Season cracking

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Season cracking is a form of corrosion that occurs in metals due to the combined effects of environmental conditions and stress. This phenomenon is particularly prevalent in certain types of metals and alloys, including brass, which is notably susceptible to this type of corrosion. Season cracking is a specific type of stress corrosion cracking (SCC), which is a process whereby a combination of tensile stress and a corrosive environment leads to the failure of a material at stress levels below the material's tensile strength.

Causes

The primary cause of season cracking is the exposure of susceptible materials to a corrosive environment while under tensile stress. The term "season" in season cracking refers to the environmental conditions, such as humidity and temperature, which can vary with the seasons and significantly affect the rate and occurrence of this corrosion process. In the case of brass, for example, the presence of ammonia can lead to season cracking. Ammonia acts as a catalyst, accelerating the corrosion process in the presence of moisture and leading to the formation of cracks.

Mechanism

The mechanism of season cracking involves the initiation and propagation of cracks due to the synergistic effects of mechanical stress and chemical attack. The process typically begins at the material's surface and progresses inward. Microscopic cracks form and propagate through the material due to the combined action of the tensile stress and the corrosive environment. Over time, these cracks can grow and coalesce, leading to significant weakening of the material and, eventually, catastrophic failure.

Prevention

Preventing season cracking involves a combination of material selection, design considerations, and environmental control. Choosing materials that are less susceptible to stress corrosion cracking in the expected service environment is a fundamental step. For applications where the use of susceptible materials cannot be avoided, applying protective coatings or treatments can reduce the risk of corrosion. Additionally, designing components to minimize tensile stresses, particularly at the surface where corrosion initiates, can significantly reduce the likelihood of season cracking. Controlling the environment, such as by reducing the presence of corrosive agents, is also crucial in preventing this type of corrosion.

Examples

One of the most famous examples of season cracking occurred in the early 20th century with the failure of brass cartridge cases. These cases were found to crack when stored in stables, where the presence of ammonia from horse urine acted as a corrosive agent, leading to the cracking of the brass cases under the residual stresses from their manufacturing process.

Conclusion

Season cracking is a significant concern in the design and maintenance of metal components and structures, particularly those made from materials like brass that are susceptible to this form of corrosion. Understanding the causes and mechanisms of season cracking is essential for engineers and designers to develop strategies to prevent its occurrence, ensuring the longevity and reliability of metal components in various applications.

Season cracking Resources
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