Abstract: Using computer simulations the influence of different standard deviations of the yield strength, ultimate tensile strength and of Kc values on the probability of the crack growth in an Al-alloy pressure vessel is studied. The state of art of non destructive materials characterization as it is achieved at the IZFP is demonstrated by experimental results of non destructive stress analysis, evaluation of strength data and of materials state characterization. The goals of future developments are clear: in case of components made by Al alloys, the combined use of the ultrasonic and eddy current techniques and in case of steel components, the additional use of micro magnetic techniques have to be enforced. The miniaturization of the sensors and the continuation of the experimental investigation of the correlation between the strength and Kc data on one side and the materials structure as characterized by non destructive tools on the other side are the next steps.
Abstract: The susceptibility of Inconel 600 to stress corrosion cracking (SCC) in the primary water of a pressurized water reactor (PWR) is strongly dependent on potential. In the present paper we assess the validity of the hypothesis that the potential dependency of SCC is related to the influence of potential on the identities of the surface films that form on Inconel 600. That is, according to the hypothesis,
SCC requires the presence of a particular surface film.
The identities of the surface films that develop at different potentials on Inconel 600 in PWR primary water (2 ppm LiOH + 1200 ppm H3BO3) at 288°C were investigated in situ by surface enhanced Raman spectroscopy (SERS).
To help identify the components of the films that grow on Inconel 600, the films that form on unalloyed nickel, chromium, and iron in 288°C PWR primary water were also investigated.
The main results of the in situ SERS investigation of the surface films are as follows. (1) No films were formed on Inconel 600 at potentials below the region of potential in which SCC occurs. (2) A chromium-rich M3O4 oxide forms on Inconel 600 in the SCC region. (3) NiO forms as the potential is increased immediately above the region of SCC susceptibility. (4) At still higher potentials, films of (Fe,Cr)2O3 and Ni3-xFexO4 form.
The results are consistent with concept that specific films affect SCC susceptibility.
Abstract: Grain boundary properties are known to affect the intergranular stress corrosion cracking (IGSCC) and irradiation assisted stress corrosion cracking behavior of austenitic alloys in high temperature water. However, it is only recently that sufficient evidence has accumulated to show that the disposition of deformation in and near the grain boundary plays a key role in intergranular cracking. Grain boundaries that can transmit strain to adjacent grains can relieve stresses without undergoing localized deformation. Grain boundaries that cannot transmit strain will either experience high stresses or high strains. High stresses can lead to wedge-type cracking and sliding can lead to rupture of the protective oxide film. These processes are also applicable to irradiated materials in which the deformation can become highly localized in the form of dislocation channels and deformation twins. These deformation bands conduct tremendous amounts of strain to the grain boundaries. The capability of a boundary to transmit strain to a neighboring grain will determine its propensity for cracking, analogous to that in unirradiated metals. Thus, IGSCC in unirradiated materials and IASCC in irradiated materials are governed by the same local processes of stress and strain accommodation at the boundary.
Abstract: Environmentally assisted cracking (EAC) consists of two distinct events viz., i) crack initiation and ii) crack propagation. On a smooth surface, the EAC initiates by the rupture or by the degradation of the surface film due to the combined action of stress and an electrochemical reaction of the materials with the environment. The mechanical properties of the surface oxide films are also important considerations when determining the susceptibility to EAC.
In this research, Micro Raman Spectroscopy(MRS) was applied for in-situ oxides characterization and for in-situ measurements of the stress in oxide film formed on the surface of 304L stainless steel during the scratching electrode and the slow strain rate test (SSRT), respectively. The passive oxide film growth formed on the bare surface was continuously monitored by MRS as a function of time. For stress measurements, Cr2O3 was focused on and Raman shift at Cr2O3 peak of Raman spectrum was measured continuously. The strain rate was 8.2×10-7 /sec. . In the initial stage of
SSRT, the Raman shift of surface film decreased gradually with strain. At 5% strain, the Raman shift of surface film increased rapidly to around the initial value of Raman shift. It is considered that the surface film was ruptured at this time. At 5% strain, the shift value of Raman peak of Cr2O3 reached to 5cm-1. This value (5cm-1) corresponds to 1.2GPa which value of tensile stress is calculated from reference data.
