Table of Contents
Brief Introduction of Stress Corrosion Cracking
Stress corrosion cracking or SCC is a slow failure mechanism of engineering materials in corrosive environments. Many ductile metals and alloys, when exposed to a corrosive environment, begin with crack initiation, propagation, and growth of that crack, and fail year after year due to stress corrosion cracking.
Stress corrosion cracking is alloy- and environment-specific, and the mechanisms vary greatly by material and environment. A metal that is SCC prone in one environment may not be attacked by SCC in another. However, the exact mechanism of stress corrosion cracking has not yet been fully elucidated.
Stress corrosion cracking is a slow and delayed failure process. SCC can originate and spread externally with little or no warning of corrosion. Cracks usually start from surface imperfections caused by corrosion, wear, or other processes.
In the steel industry, stress corrosion cracking (SCC) is a type of intergranular corrosion that causes cracks in corrosive environments. As steel is the most common industrial material, stress corrosion cracking poses a significant threat to industrial systems such as pipelines, power plants, the chemical industry, and bridges.
Various Causes of Stress Corrosion Cracking?
There are three main factors that contribute to stress corrosion failure.
- Tensile stresses (usually due to operational, thermal, or residual stresses from welding and manufacturing)
- Corrosive environments and susceptible materials in certain metallurgical conditions that promote premature failure of
- Weak materials in certain metallurgical conditions that promote premature component failure.
Types of Stress Corrosion Cracking
Different types of stress corrosion cracking are observed in the actual SCC mechanism.
1. Chloride Stress Corrosion Cracking
Commonly occurring in austenitic stainless steels in the presence of chloride ions and oxygen combined with mechanical tensile stress at elevated temperatures.
2. Corrosive Embrittlement
Common in stainless steels with high concentrations of hydrogen in corrosive environments.
3. SCC Steel Cracking
SCC Steel cracking in hydrogen sulfide environments in the petroleum and chemical industries.
4. Session Cracking
cracking of brass in an ammonia environment.
5. Craze Cracking
Cracking of polymeric materials due to applied stress and environmental reactions.
Properties of Stress Corrosion Cracking
- Stress corrosion cracking occurs at stress levels well below the yield strength of the material. Materials exposed to
- SCC are ductile, but the failure mechanism is brittle.
- Stress corrosion cracking is commonly caused by corrosion.
- At the microscopic level, intergranular and transgranular cracking are the main features of stress corrosion cracking. Intergranular cracks grow along grain boundaries, while transgranular cracks propagate across grains.
Materials Prone to Stress Corrosion Cracking
- Stainless steel (415°C to 850°C temperature range in chloride, caustic and polythionate environments)
- carbon steel (carbonates, strong caustic solutions, nitrates, phosphates, seawater solutions, acidic H2S and hot water environments)
- copper and copper alloys (environments with ammonia, amines, and water vapor)
- aluminum and aluminum alloys (environments with moisture and NaCl solutions)
- titanium and titanium alloys (in contact with seawater, fuming nitric acid, and methanol) HCl environments
- Polymer (aggressive acid and alkaline environments)
- Ceramic
How to Avoid Stress Corrosion Cracking
- Tensile stress is one of the major contributors to stress corrosion cracking, so reducing the stress level of a component reduces the potential for SCC attack. Internal stress can be largely removed by annealing the component. Eliminating or reducing aggressive species from the environment in which the
- Component is installed in a useful way to reduce SCC attacks. For austenitic stainless steels, for example, maintaining chloride levels below 10 ppm significantly reduces the potential for SCC. Selecting a stress corrosion resistant material over
- Protects the product from stress corrosion cracking. Application of
- Cathodic protection reduces stress corrosion cracking failures.
- In mildly corrosive media, the addition of phosphates and other organic and inorganic inhibitors can reduce the effects of stress corrosion cracking.
- Applying a protective layer may help.
- Stress corrosion cracking can be prevented with a shot peening process that creates residual compressive stress on the part surface.
- Control of temperature and electrochemical potential reduces the likelihood of SCC.