Evaluation of an Oxide Layer on NI-CR-MO-W Alloy Using Electrochemical Impedance Spectroscopy and Surface Analysis

High corrosion resistance under very aggressive conditions is a distinguishing property of Ni-Cr-Mo-W alloys. One such alloy, Alloy 22, is a candidate material for fabrication of the outer layer of high-level nuclear waste (HLNW) packages for the proposed HLNW repository at Yucca Mountain, Nevada, USA. We are using Electrochemical Impedance Spectroscopy (EIS), ex-situ X-Ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectroscopy (ToF SIMS) to characterize the electrochemical properties and composition of the protective oxide formed on Alloy 22 surfaces. These studies have been conducted at temperatures up to 90 C at potentials from -0.8 V to 0.8 V (vs. Ag/AgCl (sat'd KCl)) in deaerated 5 mol L{sup -1} NaCl solution. Using this combination of techniques, we can correlate the electrical (from EIS) and compositional properties (from XPS, ToF SIMS) of the oxide. At more negative potentials (-0.8 V to -0.4 V) the film exhibits a low charge transfer resistance and high capacitance, indicating the presence of a very defective film with a high concentration of electronic defects. The presence of additional elements in the equivalent circuit, corresponding to water reduction, supports this suggestion. At these potentials, surface analysis techniques show a thin oxide layer with a low concentration of Cr203. Increasing the potential (to between -0.2 and 0.2 V) leads to a major increase in overall interfacial resistance consistent with the formation of an oxide with a small concentration of electronic defects. At the same time, the surface analysis techniques show increases in the film thickness and the Cr{sub 2}O{sub 3} content. A further increase in potential to 0.8 V, in general, leads to a decrease in interfacial resistance throughout the film. When the Cr{sub 2}O{sub 3} barrier layer is degraded, then the higher oxidation states of Mo and W species (MO{sup VI}, W{sup VI}) increase in concentration and are stored in the outer part of the film (at temperatures up to 60 C). The storage of these high oxidation state ions generates a high interfacial capacitance. At high temperature (above 60 C), the XPS and EIS show that the high oxidation states of Mo and W are absent. We think this is because they dissolve from the oxide under those conditions

Crevice Corrosion on Ni-Cr-Mo Alloys

Ni-Cr-Mo alloys were developed for their exceptional corrosion resistance in a variety of extreme corrosive environments. An alloy from this series, Alloy-22, has been selected as the reference material for the fabrication of nuclear waste containers in the proposed Yucca Mountain repository located in Nevada (US). A possible localized corrosion process under the anticipated conditions at this location is crevice corrosion. therefore, it is necessary to assess how this process may, or may not, propagate if the use of this alloy is to be justified. Consequently, the primary objective is the development of a crevice corrosion damage function that can be used to assess the evolution of material penetration rates. They have been using various electrochemical methods such as potentiostatic, galvanostatic and galvanic coupling techniques. Corrosion damage patterns have been investigated using surface analysis techniques such as scanning electron microscopy (SEM) and optical microscopy. All crevice corrosion experiments were performed at 120 C in 5M NaCl solution. Initiating crevice corrosion on these alloys has proven to be difficult; therefore, they have forced it to occur under either potentiostatic or galvanostatic conditions

Hydrogen absorption into titanium under cathodic polarization: An in-situ neutron reflectometry and EIS study

Hydrogen (deuterium) absorption into sputter-coated titanium (Ti) film electrodes during cathodic polarization in heavy water (D2O) was monitored using in-situ neutron reflectometry (NR) and electrochemical impedance spectroscopy (EIS). The scattering length density (SLD) of Ti metal increased with increasing cathodic polarization, due to the penetration of deuterium through the surface oxide and into the underlying metal. The rate of D absorption estimated from the NR data showed a pattern with four distinctive regions separated by potential boundaries between -0.35 and -0.4 VSCE and around 3c-0.6 VSCE. EIS results support division of the behavior into these potential ranges. Hydrogen absorption by Tiwas observed at potentials < 3c-0.35 VSCE, where the capacitance and resistance of the TiO2 layer dramatically changed. At this point, the D content of the film quickly achieved a level of 3c900 ppm by weight (atom ratio D:Ti 3c 0.04). Decreased absorption kinetics were observed over the potential region from 3c-0.40 VSCE to -0.6 VSCE, indicating that D absorption was controlled either by a diffusion process through the TiO 2 layer or by the formation of blocking hydrides at the Ti/TiO 2 interface, at the base of the defective locations in the oxide through which the hydrogen was entering. Significant increases in the current density and SLD of the Ti film at potentials more negative than -0.6 V SCE were assigned to widespread hydrogen absorption and TiH x growth within the metal. These observations are consistent with hydrogen ingress through the oxide film, probably via weak points containing electronic defects and disorder, such as grain boundaries and triple points, at potentials as mild as 3c-0.4 VSCE, and with hydrogen penetration through continuous, intact oxide via the previously published redox transformation mechanism, at potentials more negative than -0.6 VSCE. \ua9 2013 The Electrochemical Society. All rights reserved.Peer reviewed: YesNRC publication: Ye

Effect of duty cycle and applied current frequency on plasma electrolytic oxidation (PEO) coating growth behavior

Ceramic coatings were created on the surface of 6061 aluminum alloy using a plasma electrolytic oxidation (PEO) process employing a pulsed direct current (DC) power mode in an alkaline electrolyte. The effect of electrical parameters including frequency and duty cycle on the microdischarge behavior and coating growth was investigated at constant current. Surface features of coatings were studied using scanning electron microscopy. Energy dispersive spectroscopy was employed to investigate elemental distribution on the coating surfaces and cross-sections. Applying lower duty cycles was found to result in increased breakdown voltages and microdischarges with higher spatial density and lower intensity. Further, applying a lower duty cycle was also found to promote the uniformity of silicon distribution in the coating. Based on these new findings, a new conceptual model is proposed to explain the concentration distribution of Si on the surface of coatings prepared at different duty cycles. \ua9 2013 .Peer reviewed: YesNRC publication: Ye

Spent fuel performance under repository conditions: A model for use in SR-Can

Technical Report TR-04-19, Svensk Kärnbränslehantering AB, SKB, Stockhol

Trophic relationship between the invasive parasitic copepod <i>Mytilicola orientalis</i> and its native blue mussel (<i>Mytilus edulis</i>) host

Invasive parasites can spill over to new hosts in invaded ecosystems with often unpredictable trophic relationships in the newly arising parasite-host interactions. In European seas, the intestinal copepod Mytilicola orientalis was co-introduced with Pacific oysters (Magallana gigas) and spilled over to native blue mussels (Mytilus edulis), with negative impacts on the condition of infected mussels. However, whether the parasite feeds on host tissue and/or stomach contents is yet unknown. To answer this question, we performed a stable isotope analysis in which we included mussel host tissue and the primary food sources of the mussels, microphytobenthos (MPB) and particulate organic matter (POM). The copepods were slightly enriched in δ 15N (mean Δ15N ± s.d.; 1·22 ± 0·58‰) and δ 13C (Δ13C 0·25 ± 0·32‰) with respect to their host. Stable isotope mixing models using a range of trophic fractionation factors indicated that host tissue was the main food resource with consistent additional contributions of MPB and POM. These results suggest that the trophic relationship of the invasive copepod with its mussel host is parasitic as well as commensalistic. Stable isotope studies such as this one may be a useful tool to unravel trophic relationships in new parasite-host associations in the course of invasions