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Dates: May 23, 2017 - May 26, 2017
Corrosion Fatigue is a damage mechanism caused by a combination of cyclic stress and corrosion in which a piece of equipment will crack due to the repeated application of stress in a corrosive environment. 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.
While it is commonly associated with deaerators in boiler systems, corrosion fatigue can occur in any environment with any material of construction that has sufficient cyclic stresses, corrosion pitting, and stress raisers and is not isolated to deaerators. Equipment and piping that happen to be closely associated with rotating machinery can be susceptible as well, along with other equipment that is exposed to significant pressure or thermal cycles.
Typically corrosion fatigue cracks are not branched, but rather have multiple parallel cracks. This cracking can be found with effective surface NDE techniques. To better avoid corrosion fatigue one whould carefully control the chemistry of boiler feed water and condensate, reduce stresses and stress concentrations with PWHT and contour grinding of weld irregularities, and upgrade to more corrosion resistant alloys and use coatings to reduce corrosion.
November/December 2013 Inspectioneering Journal
By 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.
January/February 2005 Inspectioneering Journal
By 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.