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Failure of components operating in various environments due to cracking remains a safety and economic problem despite the effort that has been devoted to understanding the phenomena of stress corrosion, hydrogen embrittlement, and corrosion fatigue. This article reviews our current understanding of the mechanisms of stress corrosion and corrosion fatigue in ductile alloy/aqueous environment systems, with emphasis placed on the application of this mechanistic knowledge to the formulation of remedial actions, design criteria, and relevant testing techniques.
It is concluded that many of the recent arguments associated with, for example, the applicability of the slip-dissolution or hydrogen-embrittlement mechanisms of crack advancement under static and dynamic loading conditions may be of minor importance from a practical view, since the same rate-determining steps in crack propagation can operate in these mechanisms and modes of cracking. Bearing in mind that crack advance in ductile structural-alloy/aqueous-environment systems is primarily an electrochemically related process, our understanding of the cracking mechanism suggests that, at a given potential, the propagation rate is controlled by reaction rates (for example, dissolution, hydrogen-ion reduction) on the crack tip and sides and how these are affected by the passivation rate, liquid diffusion rate, and strain rate at or near the crack tip. Knowledge of the interactions between these controlling processes leads to a quantitative understanding of the spectrum of behavior between stress corrosion and corrosion fatigue and of the criteria that dictate whether crack propagation will arrest or accelerate.
cracking mechanisms, corrosion fatigue, stress corrosion, dissolution, passivation, liquid diffusion, hydrogen embrittlement, oxide rupture rate
Unit Manager, General Electric Research and Development Center, Schenectady, N.Y.