Stress corrosion of austenitic stainless steels and protective measures

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Under normal conditions, austenitic stainless steels exhibit good corrosion resistance; however, under certain specific operating conditions, stress corrosion can occur, posing significant safety risks in engineering applications. It discusses the conditions under which stress corrosion occurs in austenitic stainless steels, the mechanism of corrosion, and the protective measures, providing a basis for addressing the issue of stress corrosion failure in such steels. Austenitic stainless steel has become an important corrosion-resistant material in the petrochemical industry, thanks to its excellent mechanical properties and weldability, as well as corrosion resistance that is unmatched by other common metals. Austenitic stainless steel is corrosion-resistant due to the formation of a very thin, adhesive, translucent film of chromium oxide on its surface. Cr and Ni are the main alloying elements that confer corrosion resistance to stainless steel. Cr and Ni enable austenitic stainless steels to form a highly dense oxide film in oxidizing media, thereby causing passivation and reducing the corrosion rate of the stainless steel in such media, which improves its corrosion resistance. Once this layer is damaged mechanically, the chromium in the steel can rapidly restore it through a chemical reaction with oxygen in the atmosphere. However, the corrosion resistance of stainless steel is specific: it remains stable in environments such as air, water, and neutral solutions, but it may suffer corrosion and damage in other media. According to statistics, in equipment corrosion, the corrosion of austenitic stainless steels accounts for about half of all types of corrosion, while stress corrosion in austenitic stainless steel materials constitutes more than two-thirds of all cases of stress corrosion in materials. Stress corrosion refers to the low-stress brittle fracture of metals that occurs under the combined action of tensile stress and a specific corrosive environment. Stress corrosion cracking is a type of corrosion that occurs at a rapid rate and causes severe damage; it is a sudden, low-stress brittle fracture that often takes place without any obvious macroscopic deformation or corrosion on the metal surface. Due to the suddenness and unpredictability of such brittle fractures, it poses a considerable risk. 1 Conditions for occurrence Stress corrosion cracking results from the combined action of tensile stress and corrosive factors. Generally, three conditions must be met for stress corrosion cracking to occur: (1) The specific material must be a stainless steel with certain chemical compositions and microstructures; such steels are susceptible to stress corrosion in certain environments ； (2) Residual tensile stress resulting from sufficiently high tensile stresses during the manufacturing process or from an unreasonable structural design of the product; tensile stress is a necessary condition for the occurrence of stress corrosion cracking ； (3) Stress corrosion cracking occurs in specific corrosive environments only under certain combinations of material and medium. 2 Generation Mechanism 01 Stress Corrosion Cracking Process The stress corrosion cracking process can be divided into 3 stages: The first stage is the one in which corrosion causes cracks or pits, that is, the stage of nucleation of crack sources resulting from stress concentration; this stage is often referred to as the incubation period or induction period ； The second stage is the crack propagation stage, that is, the stage in which the crack originates from a source or pit and progresses until it reaches the so-called ultimate stress value, which represents the maximum load that a unit area can withstand ； The third stage is the crack propagation phase of unstable pure mechanics, namely the fracture stage. The first stage is relatively less affected by stress, but it lasts longer, accounting for about 90% of the total rupture time. 02 Mechanism of stress corrosion: As for the mechanism of stress corrosion, it is still under investigation at present. Various theories have been developed to date, and each of these theories has its own applicable scenarios; they can be summarized into three categories: hypotheses and theories related to environmental factors. (1) The electrochemical corrosion theory is suitable for explaining stress corrosion that occurs along grain boundaries. It posits that grain boundaries possess higher energy than the crystal planes within the grains, thereby forming cathodes and anodes and constituting a galvanic cell for electrochemical corrosion. (2) The stress adsorption theory suggests that stress corrosion cracking occurs because special ions are adsorbed on the surface of the metal (or alloy), which reduces its surface energy and thereby lowers the stress required for the material to fail. (3) Surface film rupture theory: This theory holds that the protective film on the surface of a metal (or alloy), especially at the grain boundaries, is continuously