SUMMARY OF THE SUMMARY/POSTER PRESENTATION SESSIONS – Chaired by Samaneh Nouraei and Gary Was

Part I of the summary/poster presentation session consisted of four poster introductions on Monday before the coffee break.

Tomas Martin (University of Bristol, UK), “The effect of cold-work on oxidation of 304 stainless steel”. 304 austenitic stainless steel has excellent resistance to a wide range of atmospheric environments and many corrosive media. It is subject to pitting and crevice corrosion in warm chloride environments and to stress corrosion cracking above about 60 °C. However in pure high-temperature water around 300 °C it exhibits good stability and resistance due to the formation of a protective oxide layer. Austenitic steels have been widely used in light water reactor components, and it is known that their resistance to stress corrosion cracking can be increased using zinc injection into the primary water. Zinc injection is also beneficial for decreasing the dose rate within the cooling circuit. Above a certain level of cold-work, the high residual stresses in the subsurface area considerably effect materials’ susceptibility to stress corrosion cracking. It has been previously shown that the significant effect of cold-work on the corrosion resistance in high-temperature water can be substantially mitigated using Zn injection. In this work, cold-worked 304 stainless steel specimens with different amounts of deformation were exposed in a high-temperature water environment with presence of zinc (10-100 ppb) injected in the beginning of the simulated operational cycle. The microstructural sub-surface characterization after exposure was performed using advanced microscopy techniques and results were compared to the specimens exposed to non-Zn high-temperature water.

Victor Okoro (Queen’s University, Canada), “The effect of sulfur on the passivation and pitting of Ni- and Fe-based alloys in 280 °C mildly acidic, sulfate-containing solutions”. This study aims to investigate the effect of sulfur on the passivation and pitting of Ni- and Fe-based alloys when exposed to a sulfate solution at 280 °C with a pH280C of 4.62 and a concentration of 0.55 mol/kg. The behavior of Alloys 600, 690, and 800 were analyzed. Results showed that the presence of adsorbed sulfur at the oxide-metal interface affected the passivity of the alloys. After exposure to the sulfate solution, Alloy 600 exhibited a high susceptibility to pitting with a maximum pit depth of 8.3 µm, and sulfur was present in the oxide layer and pits. Alloy 800 showed increased spallation due to a thick Fe oxide spalling off, with a mixed oxide layer and pits observed in areas of spallation and at the interface of TiN inclusions and the matrix. In contrast, Alloy 690 formed a continuous thin film containing chromium and sulfur, providing a substantially reduced pitting susceptibility compared to the other alloys. Preliminary results are also presented comparing the environmentally-assisted cracking susceptibility for Alloys 690 and 800, and potential mechanisms are discussed.

Aidan Gunn (University of Bristol, UK), “Effect of machining process upon Alloy 690 steam generator tube microstructure and subsequent oxide passivation during simulated primary water reactor condition”. This study aims to investigate the effects of machining processes upon Alloy 690 steam generator tube material and the subsequent oxidation behavior under simulated primary water reactor (PWR) condition. This machining process is used to reduce the overall cross section and wall thickness, inducing deformation into the inner surface through grain refinement. Samples of Alloy 690 tube with machined and non-machined condition were inspected by correlative microscopy technique including EBSD and SEM to assess the crystallographic structures and stress distributions, followed by the comparison to correlative hardness measurements. Preliminary results showed that the inner diameter of the machined tube had a much greater degree of misorientation between grains, whilst presenting less depth of deformation than the non-machined material. The oxidation behavior of these samples under the simulated PWR conditions was also investigated, with the surface oxide film characterized through further advanced microscopic inspection.

