Mass transport in the crack solution as the rate-determining step in the crack growth mechanism during the corrosion fat
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INTRODUCTION
C U R R E N T L Y , models capable of predicting the crack growth rates in general during environmentally assisted cracking of metals on the basis of a detailed fundamental knowledge of the operating mechanisms are not available. Since the early work of Brown I~l and Wei, I21 fracture mechanics variables, in particular the stress-intensity factor K and the stress-intensity range ~rs have been commonly used in the presentation and analysis of stress corrosion and corrosion fatigue crack growth data. For a variety of metal environment systems, the observed behavior of CF growth rates for short and long cracks shows that the growth rates during corrosion fatigue are not uniquely controlled by 2ffs and the test frequency w. In general, according to the superposition models, three different contributions to CF crack growth can be distinguished. 13-71 (a) A purely mechanical component, which is also operating in inert environments. This contribution follows the Paris law, stating that the growth rate per cycle da/dn is proportional to the stress-intensity range z3Jr to a power n: da/dn oc (2ffs n. (b) An environmental component referred to as "true corrosion fatigue" (TCF). (c) A component, which is assumed to be essentially due to a stress corrosion mechanism. This stress corrosion fatigue (SCF) contribution is expected in cases where, during a fatigue cycle, the value of the stress intensity factor K exceeds a critical value K~scc for the initiation of stress corrosion cracking. Different types of CF behavior can result from these contributions, as illustrated schematically, for example, by McEvily and Wei. t41 In the case of the corrosion fatigue of steel in chloride solutions, there is evidence in the literature indicating that the environmental component is due to a hydrogen embrittlement mechanism. The available information has been presented in a review article by Gangloff. tS~ This author also discussed the breakdown in similitude C.J. VAN DER WEKKEN, Research Associate, is with the Delft University of Technology, 2628 AL Delft, The Netherlands. Manuscript submitted December 8, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
observed in certain situations in the behavior of short and long cracks.J8,9,1~ In a previous investigation in our laboratory, l~l measurements of the local electrochemical conditions at the crack tip during CF testing of a pipeline steel in deaerated seawater showed that even under strong external anodic polarization, there was still a cathodic overpotential for the hydrogen reaction at the crack tip, while the tip solution was acidified (pH 5.5). This result provided further support for the view that the accelerated crack growth rates observed during strong external anodic polarization were associated with hydrogen embrittlement of the material near the crack tip. The study of the electrochemical conditions in the crack also led to the conclusion that the crack-tip area could not be considered an "occluded cell": During CF cycling under external anodic polarization, acid
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