Modélisation de la tribocorrosion d'aciers inoxydables dans l'eau à haute pression et haute température

Several metallic components of pressurized water reactors (PWR) used in nuclear power plants are exposed to the combined degradation by friction and corrosion (tribocorrosion). In PWR, corrosion arises from the water pressurized at 150 bars and maintained at 300°C and results in the build up of a micrometer thick surface oxide film. Tribocorrosion is of complex nature as it involves multi-scale interactions between mechanical, chemical and material factors. Nevertheless, PWR component manufacturers are interested in understanding the deterioration mechanisms in order to select appropriate materials as well as predicting the component lifetime. This thesis is a contribution towards the elucidation of the tribocorrosion mechanisms of a specific PWR component (austenitic stainless steel control rods). The research is focused on two inter-related aspects related to the subsurface deformation induced by friction and to the high temperature and high-pressure oxidation kinetics of austenitic stainless steel under tribocorrosion conditions. The subsurface deformation is investigated by using a model tribocorrosion system consisting in an alumina ball sliding against a 304L austenitic stainless steel in sulphuric acid at ambient temperature under electrochemical control. This model system allows studying subsurface deformation in absence of a thick high temperature oxide films that can complicate the interpretation of the results. Subsurface deformation was characterized using SEM, EBDS and FIB cross-sections. Good correlation was found with the deformation found in pressurized water. Subsurface deformation was found to result in the formation of a recrystallised layer of nano-grains close to the surface and an underlying layer exhibiting a deformation gradient. The thickness of the recrystallized zone is highly affected by the presence or the absence of a few nm thick oxide in the contact. However, no obvious correlation between the thickness of the recrystallized zone, or any other measured deformation appearance, and wear could be found. The effect of oxidation was studied using a simulator (Aurore), reproducing the chemical and mechanical PWR conditions of control rods. It includes an electrochemical set-up allowing measuring in-situ and in real time the oxidation kinetic. The oxidation kinetic was found to follow a parabolic law (diffusion controlled oxide growth). Rubbing resulted in repeated scratching of the surface leading to a thinning of the oxide film and thus to an enhanced anodic oxidation. A quantitative model of the electrochemical response of stainless steel in PWR conditions was developed. Based on these results and on the third body theory of wear, a mechanistic model was proposed for describing the material behaviour in contacts operating in pressurized water. This model explains a certain number of practical observations obtained in nuclear plants or in simulators concerning chemical and materials effects on component degradation.

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

A rapidly growing population and industrialization have caused the exploitation of natural resources in keeping with fossil fuels shortage. Due to the combustion of fossil fuels, the concentration of CO2 in the earth's atmosphere, which is a potent greenhouse gas, has been increasing dramatically and contributes to the current climate change. Meanwhile, CO2 can be considered as an abundant and inexpensive carbon feedstock, especially if carbon sequestration is required for future fossil fuel-based power stations. Electrochemical reduction of CO2 to form useful chemicals or fuels is a potentially efficient method of CO2 utilization and recycling. The advantage of the electrochemical CO2 reduction is that natural renewable sources, like solar power, wind power, hydropower or nuclear power can supply electric power for CO2 conversion. Various products, such as carbon monoxide, oxalic acid, formic acid, alcohols and hydrocarbons have been reduced from CO2 First in chapter 1, the general remarks about electrochemical reduction of CO2, especially on metal electrodes are introduced. It also includes the introduction of photoelectochemical CO2 reduction and literature review study about electrocatalytic CO2 reduction with organic molecules as mediators and catalysts. In chapter 2, our interest in Mo-based electrocatalysts led us to study the activity of MoO2 for CO2 reduction. MoO2 microparticles indeed act as an active catalyst for the electrochemical reduction of CO2 in organic solvents such as acetonitrile (MeCN) and dimethylformamide (DMF). In the context of this study, it was found that the CO2 reduction activity is much higher at -20 oC than at RT, and it can be promoted by a small amount of water. The selectivity of CO2 reduction depends on the temperature, overpotential and water concentration. Chapter 3 describes the electrochemical reduction of CO2, improved and modified by using imidazolium ionic liquids (ILs) as an electrolyte in organic solvent with MoO2/Pb electrode. The important role of ILs in changing the pathway of the reduction was observed. Particularly, the use of imidazolium ILs instead of tetraalkylammonium salt as an electrolyte altered the product from oxalate to CO. Besides, the overpotential of the CO2 reduction was shifted to the less negative potentials compared to previous work, and the current density as well increased. At a low water concentration, the catalytic activity was promoted and the total Faradaic efficiency (FE) was up to 100% with more than 60% of CO formation. Furthermore, imidazolium ILs together with our MoO2/Pb electrode lowered the overpotential of CO2 reduction by about 40 mV. In addition to the high selectivity for CO formation, the imidazolium ILs can perform a role as co-catalyst for lowering the CO2 reduction potential. Finally, chapter 4 shows the possibility of photoelectrochemical CO2 reduction to extend our research with various catalysts. In this work, we could derive a photoelectrochemical method to deposit Sn/SnOx catalyst on the surface protected cuprous oxide (Cu2O) photocathodes. The deposition of Sn/SnOx catalyst was optimized on the photocathode and it exhibited reliable CO2 reduction activity in a mild aqueous condition. Sn/SnOx-Cu2O photocathode showed the onset potential of the photocurrent at +0.1 V vs. RHE, which is very promising result. At a constant potential electrolysis of -0.1 V vs. RHE, a FE of 50% for formate formation was obtained and the total FE was around 86%.

