Corrosion crack enhancement during the cathodic activation of zirconium AB<Subscript>2</Subscript> alloys

The activation of zirconium nickel alloys with and without the addition of chromium and titanium is investigated through electrochemical and optical techniques. Recent investigations in aqueous 1 M KOH indicate oxide layer growth and occlusion of hydrogen species in the alloys during the application of different cathodic scan potential programmes currently used for the activation process. In this research, several techniques, such as voltammetry, ellipsometry, energy dispersive analysis of X-rays and scanning electron microscopy, are applied to three polished massive alloys, Zr_1−x Ni _x , x = 0.36 and 0.43, and Zr_0.9Ti_0.1NiCr. Data analysis shows that the stability, compactness and structure of the passive layers are strongly dependent on the applied potential programme. The alloy activation depends on the formation of deep crevices that remain after further polishing. The microscopic observation shows an increase in crevice thickness after cathodic sweep potential cycling, which produces fragmentation of the grains and oxide growth during the activation process. This indicates metal breaking and intergranular alloy dissolution that take place together with oxide and hydride formation. In some cases, the resultant crevice thickness is one or two orders higher than the growth of surface oxide indicating localised intergranular corrosion. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Effective octadecylamine system for nanocrystal synthesis

New chemical reactions and synthetic systems are of key importance for materials fabrication. In this work, we reported a facile and effective octadecylamine (ODA) synthetic system for various nanocrystals including metals, mixed metal oxides, metal/metal oxide heterostructured nanocrystals, intermetallics, and alloys. We found that the products were mainly determined by metal ions used in our synthetic system: noble metal ions led to the formation of metals; two kinds of non-noble metal ions led to the formation of mixed metal oxides; silver ions and non-noble metal ions led to the formation of metal/metal oxide heterostructured nanocrystals; non-noble metal ions and noble metal (excluding Ag) ions led to the formation of intermetallics and alloys. The difference was attributed to different ability to attract electrons from ODA solvent among these metal ions. This effective system provides a general strategy for various nanocrystals which would find potential applications in many significant fields.     

Partially Reduced Graphite Oxide as an Electrode Material for Electrochemical Double‐Layer Capacitors

Partially reduced graphite oxide was prepared from graphite oxide by using synthetic graphite as precursor. The reduction of graphite oxide with a layer distance of 0.57 nm resulted in a reduction of the layer distance depending on the degree of reduction. Simultaneously the amount of oxygen functionalities in the graphite oxide was reduced, which was corroborated by elemental analysis and EDX. The electrochemical activation of the partially reduced graphite oxide was investigated for tetraethylammonium tetrafluoroborate in acetonitrile and in propylene carbonate. The activation potential depends significantly on the degree of reduction, that is, on the graphene‐layer distance and on the solvent used. The activation potential decreased with increasing layer distance for both positive and negative activation. The resulting capacitance after activation was found to be affected by the layer distance, the oxygen functionalities and the used electrolyte. For a layer distance of 0.43 nm and with acetonitrile as the solvent, a differential capacitance of 220 Fg^?1 was achieved for the discharge of the positive electrode near the open‐circuit potential and 195 Fg^?1 in a symmetric full‐cell assembly.     

SIMS depth profiling, line scanning and imaging analyses of the oxide layer on in-reactor corroded cladding specimens with high lateral resolution

The distribution of the reactor water components lithium and boron in the oxide layer of in-reactor corroded Zircaloy fuel rod cladding specimens was investigated by depth profiling, line scanning and imaging analyses using secondary ion mass spectrometry (SIMS). The exact thickness of the oxide layer on the specimens was measured by scanning electron microscopy (SEM). The SIMS analyses showed that lithium and boron were not homogeneously distributed in the oxide layer. The peak concentration of lithium was found close to the reactor water/oxide interface of the specimens whereas boron showed no relevant variations in the bulk of the oxide layers investigated. The concentration of both elements decreased rapidly at a significant distance close to the oxide/metal interface. Conclusions were drawn to improve the understanding of the in-reactor corrosion process of fuel rod claddings consisting of zirconium based alloys.     

