Acidic Fluoride Solutions Research Articles

Post-Synthesis of HZSM-5 by Aluminum Fluoride in an Acidic Fluoride Solution for Enhanced Reforming of Lignite Pyrolytic Volatiles into Light Aromatics

Post-Synthesis of HZSM-5 by Aluminum Fluoride in an Acidic Fluoride Solution for Enhanced Reforming of Lignite Pyrolytic Volatiles into Light Aromatics

Ta-Nb mineralization in the shallow-level highly-evolved P-poor Shihuiyao granite, Northeast China

The Shihuiyao granite district in the southern Great Xing'an Range, Northeast China, is a newly discovered area of rare-metal-bearing granites (RMG). The latest Jurassic Shihuiyao granitic pluton, emplaced into the Lower Permian Linxi Formation, comprises porphyritic granite, leucogranite, and amazonite-bearing pegmatite, all enriched in rare metals (mainly Ta, Nb, Rb). The Shihuiyao granites are peraluminous (A/CNK = 1.02–1.27), characterized by high contents of SiO2 (69.4–78.0 wt%), Al2O3 (12.7–16.9 wt%) and total alkalis (6.8–12.8 wt%), enriched in Ta, Nb, Rb, Cs, and Sn, but depleted in Ba and Sr. They have strong negative Eu anomalies (Eu/Eu⁎ = 0.02–0.42) and show the M-type REE tetrad effect. Low Nb/Ta (<4) and Zr/Hf (<8) values indicate high degree of magmatic fractionation. Whole-rock and mineral compositions variations indicate that the Shihuiyao granites are a typical low pressure and P-poor RMG, which experienced intensive magmatic fractionation followed by intensive hydrothermal reworking. The narrow range of δ7Li values (−0.86‰ to 0.88‰) and Rayleigh distillation modeling suggest that the porphyritic granite and leucogranite represent products of the same magma source, but evolved independently at depth. LA-ICP-MS UPb dating of monazite from the porphyritic granite and leucogranite yield lower-intercept ages of 150.2 ± 0.6 Ma and 147.0 ± 1.7 Ma, respectively. The 40Ar39Ar plateau ages of mica from leucogranite (146.3 ± 1.1 Ma) and an ore-bearing quartz vein (145.6 ± 1.0 Ma), and the UPb age of columbite (146.7 ± 4.7 Ma) further identify this as a latest Jurassic magmatic-hydrothermal mineralization event in NE China. Rapid crystallization and chemical quenching of magma resulted in neutralization of the acid fluoride solution, and a loss of the fluoride ligand during fluid evolution. This further triggered the subsequent crystallization of NbTa oxides. As revealed from geochemical and textural characteristics of the columbite group minerals, the high-degree of fractional crystallization and Ta-rich hydrosilicate fluid metasomatism are the main factors leading to the enrichment of TaNb within Shihuiyao RMG. In general, we conclude that rare metal-hosted magmatism took place in two episodes in NE China, namely, (i) Latest Jurassic to Early Cretaceous (150–130 Ma) peraluminous to metaluminous Ta-Nb-Sn-W RMG, and (ii) Early Cretaceous (130–120 Ma) alkaline Nb-Ta-Be-REE-Zr RMG, which highlight the prospecting fertility for Nb-Ta-REE (SnW) mineralization in NE China.

Countermeasure for corrosion and hydrogen embrittlement of Ni-Ti superelastic alloy in acidic fluoride solution by adding sodium bicarbonate and hydrogen peroxide

ABSTRACT Development of countermeasure for corrosion and hydrogen embrittlement of Ni-Ti superelastic alloy in an acidulated phosphate fluoride (APF) solution has been attempted by adding NaHCO3 and H2O2 to the solution. Upon addition of only 0.001 M NaHCO3 to a 0.2% APF solution, almost no changes in corrosion current density and corrosion potential are observed. With an increasing NaHCO3 concentration up to 0.01 M, the corrosion current density substantially decreases and the corrosion potential slightly shifts in the noble direction. The corrosion loss significantly decreases and hydrogen absorption is slightly inhibited. Upon the combined addition of 0.01 M NaHCO3 and 0.001 M H2O2, the corrosion current density is lowest and the corrosion potential shifts in the noble direction. The corrosion and hydrogen absorption are substantially inhibited. The present study clearly indicates that the combined addition is exceptionally effective as a simultaneous countermeasure for corrosion and hydrogen embrittlement of the alloy.

