the specimen after the bending test. Higher requirements: According to GB4334.4 \"Test methods for corrosion of stainless steel by nitric acid and hydrofluoric acid\", the corrosion ratio shall not be greater than 1.5. It is also possible to follow GB4334.2 \"Test methods for sulfuric acid—ferric sulfate\", with the average corrosion rate not exceeding 1.1 g/m2 h. (4) When special requirements are imposed on the medium, intergranular corrosion tests other than those specified above may be conducted, with corresponding acceptance criteria established. IV. Measures to Prevent and Control Intergranular Corrosion Based on the mechanisms of corrosion, the following measures can be taken to prevent and control intergranular corrosion in austenitic stainless steels: (1) Use ultra-low carbon stainless steels to reduce the carbon content to below 0.03%, such as 00Cr17Ni14Mo2; this prevents the formation of (Fe, Cr)23C6 in the steel, eliminates chromium-depleted zones, and thus stops intergranular corrosion from occurring. Generally, it does not have high strength and is not subject to heavy loads; for parts that require good plasticity, 0Cr18Ni9 and similar materials can be chosen from an economic perspective. (2) For stabilized stainless steel, those containing titanium and niobium are used (i.e., what we commonly refer to as stabilized stainless steel). During the steel manufacturing process, certain amounts of titanium and niobium are added; these elements have a strong affinity for carbon, which leads to the formation of TiC or NbC in the steel. Moreover, the solubility of TiC or NbC is much lower than that of (Fe, Cr)23C6, and they hardly dissolve in austenite at the solubilization temperature. In this way, even when passing through the sensitization temperature, (Fe, Cr)23C6 does not precipitate in large amounts at the grain boundaries, thereby substantially eliminating the tendency of austenitic stainless steels to suffer intergranular corrosion. Stains such as 1Cr18Ni9Ti and 1Cr18Ni9Nb can operate in the range of 500–700 °C without a tendency to intergranular corrosion. (3) Re-perform solution treatment. When welding austenitic stainless steel, the temperature of the arc molten pool exceeds 1300 °C. The temperature on both sides of the weld decreases with increasing distance; there exists a sensitization temperature range within this region. It is necessary to avoid heating austenitic stainless steels and cooling them slowly within their sensitization temperature range. If intergranular corrosion is detected, unstabilized stainless steels are generally heated to 1000–1120 °C; the holding time is 1–2 minutes per millimeter, after which they are cooled rapidly ； It is appropriate to heat stabilized stainless steel to 950–1050 °C. Even after solution treatment, the steel must be prevented from being heated at sensitization temperatures; otherwise, chromium carbides will redeposit along the grain boundaries. (4) When using the correct welding method, if the operation is not proficient or the welding material is too thick, the longer the welding time, the greater the chance of remaining in the sensitization temperature range, which in turn makes the base metal on both sides of the weld susceptible to intergranular corrosion. To reduce the sensitivity of the welded joint, the input of wire energy during welding should be minimized as much as possible. Generally, TIG welding requires less input wire energy than arc welding; therefore, TIG welding should be used for welding and repair work. For welded parts, ultra-low carbon stainless steel or stainless steel containing stabilizing elements such as Ti and Nb should be used, while for electrodes, ultra-low carbon electrodes or electrodes containing Nb should be selected. When using TIG welding, to prevent overheating of the weld joint, the operation must be performed quickly, and rapid cooling is required after welding, in order to minimize the time that the base metal on both sides of the weld remains within the sensitization temperature range. V. Post-weld treatment: Heat treatment after welding is not always necessary for the weld area. Generally, solution treatment involves holding the material at a temperature of 1100–1150 °C for a certain period of time before rapid cooling; the cooling process must be completed within 3 minutes, with the temperature dropping to the range of 925–540 °C, followed by further rapid cooling to below 425 °C ； The stabilization treatment requires holding at a temperature range of 850–880 °C for several hours before air cooling. The desired post-weld heat treatment results are closely related to various key process parameters throughout the entire heat treatment process (such as the temperature upon entering the furnace, the heating rate, the temperature differences in different parts of the workpiece during heating, the atmosphere inside the furnace, the holding time, the temperature differences in different parts during holding, the cooling rate, and the temperature upon exiting the furnace). For austenitic stainless steel vessels used in environments where intergranular corrosion may occur, solution treatment or stabilization treatment of general components can be carried out. Moreover, performing post-weld heat treatment on the welds of the entire container (which are mostly heat exchangers) presents numerous difficulties. This type of treatment is not a localized post-weld heat treatment, but rather a post-weld heat treatment of the entire welded component or the whole container. Due to the complex structural shapes of most chemical processing vessels (such as the shell-and-tube heat exchangers we commonly use). If it is required to perform post-weld solution or stabilization treatment on the welded areas of an entire shell-and-tube heat exchanger, the aforementioned key process parameters simply cannot be controlled; let alone ensuring the quality of the post-weld heat treatment. Even when attempts are made to address the issue, they often backfire; not only is the microstructure of the weld not improved, but the microstructure of the base material also suffers undue deterioration. Therefore, even for chemical reactors made of austenitic stainless steel used in intergranular corrosion environments, over 90% are used in their as-welded condition rather than after post-weld heat treatment. VI. Some observations: Chromium-nickel austenitic stainless steel is the most commonly used corrosion-resistant material; intergranular corrosion is the most common mode of failure in containers made of chromium-nickel austenitic stainless steel. Intergranular corrosion weakens the bond between grains, and in severe cases can result in the complete loss of mechanical strength. Stainless steel affected by this corrosion appears shiny on the surface, but crumbles into fine particles at the slightest tap. Since intergranular corrosion is difficult to detect, it can cause sudden failure of equipment, and its hazards are significant; therefore, it deserves our full attention. Chromium-nickel austenitic stainless steel containers are essentially formed by welding, and the areas on either side of the weld joints are zones susceptible to intergranular corrosion; these areas are always subject to corrosion and degradation before the base material. Our goal is to enhance the intergranular corrosion resistance of the weld area through post-weld heat treatment, so that it reaches the same level as that of the base material; this is also the original intention behind carrying out post-weld heat treatment. However, in practice, there are many factors to consider, such as the complex overall structural shape of the welded parts and the difficulty in maintaining consistent parameters for post-weld heat treatment. As a result, in reality, the vast majority of chromium-nickel austenitic stainless steels in service are used in their post-weld condition. It is not possible to make a general statement regarding whether solubilization or stabilization treatment should be applied to the weld areas of chromium-nickel austenitic stainless steel containers used to resist intergranular corrosion; instead, a specific analysis of the container’s structural design is necessary to determine whether heat treatment can achieve the desired results. Otherwise, even if requirements for post-weld heat treatment are established, the outcomes are often contrary to what is intended – not only will the desired effects not be achieved, but the microstructure of the base material may also be affected. To enhance the resistance of chromium-nickel austenitic stainless steel vessels to intergranular corrosion, it is necessary, based on the specific corrosive environment and corrosion mechanisms, to first select ultra-low carbon or stabilized stainless steels during material selection. During welding, the appropriate welding method should be chosen. Only by properly combining these various preventive and control measures can good results be achieved; one cannot rely solely on post-weld solution treatment or stabilization treatment.