Electrochemical Potential Oscillations during Galvanostatic Passivation of Copper in NaNO₂ Solutions and Their Role in TGSCC Mechanism

Auto-oscillations of the electrochemical potential around its average value close to +100 mVSCE under galvanostatic polarization of pure copper in 0.3M NaNO2 solution were studied in annealed state of copper and during slow strain rate tensile tests of cold-deformed copper. The oscillations occur in the potential range where the passivation current increases dramaticaly due to Cu++ cation generation in the semi-coherent oxide film on the copper substrate. Parameters of the oscillations were measured depending on the polarization conditions. It was supposed that the observed oscillations represent a nonlinear relaxation process in the passive oxide film interacting with the metal substrate at the semi-coherent oxide/metal interface. Based on the idea that cation vacancy balance at the Cu2O/Cu interface depends on the stresses induced in the metal substrate by the interface conditions, a model of the observed nonlinear phenomena was formulated

Effects of straining on oxide films and passivity of copper in nitrite solution at ambient temperature

The effects of strain rate on interactions between copper and its oxide films have been studied. The electrochemical oxidation process of copper is accompanied by the generation of vacancies in the copper substrate. Diffusion of vacancies from the oxide/metal interface or annihilation of vacancies by dislocation reactions is essential for oxidation to continue. Sufficiently slow straining without breaking the passive film on copper leads to a re-arrangement of the dislocation sub-structure at the interface, which helps to consume the oxidation-generated vacancies. The balance of slow straining and oxidation/dissolution produces environmentally-enhanced plasticity in the copper substrate, and simultaneously, accelerated corrosion. These conditions occur as long as the strain rate is low, i.e., on order of less than 10−8 s−1. The effects of slow straining on oxidation/dissolution of high purity copper were studied at various strain rates, even as low as 10−10 s−1. The electrochemical oxidation behaviour of cold-deformed copper was recorded by dynamic polarization scans. The synergistic effects of oxidation/dissolution process and straining on the mechanical properties of metal, i.e., dislocation sub-structure of copper were studied by TEM. The results are discussed in conjunction with a TGSCC model (SDVC, Selective Dissolution – Vacancy Creep). Central to the TGSCC model is the concept of vacancies generated in the process of oxidation/dissolution taking part in substrate recovery and playing a key role in the stress corrosion crack growth behaviour

Strain localization in copper canister FSW welds for spent nuclear fuel disposal

Spent nuclear fuel disposal in copper canisters in a deep geologic repository is planned in Finland and Sweden. The purpose of the copper shell is to perform as a ductile corrosion barrier to prevent radioactive substances from leaking into the environment. Therefore, the most important property of the copper shell, besides the good corrosion resistance, is its ductility. The copper canisters are sealed by friction stir welding (FSW), which results in strong welds when compared to the base materials, but the microstructural heterogeneity introduced by the welding may also lead to strain localization. Thus, the strain localization behavior of two different copper canister welds was studied by tensile tests in combination with digital image correlation (DIC). The main difference between the welds is the utilization of shielding gas to reduce oxidation during welding. The shielding gas improves the stability of the welding process, as well as reduces the number of oxide particles in the welds. It is known, that oxide particles are detrimental in copper in the presence of hydrogen. Therefore, the two welds were also thermally hydrogen charged to study hydrogen trapping in the weld material. Thermal desorption measurements (TDS) show that considerable hydrogen uptake occurs in the weld oxide zone, but it did not compromise the ductility of the copper welds in these tests. However, the DIC tests indicate considerably earlier strain localization on the retreating side of the new weld, welded with the shielding gas. This is attributed to differences in the initial state of the lid materials.Peer reviewe

A New Method for Studying Thermal Desorption of Hydrogen from Metals Based on Internal Friction Technique.

Abstract not availableJRC.(IAM)-Institute For Advanced Material

Study on hydrogen embrittlement and dynamic strain ageing on low-alloy reactor pressure vessel steels

Funding Information: The funding for the “SAFE-II” and “LEAD” projects from the Swiss Federal Nuclear Safety Inspectorate (ENSI) is gratefully acknowledged. The authors would like to express their gratitude for the experimental contributions and helpful suggestions from S. Ritter, H. Kottmann, B. Baumgartner, R. Schwenold and D. Stammbach from Paul Scherrer Institut. Publisher Copyright: © 2021 The Author(s)Tensile tests in air with hydrogen pre-charged smooth specimens and slow strain rate tests with smooth and notched specimens in hydrogenated high-temperature water (HTW) at elevated temperatures (250−288 °C) on low-alloy reactor pressure vessel (RPV) steels revealed a softening in strength and a pronounced reduction in ductility, where the magnitude of hydrogen embrittlement (HE) increased with the dynamic strain ageing (DSA) susceptibility of the RPV steels. In hydrogen pre-charged specimens and in hydrogenated HTW, shear dominated transgranular fracture by microvoid coalescence with increasing amounts of macrovoids, quasi-cleavage regions and secondary cracking were observed. Thermal desorption spectroscopy showed an increase in the concentration of trapped hydrogen in high binding energy traps (vacancies & voids) induced by straining in DSA regime. The observed hydrogen effects on fracture behaviour is a consequence of plasticity localization resulting from the interaction between DSA and hydrogen. HESIV and HELP are the dominant HE mechanisms.Peer reviewe