Corrosion fatigue is the nucleation of fatigue cracks at the stress concentrations associated with corrosion pits and/or the acceleration of fatigue crack growth by the simultaneous occurrence of corrosion in the crack. It is related to mechanical fatigue in that a cyclic stress is necessary, but corrosion fatigue failures can occur more rapidly due to the interaction with a corrosive environment. Residual stresses can also play a very significant role in the nucleation of corrosion fatigue cracks and the acceleration of their growth rates.
Deaerator vessels can be highly susceptible to corrosion fatigue related to oxygen contamination of the boiler feedwater, especially at low pH levels. The cracking has been observed predominantly in welds and heat affected zones of vessels that have not received a postweld heat treatment, due to the relatively high residual stresses at these locations.
Corrosion fatigue cracks in deaerator vessels can be detected by magnetic particle examination (MT) of the I.D. surface of the vessel. The surface should be lightly ground to assure adequate sensitivity for crack detection. Ultrasonic examination (UT) from the O.D. surface using shear wave techniques can be used to determine the depth of the cracks detected, and thus evaluate if the vessel can be economically repaired or must be replaced.
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