Corrosion Fatigue
Corrosion Fatigue is a specific type of fatigue failure mechanism that is caused by a combination of cyclic stress and a corrosive environment. When a crack initiates as a result of fatigue, corrosion typically increases crack propagation.
While corrosion fatigue is commonly associated with rotating equipment, deaerators, and cycling boilers, corrosion fatigue can affect any unit operating in a corrosive environment that has sufficient cyclic stresses and stress raisers. Stress raisers are typically found in concentrated areas of a component such as pits, notches, surface defects, or changes in metallurgy.
Corrosion fatigue usually results in multiple parallel cracks at the surface of the component. However, some corrosion fatigue cracks can also be round, especially when cracking occurs near welded joints. Cracks can be located using effective nondestructive testing techniques. Ultrasonic testing and magnetic particle testing are two of the most common methods in particular to inspect for corrosion fatigue.
Several techniques are available to mitigate corrosion fatigue. Some of these techniques include using more corrosion resistant alloys, using coatings or inhibitors, reducing stress raisers, modifying the corrosive environment, and minimizing residual stresses due to welding by performing a post-weld heat treatment.
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November/December 2013 Inspectioneering JournalBy Fernando Vicente at ABB
Service failures and safety incidents of machines, structures, and pressure equipment have been experienced in the oil and gas industry for many years without warning, with varying degrees of consequential damages to health, safety, environmental, business, and reputation. Unfortunately, equipment failures will occur no matter how effective a plant’s reliability program is.
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January/February 2005 Inspectioneering JournalBy John Reynolds at Intertek
Corrosion fatigue is closely related to mechanical and vibration fatigue cracking, except that it is initiated and accelerated by a corrosion mechanism, especially one that gives rise to pitting, from which cracks often initiate. But that corrosion mechanism need not be very severe in order to give rise to corrosion fatigue. Probably the best known case of corrosion fatigue, in a lightly corrosive environment, stems from deaerators in boiler systems.
