Corrosion hazards of austenitic stainless steels when used in Cl–media

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Corrosion hazards of austenitic stainless steels when used in Cl–media / Shu Runtao 1. Overview of austenitic stainless steels Austenitic stainless steels are typically represented by 304, 321, 304L, and 316L; due to the differences in their alloying elements, they exhibit resistance to corrosion in various media environments, and are widely used in industries such as petroleum, chemicals, pharmaceuticals, power generation, and civil engineering. Compared to 304, 321 contains titanium added to the material to improve weldability. Due to the higher reactivity of titanium metal compared to carbon, titanium acts to stabilize chromium in the weld heat-affected zone, thereby preventing intergranular corrosion in that area caused by chromium deficiency. The corrosion resistance of 304 and 321 is comparable under most medium conditions; however, in environments subjected to severe acid scouring corrosion, knife-edge corrosion occurs at the weld edges of the 321 material. 304L improves the weldability of the material by further controlling carbon content; however, the reduced carbon level results in lower strength compared to 321. 316L (00Cr17Ni14Mo2) austenitic steel is a super-low carbon austenitic stainless steel containing Mo, and it exhibits good resistance to uniform corrosion and pitting corrosion in many media conditions. An increase in Ni content (14%) facilitates the stabilization of the austenite phase. 316L is significantly superior to 304 (0Cr18Ni9) and 321 (0Cr18Ni10Ti) stainless steel materials in terms of resistance to intergranular corrosion, high-temperature sulfur, high-temperature naphthenic acid, and pitting corrosion. Based on numerous experiments and practical applications, it has been shown that the stress corrosion resistance of 316L in Cl–corrosive environments is only comparable to that of 304 and 321 materials. In engineering applications, the probability of failure due to stress corrosion exceeds 50%. When the medium contains more than 10 ppm of Cl–, the hazard associated with stress corrosion becomes quite significant, as Cl– can accumulate in certain areas such as under scale in circulating water, in the gaps between heat exchange tubes and tube sheets, due to mechanical damage, and in areas with stress concentration in the heat-affected zones of welds. It should be noted that austenitic stainless steel materials that have undergone solution treatment or stabilization do not exhibit significant destructive stress corrosion behavior in the absence of processing stresses and welding stresses. 2. Corrosion mechanism of metal materials by Cl— 2.1 Pitting corrosion All metal materials contain non-metallic inclusions to varying degrees, such as sulfides, oxides, etc. These non-metallic compounds on the surface of the material will rapidly develop into pitting corrosion under the corrosive action of Cl—. Once pits are formed, due to the effect of the closed cell, Cl— ions outside the pits will migrate into them, while the positively charged metal ions inside the pits will migrate outward, thereby causing electrochemical corrosion. Due to the very small atomic radius of Cl—, any non-metallic inclusions in the metal as well as welding defects become sources of corrosion caused by Cl— penetration. For stainless steel materials with low alloy content and without molybdenum, although they have a relatively dense oxide film on their surface, pitting corrosion can occur easily under the influence of Cl−, which in turn induces stress corrosion. In stainless steel materials, those with Mo added exhibit better resistance to pitting corrosion compared to those without Mo; the higher the Mo content, the better the resistance to pitting corrosion. Pitting corrosion is the origin of stress corrosion; when the Mo content in steel is ≥3%, it is sufficient to prevent Cl− from penetrating into the material matrix. In austenitic stainless steels, the main role of Ni is to form and stabilize austenite, enabling the steel to achieve a fully austenitic structure, enhancing the material’s toughness, and also providing good resistance to oxidation and corrosion. However, Ni in ordinary austenitic steel does not provide resistance to pitting corrosion in a Cl–corrosive environment. 2.2 Crevice Corrosion: Crevice corrosion operates on the same principle as pitting corrosion; it is a corrosion phenomenon that occurs due to the accumulation of Cl— ions as a result of the presence of closed electrochemical cells within the crevices. This type of corrosion generally occurs in the gaps of flange gaskets, lap joints, bolt nuts, as well as in the gaps between heat exchange tubes and tube sheet holes. Gap corrosion is closely related to the concentration of the liquid that remains stationary in these gaps; once such an environment for gap corrosion exists, the likelihood of stress corrosion occurring is quite high. 2.3 Stress corrosion: Cl– causes extremely severe stress corrosion damage to austenitic stainless steels. The important factors affecting stress corrosion in austenitic stainless steels are temperature, the medium, the shape/size and distribution of non-metallic inclusions, as well as the influence of processing stresses. The fracture direction of stress corrosion is generally perpendicular to the direction of the stress, and it propagates in a dendritic pattern. Stresses originate from residual stresses resulting from cold deformation, welding, and metal impact; the occurrence of these stresses disrupts the stable structure within the metal, causing dislocations to form slip steps under the action of the stress. The presence of these slip steps provides an opportunity for Cl– to adsorb and penetrate. Chloride stress corrosion resistance test [1]: In the aforementioned corrosive environment, both ultra-pure ferritic stainless steels and duplex stainless steels withstood a testing period of over 1000 hours without experiencing fracture. It can be seen that ordinary austenitic stainless steels are not resistant to chloride stress corrosion. 