Industrial Corrosion and Protection -- 12 Stress Corrosion Cracking

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Industrial Corrosion and Protection--12 Stress Corrosion Cracking. Immersive reading in a novel reader. Definition of stress corrosion cracking: In a broad sense, stress corrosion cracking includes hydrogen embrittlement, but hydrogen embrittlement is usually treated separately from stress corrosion cracking. The definitions of stress corrosion cracking and hydrogen embrittlement are as follows. 1. Stress corrosion cracking. Under the combined effects of stress and a specific corrosive environment, metallic equipment and components exhibit brittle cracking below their material strength limit. This phenomenon is known as stress corrosion cracking or stress corrosion fracture, abbreviated as SCC. 2. Hydrogen embrittlement. The brittle failure phenomenon that occurs in alloys under tensile stress due to the absorption of hydrogen (including hydrogen generated by corrosion reactions). Based on their mechanism, stress corrosion can be divided into two types: anodic dissolution type and hydrogen-induced cracking type. If the cathodic process corresponding to anodic dissolution in a stress corrosion system is an oxygen absorption reaction, or although the cathodic process is hydrogen evolution, the hydrogen that enters the metal is not sufficient to cause hydrogen-induced cracking, then the nucleation and propagation of stress corrosion cracks are controlled by the anodic dissolution process of the metal; this is known as anodic dissolution-type stress corrosion. If the cathodic process corresponding to the dissolution (corrosion) of the anode metal is the hydrogen evolution reaction, and atomic hydrogen can diffuse into the metal and control crack nucleation and propagation, this type of stress corrosion is referred to as hydrogen-induced cracking stress corrosion. It can be seen that the main difference between stress corrosion cracking and hydrogen embrittlement is as follows: the former is caused by directed anodic dissolution, while the latter results from cathodic hydrogen absorption. Therefore, under stress, stress corrosion cracking is the type of cracking that is accelerated by external current anodic polarization, while hydrogen embrittlement is the type of cracking that is accelerated by external current cathodic polarization ； Metal damage caused by stress corrosion cracking is not a simple sum of the individual effects of mechanical failure and corrosion damage. Because in a corrosive medium, cracking occurs at stresses well below the material’s yield limit; even a medium with very low corrosivity can cause corrosion cracking under stress. As the main components of the medium are chlorides, hydroxides, nitrates, and oxygenated waters, this phenomenon is referred to as chloride cracking, alkali cracking, and nitrate cracking, respectively. The conditions for stress corrosion cracking: For equipment and components to experience stress corrosion cracking, specific conditions related to the material, the environment, and stress must all be met. First and foremost, there must be medium conditions that facilitate local corrosion, thereby controlling the anodic dissolution process of the metal. The main effect of stress is to damage the passivation film and inhibit its re-passivation, thereby promoting localized corrosion. (1) Materials: These materials form a good protective film in their surrounding environment, offering excellent resistance to general corrosion. Under such conditions, the cracking process only occurs when this protective film is locally damaged as a result of the combined effect of stress and corrosion. If the film formed on the material surface does not provide sufficient protection and general corrosion is severe, stress corrosion cracking will not occur. Therefore, stress corrosion cracking generally occurs only in materials with good resistance to general corrosion. (2) Stress: When stress is present, the greater the stress, the shorter the time required for cracking to occur. Generally, stress corrosion cracking occurs when the stress is below the yield strength of the material. The main sources of stress that cause stress corrosion cracking in stainless steel are shown in Table 4-25. It is reported that stress corrosion cracking caused by residual stresses generated during the manufacturing process accounts for 81.5% of all cases of stress corrosion cracking. (3) Environment: Stress corrosion cracking can occur when the environment contains certain ions or molecules that have a special effect on the occurrence of stress corrosion cracking. Generally speaking, any medium that can cause pitting and crevice corrosion in stainless steel is likely to cause stress corrosion cracking in it as well. Morphological characteristics (1) Visual appearance: The visual features of stress corrosion cracking can be summarized as follows: 1. Cracks appear in localized areas of the equipment or components, rather than across the entire surface in contact with the corrosive medium. The number of cracks varies – sometimes there are many, sometimes fewer, or even just one crack; 2. Cracks are generally deep and narrow, and their direction is closely related to the stress acting on the equipment or components. Typically, the cracks run perpendicular to the direction of the principal stresses, but in some cases, branched cracks can also be observed; 3. The equipment and components do not exhibit significant plastic deformation; 4. It should be noted that there is also a type of invisible stress corrosion cracking, where no cracks are visible on the surface of the equipment or components, but corrosion pits are present. However, when such components are cut open, stress corrosion cracks can be found starting from the bottom of these corrosion pits ； Stress corrosion cracking of several commonly used materials; the resistance of ferritic stainless steels to stress corrosion cracking in chloride solutions. Ferritic stainless steels exhibit much higher resistance to stress corrosion cracking in chloride solutions compared to austenitic stainless steels, and corrosion cracking generally does not occur unless very high stress is applied. The reason is that cross-sliding occurs easily during deformation, resulting in a network of dislocation structures; no linear etching grooves are formed in the material, hence it resists transgranular fracture. However, after sensitization heating (such as welding), ferritic stainless steels can suffer from intergranular stress corrosion cracking in high-temperature and high-pressure water, which originates either from intergranular corrosion or from pitting ； Stress corrosion cracking in austenite-ferrite duplex stainless steels: Austenite-ferrite duplex stainless steels exhibit excellent resistance to stress corrosion in many media, and their stress corrosion behavior is influenced by various factors such as the steel’s composition, heat treatment conditions, and the properties of the surrounding medium. Stress corrosion resistance in CI-containing media: The stress corrosion resistance of duplex stainless steels in MgCl2 solutions is much higher than that of austenitic stainless steels such as 316L and 304L. At lower stress levels, duplex stainless steel is less prone to stress corrosion cracking.