Stress corrosion cracking (SCC) is the process by which engineering materials gradually degrade in a corrosive environment. Every year, many ductile metals and alloys fail due to stress corrosion cracking, which begins with the initiation, propagation and propagation of cracks to their destructive limits when exposed to a corrosive environment. Because stress corrosion cracking is alloy and environment specific, its mechanisms vary widely. A metal that exhibits SCC properties in one environment may be less susceptible to SCC in another. However, the actual stress corrosion cracking process has not been studied in depth.
Stress corrosion cracking is a slow and progressive failure process. SCC can start and spread with little or no external signs of corrosion. Surface imperfections caused by corrosion, wear, or other factors can lead to the formation of cracks.
Stress corrosion cracking (SCC) is a type of intergranular corrosion that occurs in the steel industry, leading to crack initiation in corrosive environments. Considering that steel is the most commonly used industrial material in facilities such as pipelines, power plants, chemical industries, construction, and more, stress corrosion cracking is a major concern.
Causes of stress corrosion cracking:
Failure of stress corrosion cracking methods is primarily caused by three factors:
- Tensile Strength: (usually due to stresses imposed by operations, thermal stresses, or residual stresses from welding and fabrication)
- Corrosive environment:
- Sensitive materials in certain metallurgical states can cause premature component failure.
- Temperature and time are two other factors that can cause stress corrosion cracking.
The presence of cracks and other defects on parts accelerates the deformation process of SCC. SCC failure is rapid and catastrophic in nature and typically occurs at stress levels significantly below the yield stress. Here are some common examples of SCC:
- Seasonal cracking of brass in ammonia-rich environments
- Stainless steel can undergo sensitization and stress corrosion cracking in the presence of caustic alkalis, chlorides and polythionic acid.
Classification of stress corrosion cracking：
Depending on the actual SCC process, various forms of stress corrosion cracking have been found.
- Chloride Stress Corrosion Cracking: This type of cracking is prevalent in austenitic stainless steels in the presence of chloride ions and oxygen, combined with high-temperature tensile mechanical stress.
- Alkali embrittlement: common in stainless steel with high hydrogen content in alkaline environments.
- In the petroleum and chemical industries, SCC cracking of steel occurs under hydrogen sulfide conditions.
- Session Cracking: Brass cracks in an ammonia environment.
- Cracking: Cracking of polymeric materials due to applied stress and environmental reactions.
Stress corrosion cracking characteristics
SCC has the following unique features:
Stress corrosion cracking failure occurs at stress levels well below the material’s yield stress.
- Although SCC materials are ductile, their failure mechanisms are brittle.
- Corrosion is the most common cause of stress corrosion cracking.
- Intergranular and transgranular cracks are the main characteristics of stress corrosion cracking at the microscopic scale. Intergranular cracks form at grain boundaries, while transgranular cracks form across grains.
- Materials affected by stress corrosion cracking
Stress corrosion cracking attacks can occur in the following materials:
- Stainless steel (temperature range 415°C to 850°C in chloride, caustic and polythionate environments)
- Carbon steel (carbonate, caustic alkali solution, nitrate, phosphate, seawater solution, acidic H2S, high temperature water environment)
- Copper and copper alloys (in environments containing ammonia, amines, and water vapor)
- Aluminum and aluminum alloys (in humid environments with sodium chloride solution)
- Titanium and titanium alloys (in the presence of seawater, fuming nitric acid, methanol-HCl)
- Polymers (in acidic and alkaline environments)
- ceramics
Stress corrosion cracking in welding
The main cause of stress corrosion cracking is residual stress generated during welding and manufacturing processes. Residual stresses generated by welding are critical to stress corrosion cracking of metal alloys. Welding stress corrosion cracking is caused by uneven temperature fluctuations during the welding process. In addition, when welding certain steel grades, the solid-state transformation of austenite to martensite during cooling can produce high residual stresses. Martensite is produced by rapid cooling of austenite containing carbon atoms in carbon and low-alloy steels. In this case, the carbon atoms are unable to diffuse out of the crystal structure and form cementite. This increases the volume of the metal, resulting in high residual stresses.
stress corrosion cracking mechanism
Various stress corrosion cracking mechanisms are common in industry depending on the type of material and environment. Some well-known SCC mechanisms include:
- force electrochemical model
According to this stress corrosion cracking process, there are pre-existing regions in the alloy microstructure that are sensitive to anodic dissolution.
- Film rupture model
The film rupture SCC mechanism is well known for alloys with surface passivation layers. In this technique, corrosion is initiated after plastic deformation. The plastic pressure ruptures the film, exposing the underlying metal to the corrosive atmosphere. Soon after, these components begin to experience localized SCC attack, and the cycle continues, causing further crack development.
- adsorption phenomenon
The SCC mechanism takes into account material embrittlement near the corrosion zone.
- Pre-existing activity path model
Intermetallic compounds and compounds are formed in pre-existing pathways such as grain boundaries, which are susceptible to SCC.
Stress Corrosion Cracking Prevention Measures
Since the process of stress corrosion cracking is not fully understood, prevention techniques are based on actual experience. Generally speaking, one or more of the following measures can help reduce the risk of squamous cell carcinoma:
- Since tensile stress is the primary factor in stress corrosion cracking, reducing stress levels in the part can reduce the likelihood of SCC attack. Residual stresses can be greatly reduced by annealing the components.
- One technique for minimizing SCC attacks is to eliminate or reduce invasive species in the environment where components are placed. For example, with austenitic stainless steels, keeping chloride concentrations below 10 ppm can significantly reduce the risk of SCC.
- Stress corrosion cracking resistant materials will ensure protection against stress corrosion cracking.
- Using cathodic protection can help reduce stress corrosion cracking.
- Phosphates and other organic and inorganic inhibitors can minimize the effects of stress corrosion cracking in moderately corrosive environments.
- In some cases, it may be useful to apply a protective coating.
- The use of shot peening to create residual compressive stresses on component surfaces can help minimize stress corrosion cracking.
- Reducing temperature and electrochemical potential can minimize the risk of SCC.
Tianjin Anton Metal Manufacture Co., Ltd. is a company specializing in the production of various nickel-based alloys, Hastelloy alloys and high-temperature alloy materials. The company was established in 1989 with a registered capital of 10.0 million, specializing in the production and sales of alloy materials. Anton Metal’s products are widely used in aerospace, chemical industry, electric power, automobile, nuclear energy and other fields, and can also provide customized alloy material solutions according to customer needs. If you need to know the price consultation of alloy materials or provide customized alloy material solutions, please feel free to contact the sales staff.
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Post time: Jan-09-2024
