Surface Chromium Research Articles

Characterization of Surface Chromia Species on CrOx/TiO2 Catalysts

<p>Titania supported chromia catalysts have been employed for selective catalytic reduction (SCR) process because of their higher activity and selectivity. The catalytic activity of the titania supported chromia system is owing to the stabilization of the anchored chromia species having multiple chemical and molecular states on the surface<strong>. </strong>In the present work an attempt has been made to prepare titania supported chromia catalysts, from TiCl<sub>4</sub> and CrO<sub>3</sub> as precursors for the support and active phase respectively. Characterization of the catalysts have been made using FTIR spectroscopy and thermo analytical techniques such as thermogravimetric analysis (TGA), evolved gas analysis (EGA) and temperature programmed reduction (TPR). The EGA, TGA and FTIR results indicate that the surface hydroxyl groups of TiO<sub>2</sub> (gel) has an influence on the chemical state of the chromium. Due to the surface anchoring, Cr<sup>+5</sup> species have been detected on the surface of the fresh catalyst which decomposes to lower valence state on calcination. EGA results indicate that Cr<sup>+3</sup> species having higher degree of coordinatively unsaturated centers is stabilized by gel titania as support. EG analysis and FTIR studies illustrate the coverage of the surface with similar type of sites at 10 wt.% chromia loading. TPR studies demonstrate the presence different kinds of surface chromia species with respect to chromia content.</p>

Atomic level characterization in corrosion studies.

Atomic level characterization brings fundamental insight into the mechanisms of self-protection against corrosion of metals and alloys by oxide passive films and into how localized corrosion is initiated on passivated metal surfaces. This is illustrated in this overview with selected data obtained at the subnanometre, i.e. atomic or molecular, scale and also at the nanometre scale on single-crystal copper, nickel, chromium and stainless steel surfaces passivated in well-controlled conditions and analysed in situ and/or ex situ by scanning tunnelling microscopy/spectroscopy and atomic force microscopy. A selected example of corrosion modelling by ab initio density functional theory is also presented. The discussed aspects include the surface reconstruction induced by hydroxide adsorption and formation of two-dimensional (hydr)oxide precursors, the atomic structure, orientation and surface hydroxylation of three-dimensional ultrathin oxide passive films, the effect of grain boundaries in polycrystalline passive films acting as preferential sites of passivity breakdown, the differences in local electronic properties measured at grain boundaries of passive films and the role of step edges at the exposed surface of oxide grains on the dissolution of the passive film.This article is part of the themed issue 'The challenges of hydrogen and metals'.

Evaluation of ion implantation for anti-thrombogenic coronary stent in vitro and in vivo

Abstract The aim of this study was to evaluate the effect of ion implantation on the surface of anti-thrombogenic coronary stent. Nitrogen (N) was implanted into the cobalt–chromium surface by using an ion source under a plasma environment to enhance its hardness and surface modification. The N ion dosage was 1 × 1015 ions/cm2. All analysis results of the ion implanted stent were compared with those of bare metal stent. The N component and distribution were confirmed by auger electron spectroscopy. Microhardness was significantly increased after 40 min of implantation (415.3 ± 12.38 HV, 18.9 ± 2.62%). The surface was altered to hydrophobic status through ion implantation (30.6 ± 1.12% in contact angle increment). Platelet adhesion, and smooth muscle cell migration and proliferation were prevented in the ion implanted group (43.9%, 11.2%, and 45.1%, respectively). To verify the in vitro result, stents were implanted to rabbit iliac artery and isolated at 4 weeks post implantation. Then, the stents were subjected to histological analysis. No significant differences in injury score, internal elastic lamina, lumen area, and restenosis rate were found. However, the fibrin score was more significantly decreased in the ion beam-implanted group (2.5 ± 0.15) than in the bare metal stent group (1.8 ± 0.31, n = 10, p

High-speed 3D profilometry employing HSI color model for color surface with discontinuities

Structured-light projection methods are facing two remaining challenges. One is a high-speed optical metrology on objects with chromatic surfaces and the other one is avoiding phase errors caused by height steps or spatially isolated surfaces. To overcome them, this paper provides an effective profilometry with a single-shot image by employing the HSI color model to compose the colorful pattern. Three sinusoidal fringes with different phase steps are encoded in RGB channels respectively. The hue component of a deformed pattern is applied to reconstruct the 3D shape of an object. The saturation and the intensity are utilized to correct the hue demodulation. Besides, an effective color calibration procedure is developed to compensate the hue error. Experimental results verify the feasibility of the developed method.

