The Mechanism and Modeling of Intergranular Stress Corrosion Cracking of Nickel-Chromium-Iron Alloys Exposed to high Purity Water

Abstract

This investigation combines microstructural characterization, fracture mechanics analyses, atomistic modeling, and experimental crack growth rate data to better elucidate the mechanism of stress corrosion cracking of nickel-based alloys exposed to high temperature, high purity deaerated water.Additionally, this paper develops a mechanistically based equation that is suggested to be generally applicable to SCC of Alloy 600-type alloys exposed to high purity water.Results show that stress corrosion crack tips are truly intergranular, sharp (~5-10 nm crack tip openings), and are well described by moving crack fracture mechanics.These findings, combined with the clear dependency of the crack growth rate on the electrochemical potential and the constancy of the apparent activation energy (see Morton's paper in these proceedings) suggest that the stress corrosion crack growth rate in high purity water is governed by the supply rate of an embrittling species to the crack tip process zone and by the tearing resistance of the material immediately in front of the crack tip (i.e. the local J-R curve).Consideration of both hydrogen and oxygen embrittlement show that both mechanisms are feasible, although there is somewhat more support for a hydrogen mechanism.An example of the crack growth rate model and data fitting procedures are given for Alloy 600 heat affected zone (HAZ) material.Results show that the fitting procedure can have a large effect on model parameters and subsequent extrapolations.For the data considered, nonlinear curve fitting in real space (vice log space) resulted in the most accurate fit.The Alloy 600 HAZ modeling shows that the apparent activation energy for crack growth is lower than is typically reported (91.2 kJ/mol 27.4| 95% kJ/mol vice ~130 kJ/mol), the crack growth rate is weakly dependent on the applied stress intensity factor (SCCGR K 1 ), and the effect of electrochemical potential is significant (~3.6X near Ni/NiO).