It is well known that a material subjected to a cyclic load far below the ultimate tensile stress can fail, a process called fatigue.
If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.
The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-corrosion cracking, but not branched. The picture shows a primary corrosion-fatigue crack that in part has been widened by a secondary corrosion reaction