Niobium and tantalum in hydrothermal fluids: Thermodynamic description of hydroxide and hydroxofluoride complexes

Available experimental data on the solubility of β -Nb2O5 and β -Ta2O5 in the Nb–O–H–F and Ta–O–H–F systems were critically evaluated, and the thermodynamic properties of hydroxide and hydroxofluoride complexes were estimated in a wide range of temperatures (0–600 °C) and pressures (1–3000 bar). In near neutral pH solutions, the solubility of the oxides is governed by uncharged aqueous hydroxides, Nb(OH)5 and Ta(OH)5, and is rather low even at high temperatures (1–9 ppm for Nb and 0.1–0.9 ppm for Ta at 600 °C, 1000 bar). In acid HF bearing solutions, the solubility is governed by Nb(OH)2F3 and Ta(OH)2F3 complexes and increases sharply with increasing HF molality: at mHF = 0.1 mol·kg−1, 600 °C and 1000 bar, the Nb and Ta concentrations reach 80 and 30 ppm respectively. In alkaline fluoride bearing solutions, the HNbO3F− and HTaO3F− - oxyfluoride anions control the solubility of the oxides. The temperature and pressure dependences of the oxides solubility governed by these fluoride complexes are rather weak; therefore, the neutralization of acid fluoride solution and the loss of fluoride ligand during fluid evolution can be the main factors of Nb and Ta deposition.

Effects of Fluoride on NiTi Orthodontic Archwires: An X-ray Diffraction Study:

Objective: To explore the changes in nickel-titanium (NiTi) archwires after exposure to fluoride-containing acetic acid solutions. Materials and methods: Preformed rectangular NiTi wires were immersed in either an acidulated fluoride solution, a neutral fluoride solution or distilled water (control) for 1.5 hours at 37°C. After immersion, the unloading force values at four deflection points were measured by three-point bending test. Scanning electron microscopy and X-ray diffraction analysis were also performed to characterize the effects of the fluoride exposure on the wire. Results: Unloading force values of NiTi orthodontic wires were significantly decreased after exposure to both fluoride solutions (p < 0.001). Corrosive changes in surface topography were observed for both fluoride solutions. Wires exposed to acidic fluoride appeared as more severely affected. X-ray diffraction analysis showed no change in crystal lattice of NiTi wires in both solutions. Conclusion: The results suggest that using topical fluoride agents with NiTi wire could decrease the functional unloading mechanical properties of the wire and contribute to prolonged orthodontic treatment.

Preparation of mesoporous Beta zeolite by fluoride treatment in liquid phase. Textural, acid and catalytic properties

Abstract Beta zeolite (nominal Si/Al = 19) has been treated with HF/NH4F solutions under different HF concentration, temperature and reaction time. It has been found that the crystallinity of the resulting materials is >80% for HF concentration up to 0.5 M, but it drops below this value for the most severe condition (HF = 1 M and 40 °C). The treatment dissolves selectively aluminum, in such a way that the Si/Al increases to 34 for the most attacked material. As a consequence of these treatments, the mesopore volume increases by up to 80% respect to the parent Beta for samples retaining high crystallinity, corresponding to newly created mesopores in the range of 5–20 nm. Detailed STEM-HAADF studies reveal that the chemical attack of the Beta crystals by the acidic fluoride solution progresses slowly along the crystals, leaving behind highly corroded and porous “sponge-like” crystals where the structure of the zeolite is nevertheless retained. Pyridine adsorption/desorption experiments show that a fraction of the extraframework Al present in the starting Beta is reinserted in framework sites, a process that would be mediated by aluminum-fluoride soluble species. As a consequence of this process, the Bronsted/Lewis ratio of the strong acid sites increases from 1 in the starting Beta to close to 3. The concentration of total and strong Bronsted acid sites associated to framework Al increases with the treatment. However, the activity of the samples in the isomerization/disproportionation of m-xylene is lower than that of the parent Beta zeolite, a result that could be due to the higher B/L ratio.