Partially reduced graphite oxide as an electrode material for electrochemical double-layer capacitors

Partially reduced graphite oxide was prepared from graphite oxide by using synthetic graphite as precursor. The reduction of graphite oxide with a layer distance of 0.57?nm resulted in a reduction of the layer distance depending on the degree of reduction. Simultaneously the amount of oxygen functionalities in the graphite oxide was reduced, which was corroborated by elemental analysis and EDX. The electrochemical activation of the partially reduced graphite oxide was investigated for tetraethylammonium tetrafluoroborate in acetonitrile and in propylene carbonate. The activation potential depends significantly on the degree of reduction, that is, on the graphene-layer distance and on the solvent used. The activation potential decreased with increasing layer distance for both positive and negative activation. The resulting capacitance after activation was found to be affected by the layer distance, the oxygen functionalities and the used electrolyte. For a layer distance of 0.43?nm and with acetonitrile as the solvent, a differential capacitance of 220?Fg(-1) was achieved for the discharge of the positive electrode near the open-circuit potential and 195?Fg(-1) in a symmetric full-cell assembly.     

Static electrification of solid oxide in liquid metal and electrical double layer at the interface

Static electrification of a solid oxide, say a semiconducting oxide in liquid metal, is mainly due to electron transfer between two phases. Excess electrons in the liquid metal phase provided by the oxide give rise to an electrical double layer at the interface. The electrical double layer may be divided into three parts, an immobile inner layer, a compressed diffuse layer, and a flat layer extending into the bulk liquid metal. Differential potential analysis and the induced emf method were used to measure the potential of the compressed diffuse layer and the excess electron density of the flat layer, respectively. Results show that most oxides in liquid metals carry positive charges on their surfaces and the potentials of the compressed diffuse layer are in the range of 3 to 42 microV. Such a low potential implies that the diffuse layer is considerably compressed. The excess electron densities of the flat layer are on the order of 10(22) electrons/m(3) of Hg and their contributions to surface charges of oxide are in the range of 10(17) to 10(18) charges/m(2) for the oxide/mercury systems with a solid density of 0.3 wt% at room temperature.     

Effect of atmospheric nitrogen on the oxidation mechanism of aluminum alloys with rare-earth metals

Interaction (25–620°C) of aluminum and its alloys with an atmosphere saturated with nitrogen was studied to determine the role played by rare-earth metals in the mechanism by which nitride phases are formed in oxidation of Al + REM alloys in air. The ellipsometric method and Auger electron spectroscopy were used to determine that, under the given experimental conditions, metallic aluminum is subjected to the greatest extent to the nitridation process, which is competing with the oxidation process. The process is initiated by the conversion of the amorphous oxide film to γ-Al₂O₃. The surface of Al + REM alloys interacts with nitrogen at the outer part of the oxide layer. The rare-earth metal actively reacts with impurity oxygen present in the atmosphere under study and hinders formation of nitride/oxynitride layers.     

Complex surface analytical investigations on hydrogen absorption and desorption processes of a TiMn<Subscript>2</Subscript>-based alloy

Metal hydrides are one of the most promising technologies in the field of hydrogen storage due to their high volumetric storage density. Important reaction steps take place at the very surface of the solid during hydrogen absorption. Since these reaction steps are drastically influenced by the properties and potential contamination of the solid, it is very important to understand the characteristics of the surface, and a variety of analytical methods are required to achieve this. In this work, a TiMn₂-type metal hydride alloy is investigated by means of high-pressure activation measurements, X-ray photoelectron spectroscopy (XPS), secondary neutral mass spectrometry (SNMS) and thermal desorption mass spectrometry (TDMS). In particular, TDMS is an analytical tool that, in contrast to SIMS or SNMS, allows the hydrogen content in a metal to be quantified. Furthermore, it allows the activation energy for desorption to be determined from TDMS profiles; the method used to achieve this is presented here in detail. In the results section, it is shown that the oxide layer formed during manufacture and long-term storage prevents any hydrogen from being absorbed, and so an activation process is required. XPS measurements show the oxide states of the main alloy elements, and a layer 18 nm thick is determined via SNMS. Furthermore, defined oxide layers are produced and characterized in UHV using XPS. The influence of these thin oxide layers on the hydrogen sorption process is examined using TDMS. Finally, the activation energy of desorption is determined for the investigated alloy using the method presented here, and values of 46 kJ/mol for hydrogen sorbed in UHV and 103 kJ/mol for hydrogen originating from the manufacturing process are obtained.     