3. Cases of corrosion of austenitic stainless steel in Cl– environments. Although austenitic stainless steel is susceptible to pitting corrosion, crevice corrosion, and stress corrosion in Cl– environments, both pitting corrosion and crevice corrosion can evolve into forms of stress corrosion, ultimately leading to destructive corrosion failure of the equipment. Here are a few specific examples of corrosion-induced failure in austenitic stainless steels for discussion. 3.1 Stress corrosion caused by welding issues 3.1.1 Stress corrosion in the heat-affected zone of welds Failure in the welded areas of austenitic stainless steels first occurs in the heat-affected zone of the welds, and then spreads toward the center of the weld as well as on both sides of the base material. The thermal expansion coefficient of austenitic stainless steel is 1.35 times that of ferritic steel. Under the effect of thermal expansion in the welding pool, the fluidity of the molten steel increases; upon cooling, it is subject to contraction forces within the welding pool, resulting in significant contraction deformation and certain tensile stresses. As a result, the likelihood of stress corrosion increases. Furthermore, the sensitization temperature for austenitic stainless steel is 650°C; if rapid cooling is not carried out immediately after welding, the weld zone and heat-affected zone will also experience a decrease in their corrosion potential due to chromium deficiency, allowing Cl– ions to adsorb in these areas. This can further lead to pitting corrosion progressing into stress corrosion cracks. In a certain facility, the medium used in the first-effect evaporator is citric acid at 125°C, and 316L material is selected for its construction; the actual operating period is only 4 to 6 months, with corrosion occurring primarily at the edges of the welds. Citric acid itself does not have a strong corrosive effect, but the process water contains 200–300 ppm of Cl− in citric acid. Given that corrosion occurs primarily in the weld fusion zone and heat-affected zone, it is the chloride ions that are responsible for the stress corrosion of 316L stainless steel. During welding, 316L material develops chromium depletion at the edges of the weld seam, which creates voids in the lattice near the weld seam and provides conditions for chloride ions to accumulate; as a result, the equipment suffers from corrosion and damage in a short period of time. The corrosive failure of this material is characterized by stress corrosion, which occurs after pitting corrosion caused by chloride ions and is further exacerbated by welding stresses. 3.1.2 Stress corrosion caused by welding spatter: If the base material is not protected during welding, allowing spatter and weld beads to merge with it, this will generate significant cooling contraction stresses, and it will cause the base material in the area where the spatter and weld beads are attached to develop in the form of transverse columnar crystals. Although the weld beads and spatter can be ground smooth using a grinder after welding, the microstructure of the material in that area has changed; the grains become larger and more directional, which makes it highly susceptible to pitting and stress corrosion damage. For example, a 316L cyclohexane wastewater separator in a certain facility developed corrosion and leakage after just one year of use. During the inspection, a large number of traces left over from the grinding of weld beads and spatter during the manufacturing process were found inside the tank. At the weld beads and spatter areas, corrosion pits can be seen with the naked eye in most cases; although some areas show no visible pits, tiny pitting corrosion has actually occurred there. Tests have shown that under the corrosive conditions to which this equipment is exposed, only trace amounts of acetic acid and chloride ions are present, with the chloride ion concentration at the inlet being only 2–5 ppm. It can be said that this corrosive environment is not sufficient to pose a stress corrosion risk to 316L in such a short period of time; in fact, no corrosion cracks were found on either side of the weld joint, where stress corrosion occurs quite easily. The image below shows a separation tank made of 316L suffering from corrosion and leakage in an environment with only a small amount of chloride ions, as a result of welding issues. Figure 1 shows the through-crack formed in a chloride environment after a rough arc start during welding. Figure 2 shows the destructive effect of excessive spatter during welding on pitting corrosion of the equipment. 