Mechanisms of Environmental Cracking in Systems Peculiar to the Power-Generation Industry. Final Report

摘要

The mechanisms of cracking were quantified in an attempt to provide a sounder planning basis for other design-oriented programs and to indicate possible causes of, and remedies against, environmentally-controlled cracking. The slip-dissolution mechanism of subcritical crack propagation was shown to be valid quantitatively < 100 exp 0 C for sensitized 304 in dilute sulphate solutions and for a 3CrMo turbine disc steel in neutral and alkaline environments. The model also explains the cracking response of stainless steel and carbon steel at temperatures up to 288 exp 0 C. The rate determining steps were identified by a transient electrochemical technique to be the bare surface dissolution rate, passivation rate, oxide rupture rate, and liquid diffusion rates in the crack tip region. The operating conditions where each predominates are related to the engineering variables such as stress, oxygen content, degree of sensitization, etc. Insights are offered into the reasons why startup deaeration in boiling water reactors (BWRs) can be beneficial; why turbine steels crack in both an alkaline environment and condensing steam and how this can be alleviated; why design criteria based solely on data from static tests may be in error when operating components are undergoing slight amounts of resonant vibration; how accelerated laboratory data involving rapid straining is related quantitatively to the condition expected in an operating plant, and, finally, why failure diagnoses narrowly defined as stress corrosion or corrosion-fatigue may be restrictive in terms of offering a possible remedial action. 194 figures, 5 tables. (ERA citation 08:005625)