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

Abstract

Abstract. 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' ∆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 K max 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. Review of Mechanisms and Some General Aspects of Corrosion FatigueFor many materials, embrittling environments (e.g. aqueous, gaseous hydrogen, liquid metals) can increase rates of fatigue crack growth by up to several orders of magnitude compared with the rate in inert environments, with the degree of embrittlement depending on variables such as ∆K, cycle frequency, and the 'potency' of the environment. In addition, the fracture path and fracturesurface appearance can be markedly different in inert and embrittling environments [1,2]. Fatigue in inert environments generally produces transgranular fractures exhibiting 'ductile' striations whereas intergranular or cleavage-like fractures with 'brittle' striations are observed after fatigue in embrittling environments (although brittle striations are sometimes difficult to resolve owing to their shallow profile and post-fracture corrosion/film formation (Fig. 1).Fatigue crack growth at intermediate ∆K, for both inert and embrittling environments, generally involves plastic blunting and crack growth during the rising load part of each stress cycle, with resharpening of the crack tip occurring by deformation just behind crack tips during unloading. The difference between crack growth in inert and embrittling environments is that slip is more localised for the latter, resulting in less blunting and greater crack-growth increments for a given crack-tipopening displacement (Fig. 2). This figure, and others like it in the literature [e.g. 3,4], omits an important feature, namely, the formation of nanovoids ahead of cracks. Nanovoid formation at intermediate ∆K levels is often overlooked because the presence of nano-scale dimples on fracture surfaces is clearly resolved only when 'state-of-the-art' high-resolution SEM is used [5] or, even better, when TEM is used to examine well-shadowed replicas under ideal conditions [6] (Fig. 3).The localisation of plasticity and increased crack growth rates in both aqueous and gaseous hydrogen environments for many materials (e.g. Fe, Mg, Ni, Ti) are genera...