Predicting the kinetics of hydrogen generation at the tips of corrosion fatigue cracks

A model has been developed to predict the rate of generation of hydrogen atoms at the tips of fatigue cracks for steel cathodically protected in marine environments. The model incorporates crack-tip chemistry, scraping electrode measurements, and crack-tip deformation. The current density for generation of hydrogen atoms by reduction of water at the crack tip has been calculated for a range of electrochemical and mechanical variables (electrode potential, cyclic frequency, waveform, ΔK, and R value). The crack-tip current density is always greater than on adjacent crack walls and tends to increase with decreasing (more negative) potential. However, at potentials more negative than about-900 mV (SCE), at a cyclic frequency of 0.1 Hz, cathodic reduction of water on the external surface of the steel is predicted to be the dominant source of hydrogen atoms. Decreasing the frequency reduces the crack-tip current density and further emphasizes the dominance of bulk charging. There is little difference in hydrogen charging current densities at the crack tip for sinusoidal, triangular, or positive sawtooth waveforms, but square-wave loading provides a greater charging current. Increasing ΔK and R value have only a small effect on crack-tip current density but increase the area of the active surface and thus lead to more significant charging. Hydrogen-atom concentration profiles in fracture mechanics specimens and in tubular sections have been calculated for conditions in which bulk charging of the steel with hydrogen is dominant. To ensure that crack growth rates are “steady-state” values, test times have to be long enough to establish steady conditions of hydrogen charging. Crack growth data from fracture mechanics specimens may not be directly relevant to cracking in tubular sections because of hydrogen concentration gradients in the latter.