Corrosion Fatigue

Corrosion-fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated.  If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.

• Corrosion fatigue models

• Protection checklist

In a corrosive environment the stress level at which it could be assumed a material has infinite life is lowered or removed completely. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue. Corrosion fatigue and fretting are both in this class. Much lower failure stresses and much shorter failure times can occur in a corrosive environment compared to the situation where the alternating stress is in a non-corrosive environment.  (reference)

The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-corrosion cracking, but not branched. The picture shown here reveals a primary corrosion-fatigue crack that in part has been widened by a secondary corrosion reaction. The corrosive environment can cause a faster crack growth and/or crack growth at a lower tension level than in dry air. Even relatively mild corrosive atmospheres can reduce the fatigue strength of aluminum structures considerably, down to 75 to 25% of the fatigue strength in dry air. (photo courtesy)

 No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Control of corrosion fatigue can be accomplished by either lowering the cyclic stresses or by various corrosion control measures (see checklist).

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Corrosion Fatigue Models

•  Corrosion incidents of aircraft and helicopters
•  Structural integrity
•  Pitting corrosion incidents
•  PCF models
•  Validity of PCF models

Protection Possibilities Checklist

•  Minimize or eliminate cyclic stresses
•  Reduce stress concentration or redistribute stress (balance strength and stress throughout the component)
•  Select the correct shape of critical sections
•  Provide against rapid changes of loading, temperature or pressure
•  Avoid internal stress
•  Avoid fluttering and vibration-producing or vibration-transmitting design
•  Increase natural frequency for reduction of resonance corrosion fatigue
•  Limit corrosion factor in the corrosion-fatigue process (more resistant material / less corrosive environment).