Stress Corrosion Cracking (SCC) is the brittle cracking of metal under the combined influence of stress and a corrosive medium in specific environments.Throughout the history of modern industrial development, there have been a plethora of cases involving sudden and catastrophic accidents caused by stress corrosion. In the 1800s, the alkali brittle cracking of British boilers first incited research into the mechanics and forms of stress corrosion. In 1925, Moore performed the first systematic analysis of stress corrosion in the laboratory, discovering that the seasonal cracking of brass was caused primarily by intergranular stress corrosion in the presence of ammonia. Brown later introduced fracture mechanics theory to the field of material corrosion, laying the foundation for mechanical studies of stress corrosion in the 1960s. In 1967, Parkins, the main founder of stress corrosion research and theory, then combined these mechanics with electrochemical measurement methods to establish the core methods for study of the mechanisms of stress corrosion. This allowed for numerous studies to be conducted on the forms and systems of environment-sensitive cracking under the influence of various states of stress and in various media, contributing greatly to modern stress corrosion theory.
During this period, the incidence of stress corrosion faults had been expanding both in volume and scope; affected industries included petroleum mining, chemical and marine engineering, and nuclear energy. As such, stress corrosion research developed radically, gradually adopting one of the most distinguished research areas within the field of material corrosion and protection. More recently, scientific understanding of mechanical-chemical behavior and the mechanisms of SCC at the micro and nano-scales has seen some development. It has also been recognized that stress corrosion also occurs in non-metallic materials; environmental cracking in polymer materials has the same characteristics and rules as stress corrosion in metals.
SCC is best characterized as the cracking of metal placed under tensile stress for a period of time, within a specific corrosive medium. From the perspective of fracture mechanics, it takes a certain amount of time for the crack to initiate, expand, and finally reach critical stress, resulting in an unstable fracture. The minimum stress causing SCC is far less than the strength, b, of the material, and there may be no apparent macro-plastic deformation before fracture.
Stress corrosion mechanisms can be divided into two types: hydrogen-induced cracking and anodic dissolution. In hydrogen-induced cracking, the cathodic process is a hydrogen evolution reaction, in which hydrogen entering the metal causes cracking. An example is the SCC of ultra-high strength steel in water. In anodic dissolution, the cathodic process is an oxygen uptake reaction and not related to hydrogen. Stress corrosion is from anodic dissolution. Examples include brass in ammonia solution or titanium in methanol solution. It should be noted that in the process of anodic dissolution a hydrogen evolution reaction still takes place at the cathode, but it remains less than the critical value required for hydrogen-induced cracking. This form of SCC should still be considered as anodic dissolution. An example of this is seen when austenitic stainless steel is stress corroded in hot salt solution. The amount of hydrogen entering the metal is too low to cause hydrogen-induced cracking, so it instead promotes SCC via anodic dissolution.
It has been proposed that the mechanisms of anodic-dissolution-based SCC can be further subdivided into the following theories and models: (1) the slip dissolution mechanism or membrane rupture theory (2) the preferred dissolution mechanism, including the intergranular preferential dissolution model and tunnel corrosion model (3) the medium-induced cleavage mechanism, and (4) the theory of corrosion-induced plastic deformation leading to SCC cracking.
There is not yet any consensus on the mechanisms of hydrogen-induced cracking. The common point of various theories is that hydrogen atoms are accumulated in high stress regions by stress-induced diffusion, and the material is broken when the hydrogen concentration reaches a critical value. Theories and models on hydrogen enrichment include: (1) hydrogen pressure theory (2) adsorption hydrogen reducing surface energy theory (3) weak bond theory, and (4) hydrogen promoting local plastic deformation theory.
For more than two decades, little progress has been made in the study of stress corrosion mechanisms. This is attributable to the lack of observation and research on mechanical-chemical behavior at crack tips on the micro and nano scales. However, with the more recent development of micro-area electrochemical testing technology, understanding of stress corrosion is projected to advance again. Micro-area electrochemical testing can be used to compare the electrochemical reactivity of the sample surface under the presence or absence of stress, and the local electrochemical reaction characteristics at different positions of the crack tip can also be obtained. These micro-area measurement techniques include scanning vibration reference electrode technology (SVET), local electrochemical impedance spectroscopy (LEIS), scanning Kelvin probe technology (SKP), and capillary microelectrode technology. Such techniques allow for unprecedented insight into the processes of stress corrosion initiation and development at the atomic level.This correlates to an improved understanding of local corrosion processes in the aqueous solution at the crack tip, including the behaviors of ions, passivation film, sediments and corrosion products. Combined with finite element analysis and other numerical simulation methods, we are better equipped than ever to analyze the effects of mechanical and electromechanical factors on the mechanisms of the SCC process.
Increasingly advanced morphological observation methods have also been used in crack tip behavior analysis, such as focused ion-beam tomography (FIB) and X-ray tomography. Through advanced, three-dimensional in situ structural analysis, information regarding the initiation of and changes to the crack tip structure can be further obtained, providing further insight to the evolution process and mechanisms of SCC. With recent technological advancements, even spatial radiotracer methods combined with electrochemical methods are possible.
Research on SCC in China remains in sync with international progress. The primary body of research in China focuses on the influence of environmental factors, loading conditions, and heat treatment methods on the SCC process. Focus can remain on problems with practical application, such as those seen with pipeline steel, nickel-based alloys in nuclear plants, and the SCC of steel for pressure vessels. Various advanced research and analysis methods are being gradually applied in said research. However, there remains a lack of distinctive research direction and few distinguished fields of study, which, to some extent, limits the originality and innovation of results in the study of SCC and crack tip behavior in China.
Due to the complexity of the stress corrosion process, it is difficult to establish a unified stress corrosion model and associated mechanisms explaining the SCC phenomenon. Theoretical research is limited in application. Therefore, greater attention should be devoted to solving issues as they materialize in practice. Through the study of specific stress corrosion failure cases under actual service conditions, simulations, and acceleration tests conducted in laboratory settings, the SCC process of a certain system can be studied in depth, and a suitable stress corrosion model established to effectively analyze and explain it. By finding the laboratory accelerated test conditions that can reproduce the actual stress corrosion process, an accurate assessment of stress corrosion life and targeted protection measures can be achieved. This has been the foremost strategy for stress corrosion research in recent years.
The petroleum industry has so far used industrial systems with the largest variety of metal materials, the most complex corrosive media, and a large variety of complex loads, resulting in a large number of stress corrosion accidents. Petroleum is a flammable, liquid, organic mineral that is found in the porous media of underground rock and consists of various hydrocarbons and their derivatives. Natural gas is a flammable gas that also exists in porous underground rock and is similarly composed of hydrocarbons and their derivatives. The oil industry centers on exploration, drilling, development, oil recovery, gathering, refining and storage. Industrial processes are divided into two major sections: oil exploration and mining (upstream) and petrochemical refinement (downstream). Every aspect of the oil industry is closely linked to metallic materials, especially various steels, exposed to extremely harsh environments and various loads of stress.
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