99 Diseases of Pressure Equipment: Corrosion Fatigue (Deaerator Cracking)

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. In the late 80’s, a number of serious cases of deaerator failures (some including fatalities) occurred in the pulp and paper and petrochemical industries, giving rise to significant research and field inspection efforts to find and understand the cause of the failures. High residual stresses and significant stress raisers were two big factors in the cause of corrosion fatigue. Now most of the industry takes steps to more carefully control the chemistry of boiler feedwater and condensate, as well as reducing stresses and stress concentrations with PWHT and contour grinding of weld irregularities for new deaerators. Mitigation of corrosion fatigue can also include upgrading to more corrosion resistant alloys and use of coatings to reduce corrosion. Corrosion fatigue is not isolated to deaerators, and can occur in any environment with any material of construction that has sufficient cyclic stresses, corrosion pitting, and stress raisers. Equipment and piping closely associated with rotating machinery can be susceptible, as well as other equipment that is exposed to significant pressure or thermal cycles.

Corrosion fatigue cracking can be found with effective surface NDE techniques, but WFMT is usually used on deaerators because the multiple cracks may be tight and fine. Typically corrosion fatigue cracks are not branched, like you find in many cases of stress corrosion cracking, but have multiple parallel cracks.

Do you have an effective inspection and possible mitigation program for your deaerators and have you identified any other equipment that might be susceptible to corrosion fatigue because of cyclic stresses in a corrosive environment?