Effect of Mechanical Parameters on Primary Water Stress Corrosion Cracking of Nickel-base Alloys Junhyun Kwon*, Yong-Sun Yi, Joung-Soo Kim, Whungwhoe Kim Korea Atomic Energy Research Institute, 150 Deokjin, Yuseong, Daejeon 305-353 E-mail: jhkwon@kaeri.re.kr* 1. Introduction Primary water stress corrosion cracking (PWSCC) of nickel-base alloys has become one of the serious problems in the primary water loops of pressurized water reactors (PWRs). Several mechanisms have been proposed to account for the PWSCC behavior of nickel- base alloys in the high temperature water (~300 o C). In this study, we consider the slip oxidation model as a potential PWSCC mechanism, and apply this model to the estimation of the crack growth rates quantitatively. A film-rupture / slip-oxidation (SO) process has been proposed to account for SCC of Alloy 600 as a PWSCC mechanism. The rate controlling factors affecting the crack propagation in the SO model are the rupture rate of the oxide film, the rate of repassivation, and the rate of the diffusion of the dissolved metal ions away from the crack tip. The SO model, as proposed by Ford and Andresen [1], has been successful in predicting the crack propagation rate in austenitic stainless steels and low- alloy steels in 288 o C water, symptomatic of boiling water reactor systems. Although there are differences in environmental conditions between PWR and BWR, the dissolution of metal atoms plays an important role in the intergranular SCC of any alloys. The primary objective of the present study is to evaluate the significance of mechanical parameters affecting PWSCC of Alloy 600. Because of the multiplicity of the interacting variables, it is difficult to single out the critical parameter affecting PWSCC. The concept of the crack-tip mechanics, which enables us to determine the crack-tip strain rate analytically, was applied in deriving the crack growth rate (CGR). Emphasis is placed on identifying the parameters that affect the cracking behavior, as well as calculating the CGR of Alloy 600. 2. Modeling of stress corrosion cracking The SO model relates a crack propagation to the amount of the metal dissolution that occurs on the bare surface when a protective oxide film is mechanically ruptured. This model consists of three processes; 1) rupture of the oxide film, 2) anodic dissolution of the bare metal, 3) reformation of an oxide film (repassivation). Thus, the crack growth rate is equivalent to the rate of a metal dissolution at the crack tip. The average CGR is given by: ( ) ( )( ) ( ) m ct m f m o o m 1 t i F z M a ε ⋅ ε − ⋅ ρ = & & (1) where, M and ρ are the atomic weight and mass density of the crack tip metal, respectively, F is the Faraday constant, z is the number of electrons involved in the oxidation of the metal atom, i o is the current density dissolved from a bare surface, t o is the time at which repassivation starts, ε f is the fracture strain of the oxide-film, and m is the slope of the current-density curve. The factor m is a complex function of several parameters such as electrochemical potential, solution conductivity, pH, etc. The crack-tip strain rate ct ε& in Eq. (1) is an engineering parameter which represents the mechanical contribution to the CGR. A mechanically-based ct ε & equation was derived by Shoji [2] using the plastic strain distribution ahead of a growing crack tip, developed by Gao [3] et al., which is given by: ( ) 1 n / 1 2 y I o o I I y ct K r ln r a K K 2 1 n E n − ⎪ ⎭ ⎪ ⎬ ⎫ ⎪ ⎩ ⎪ ⎨ ⎧ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ σ λ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − σ β = ε & & & (2) where β and λ are dimensionless constants, σ y the yield strength, n the strain-hardening exponent in the Ramberg- Osgood power law, E the Young’s modulus, K I the stress intensity factor, and r o the characteristic distance. In Eq. (2), the variable r o represents the characteristic distance ahead of a crack tip where the strain rate should be defined. While the physical significance of r o still remains unsettled, the best estimate of r o is in the order of several µm, which was derived from the inverse analysis. The CGR equation can be obtained by combining two Eqs. of (1) and (2). The CGR equation derived here takes into account all the factors affecting the SCC behavior – load, material properties, and electrode kinetics. Although the SO mechanism may not work in the SCC of Alloy 600, the application of the theoretical equation enables us to predict the CGR in a quantitative way, at least to provide the numerical tool for a future work. Transactions of the Korean Nuclear Society Autumn Meeting Busan, Korea, October 27-28, 2005