These characteristics of oxide film will be implemented into the theoretical formulation of EAC and their implication to EAC growth rate will be discussed.
Abstract: The electronic properties of the interfacial oxide film formed on 304L stainless steel in high temperature water are investigated by contact electric resistance (CER) measurements. Tests are performed in pure water with a wide range of dissolved oxygen (DO) content at 200, 250, and 288°C. The electrochemical potential (ECP) moves in the noble direction and CER increases when increasing DO. Results show that DO content has a dominant effect on the electronic properties of
oxide film. The change of oxide film properties can also be attributed to the variation of the electrochemical potential, which is directly affected by DO content. Critical potentials exist for the formation and reduction of oxide films in high temperature water. Multiple steps are found for the reduction of oxide films due to de-aeration in 200, 250, and 288°C water, implying the presence of multiple-layer interfacial oxide films. The film reduction process is relatively slower than the film formation process. Present results show that even in high purity water, a moderate change of DO
content can result in different surface conditions. Dissolved hydrogen has a moderate effect on interfacial surface films in deaerated water. In-situ monitoring of the oxide film properties by CER technique provides information on the interfacial reactions that are related to the SCC behavior of materials in high temperature water environments.
Abstract: A possible approach to describe the role of the environment in the phenomena behind crack initiation and crack propagation in stress corrosion cracking (SCC) is to assume that the transport of species through the oxide film on the material surface is one of the rate-controlling factors. The transport rates of ionic and electronic defects through the oxide film are, in addition to the environment,
also affected by the stress and strain applied to the bulk material.
In this paper, the surface oxide film formed on AISI 316L steel in slow strain rate tests (SSRT) in simulated BWR condition has been analyzed by using Electron Spectroscopy for Chemical Analysis (ESCA). The obtained film composition and structure have been combined with in-situ contact electric resistance (CER) measurements in order to evaluate the changes in oxide film electric properties during straining in the above environment. The results show that oxide film resistance of the strained part exhibits a maximum at around 2% of strain, which seems to correlate with a maximum in the Cr(III) concentration in the inner layer of the oxide. The implications of these results to SCC are discussed based on Mixed-Conduction Model (MCM).
Abstract: Hydrogen embrittlement causes a crack initiation and brittle fracture. Therefore, it is important to conduct theoretical analysis to predict hydrogen distribution on the basis of reasonable physical model. In this paper, numerical analysis on the hydrogen diffusion and concentration at the localized stress field was conducted to clarify the behavior of hydrogen concentration and the sensitivity of hydrogen embrittlement. Furthermore, on the basis of this analysis, the effect of post weld heat treatment on the release of concentrated hydrogen was analyzed and optimum condition of post and pre heat treatment on the welding was investigated.
Abstract: Previously, we proposed stress corrosion cracking model on the basis of interaction of dislocation and hydrogen around a crack tip to predict discontinuous cleavage-like fracture during stress corrosion cracking (SCC) for ductile fcc alloys. Furthermore, we conducted numerical analyses using this proposed model. In the analysis, hydrogen was treated as a static cluster. However, actually, both of hydrogen and dislocations move with interaction each other. Therefore, in this paper, a physical model of dislocation and hydrogen dynamics with interaction was proposed. And the behavior of interaction between dynamic dislocations and hydrogen was investigated. On the basis of this analysis, the discontinuous cleavage-like fracture during SCC for ductile fcc alloys was clarified.
Abstract: Primary water stress corrosion cracking (PWSCC) of Alloy 600 has been a great concern to the nuclear power industry. Reliable PWSCC growth rate data, especially at temperatures in the range of 290-330°C, of the alloy are required in order to evaluate the lifetime of power plant components. In this study, three tests were carried out in simulated pressurized water reactor (PWR) primary water at 325°C at different dissolved hydrogen (DH) concentrations using standard one-inch compact tension (1T-CT) specimens. The initiation and growth of cracks as well as insights into the different PWSCC mechanisms proposed in the literature were discussed. The experimental results show that the detrimental effects of hydrogen on
crack initiation and growth reached a maximum at a certain level of DH in water. The
experimental results were explained in terms of changes in the stability of the surface oxide films under different DH levels. The experimental results also support the assumption that hydrogen absorption as a result of cathodic reactions within the metal plays a fundamental role in PWSCC.