SUMMARY OF THE SUMMARY/POSTER PRESENTATION SESSIONS – Chaired by Mike Wright and Gary Was

Part I of the summary/poster presentation session on Monday morning consisted of four brief summaries covering a range of topics, from how the opening of a corrosion fatigue crack affects the oxides formed with the crack, through the effect of delta ferrite content on SCC behavior, an assessment of NobleChem mitigation and ended with a summary of work looking at the acceleration of thermal aging in stainless steel by neutrons. After the short talks the posters were presented by the authors during the coffee break.

The session started with a presentation by Kuniki Hata (JAEA, Japan), entitled “Oxidation in the vicinity of crack tips of load-applied CW316L stainless steel immersed in high-temperature water at 290°C”. His summary focused on the oxides formed in the immediate wake of a corrosion fatigue crack growing in cold-worked 316L under (oxygenated) BWR coolant conditions. Specifically, he contrasted the behavior when the crack was held at constant load, keeping the crack open for extended periods (50, 100, and 224 h), with the situation of an unloaded, closed crack. It was not explicitly mentioned, but the context is presumably the oxide crack closure effects that may be a cause of corrosion fatigue retardation under certain conditions. Hata described the characteristics of the oxides as analyzed by SEM-EDX and TEM-EDX, but drew particular attention to the thickness, and apparently rapid grow kinetics, of the inner oxide layer within the loaded, open crack-tip. It was noted that the apparent oxide growth rate was two orders of magnitude higher than expected for formation of an Fe-Cr spinel on a free surface exposed to the bulk environment. Based on FEM analysis of stress and strain in the loaded C(T) specimen, the authors speculated that localized plastic deformation and elastic strain in the crack wake could promote (inner layer) oxidation.

Jiamei Wang (SJTU, China) presented the next summary with the title “Effect of δ-ferrite on the stress corrosion cracking behavior of 321 stainless steel”. The test environment for the SCC testing was simulated BWR water plus chloride contamination up to 400 ppb for parts of the tests. Two (non-cold-worked) samples of 321 were tested, one solution annealed and one annealed followed by a 720°C/30 min heat treatment. Neither material showed signs of any significant sensitization and the ferrite content was 1.2 and 1.8%, respectively. The authors described the growth rates observed as “decreased” compared to industry disposition curves but the measured growth rates seem as expected, compared to an annealed 304L under similar test conditions. The growth rate increases an order of magnitude on addition of 400 ppb chloride. As an aside, for non-sensitized material this seems (to me) to indicate more vulnerability to chloride contamination than previous work might suggest (see Tice et al., EnvDeg. Conf. 2013, Asheville, NC, USA). Returning to the authors’ narrative, the low crack growth rates were attributed to a combination of δ-ferrite creating a more highly branched crack and δ-ferrite acting as a source of Cr to facilitate the formation of protective Cr-rich oxides at γ/δ boundaries. They also noted that Ti(CN) particles in the γ/δ boundary tended to blunt the crack-tip and inhibit oxidation, and thus impede the total crack growth.

Stefan Ritter (PSI, Switzerland) presented an “Assessment of the SCC mitigation capabilities of the NobleChem technology in simulated BWR environment”. He provided a brief background covering how NobleChem treatment creates a catalytic surface of distributed nano-particles of Pt on vessel internals (or test specimens) that enhance hydrogen-oxidant recombination and thus achieves lower ECP for a given dissolved hydrogen level. He explained that there is currently no accepted characteristic of the Pt particles or their distribution that can be measured and used to predict that a sufficiently low ECP is achieved. The work summarized by Stefan was recent testing at PSI showing that the Pt surface loading alone is not sufficient to assess the SCC mitigation capabilities. Pt particle size and inter-particle distances correlate better with the degree of depression of the ECP. To verify that SCC mitigation can actually be achieved, a series of SCC initiation tests (i.e., constant extension rate tensile tests) with Pt-treated and non-treated Alloy 182 specimens was also reported.

The last presentation was given by Yutaka Watanabe (Tohoku University, Japan) and was entitled “Acceleration of thermal ageing in stainless steel welds by neutron irradiation”. It started by stating the background to the work, which is that both cast stainless steels and weld metals are subject to thermal aging (and possible embrittlement) at reactor operating temperature due to their delta-ferrite content. The aim of the work summarized was to determine if thermal aging was accelerated by relatively low flux and low neutron irradiation damage (0.003 to 0.75 dpa) to fill a knowledge gap regarding superposition effects. Irradiation of thermally aged 316L weld metal (to 0.003 and 0.75 dpa) and 308L weld metal (to 0.75 dpa) were performed at the Halden reactor to compare with material subjected only to thermal aging. Microstructural evolution (Ni, Si, Mn, and Mo clustering, as well as spinodal decomposition) was characterized using Atom Probe Tomography. The aging of the materials was checked with hardness measurements. The results of the study indicate that there is an effect of superposition even at low flux and dpa.

