Response of short cracks in high-strength steels to fatigue loadings. Part 2: Behavior in seawater under cathodic polarization

Experiments have been performed to determine the fatigue crack growth rate (FCGR) of short cracks (length from 0.1 to several nm) of five steels with yield stress in the range 370--570 MPa while cathodically polarized in natural seawater. Attention was focused on regions I and II of the classical FCGR-stress intensity range curve with particular consideration being given to the near-threshold behavior of short cracks. Single-edge notched, three-point bend specimens and a direct current potential drop crack monitoring system were employed; and test parameters were selected to simulate conditions experienced by deepwater offshore structures. These included a stress ratio of 0.5, a frequency of 0.3 Hz, and three levels of cathodic polarization (-800, -950, and -1,100 mV, SCE). Crack growth rates were evaluated in terms of environment (air versus seawater), potential, material, and crack length. It was determined that cathodic polarization was generally beneficial with regard to FCGR compared to the freely corroding case, even at the most negative potential considered (-1,100 mV, SCE), in contrast to what occurs for macro-cracks. The results are discussed within the context of design of offshore structures for resistance to fatigue