The effects of ball milling time and surface enriched chromium on microstructures and corrosion resistance of AISI 304 stainless steel

This study showed that the increasing of ball milling time caused a beneficial surface chromium enrichment of AISI 304 stainless steel. In addition, grain size decreased with the increasing of ball milling time. The electrochemical results showed that oxidation reaction transition of Cr2O3 to CrO3 was significantly inhibited by surface enriched chromium, which improved corrosion resistance of AISI 304 stainless steel. Moreover, higher surface enriched chromium reduced acceptor concentration in passive film, which also improved corrosion resistance of AISI 304 stainless steel. Moreover, more chromium oxides improved corrosion resistance of AISI 304 stainless steel in borate buffer solution.

High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst

Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800°C. For the catalysts treated at temperatures lower than 300°C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300°C. The geminal silanols almost disappeared on the catalysts at 120°C, while that for the silica gel occurred at 300°C. Further increasing the calcination temperatures from 300 to 800°C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.

Comparison of adsorption affinity of ionic polyacrylamide for the surfaces of selected metal oxides

The influence of metal oxide type on the adsorption affinity of anionic polyacrylamide for the solid surface was studied. There were selected three metal oxides: chromium(III) oxide, zirconium(IV) oxide and aluminium(III) oxide. The adsorption behaviour of high-molecular weight polymer containing 30% of carboxyl groups was determined based on the results of spectrophotometric, electrokinetic and turbidimetric measurements. The anionic polymer exhibits the greatest adsorption on the surface of chromium(III) oxide whereas PAM affinity for the aluminium(III) oxide is the lowest. The polymer presence has a pronounced impact on the electrokinetic properties of examined suspensions manifesting in changes in pHpzc and pHiep values of PAM-containing systems compared to those without polyacrylamide. The anionic PAM addition at pH 6 has the greatest effect on the stability of metal oxide aqueous suspensions.

The effect of surface enriched chromium and grain refinement by ball milling on corrosion resistance of 316L stainless steel

Surface chromium enrichment on the 316L stainless steel was obtained by ball milling technique. The present study showed that chromium enrichment on the surface of the 316L stainless steel after ball milling promoted strain-induced α′-martensite transformation. The electrochemical test results showed that nanocrystallization and surface chromium enrichment nullified the adverse influence of α′-martensite on the corrosion resistance. In addition, more chromium on the surface of 316L stainless steel improved its corrosion resistance. More chromium oxides improved corrosion resistance of 316L stainless steel in borate buffer solution. This was attributed to the fact that more chromium reduced acceptor concentration in passive film, which enhanced the corrosion resistance of 316L stainless steel.

Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery

The erosion response under cavitation of different steel grades was assessed by studying the erosion rate, the volume removal, the roughness evolution, and the accumulated strain energy. A 20 kHz ultrasonic transducer with a probe diameter of 5 mm and peak-to-peak amplitude of 50 μm was deployed in distilled water to induce damage on the surface of commercial chromium and carbon steel samples. After a relatively short incubation period, cavitation induced the formation of pits, cracks, and craters whose features strongly depended on the hardness and composition of the tested steel. AISI 52100 chromium steel showed the best performance and is, therefore, a promising design candidate for replacing the existing fluid machinery materials that operate within potential cavitating environments.Graphical

Experimental patch testing with chromium-coated materials.

