Corrosion Fatigue

Corrosion fatigue

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Module Six of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

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 reference))

http://corrosion-doctors.org/Forms-fatigue/fatigue.htm

12.08.2010

Corrosion fatigue

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The fatigue fracture is brittle and the cracks are most often transgranular, as in stresscorrosion cracking, but not  branched. The picture shown here reveals a primary corrosionfatigue 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  

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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 desi gn Increase natural frequency for reduction of resonance corrosion fatigue Limit corrosion factor in the corrosion-fatigue process (more resistant material / less corrosive environment).

http://corrosion-doctors.org/Forms-fatigue/fatigue.htm

12.08.2010