SIMS investigations on the growth mechanisms of protective chromia and alumina surface scales

The oxidation behaviour of the oxide-dispersion strengthened (ODS) high-temperature alloys MA 956 (an aluminium oxide former) and MA 754 (a chromium oxide former) has been compared with that of two model alloys, Fe-20Cr-5Al and Ni-25Cr. The morphology and composition of the oxide scales were investigated by metallography, X-ray diffraction analysis and scanning electron microscopy. For analysis of the oxide layer growth mechanisms, twostage oxidation experiments with¹⁸O as tracer were used, the distribution of the oxygen isotopes in the oxide scale being determined by SIMS. The ODS alloys show a more selective oxidation than the two model alloys; moreover, the protective oxides on the ODS alloys have a lower growth rate and better adhesion than those on the two model alloys. From the SIMS investigations it can be deduced that the improved properties of the layers on the ODS alloys result from a change in the transport processes in the protective layer; whereas the aluminium and chromium oxide films on the conventional alloys grow by cation and anion transport, the scales on the ODS alloys grow almost exclusively by anion transport. It is shown that the observed properties of the oxide scales on the ODS alloys can be explained by this change in transport mechanism.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Growth of anodic oxide films on titanium-nickel alloys and breakdown of alloy passivity with halide ions

Some characteristics of anodic oxide films growth on titanium-nickel alloys in the borate buffer solution and local breakdown of alloy passivity in the chloride and bromide solutions were determined from the experimental results. In particular, the dependences of anodic oxide film thickness at a given formation potential, anodizing constant, and potential of passivity breakdown (anodic-anionic activation) on the nickel content in the alloys are obtained.     

Oxide‐Nanotrap‐Anchored Platinum Nanoparticles with High Activity and Sintering Resistance by Area‐Selective Atomic Layer Deposition

An area‐selective atomic layer deposition (AS‐ALD) method is described to construct oxide nanotraps to anchor Pt nanoparticles (NPs) on Al₂O₃ supports. The as‐synthesized catalysts have exhibited outstanding room‐temperature CO oxidation activity, with a significantly lowered apparent activation energy (ca. 22.17 kJ mol⁻¹) that is half that of pure Pt catalyst with the same loading. Furthermore, the structure shows excellent sintering resistance with the high catalytic activity retention up to 600 °C calcination. The key feature of the oxide nanotraps lies in its ability to anchor Pt NPs via strong metal–oxide interactions while still leaving active metal facets exposed. Our reported method for forming such oxide structure with nanotraps shows great potential for the simultaneous enhancement of thermal stability and activity of precious metal NPs.     

Role of anionic impurities in the formation of the active state of catalysts based on transition metals

Experimental data concerning the structure of transition metal-based catalysts are analyzed. Anionic impurities in the oxide precursor markedly modify both its structure and the structure of the catalyst resulting from its reduction. The modifying anionic impurities exert a significant effect on the local environment of the transition metal cations (particularly on that of Jahn-Teller cations) and even on the very possibility of existence of a mixed oxide precursor. In some cases, the changes in the local environment of the cations under the action of modifying ions show themselves as radical changes in the catalyst reduction kinetics. The formation of epitaxial bonding between the particles of the active metal and the surface of the oxide support, as well as the decoration of the particles with an amorphous oxyhydroxide layer, can be favorable for the stabilization of the active metal particles in the reduced state. Presumably, the activation of hydrogen molecules and the substrate being hydrogenated (e.g., CO) occurs on the surface of metal particles completely covered by a thin layer of the amorphous oxyhydroxide. The experimentally observed high activity of these catalysts is unlikely to be solely due to the increase in the specific catalytic activity of the remaining uncovered surface of the metal. It should also be attributed to the high inherent catalytic activity of the metal particles decorated with the oxyhydroxide layer.     