EPFL2015

Helium effects on irradiation assisted stress corrosion cracking susceptibility of 316L austenitic stainless steel

Ignasi Villacampa Roses

The formation and growth of cracks by irradiation assisted stress corrosion cracking (IASCC) in light water reactor internals is a critical issue for a safe long-term operation of nuclear power plants. The IASCC susceptibility at relatively low dose is dominated by conventional mechanisms such as radiation-induced segregation and radiation hardening. However, the ageing of the nuclear fleet combined with the increase of their life-span reveals other mechanisms that could play an important role on IASCC susceptibility. Recent studies show that a huge amount of helium (He) can be accumulated in reactor internal components of pressurized water reactors (PWR) after long-term operation. This occurrence could significantly increase the IASCC susceptibility at high doses. The main objective was to investigate the He effects on IASCC susceptibility of the solution-annealed (SA) and 20% cold-worked (CW) austenitic steel 316L. SA and CW miniaturized tensile samples and a SA plate were homogeneously implanted up to 1000 appm He. He was implanted at ~45 MeV and 300°C at the cyclotron in CEMHTI/CNRS (France). Slow strain rate tests (SSRT) in air and in high-temperature water were then carried out on SA, CW, post-implantation annealed (PIA) and as-implanted samples; instrumented nanoindentation tests were performed at room temperature on non-implanted and implanted specimens; and scanning electron microscope and transmission electron microscope were employed for fractography and microstructural characterization. The results of SSRTs in high-temperature air and in hydrogenated high-temperature water showed that homogenized implanted He up to 1000 appm corresponding to a displacement damage of about 0.16 dpa (displacement per atom) does not produce intergranular cracking and IASCC. However, the deformation microstructure of non-implanted SA/CW is characterized by high dislocation density arranged in cell walls separated by relatively dislocation-free regions, while the as-implanted SA samples (> 0.05 dpa) exhibit a more planar deformation microstructure. Characterization of as-implanted CW and PIA samples did not show any significant implantation effect on the deformation microstructure. PIA of the He implanted plate was carried out from 650 to 1000°C for 1h to increase the helium grain boundary coverage, and to reproduce the observed microstructure of the replaced reactor internal components that showed IASCC and use the results for the PIA of tensile samples. The transmission electron microscope investigations showed that an increase of the annealing temperature causes an average bubble size increase and density decrease. The average He bubble size and distribution in the grain interior and on the grain boundary were similar in the range of temperatures studied. In both cases, bubbles grew by the Ostwald ripening mechanism. No preferential He build-up took place on the grain boundaries. The increase of yield stress produced by He bubbles was calculated with three distinct dislocation-localized obstacle and compared to the tensile test results. The bubble strength estimated for these models was used to assess their validity and to compare the results to published data. Finally, nano-indentation tests in SA, CW, as-implanted SA and PIA (650 to 850°C for 1h) samples did not show He effects on grain boundaries strength. However, the average hardness of the grain interior increased with the He implantation and decreased increasing the PIA temperature.

EPFL2018

Chemical-mechanical polishing of tungsten

Jelena Stojadinovic

The chemical-mechanical polishing process (CMP) is an essential part of the production of integrated circuits. Metal to be polished in the CMP reacts with the oxidants from the aqueous suspension (slurry) and the passive film is formed on the metal surface. Suspended abrasive nano particles from the slurry cause detachment of the formed friable passive films. The CMP represents the tribocorrosion process where material deterioration results from the interaction of wear and corrosion which takes place in tribological contact exposed to an aggressive environment. Due to increasing number of the materials to be polished in the modern integrated circuits it is necessary to gather fundamental understanding of the removal mechanism of each material. The aim of this work is to elucidate the removal mechanisms of tungsten (W), as a typical microelectronic metal (chosen as a model system), under different oxidising conditions (represented by electrochemically imposed potential). Also, we want to evaluate the effect of the slurry composition (chelating agents lactic and phosphoric acid) on material removal. To achieve this electrochemical techniques (chrono amperometry, cyclic voltametry, passivation kinetics measurements under mass transport control), and tribocorrosion techniques (tests on tribometer with electrochemical cell with reciprocating sliding motion of alumina ball) were used. Surface analysis techniques (SEM, XPS, AES) were used to assess worn and unworn surfaces' morphology and chemistry. In addition, electrochemical tests and polishing tests on the CMP machine were performed with technical CMP slurries. The sliding action of the alumina ball on tungsten in the 0.01 M H2SO4 solution was found to cause three distinct wear responses depending on imposed electrode potential. Below the first threshold potential the wear is low due to negligible oxide film formation. In the following wear region, up to the second threshold potential, materials deterioration in the rubbed area proceeds by cyclic mechanical removal of the WO3 passive film followed by repassivation of tungsten. The amount of anodic oxidation corresponds to the overall removed metal volume. Beyond the second threshold potential a compact tribolayer of WO3 (more than 400 nm thick), probably formed by agglomeration of oxide particles detached from the metal surface, forms and covers the wear track. Similar tribocorrosion behavior of tungsten was also observed in the presence of 0.1 M H3PO4. The presence of chelating agents, lactic acid and phosphoric acid, in the 0.01 M H2SO4 solution does not change the volume of material wastage at potentials below the second threshold potential. Beyond this electrode potential, lactic acid promotes WO3 dissolution and thus impedes the third body formation and enhances wear. Electrochemical factors have a strong influence on the tungsten CMP removal rate. It was found that CMP removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A good correlation of tribocorrosion results and CMP practice was observed. Tribocorrosion models can be successfully applied to the tribocorrosion of tungsten and as well can be fairly used for modelling the material removal in CMP.

EPFL2009