Mechanism

In addition to the problems that can occur with stress corrosion cracking (SCC) due to the effects of static stress, in many situations, the applied stresses are cyclic due to mechanical or thermal effects. Under so called non corrosive conditions, cyclic or varying stress levels can result in fatigue damage at surfaces, leading to the growth of fatigue cracks and eventual failure.

Under corrosive conditions, fatigue cracks may be initiated, at corrosion pits for example, at a much earlier stage during service life and these cracks may well grow faster in the corrosive environment. The combined action of corrosion and fatigue will therefore be responsible for failure after a smaller number of stress cycles than would be expected in a non-corrosive situation.

In ferrous alloys, a fatigue (endurance) limit is obtained under non-corrosive fatigue conditions. At stress levels below this limit, fatigue failure is not expected. Corrosive conditions remove this limit, as shown schematically in Figure 1, giving the possibility of failure even at low levels of stress.

Corrosion fatigue tends to produce a number of growing cracks, rather than the more normal single crack in fatigue under conditions where corrosion has no influence. The corrosion fatigue cracks usually develop normal to the main tensile stress and do not branch, whereas SCC cracks are highly branched.

Corrosion fatigue fracture surfaces may or may not be coated with corrosion product depending on the relative effects of corrosion and stress. There will be more evidence of corrosion at lower stress levels or lower frequencies of stress cycling, because of the increased time of exposure. Unlike stress corrosion cracking, corrosion fatigue is a general problem and is not confined to a specific alloy environment combination.