Chromium coatings on metal alloys can be decorative, and prevent corrosion and metal ion release. We recently showed that handling of a chromium-containing disc resulted in chromium deposition on the skin. To examine patch test reactivity to chromium-coated discs. We included 15 patients: 10 chromium-allergic patients, and 5 patients without chromium allergy. All were patch tested with potassium dichromate, cobalt chloride, nickel sulfate, and nine different metallic discs. The chromium-allergic patients were also patch tested with serial dilutions of potassium dichromate. Positive/weaker reactions were observed to disc B (1 of 10), disc C (1 of 10), and disc D, disc E, and disc I (4 of 10 each). As no controls reacted to any of the discs, the weak reactions indicate allergic reactions. Positive patch test reactions to 1770 ppm chromium(VI) in the serial dilutions of potassium dichromate were observed in 7 of 10 patients. When the case group was narrowed down to include only the patients with a current positive patch test reaction to potassium dichromate, elicitation of dermatitis by both chromium(III) and chromium(VI) discs was observed in 4 of 7 of patients. Many of the patients reacted to both chromium(III) and chromium(VI) surfaces. Our results indicate that both chromium(VI) and chromium(III) pose a risk to chromium-allergic patients.

Potential-pH Diagrams for Pb Adsorption on Ni, Fe and Cr Surfaces at 25 and 300°C

The Potential-pH relations for equilibrium adsorption of lead on chromium and iron surfaces in aqueous solutions containing dissolved Pb are calculated from recent DFT data on energies of Pb adsorption on Cr and Fe surfaces. Using these relations and relations calculated previously for Pb adsorption on Ni, Pourbaix diagrams showing the potential and pH conditions of adsorption of Pb on Ni, Cr and Fe by reduction of its dissolved species above the equilibrium potential of deposition of bulk Pb (underpotential deposition or UPD) are constructed at 25°C and 300°C. Overlap of the domains of stability of adsorbed Pb and Ni, Cr and Fe oxides is observed, suggesting that the formation and properties of the passive film might be affected by the presence of adsorbed Pb. This could be the cause of lead-induced stress corrosion cracking (Pb SCC) of Ni-based alloys used as tubing materials of steam generators in pressurized water nuclear reactors.

Catalytic dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) to 2,3,3,3-tetrafluoropropene (HFO-1234yf) using in-situ fluorinated chromium oxyfluoride catalyst

As part of an effort to prepare a refrigerant with zero ozone depletion potential (ODP) and with a low global warming potential (GWP), the catalytic dehydrofluorination of 1,1,1,2,3-tetrafluoropropane (HFC-245eb) is performed to produce 2,3,3,3-tetrafluoropropene (HFO-1234yf). Cr2O3 prepared by the sol-gel method (Cr2O3(SG)) exhibits good catalytic activity attributable to the formation of surface chromium oxyfluoride, which exhibits better catalytic activity with high-valent chromium and a larger fluorine content. The formation of chromium oxyfluoride from chromium oxide is achieved by in-situ fluorination in the early stage of dehydrofluorination, avoiding the conventional fluorination method using toxic HF.

A Promising Approach for Improving the Coating Stability and In Vivo Performance of Biodegradable Polymer-Coated Sirolimus-Eluting Stent.

Several developments are in progress for improving the performance of drug-eluting stents (DESs) including use of biodegradable polymers, polymer-free DES, fully bioabsorbable stents, and so on. The commercially available DESs still suffer from polymer defects that could affect the performance of a DES through a series of adverse events such as coating delamination and/or peeling-off that lead to non-uniform local drug distribution, restenosis, and thrombosis. The goal of this work was to enhance the stability of drug-in-polymer matrix coating on a stent metal surface through surface modification. The cobalt–chromium (Co–Cr) surface was chemically modified using poly(dopamine) (PDA) nano-coating and poly(L-lactide) (PLLA) nano-brush in order to be applied to a biodegradable polymer-coated DES. The biodegradable polymer loaded with sirolimus was coated using an ultrasonic spray coating instrument. The coating morphology on all samples showed a very smooth and uniform coating. The stability of the coating was evaluated for 2 months under the circulation system in which the drug-in-polymer coating on the PLLA brush-modified stent presented the most stable coating behavior as compared to other samples. The in vitro sirolimus release study from both unmodified and modified stents was studied in phosphate-buffered saline (PBS), and the modified stents showed slower sirolimus release profile as compared to unmodified stents. In vivo study was performed in a porcine coronary artery injury model for 28 days. The percentage of in-stent restenosis area (ISR) for PLLA brush-modified sirolimus-eluting stent (SES) decreased significantly as compared to unmodified SES and bare metal stent (BMS). This study demonstrated that the modification of stent surface using PLLA brushes affects in vitro and in vivo performance effectively to be applied for biodegradable polymercoated DES.

