What Does Corrosion Fatigue Limit Mean?
Corrosion fatigue limit is the maximum stress that a corroded material can withstand for a given number of stress reversals. It is the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.
Both fatigue life (number of cycles until failure) and fatigue limit (maximum value of alternating stress, which the metal may withstand without failure) are reduced in the presence of a corrosive environment as compared to a neutral environment (dry air).
For many steels, fatigue limits range between 35% and 60% of the tensile strength.
Corrosionpedia Explains Corrosion Fatigue Limit
Corrosion fatigue limit is fatigue limit in a corrosive environment. It is the point after which mechanical degradation occurs under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress, and are exposed to harmful environments during their service life. The environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminum alloys and titanium alloys.
The fatigue process is thought to cause rupture of the protective passive film, which accelerates corrosion. The introduction of a corrosive environment often eliminates the normal "fatigue limit" of a ferrous alloy, thereby creating a finite life regardless of stress level. During very high frequency loading there is less time for the corrosion process to act. At lower frequencies, the corrosion process is more pronounced and can cause local attack that acts as a stress concentration site and thus contributes to a shorter life.
Corrosion-fatigue damage can be prevented or reduced by:
- Decreasing tensile stresses
- Use of stress-relief annealing
- Modifying component design
- Applying mechanical surface treatments such as peening, plating, nitriding
- Avoiding notches, accidental dents and rough surface finish
- Removing environmental factors
- Use of corrosion inhibitors & coatings
- Cathodic protection
As corrosion is time dependent, high fluctuating stress rates may result in fatigue failure before corrosion damage occurs. Laboratory test data on corrosion fatigue is difficult to generalize due to the added dimension of a corrosion environment in addition to a stressing regime.