Corrosion fatigue is the generation of cracks due to the combined effects of cyclic stresses and corrosion. A significant difference between corrosion fatigue and traditional fatigue is that for the former, a fatigue limit, which is the maximum stress below which a metal can endure an infinite number of stress cycles, does not exist. The level of stress is generally much less than the yield strength of a metal; though, in general, higher stresses increase crack growth rate and the number of cracks initiated. If a corrosive environment is not taken into consideration at the design stage or is an unknown factor, this can lead to premature catastrophic failure.

In 2013, during take-off, an aircraft engine experienced a flash, smoke and loss of power which was the result of a turbine failure due to corrosion fatigue. Luckily the crew were able to perform an emergency stop and taxi the aircraft clear of the runway using the second engine for power. There were no injuries. The NTSB determined that the likely cause of failure was attributable to corrosion fatigue. At a refinery in Michigan in 2010, a segment of a 30” pipeline ruptured during a planned shutdown and was not discovered for 17 hours. This resulted in a leak of 843,444 gallons of crude oil to the surrounding wetlands. Clean up costs exceeded 767 million dollars. There were no fatalities, but 320 people had symptoms that were consistent with exposure to crude oil.

Figure 1 – Ruptured pipe line – Refinery in Michigan in 2010.

Corrosion fatigue occurs when a material is subjected to repeated or fluctuating stress in the presence of a corrosive environment, such as saltwater, chemicals, or high humidity. This combination weakens the material over time and causes microscopic cracks to form and propagate faster than they would under normal fatigue conditions.

Here’s a breakdown of the processes involved:

1. Cyclic Stress: Repeated mechanical stress can lead to the formation of cracks at the surface of the material, especially in areas with high stress concentrations i.e. change in section, radii, welds etc.
2. Corrosion: The corrosive environment accelerates material degradation by breaking down any protective oxides, leading to localised pitting or crack initiation. In environments like seawater or acidic solutions, corrosion can be significant.
3. Crack Propagation: As the material undergoes cyclic loading, cracks continue to grow and propagate due to both mechanical stress and corrosive attack, leading to eventual failure. The cracks can propagate more quickly than in normal fatigue conditions due to the enhanced effect of corrosion.

Corrosion fatigue cracks are typically straight, unbranched, wedge-shaped (see Figure 2) and are frequently observed in parallel families. They may be oriented longitudinally or transversely or skewed, depending on the orientation of the maximum cyclic stress.

Figure 2 – Corrosion fatigue cracks

Mitigating the risks of corrosion fatigue includes proper design, materials selection, regular inspection, and maintenance practices. Here are some strategies that can help prevent corrosion fatigue:

1. Materials Selection: Choosing materials that are resistant to corrosion in specific environments is essential. Stainless steel, for example, is highly resistant to corrosion and is often used in marine and chemical applications.
2. Protective Coatings: Coatings like galvanisation, anodising, and paint can provide an additional layer of protection against corrosive agents, helping to extend the life of materials exposed to aggressive environments.
3. Design Considerations: Designers can minimise the risk of corrosion fatigue by avoiding sharp corners, stress concentrators, and areas that are difficult to inspect or maintain. Additionally, ensuring proper drainage and ventilation can prevent the accumulation of corrosive substances.
4. Corrosion Monitoring and Inspection: Regular monitoring for signs of corrosion and fatigue, such as visual inspections, ultrasonic testing, and radiography, can help detect issues early. Predictive maintenance strategies can be implemented to identify components at risk of failure before it occurs.
5. Environment Control: If possible, controlling the environmental factors that contribute to corrosion, such as humidity, temperature, and chemical exposure, can help reduce the risk of corrosion fatigue.

Corrosion fatigue is a serious and often underestimated risk in engineering and manufacturing, posing a significant threat to the integrity and safety of structures and components. By understanding how corrosion fatigue occurs and implementing preventive measures during design, material selection, and maintenance, engineers can help minimize its impact and prevent potentially disastrous failures.

For more information or expert advice on corrosion fatigue and its prevention, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.