Dr. Dmitri Kopeliovich
Corrosion Fatigue (CF) is the metal cracking caused by combined action of a cycling loading and a corrosive environment.
Corrosion fatigue is similar to Stress corrosion cracking in many aspects (combine action of stress and corrosion, mechanisms of crack formation, brittle fracture, prevention measures). The principal difference between these two types of environment enhanced cracking is in the character of loading, which is static in Stress corrosion cracking and alternating/repeated/cycling/periodically fluctuating in corrosion fatigue.
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 corrosive environment as compared to the fatigue in neutral environment (dry air).
Effect of corrosive environment on fatigue may be expressed by the Damage ratio:
Damage ratio = σcf / σf
σcf - corrosion fatigue strength;
σf - fatigue strength in neutral environment.
Damage ratio may vary from 0.2 (for Carbon steels in sea water) to 1.0 (for copper in sea water). Damage ratio of Stainless steels in sea water is 0.5.
Corrosion fatigue cracks propagate generally by transgranular mechanism (through the grains along crystallographic planes).
Corrosion fatigue cracks are generally not brunched (a few secondary cracks may form in the regions adjacent to the main fatigue crack).
Corrosion fatigue is characterized by brittle fracture.
Corrosion fatigue (CF) is associated with two different mechanisms:
According to the mechanism of anodic dissolution (slip dissolution, stress enhanced dissolution, active path corrosion) cracks initiate at the surface sites of localized concentration of tensile strength (trenches, pits).
A crack progresses along a specific path (active path), which is composed of specific crystal planes within the grains.
The corrosion fatigue crack propagates by the repetitive process, cycle of which consists of the following stages:
- Brittle passive oxide film is ruptured at the crack tip under tensile stress resulting in exposure of fresh metal.
- The bare metal surface undergoes anodic dissolution.
- As a result of the corrosion process the crack tip surface is re-passivated forming a new protective oxide film.
The mechanism of anodic dissolution is mainly referred to corrosion fatigue of Carbon steels and Stainless steels in water and also corrosion fatigue of Aluminum alloys and Titanium alloys in aqueous chloride solutions.
In contrast to anodic dissolution mechanism Hydrogen assisted corrosion fatigue (Hydrogen Environment Embrittlement / HEE) is enhanced by cathodic reaction: H+ + e- = H occurring on the crack tip surface.
The atomic hydrogen dissolves in the metal where its ions interact with the dislocations of the crystal lattice causing decrease of the metal ductility (hydrogen embrittlement).
Hydrogen cracking effect is increased in harder materials and at higher temperatures.
Hydrogen assisted corrosion fatigue may be prevented by selection of suitable materials and maintaining the environment solution at neutral or basic PH (PH>6). In contrast to anodic stress corrosion hydrogen cracking is enhanced by Cathodic protection.
Hydrogen assisted corrosion fatigue is mainly referred to ferritic and martensitic steels. This mechanism also explains corrosion fatigue of Nickel alloys, Aluminum alloys and Titanium alloys in electrolytic and gaseous environments.