293 research outputs found
An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium
In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide
The influence of modified water chemistries on metal oxide films, activity build-up and stress corrosion cracking of structural materials in nuclear power plants
Localised electrochemical processes on laser powder bed fused 316 stainless steel with various heat treatments in high-temperature water
Design-Based Confidence Sequences for Anytime-valid Causal Inference
Many organizations run thousands of randomized experiments, or A/B tests, to
statistically quantify and detect the impact of product changes. Analysts take
these results to augment decision-making around deployment and investment
opportunities, making the time it takes to detect an effect a key priority.
Often, these experiments are conducted on customers arriving sequentially;
however, the analysis is only performed at the end of the study. This is
undesirable because strong effects can be detected before the end of the study,
which is especially relevant for risk mitigation when the treatment effect is
negative. Alternatively, analysts could perform hypotheses tests more
frequently and stop the experiment when the estimated causal effect is
statistically significant; this practice is often called "peeking."
Unfortunately, peeking invalidates the statistical guarantees and quickly leads
to a substantial uncontrolled type-1 error. Our paper provides valid confidence
sequences from the design-based perspective, where we condition on the full set
of potential outcomes and perform inference on the obtained sample. Our
design-based confidence sequence accommodates a wide variety of sequential
experiments in an assumption-light manner. In particular, we build confidence
sequences for 1) the average treatment effect for different individuals
arriving sequentially, 2) the reward mean difference in multi-arm bandit
settings with adaptive treatment assignments, 3) the contemporaneous treatment
effect for single time series experiment with potential carryover effects in
the potential outcome, and 4) the average contemporaneous treatment effect in
panel experiments. We further provide a variance reduction technique that
incorporates modeling assumptions and covariates to reduce the confidence
sequence width proportional to how well the analyst can predict the next
outcome
The properties of and transport phenomena in oxide films on iron, nickel, chromium and their alloys in aqueous environments
Effect of hydrogen on electrochemical behavior of additively manufactured 316L in pressurized water reactor primary water
The electrochemical behavior of laser powder bed fusion (LPBF) 316 L stainless steel subject to different heat-treatments (solution annealing and hot isostatic pressing) is compared to nuclear-grade wrought 316 L in pressurized water reactor primary water at 288 °C (with and without dissolved hydrogen) using current-time transients, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Analysis of spectra by the Mixed-Conduction Model revealed slower corrosion rates of LPBF 316 L than wrought 316 L, the effect being more pronounced in the presence of dissolved hydrogen. The characteristics of the barrier layer and the oxide film/coolant interface were irreversibly altered upon removal of dissolved hydrogen
Mechanistic understanding of the localized corrosion behavior of laser powder bed fused 316L stainless steel in pressurized water reactor primary water
The laser powder bed fused (LPBFed) stainless steels showed anomalous and localized corrosion behavior in the nuclear reactor high-temperature water compared to their wrought counterparts, which affects their performance during plant operation. In this study, advanced microstructural characterization was performed on LPBFed 316 L sample along with wrought 316 L sample after corrosion tests to understand the underlying mechanisms. The results showed that an inhomogeneous/discontinuous inner oxide layer formed on LPBFed 316 L, in contrast to the continuous inner oxide layer on the wrought 316 L specimen. This discontinuous inner oxide layer was identified to consist of Cr-enriched nano-sized spinel oxide and the barrier layer features a Ni-enriched hexagonal close-packed Laves phase. Localized/preferential oxidation was found to occur along the cellular walls which were tangled with high density dislocations and decorated with Mn and Si-enriched nano-sized precipitates, and the nano-precipitates were observed in the core of dispersed Cr-enriched inner oxide crystals
Electric and electrochemical properties of surface films formed on copper in the presence of bicarbonate anions
Effect of hydrogen on electrochemical behavior of additively manufactured 316L in pressurized water reactor primary water
The electrochemical behavior of laser powder bed fusion (LPBF) 316 L stainless steel subject to different heat-treatments (solution annealing and hot isostatic pressing) is compared to nuclear-grade wrought 316 L in pressurized water reactor primary water at 288 °C (with and without dissolved hydrogen) using current-time transients, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Analysis of spectra by the Mixed-Conduction Model revealed slower corrosion rates of LPBF 316 L than wrought 316 L, the effect being more pronounced in the presence of dissolved hydrogen. The characteristics of the barrier layer and the oxide film/coolant interface were irreversibly altered upon removal of dissolved hydrogen
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