Corrosion fatigue

A form of fatigue cracking in which cracks develop under the combined affects of cyclic loading and corrosion. Cracking often initiates at a stress concentration . Cracking can initiate at multiple sites.

In cycling boilers, the damage usually appears first on the water side of buckstay attachments . The cracking pattern may be circular cracks surrounding the weld between the buckstay attachment and the waterwall tube. In cross-section, the cracks tend to be bulbous with numerous lobes . The crack tips themselves may be somewhat blunted but are oxide filled and transgranular.

Critical Factors

a) The critical factors are the material, corrosive environment, cyclic stresses and stress raisers.

b) Cracking is more likely to occur in environments that promote pitting or localized corrosion under cyclic stress due to thermal stress, vibration or differential expansion.

c) Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.

Corrosion promotes failure at a lower stress and number of cycles than the materialsí normal endurance limit in the absence of corrosion and often results in propagation of multiple parallel cracks.

d) Crack initiation sites include concentrators such as pits, notches, surface defects, changes in section or fillet welds.