AES- und XPS-Untersuchungen zum Einflu? von Chrom, Nickel und Molybd?n auf das Korrosionsverhalten von rostfreien St?hlen in S?uren

Summary The alloys Fe17.8Cr, Fe16Cr2.4Mo and Fe18Cr14Ni2.5Mo (at%) were polarized in 0.5 mol/l H₂SO₄ or in 0.1 mol/l HC1 + 0.4 mol/l NaCl. The composition of the oxide layer and of the metallic layer beneath the oxide and the kinetics of the passive layer formation were determined by AES and XPS. In the active region, selective dissolution of Fe leads to an enrichment of Cr, Ni and Mo at the metal/electrolyte interface. In the passive region, the thickness of the rapidly formed passive layer is determined by the potential. The chromium content of the passive layer approaches a stationary, high value. The passive layer essentially consists of the anions O^2- and OH^– and of the cations of Cr, Fe, Mo, whereas Ni — and less pronounced Mo — are enriched below the layer.     

Influence of Electrolyte on the Electrode/Electrolyte Interface Formation on InSb Electrode in Mg-Ion Batteries

Achieving the full potential of magnesium-ion batteries (MIBs) is still a challenge due to the lack of adequate electrodes or electrolytes. Grignard-based electrolytes show excellent Mg plating/stripping, but their incompatibility with oxide cathodes restricts their use. Conventional electrolytes like bis(trifluoromethanesulfonyl)imide ((Mg(TFSI)₂) solutions are incompatible with Mg metal, which hinders their application in high-energy Mg batteries. In this regard, alloys can be game changers. The insertion/extraction of Mg²⁺ in alloys is possible in conventional electrolytes, suggesting the absence of a passivation layer or the formation of a conductive surface layer. Yet, the role and influence of this layer on the alloys performance have been studied only scarcely. To evaluate the reactivity of alloys, we studied InSb as a model material. Ex situ X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy were used to investigate the surface behavior of InSb in both Grignard and conventional Mg(TFSI)₂/DME electrolytes. For the Grignard electrolyte, we discovered an intrinsic instability of both solvent and salt against InSb. XPS showed the formation of a thick surface layer consisting of hydrocarbon species and degradation products from the solvent (THF) and salt (C₂H₅MgCl−(C₂H₅)₂AlCl). On the contrary, this study highlighted the stability of InSb in Mg(TFSI)₂ electrolyte.     

金属│固体电解质│含水凝胶│石墨体系的研究

应用点缺陷模型和双层模型分析了金属│固体电解质│含水凝胶│石墨体系贮藏和放电时金属表面氧化膜的变化.在固体电解质含有微量水分的情况下,微量水分有助于金属表面形成松层,但由于水分子与紧密钝化膜的持续作用,金属表面松层厚度会不断增长,孔率降低,使得电池的电压和放电电流下降.松层的孔率强烈影响膜溶解行为,与紧密钝化膜的电学特性一样,松层也是一种半导体.     

A model for anodic hydrous oxide growth at iridium

A detailed investigation of the electrochemistry of Ir in 0.5 M H₂SO₄ has been used as an experimental basis for a model for oxide growth at Ir. It appears that a compact oxide (probably IrO₂) is formed initially. At potentials above + 1.2 V vs. RHE, the outer monolayer of this compact oxide is oxidised and becomes hydrated. The hydrated surface layer inhibits further oxidation of the compact oxide and therefore only one monolayer of hydrous oxide can be formed at constant potential. To obtain more hydrous oxide than this, the compact oxide must be continually reduced to Ir metal and reformed, by cycling of the potential. On each cycle, the hydrated surface layer of the compact oxide remains after reduction of the compact oxide. Thus, this material accumulates as a hydrous oxide layer.     