Six further summary presentations were given in part II of the summary/poster presentation session on Tuesday before the coffee break.

Takumi Terachi (INSS, Japan) presented his poster “Long-term SCC initiation tests of Alloy TT690 in simulated PWR primary water”. Crack initiation tests on 20% cold-worked Alloy TT690 using blunt notch C(T) specimens were carried out in simulated PWR primary water conditions at 360°C. Crack nucleation on the blunt notch surface was observed after 45’000 h. Although the largest crack was less than 20 µm, SEM observations indicated that it was growing. Tests with excessive applied stress conditions with dynamic deformation caused the nucleation of cracking in a relatively short time. The results indicated that the possibility of SCC initiation in Alloy TT690 should be studied quantitatively.

Hsiao-Ming Tung (INER, Taiwan) summarized the “Effects of cold-work degrees on stress corrosion cracking behavior of Alloy 600 in simulated BWR water environments”. The effects of cold-work on the SCC behavior of Alloy 600 were investigated in simulated BWR water environment using crack growth rate tests. The crack growth rates of the specimens increased with increasing degree of cold-work and fracture was predominantly IG. When the environment was changed to hydrogen water chemistry (HWC), the crack growth rate of the cold-worked specimens significantly decreased. Microstructural characterization showed that the specimens with higher degree of cold-work possessed higher fraction of random high-angle grain boundaries and larger Kernel average misorientation values near the grain boundaries.

Colin Judge (CNL, Canada) gave a short talk on “Coupling multi-scale mechanical testing techniques with FIB and TEM characterization to reveal mechanisms of embrittlement of high dose ex-service components”. In addition to performing large-scale component testing of ex-service material to ascertain fitness-for-service, a series of in-situ small-scale testing is completed on X-750 and Zr-2.5Nb material. The mechanisms of fracture were investigated with FIB preparation of TEM lamella directly from bulk fracture surfaces and small-scale test specimens to identify the mechanisms of embrittlement.

Haozhan Su (SJTU, China) advertized her poster on the “SCC behavior of 304L HAZ of a dissimilar weld joint 304L/82/A105N in high-temperature water”. Crack growth rate tests on 340L HAZ showed that the HAZ has a high SCC susceptibility and the crack propagated from the weld region (strictly close to the fusion line) to 304L HAZ in the form of IG cracking. The crack initiation behavior of the whole weld joint sample was also studied through SSRT test, and micro-cracks of 5~20 μm were observed near the fusion line between 304L and Alloy 82. Electron Back-Scattered Diffraction (EBSD) and TEM-EDS results showed that IGSCC in the HAZ of 304L was caused by the higher residual strain rather than by Cr depletion on the grain boundary in the front of the crack-tip.

Aleksandra Treichel (PSI, Switzerland) showed first results of the thesis project on the “Effect of temperature and surface treatment on SCC initiation in Alloy 182 weld metal under BWR conditions”. Temperature effects on the SCC initiation behavior in Alloy 182 weld metal were studied and because SCC mitigation methods often involve surface modifications, different surface treatments (electropolishing and grinding) were also examined for their SCC initiation properties. Preliminary results on the influence of temperature and surface condition on the SCC initiation behavior of Alloy 182 weld metal in hydrogenated high-purity, high-temperature water using accelerated constant extension rate tensile tests with flat tapered specimens showed a clear trend for higher SCC initiation susceptibility in Alloy 182 towards higher temperatures, whereas the influence of the surface treatment was not fully conclusive.

Hans-Peter Seifert (PSI, Switzerland) reported “About the relevance of homogeneous LAS specimens for assessment of RPV cracks in the context of a recent crack crevice chemistry model and chloride effects”. Recent, very sophisticated crevice chemistry modeling by VTT suggests a very strong enrichment of chloride in homogeneous LAS C(T) specimens due to the high Fe cation concentration that is much higher than in a cladding penetrating crack which reaches the fusion boundary. It was thus argued that the homogeneous LAS specimens are not representative and unduely conservative for these situations. Direct evidence from Alloy 182-RPV steel specimens is given that clearly discounts this argument, although differences in crack crevice chemistry probably exist. Microsampling tests showed orders of magnitude lower enrichment than predicted by the model. The model in its present form does not consider the dominant effect of MnS inclusions, crack growth and the interrelation between crack crevice chemistry and crack growth (and vice versa) in LAS. Furthermore, the model predicts only a small chloride enrichment if chloride and sulfate are simultaneously present in the bulk environment. This prediction was used to check the model by tests with sulfate, chloride and sulfate/chloride combinations. The observed behavior is in contradiction to the model predictions in its present form.