Low-pressure diffusion chromising of thin low-carbon steel sheet for improved surface and bulk properties

Multi-component alloying of thin metal sheets offers the particular possibility to produce novel materials with adjusted bulk and surface performance characteristics. Its application is of great interest for the development of advanced materials with tailored properties such as steel based materials for the use as bipolar plates for proton exchange membrane fuel cells. Within this work 0.13-mm thick cold-rolled sheets of low-carbon steel EN 1.0338 were double-sided chromised by means of low-pressure diffusion metallization method. Samples with chromium diffusion coatings as well as bulk-chromised samples, i.e., samples fully chromised over the whole strip thickness, were prepared. For the diffusion coefficient of chromium in γ-iron phase at 1423 K a value of 1.80·10−14 m2 s−1 was determined. As long as the samples did not reach the bulk-chromised state, the chromium concentration remained less than 12.5 wt%. The bulk-chromised sheets with surface chromium concentration of approximately 22 wt% showed the best corrosion resistance in 0.5% sodium chloride and 0.05 M sulphuric acid solutions electrochemical tests. Moreover, they exhibited the smallest interfacial contact resistance of approx. 31 mΩ cm2 at a contact pressure of 140 N cm−2.

Formation of chromium composite electrochemical coatings from sulfate oxalate solutions based on Cr(III)

The mechanism responsible for the inclusion of Al2O3 and SiC nanoparticles, the mixture of Nb2N and Ta2N (1: 1), MoS2, Cr2O3, and SiO2 with diverse electric conductivity, hydrophilicity, and resistance to solution components in chromium deposits from the sulfate–oxalate suspension solutions based on Cr(III) was studied. The main factors that determine the formation of chromium composite electrochemical coatings, their composition, and surface morphology were determined. The film on the surface of the growing deposit of the intermediates of the reduction of chromium ions plays the key role in the formation of composite coatings from Cr(III) and Cr(VI) sulfate–oxalate suspension solutions. The film can play the role of a structural mechanical barrier that hinders the incorporation of particles in the deposit, or it can fix the particles on the electrode surface by creating hydroxo bridges with chemisorbed hydroxide compounds on the particle surface.

Dynamics of CrO3–Fe2O3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

A series of supported CrO3/Fe2O3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O═)2Cr6+O2 species and a bulk Fe2O3 phase containing some Cr3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 ,and surface dioxo (O═)2Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3–-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts ...

Using Neutron Reflectometry to Discern the Structure of Fibrinogen Adsorption at the Stainless Steel/Aqueous Interface

Neutron reflectometry has been successfully used to study adsorption on a stainless steel surface by means of depositing a thin steel film on silicon. The film was characterized using XPS (X-ray photoelectron spectroscopy), TOF-SIMS (time-of-flight secondary ion mass spectrometry), and GIXRD (grazing incidence X-ray diffraction), demonstrating the retention both of the austenitic phase and of the required composition for 316L stainless steel. The adsorption of fibrinogen from a physiologically-relevant solution onto the steel surface was studied using neutron reflectometry and QCM (quartz crystal microbalance) and compared to that on a deposited chromium oxide surface. It was found that the protein forms an irreversibly bound layer at low concentrations, with maximum protein concentration a distance of around 20 Å from the surface. Evidence for a further diffuse reversibly-bound layer forming at higher concentrations was also observed. Both the structure of the layer revealed by the neutron reflectometry data and the high water retention predicted by the QCM data suggest that there is a significant extent of protein unfolding upon adsorption. A lower extent of adsorption was seen on the chromium surfaces, although the adsorbed layer structures were similar, suggesting comparable adsorption mechanisms.

