Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a fracture process that involves the combined and simultaneous action of a tensile stress and a corrosive environment. SCC occurs when the tensile stress and a specific environment are able to cause failure by their combined action, but are insufficient to cause failure by either one acting alone. In fact, the tensile stresses are usually below the metal’s yield strength. Furthermore, the metal would suffer only minimal corrosion in the absence of the applied stress.
There are three requirements for SCC to occur:
- A susceptible metal.
- Tensile stresses applied to the metal.
- A specific environment containing an aggressive species that promotes SCC.
Elimination of any of these three factors will prevent SCC.
SCC occurs in specific combinations of metals and environments. The number of metal environment combinations that promote SCC is fewer than the number of metal environment combinations that will result in corrosion. Other environmental factors, such as pH and temperature, can also influence the severity of SCC.
The source of the tensile stress may be externally applied stress or residual stresses. Externally applied stresses arise from applied mechanical loads such as tensile or bending loads. Residual stress is an internal stress that exists in a metal without an external load being applied. Residual stresses can result from cold working, heat treating, or welding.
Other than metal composition and the specific corroding species in the environment, other factors that influence SCC include total tensile stress on the metal, metal microstructure, and metal yield strength.
Increasing the yield strength of a metal is one way to improve its resistance to SCC because the threshold stress for SCC increases as the yield strength increases. The yield strength can be increased through alloying, heat treating, cold-working, and combination of these approaches. There is one very important consideration when increasing the yield strength. The increase in strength must not be accompanied by a significant reduction of the metal’s toughness, because decreasing the toughness will have a detrimental effect on a metal’s resistance to SCC and on its fracture toughness.
SCC can be controlled by any of the following three approaches: 1) design, which includes selection of the mechanical and materials aspects of components, 2) controlling the materials, 3) and controlling the environment.
Take our Corrosion of Metals course if you’d like to learn more about stress corrosion cracking and how to prevent it from occurring. Also, the books Corrosion and Corrosion Control (4th edition) by R.W. Revie and H.H. Uhlig and Corrosion: Understanding the Basics by ASM International are good resources for information about the SCC and the different corrosion mechanisms.
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