Adrianna Mackiewicz (PSI, Switzerland), “Effect of Zn on the oxide film properties of Alloy 182 exposed to BWR environment”. For many years now, boiling water reactors (BWRs) have been using zinc (Zn) injection into the coolant to reduce the radioactive corrosion build-up caused by neutron-activated Co60 isotopes. Some studies have indicated that Zn might also positively influence stress corrosion cracking (SCC) mitigation of nickel-based alloys and stainless steels. However, the potential effects of Zn injection regarding their SCC behavior require further quantification and resolving of the oxidation kinematics. Therefore, the impact of Zn injection on the SCC behavior of a nickel-based weld metal (Alloy 182) in BWR environment is investigated in the framework of a research project at PSI. Tensile tests are performed in simulated BWR environment studying the SCC initiation behavior (which are presented in the talk at this meeting). Additionally, specimens are exposed to Zn-free and Zn-containing BWR water in autoclaves with recirculating loops and their oxide films are studied. Different microstructural characterization techniques (e.g., electron microscopy, electron dispersive X-ray spectroscopy, Raman spectroscopy, etc.) are used to identify zinc incorporation into the oxide film. The first results are presented and discussed at the current poster. They show that Zn is present in the oxide film and that the oxide film properties vary after the Zn treatment.

Five further short summary talks were given in part II of the summary/poster presentation session on Tuesday before the coffee break.

Stacy Moore (University of Bristol, UK), “Real-time and in-situ characterization of localized corrosion events”. Dr. Moore summarized work on localized corrosion of type 316 stainless steel in 3.5% aqueous sodium chloride solution. She noted that pitting occurred at semi-random locations and times, making direct observation difficult. To make direct observation more likely, a microelectrode set-up was used to reduce the frequency of events and the area of interest. Preliminary results allowed for the direct observation of pitting corrosion.

Katsuki Onuma (Tohoku University, Japan), “Evaluation of SCC susceptibility of carbon steel assuming the environment in PCV on decommissioning process of Fukushima Daiichi nuclear power station”. Dr. Onuma introduced his work on the use of sodium pentaborate injected into the PCVs to secure the subcritical condition of the fuel debris at Fukushima Diachi Nuclear Power Station. Carbon steel is self-passivating in sodium pentaborate, but the objective is to determine the SCC susceptibility to maintain PCV soundness. Results showed that microcracks were formed around the active-passive transition at potentials between that for degassing and for aeration. Crack propagation will be determined next.

Po-Fu Lin (National Tsing-Hua University, Taiwan), “The effect of Pt doped TiO2 coating on the corrosion mitigation of carbon steels under ultraviolet light irradiation”. Dr. Lin investigated the atmospheric corrosion of pre-oxidized carbon steels and conductive ITO glass with TiO2 and subjected to ultraviolet light to trigger the photocatalytic effect. TiO2 was also doped with two different amounts of Pt to promote electron transfer of the TiO2. Results were shown for carbon steel annealed at 400 °C and also annealed at 600 °C and for the three TiO2 conditions.

Liberato Volpe (UKAEA, UK), “Comparison between LiOH and KOH as pH moderator for coolant water environments for fusion applications”. Dr. Volpe provided a summary of work on Eurofer97 to determine its oxidation and EAC behavior in KOH and LiOH. Oxidation behavior was determined by exposure to 300 °C water for 500 h in a static autoclave and revealed formation of a double layer oxide enriched in Fe in the outer layer, and Cr in the inner layer. Following irradiation for 1 FPY, KOH generated three long-lived, gamma-emitting nuclei and none for LiOH. The next step is to determine the EAC behavior in both, cyclic and non-cyclic tests and develop models to calculate ECP for the breeder blanket section.

Zdeněk Fulin (CVR, Czech Republic), “ECC-SMART Joint European Canadian Chinese development of SMALL modular reactor technology”. Dr. Fulin presented the ECC-SMART program designed to develop a supercritical water SMR, including a pre-licensing study and guidelines for the demonstration of the safety of the concept. The project is coordinated by CVR and includes CIEMAT, KIT, BME, JSI and ENEN. Materials under study include 310S, 800H and AFA that will be tested in different geometries in tensile, NI and SSRT tests in air vs. SCW and at RT and 380 °C.

There is also one presentation file included in the meeting minutes without summary presentation or talk:

Hans-Peter Seifert (PSI, Switzerland), “SCC of low-alloy steels in neutral or slightly alkaline high-temperature water (steam-water cycles)”. Outline: Introduction; SCC in pure, neutral or slightly alkaline HTW, e.g., in BWR and PWR environment; Anodic SCC in oxygenated HTW with anionic impurities/increased acidic conductivity; Hydrogen-assisted SCC in oxygenated and oxygen-free, neutral or slightly alkaline HTW?; Conclusions; References.