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7- Fatigue Corrosion |
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Corrosion-fatigue differs from SCC by the fact that the applied stresses are no longer static, but cyclic (periodically fluctuating or alternating loads).
In the case of steels, the conventional fatigue
limit determined from so-called Wöhler curves (applied stress
as a function of cycles to failure
δ = f(N)) does not exist
for tests performed in
a corrosive medium. Whatever the stress level, failure will eventually
occur after a finite number of cycles.
The cracks are generally transgranular in nature, with little tendency
for branching. However, a few small secondary cracks may appear in
the vicinity of the main crack.
Although there is no direct relationship between the sensitivity
to corrosion-fatigue and the mechanical properties of the material,
high strength alloys tend to be most highly prone.
Corrosion-fatigue damage can be prevented or reduced by decreasing the tensile stresses, either by the use of stress-relief annealing, by modifying component design, or by applying mechanical surface treatments such as peening, to introduce surface compressive stresses. Improvement of the surface condition by polishing is generally beneficial. Corrosion inhibitors are highly effective.
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