Embrittlement mechanisms can be classified as follows:

  • Environmentally induced cracking
  • Stress corrosion cracking
  • Hydrogen-induced cracking (hydrogen embrittlement)
  • Corrosion fatigue
  • Liquid- and solid-metal embrittlement

Let’s briefly review each of these mechanisms.

Environmentally Induced Cracking

Environmentally induced cracking refers to a corrosion-cracking phenomenon generally caused by a combination of conditions that manifest themselves in some form of corrosion-related failure. This typically occurs when stresses are present in the material from cold work (residual stress), welding, grinding, thermal treatment or service conditions. To be “effective” the stresses must be tensile.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment (Fig. 1). The various types include: sulfide stress cracking, chloride-induced SCC, caustic-induced SCC and hydrogen-induced cracking.

The cracks form and propagate at approximately right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload (Fig. 2). Usually, most of the surface is not attacked, but the areas that are exhibit fine cracks penetrating into the metal. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance and are classified as a catastrophic form of corrosion. The detection of fine cracks can be very difficult and the failure not easily predicted.

Choosing the proper alloy is one of the most important considerations to negate the effects of stress corrosion cracking. It is relatively simple to choose a component with adequate strength and good (general) corrosion resistance. Knowing the particular type of stress corrosion cracking at work, however, is an important step in achieving a resistant material. In certain environments, it may be necessary to choose a material that will experience some general corrosion since general corrosion is visually evident. With proper preventive maintenance, general corrosion can be seen and parts replaced as necessary. On the other hand, SCC is rarely visually apparent and often occurs without warning. When it does, a catastrophic failure may occur.

Other methods include theremoval of the corrosive environment or changingthe manufacturing process or design to reduce the tensile stresses. Corrosion can be effectively controlled by a combination of good design, careful selection of stress-corrosion-resistant grades of stainless steel and effective management, including maintenance and inspection. Specific steps can be taken to prevent the onset of SCC and to minimize its consequences when it does occur.

  1. Careful consideration of the potential for SCC during the design and fabrication of components
  2. Careful selection of appropriate material grades
  3. Carefully maintaining the chemical balance of the environment
  4. Ensuring that the potential for (organic or inorganic) contamination is minimized
  5. Maintaining proper environmental conditions (e.g., air quality)
  6. Regular inspections of components for signs of corrosion and SCC

References

1. Herring, Daniel H., “Fastener Failures Due to Stress Corrosion Cracking,” Fastener Technology International, August 2010
2. Corrosion Doctors (corrosion-doctors.org)
3. Spence, Thomas, “Selecting the Right Fastener,” Materials Newsletter, Flowserve (www.flowserve.com)