11622Mechanisms of CF Crack Growth

Although a number of mechanisms have been forwarded, it is still uncertain as to the exact mechanism by which corrosion fatigue cracks propagate. However, two main categories exist: anodic dissolution and absorption-related mechanism.

Anodic Dissolution This mechanism presupposes that crack advance occurs as a result of dissolution at the crack tip. As with SCC, local cells are set up either as a result of galvanic action, that is, cathodic matrix and anodic grain boundaries, or due to the formation of bare (crack tip) metal surfaces adjacent to protective films on the walls of the crack. Corrosion rates measured using static techniques suggest that crack advance cannot be wholly accounted for by anodic dissolution. However, the fact that environments within crack enclaves differ from bulk solution composition, along with changes in crack tip potential, and that transient increases in corrosion currents occur when bare metal surfaces are formed, suggests that a calculation of the environmental contribution via the Faradaic equation is possible [28] and,

zF p where M is the molecular weight, ia is the anodic corrosion current, z is the number of electrons involved in the reaction, F is Faraday's constant, and p is the density of the metal-alloy.

Adsorption-Related Mechanism This mechanism suggests that adsorption of hydrogen directly at the crack tip lowers the surface energy of the material and, subsequently, lowers the fracture stress of the material:

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