Surface characteristics of titanium–silver alloys in artificial saliva

Titanium and its alloys are widely used in biomedical and dental fields because of their excellent corrosion resistance and biocompatibility. It is well known that titanium is protected from corrosion because of the stability of the passive film that controls and determines the corrosion resistance and biocompatibility of titanium and its alloys. The purpose of this study was to evaluate the electrochemical properties of titanium–silver alloys and the surface characteristics of passive film in artificial saliva. We designed titanium–silver alloys with silver contents ranging from 0 to 5 at.%, in 1% increments. These alloys were arc‐melted, homogenized, hot‐rolled to 2 mm thickness, and finally solution heat‐treated for 1 h and quenched. Potentiostatic testing was performed, and the open circuit potentials of the alloys were measured in artificial saliva, at 37 °C. The passive films of the titanium–silver alloys were analyzed via XPS. Titanium–silver alloys maintained low current density and showed stable passive region and also had high open circuit potential as compared with pure titanium. The open circuit potential of titanium–silver alloys increased as silver addition increased. With regard to the fraction of oxygen species, a component of over 80% was found to be comprised of oxide. Therefore, the titanium surface mainly consisted of titanium oxide and, on the titanium–silver alloys, this film was composed of TiO₂, Ti₂O₃, and TiO. As silver content increased, the TiO₂ fraction also increased, as did the thickness of the titanium oxide layer formed. Copyright © 2005 John Wiley & Sons, Ltd.     

Regularities of Electrochemical Formation and Properties of Oxide Films on Titanium–Iron Alloys

Regularities governing the formation and properties of oxide films on titanium–iron alloys containing 10, 20, 32, 45, 60, or 80 at. % Fe are studied by the voltammetry and chronoamperometry methods coupled with local measurements of activation potentials. Dependences of the oxide film thickness on potential are found. Activation potentials for samples with oxide films formed in air and at 5 V decrease with increasing iron content in alloys.     

Investigation of high-temperature-oxidation of metals by infrared-reflection-absorption spectroscopy (IRAS)

The potential of infrared-reflection-absorption spectroscopy (IRAS) for the investigation of high-temperature oxidation of metals and alloys is demonstrated. The measurement of the reflectivity of p-polarized light under a suitable angle of incidence results in a characteristic minimum caused by the formed oxide (Berreman-effect). Interference fringes in the non-absorbing region of the spectrum are used for thickness determination. In the case of complex oxide layer structures a fitting procedure is described which takes into account the measured weight gain. As examples spectra of oxidized Cr, Ni and of a Ni-based superalloy (SC 16) are presented.     

The negative adsorption of chromium atoms on alloy-oxide film boundaries during oxidation in air and anodic passivation of Fe-Cr and Ni-Cr alloys

Equilibrium of Cr atoms between the surface layer and bulk of a binary alloy was analyzed. The Gibbs adsorption equation was used to obtain the dependence of the adsorption activity of atoms in the surface layer on their activity in the bulk. An approximate thermodynamic method was used to calculate the adsorption of Fe (Ni) and Cr atoms in the surface layers of Fe-Cr and Ni-Cr alloys. According to calculations, there was negative adsorption, X _Cr ≪ 1, in the surface layer of the alloys caused by a large difference between the Gibbs surface energies of Cr and Fe (or Ni). The negative adsorption of Cr shifted chemical reaction equilibria on the alloy-oxide film boundary both in oxidation in air and in anodic passivation, 3FeO (NiO) + 2Cr = Cr₂O₃ + 3Fe(Ni), toward oxide film enrichment in the FeO (or NiO) oxide. A unified method for calculating the composition of oxide films on alloys was used for both processes. The method was based on the use of the initial data on the Gibbs surface energy of metals constituting alloys. The calculated oxide film compositions were close to the experimental X-ray photoelectron spectroscopy data.