Corrosion Behaviour of Uncoated and Ceramic Coated 9Cr–1Mo Steel in Molten LiCl–KCl–UCl3 Salt

9Cr–1Mo steel has been proposed as the container material for the electrorefining process of pyrochemical reprocessing of spent metallic fuels from the future sodium cooled fast breeder reactors. The electrorefining process was carried out using LiCl–KCl–UCl3 molten salt as the electrolyte at 500 °C under argon atmosphere. To protect the electrorefining vessel, made of 9Cr–1Mo steel from the attack by the electrolyte, yttria stabilized zirconia (YSZ) ceramic coating was deposited on 9Cr–1Mo steel by thermal spray process. The corrosion behavior of 9Cr–1Mo steel with and without YSZ coating was evaluated in LiCl–KCl–UCl3 molten salt at 600 °C under argon atmosphere for various durations (100, 250, 500 and 1000 h). The results revealed that with increase in exposure time, the weight loss of uncoated 9Cr–1Mo steel samples increased, while YSZ coated 9Cr–1Mo steel samples exhibited insignificant weight loss. SEM examination of exposed uncoated 9Cr–1Mo steel samples showed Cr depletion on the surface and formation of chromium based oxides as corrosion products. After the corrosion experiment, SEM/EDS and XRD analyses of both coated and uncoated samples showed the presence of Fe–Cr and UO2.

Investigating the CrOx/Al2O3 dehydrogenation catalyst model: I. identification and stability evaluation of the Cr species on the fresh and equilibrated catalysts

In this study a series of the fresh and equilibrated CrOx/Al2O3 catalysts with the Cr loading from 0.58wt% to 13.2wt% were in detail studied using combined techniques, including Raman spectroscopy, TEM, XPS, O2 chemisorption, TPR, XRD and chemical analysis, to identify surface chromium species and to evaluate their stability under the cyclic dehydrogenation-regeneration (redox) conditions. The study showed that the stable chromium species present in the equilibrated CrOx/Al2O3 catalyst can be achieved through complete surface restructuring of initial chromium species in the fresh one under the redox conditions. This study also showed that the fresh CrOx/Al2O3 catalyst contained not only previously known chromium species of ‘isolated’ mono/poly aluminum chromate(s), amorphous and crystalline Cr2O3 clusters with different sizes, but also chromium-chromate (Cr6+) species on the surface of Cr2O3 clusters and aluminum-chromium-chromate (Cr6+) species coupled with Cr2O3 clusters. The ‘isolated’ aluminum chromate(s) and small Cr2O3 clusters on the catalyst surface were not stable under the cyclic redox conditions and disappeared after the break-in-period of the catalyst operation. The paper presents the mechanism of the surface restructuring of chromium species under the redox conditions and the revised model for the CrOx/Al2O3 catalyst with the consideration of the change of surface chromium species caused by the cyclic redox aging. The model can be applied to the commercial CrOx/Al2O3 catalysts.

Development of in situ soft colloidal probe atomic force microscopy for probing the adhesion between wood extractives and model surfaces

A new method to attach colloidal particles in situ directly onto atomic force microscopy (AFM) cantilevers was developed and used as colloidal probes. In this method, cantilever surfaces were hydrophobically functionalised and the colloids adhered without glue in an aqueous environment. Experiments measuring the interaction forces between colloidal probes, composed of either wood extractives or a range of model compounds normally present in wood extractives, and glass, cellulose, and chromium surfaces were carried out. Wood extractive colloids exhibited the strongest adhesion to glass control surface (7.0 mN m−1), followed in decreasing order by triolein, abietic acid and oleic acid colloids (3.9 mN m−1, 2.2 mN m−1 and 1.2 mN m−1 respectively). Adhesion onto a cellulose surface was strongest for triolein colloids (8.1 mN m−1) followed by oleic acid colloids (6.1 mN m−1), while adhesion forces for abietic acid and wood extractives colloids were similar (4.9 mN m−1 and 4.3 mN m−1, respectively). When attached to a chromium surface adhesion forces of abietic acid and wood extractives were strongest and similar (3.6 mN m−1 and 3.9 mN m−1, respectively); the weakest interaction at this surface was recorded for oleic acid colloids (0.8 mN m−1). Similar adhesion forces for wood extractive colloids and abietic acid colloids (onto cellulose and chromium surfaces) reinforces the proposal that abietic acid occupies the outer layer of wood extractive colloids. The differences in the interaction between the colloids and the cellulose and chromium surface helps to explain the transfer of resin acid exclusively over triolein or oleic acid from a cellulose paper surface onto chromium plated printing cylinders.