3.2 Stress corrosion caused by work hardening 3.2.1 Stress corrosion caused by collisions and hammering After mechanical collisions and hammering, stainless steel experiences significant compressive stresses; the radial direction in which these stresses are applied is the cause of the formation of dendritic stress corrosion cracks. In numerous engineering applications, we have observed many similar forms of corrosion. Therefore, when manufacturing such stainless steel, Article 108[2] of the Specifications emphasizes the necessity of using specialized facilities and equipment. It is necessary to prevent not only iron ion contamination but also forceful assembly and mechanical impact. 3.2.2 Stress corrosion caused by cold deformation: In halide media, the main reason why common austenitic stainless steels such as 316L, 304, and 321 are prone to stress corrosion lies in manufacturing factors. Theoretically, such materials undergo a cold deformation phase transformation at large deformations; that is, during expansion joining and bending, the austenite structure in the material transforms into martensite, causing the yield strength of the deformed area to increase significantly. When the deformation reaches 20% or more, this yield strength can rise several times to levels close to the tensile strength, whereas the increase in tensile strength is only around 20%, resulting in a yield-to-strength ratio that is close to 1. This is because when the cold deformation is significant, some of the unstable austenite undergoes martensitic transformation, which is what is commonly referred to as cold work hardening. Austenitic stainless steels are used in environments subject to stress corrosion; all areas that have undergone significant cold deformation must undergo solution treatment or stabilization treatment, such as the heads and the R sections of U-tubes. Compared to 304, 316L, despite the increase in Ni levels due to the addition of Mo – raising Ni from 9% to 14% – which enhances the austenite stability of the steel and significantly improves work hardening, still suffers from the risk of stress corrosion. 3.2.3 Stress corrosion of heat exchange tubes caused by expansion joint stresses: After strength expansion and mechanical pressing, significant additional stresses are generated at the step areas of the heat exchange tubes. According to literature, the additional stress resulting from mechanical pressing can reach over 100 MPa【3】, while the additional stress due to strength expansion is even greater. Therefore, the stress corrosion failure of heat exchangers made of this material occurs primarily in the heat-affected zone at the weld edges of the tube ends and at the steps formed by mechanical expansion. The stress corrosion failure of austenitic stainless steel heat exchangers produced by most domestic machinery manufacturers occurs almost entirely at the steps where the heat exchange tubes are expanded and in the heat-affected zone of the tube ends, with the microscopic corrosion pattern taking the form of dendritic cracks. The hazards caused by expansion joint stress are far greater than those caused by welding stress, and stress corrosion occurs preferentially to welding stress. Abroad, to avoid the hazards caused by welding stress, flexible strength expansion jointing is generally used as a connection method. Abroad, the dimensional accuracy of the holes at the pipe ends and the outer diameter of the pipes is much higher than that of most manufacturing plants in China. Furthermore, the expansion joining methods used abroad are liquid-bag type flexible expansion joining, which employs a approach of pre-expansion followed by forced expansion in order to minimize the additional stress generated during expansion; as a result, the probability of corrosion-induced failure is much lower compared to mechanical expansion joining used domestically. However, in applications where Cl— is used, austenitic stainless steel is generally not employed; instead, materials such as duplex stainless steels, like SAF2205, are used, but they are much more expensive. If a unit has 12 stainless steel heat exchangers, with the tube bundle made of 1Cr18Ni10Ti; the fluid flowing inside the tubes is circulating water at an inlet temperature of 20°C, while outside the tubes there is oil and gas at an inlet temperature of 130°C. The connection method for the heat exchange tubes is strength welding combined with expansion jointing. After one year of use, severe corrosion and leaks occurred multiple times at the pipe end. Subsequent sampling and analysis revealed that the corrosion leakage occurred entirely at the expansion joint of the pipe; cracks formed at the step between the expanded and unexpanded sections, and the tips of these cracks had partially penetrated through. Under these operating conditions, the Cl— content in the circulating water is 35–130 ppm, while the Cl— content in the scale reaches over 16,000 ppm. 4. Conclusion In summary, although austenitic stainless steels such as 304, 321, and 316L possess good resistance to uniform corrosion, they are not suitable for use in environments containing halide ions such as Cl−. The hazard of stress corrosion can cause unexpected problems that disrupt the normal operation of equipment. References: [1]: Stainless Steel, edited by Lu Shiying et al., Atomic Energy Press, published in 1995. Face P137. 【2】: Lu Shiying et al., Analysis of the failure causes of 12 1Cr18Ni10Ti stainless steel heat exchangers, \"Stainless Steel\", Issue 4, 2000, p. 31. 【3】: \"Regulations on the Safety Inspection of Pressure Vessels\", China Labor and Social Security Publishing House, published in October 1999.