The effect of crack- tip strain rate and potential on the propagation rate of stress corrosion crack for 321 stainless s
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The theoretical calculation shows that when the crack-tip strain rate changes from 1 0 - 4 / S to 10-2/S, the crack propagation rate changes from 0.01 m m / h to 3 m m / h at natural corrosion potential ( - 0 . 3 5 V(SEC)). If the crack-tip strain rate is above 1 0 - 2 / S , the crack propagation rate should correspond to the upper bound determined by the maximum metal dissolution rate. When the crack-tip strain rate is below 10 -4 s -~, the crack propagation rate is below 0.01 m m / h . The sensitive potential E,. to cracking is - 0 . 3 5 V(SEC); above the sensitive potential Ec, the crack propagation rate varies slowly with potential. However, below Ec, the crack propagation rate decreases rapidly with potential decreases. The crack propagation rate of 321 stainless steel in 42 pct MgC12 solution has been measured using a slow strain rate test technique. The theoretical calculation is consistent with the experimental results.
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INTRODUCTION
susceptibility of austenitic stainless steels to stress corrosion cracking (SCC) in boiling MgCI2 solution has been studied by many workers. 1~-71 The importance of crack-tip strain rate and potential during SCC has been recognized for many years. I8'91Several authors t9-~21showed that there was a critical range of applied strain rate and potential to SCC. For a variety of systems, SCC occurs only if the strain rate is within a critical range, limited by the upper and lower bound critical strain rates e j and e2. tgl Many workers t8,131investigated the relationship between applied strain rate and crack-tip strain rate. The results show that when the applied strain rate is above 10-6/s, the crack-tip strain rate is two orders of magnitude higher than the applied strain rate. The processes of SCC have long been associated with the disruption of a passive film at the crack tip of an advancing crack, as a result of slip processes followed by anodic dissolutions of the exposed bare metal. 114,15,161The model of slip-bare metal dissolution-repassivation has been suggested to evaluate the processes of SCC for stainless steel in boiling MgC12 solution, t~4J A significant limitation in previous studies of SCC is the lack of quantitative theoretical research. Obviously, the theoretical analysis of the effect of the crack-tip strain rate and potential on the SCC is of great importance for understanding the SCC processes.
D. LI, Postdoctoral Research Assistant, and X. MAO, Associate Professor, are with the Department of Mechanical Engineering, The University of Calgary, Calgary, Alberta T2N IN4, Canada. Manuscript submitted December 9, 1991. METALLURGICAL TRANSACTIONS A
Obtaining the decay law of current with time on the slip step at the crack tip is a key issue to establish the relationship between the crack propagation rate and cracktip strain rate as well as potential. Many authors have studied the repassivation kinetics of bare metal surfaces using the rapid scratching 1~8-24J and straining e
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