Stress corrosion (learn about it)
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Stress corrosion (or stress corrosion cracking) refers to the brittle cracking of metal that occurs under the simultaneous action of a specific corrosive medium and a certain level of tensile stress. Stress corrosion must meet three conditions at the same time, namely a certain level of tensile stress, a specific corrosive medium and steel that is sensitive to stress corrosion in the corrosive medium. Under the action of internal pressure, thermal stress, welding residual stress, etc., the furnace tube will have a certain level of tensile stress conditions. Most steel materials will suffer from stress corrosion in the environment of chloride ions and hydroxide ions. For example, austenitic stainless steel is prone to stress corrosion in the chloride ion environment. Furnace tubes that suffer from stress corrosion cracking generally do not show obvious signs of plastic deformation, and generally show transgranular fracture. To prevent stress corrosion, we should consider three aspects: stress, medium and material. Welding residual stress should be eliminated as much as possible to prevent the superposition of thermal stress and reduce the level of tensile stress. The concentration of the stress corrosion medium should be reduced as much as possible. When the chloride ion concentration is difficult to eliminate, material considerations should be considered. For example, using high-nickel alloy steel (such as Inconel) or using it as a protective layer can reduce the sensitivity of stress corrosion.
Mechanically, it can be generally divided into two explanation theories (1) cathode hydrogen embrittlement type (2) anode dissolution type. Each theory has its own* * explanation, but there is a perfection that cannot be explained.
Stress Corrosion of Metal Materials Stress corrosion cracking of metal materials refers to the phenomenon of corrosion cracking caused by the combined action of static tensile force and corrosive media. It is different from damage caused by stress alone. This kind of corrosion can also occur under extremely low stress conditions. ； It is also different from damage caused simply by corrosion. Extremely weak corrosive media can also cause corrosion cracking. It often develops rapidly and suddenly without warning, easily causing serious accidents. It is therefore a very harmful form of destruction. According to the electrochemical reaction in the crack development process, stress corrosion can be divided into two basic categories:: Anodic reaction sensitive type and cathodic reaction sensitive type. Anodic reaction-sensitive stress corrosion means that the formation and development process of this type of stress corrosion crack is based on the anodic dissolution of the metal at the crack, and the growth rate of the crack is also determined by the anodic dissolution rate of the metal. Cathodic reaction-sensitive stress corrosion refers to the brittle damage caused by the absorption of hydrogen by the cathode during this type of reaction. It is also called hydrogen embrittlement stress corrosion, also known as hydrogen embrittlement. Stress corrosion usually refers to anodic reaction-sensitive stress corrosion. The characteristics of stress corrosion in metal materials can be explained from four aspects: 1. Stress generation. The stress viewed from above is mainly the static part. It can be the stress caused by external loads or assembly forces (such as bolt tightening force, expansion force, etc.), or it can be the internal stress generated during processing, heat treatment, welding, etc. of components. Regardless of the source, the stress that causes stress corrosion cracking must have a tensile stress component; compressive stress will not cause stress corrosion cracking. Furthermore, this stress is usually relatively mild. If it is not in a corrosive environment, such a small stress will not cause mechanical damage to the component. The stress value that constitutes damage should be determined according to the specific conditions such as materials and corrosive media. 2. Corrosion media The materials and media that cause stress corrosion are not arbitrary. Stress corrosion will only occur when there is a certain combination of the two. The corrosive media that cause stress corrosion of ordinary steel include: Hydroxide solution ； Aqueous solutions containing nitrates, carbonates, and hydrogen sulfide ； Sea water, sulfuric acid-nitric acid mixture ； Molten zinc, lithium ； hot ferric chloride solution ； Liquid ammonia. The media that cause stress corrosion of austenitic stainless steel include: Acidic and neutral chloride solutions ； seawater ； molten chloride ； hot fluoride solution ； hydroxide solution. 3. Materials It is generally believed that extremely pure metals will not cause stress corrosion damage, and will only occur in alloys or metals containing impurities. 4. Destruction process a. Breeding stage. This is a period of time before stress corrosion cracks occur, preparing for the nucleation of cracks. b. Crack stable expansion stage. Under the combined action of stress and corrosive media, the crack expands slowly c. Crack instability expansion stage. This is the final mechanical destruction. In addition, another characteristic of stress corrosion cracking of metal materials is that the cracking of the metal has nothing to do with the thickness of the metal itself. The common situation of slow corrosion with large thickness (uniform corrosion) does not apply here. Therefore, it is almost ineffective to delay stress corrosion cracking by increasing metal thickness. Stress corrosion cracking of metal materials refers to the phenomenon of corrosion cracking caused by the combined action of static tensile force and corrosive media. It is different from damage caused by stress alone. This kind of corrosion can also occur under extremely low stress conditions. ； It is also different from damage caused simply by corrosion. Extremely weak corrosive media can also cause corrosion cracking. It often develops rapidly and suddenly without warning, easily causing serious accidents. It is therefore a very harmful form of destruction. According to the electrochemical reaction in the crack development process, stress corrosion can be divided into two basic categories:: Anodic reaction sensitive type and cathodic reaction sensitive type. Anodic reaction-sensitive stress corrosion means that the formation and development process of this type of stress corrosion crack is based on the anodic dissolution of the metal at the crack, and the growth rate of the crack is also determined by the anodic dissolution rate of the metal. Cathodic reaction-sensitive stress corrosion refers to the brittle damage caused by the absorption of hydrogen by the cathode during this type of reaction. It is also called hydrogen embrittlement stress corrosion, also known as hydrogen embrittlement. Stress corrosion usually refers to anodic reaction-sensitive stress corrosion. The characteristics of stress corrosion in metal materials can be explained from four aspects: 1. Stress generation. The stress viewed from above is mainly the static part. It can be the stress caused by external loads or assembly forces (such as bolt tightening force, expansion force, etc.), or it can be the internal stress generated during processing, heat treatment, welding, etc. of components. Regardless of the source, the stress that causes stress corrosion cracking must have a tensile stress component; compressive stress will not cause stress corrosion cracking. Furthermore, this stress is usually relatively mild. If it is not in a corrosive environment, such a small stress will not cause mechanical damage to the component. The stress value that constitutes damage should be determined according to the specific conditions such as materials and corrosive media. 2. Corrosion media The materials and media that cause stress corrosion are not arbitrary. Stress corrosion will only occur when there is a certain combination of the two. The corrosive media that cause stress corrosion of ordinary steel include: Hydroxide solution ； Aqueous solutions containing nitrates, carbonates, and hydrogen sulfide ； Sea water, sulfuric acid-nitric acid mixture ； Molten zinc, lithium ； hot ferric chloride solution ； Liquid ammonia. The media that cause stress corrosion of austenitic stainless steel include: Acidic and neutral chloride solutions ； seawater ； molten chloride ； hot fluoride solution ； hydroxide solution. 3. Materials It is generally believed that extremely pure metals will not cause stress corrosion damage, and will only occur in alloys or metals containing impurities. 4. Destruction process a. Breeding stage. This is a period of time before stress corrosion cracks occur, preparing for the nucleation of cracks. b. Crack stable expansion stage. Under the combined action of stress and corrosive media, the crack expands slowly c. Crack instability expansion stage. This is the final mechanical destruction. In addition, another characteristic of stress corrosion cracking of metal materials is that the cracking of the metal has nothing to do with the thickness of the metal itself. The common situation of slow corrosion with large thickness (uniform corrosion) does not apply here. Therefore, it is almost ineffective to delay stress corrosion cracking by increasing metal thickness.
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