damaged during the corrosion process, allowing corrosion cracks to develop until the material is destroyed. (4) Wedging effect of corrosion products: The corrosion products of a metal (or alloy) deposit in the cathodic region behind the crack tip, acting as a wedge that exerts stress on the crack ； When the stress caused by the sediment reaches a critical value, it causes the cracks to propagate forward ； The newly formed cracks absorb more electrolyte solution, allowing the anodic corrosion at the crack tips to continue; this generates additional soluble metal ions, which diffuse to the cathode region and deposit as metal oxides, thereby causing the cracks to expand further. This process repeats itself until rupture occurs. Assumptions and theories regarding metal factors (1) Dislocation theory: The susceptibility of materials to stress corrosion cracking is related to the distribution of dislocations within the material. (2) Precipitation theory: In environments where stress corrosion occurs, the material, as a result of stress or corrosion reactions, experiences precipitation in certain areas; this reduction in potential turns those areas into anodes, creating conditions conducive to corrosion. (3) According to the slip step theory, for stress corrosion to occur in a material, it must undergo a certain degree of plastic deformation, which results in the formation of slip steps on the material’s surface. This destroys the protective layer on the metal surface and creates new active areas. In the dielectric, these active regions, together with other areas that have intact protective films, form small positive peaks and large positive electrodes, accelerating the degradation process. (4) Tunnel corrosion theory: In environments that induce stress corrosion, metal (or alloy) develops corrosion pits in a certain direction along a specific slip plane, which then extend to form tunnel-like structures. Under stress, these tunnels connect with each other, reducing the cross-sectional area; the stress gradually increases until it exceeds the yield limit and even reaches the strength limit, leading to failure. Assumptions and theories regarding stress factors: The key idea of this theory is that hydrogen generated by corrosion reactions under stress diffuses to the front edge of the expanding crack, where it forms highly activated hydrides or hydrogen-strained ferrite (or bccα‘, α’ martensite) perpendicular to the direction of the stress, thereby causing brittleness in the metal at that location. As stress corrosion progresses, hydrogen is continuously generated and diffused to the crack tip, causing the crack to continue expanding forward. The initiation and propagation of metal corrosion fracture occur along the sensitive pathways formed by the diffusion and reaction of hydrogen in steel. 3 Protection Measures 01 Use of stainless steel materials resistant to stress corrosion. In recent years, various types of stainless steels resistant to stress corrosion have been developed, including high-purity austenitic chromium-nickel steels, high-silicon austenitic chromium-nickel steels, high-chromium ferritic steels, and ferritic-austenitic duplex steels. Among them, ferrite-austenite duplex steel has the best resistance to stress corrosion. 02 Improve the design structure: Try to avoid sudden changes in the shape of the structure in order to reduce local high stresses. The ends of the connectors should be rounded off to ensure a smooth transition, and larger radii of curvature as well as higher smoothness should be used at the connection points. 03 Stress relief treatment: Post-weld heat treatment is an effective method for reducing residual stresses. The typical temperatures used for stress relief treatment are not suitable for austenitic steels, as they fall within the sensitization range of these metals; therefore, the heat treatment temperature must be increased to around 900 °C in order to achieve good stress relief effects. Since this temperature is the same as the stabilization treatment temperature for austenitic steel containing stabilizing elements, treating at this temperature achieves two goals at once. Furthermore, shot peening can be applied to its surface to apply a certain compressive stress, thereby counteracting the tensile stress generated. 04 Protection using electrochemical methods: Cathodic protection with an external current or sacrificial anode cathodic protection can prevent stress corrosion, and it can also halt the progression of cracks once they form. 05 Adding corrosion inhibitors: Where process conditions permit, adding a certain amount of corrosion inhibitors can also help to slow down the rate of stress corrosion in austenitic stainless steels to some extent. 4 Conclusion Corrosion and protection is an independent discipline that studies the corrosion processes of structural materials as well as the mechanisms for controlling corrosion, with the aim of taking measures to extend the service life of these materials. Choosing the right materials and implementing appropriate protective measures are very important for extending the service life of equipment.