Effects of Waveform and Cycle Period on Corrosion-Fatigue Crack Growth in Cathodically Protected High-Strength Steels

The processes involved in corrosion fatigue in general are briefly outlined, followed by a brief review of recent studies on the effects of cycle frequency (rise times) and electrode potential on crack-growth rates at 'intermediate' Delta K levels for cathodically protected high-strength steels. New studies concerning the effects of fall times and hold times at maximum and minimum loads on crack-growth rates (for Km a. values below the sustained-load SCC threshold) are presented and discussed. Fractographic observations and the data indicate that corrosion-fatigue crack-growth rates in aqueous environments depend on the concentration of hydrogen adsorbed at crack tips and at tips of nanovoids ahead of cracks. Potential-dependent electrochemical reaction rates, crack-tip strain rates, and hydrogen transport to nanovoids are therefore critical parameters. The observations are best explained by an adsorption-induced dislocation-emission (AIDE) mechanism of hydrogen embrittlement.