Stress and electrochemical corrosion of titanium produced by laser rapid forming

Objective: To investigate the stress corrosion and electrochemical corrosion resistance of titanium produced through laser rapid forming (LRF), and to provide a basis for their clinical application. Methods: Forged commercial pure titanium (CP-Ti) was used as control group and LRF pure titanium was used as LRF group. All samples were placed in acidic artificial saliva containing fluorine (pH=7), and loaded with a stress of 1.2 × σ(0.2) Pa (σ(0.2) represents the yield strength of material). Stress corrosion resistance of specimens that have been soaked for 30 days was analyzed by naked eye observation, X-ray diffraction analysis and scanning electron microscopy. For samples placed in artificial saliva, neutral fluoride solution (pH=7) and acidic fluoride solution (pH=3) (4 test pieces in each corrosive medium), and their electrochemical corrosion resistance was evaluated by free corrosion potential (Ecorr) measurements, corrosion current (Icorr), electrochemical impedance spectroscopy (EIS), and anodic polarization curves. Results: With the prolongation of immersion time, the corrosion products gradually increased. The stress corrosion of CP-Ti group was significantly more than that of LRF-Ti group, and the pit diameter was significantly larger than that of LRF-Ti group. The electrochemical corrosion results showed that the Ecorr (-469 mV) of LRF-Ti in artificial saliva was higher than that of CP-Ti (-555 mV), and the Ecorr (-925 mV) of LRF-Ti was higher than that of CP-Ti (-943 mV) in neutral fluoride solution. In acid fluoride solution, the Ecorr (-943 mV) of LRF-Ti was higher than that of CP-Ti (-956 mV). The Ecorr of the same metal was the highest in artificial saliva and the lowest in acid fluoride solution; the Icorr of the same metal was the lowest in artificial saliva and the highest in acid fluoride solution. Conclusions: Under the same corrosion conditions, LRF Ti demonstrated better stress and electrochemical corrosion resistance than CP-Ti.

Evaluation of degradation in the performance of orthodontic wires after immersing in acidulated phosphate fluoride prophylactic agent: An in vitro study

Introduction: Fluoride prophylactic agents, such as acidulated phosphate fluorides (APFs), are being used extensively to prevent demineralization of white spot lesion around orthodontic brackets. However, corrosion has been reported in the acid fluoride solution prophylactic agents altering the mechanical properties of wires. Aim: The aim of the study was to evaluate the degradation in performance of four major orthodontic wires after short-term immersion in APF oral prophylactic agent. Materials and Methods: Four types of orthodontic wires were selected, namely, nickel-titanium wire, beta-titanium wire, stainless steel wire, and cobalt-chromium-nickel wire. These were immersed in a solution of APF-gel or APF-containing mouthwash. Surface changes of treated wires were observed under scanning electron microscope (SEM). The tensile strength of the wires was measured using universal testing machine. The data obtained were noted and subjected for statistical analysis using factorial ANOVA test. Results: SEM showed greater surface corrosion in nickel-titanium and beta-titanium wires and less surface corrosion in stainless steel and cobalt-chromium-nickel wires. Cobalt-chromium-nickel wires showed a higher mean tensile strength followed by stainless steel, beta-titanium, and nickel-titanium wires, respectively, when used with control and mouthwash immersion. With APF-gel, higher mean tensile strength was recorded in stainless steel wires. Conclusions: There was degradation in the performance of orthodontic wires after short-term immersion in the APF prophylactic agent. Greater surface corrosion was observed in nickel-titanium and beta-titanium wires, and less surface corrosion was observed in stainless steel and cobalt-chromium-nickel wires.

Niobium in hydrothermal systems related to alkali granites: Thermodynamic description of hydroxo and hydroxofluoride complexes

Available experimental data on the solubility of Nb2O5 and the stability constants for particles of an aqueous solution in the Nb–O–H–F system were processed. As a result, a set of thermodynamic properties for 25°C and 1 bar was obtained, in addition to the equation parameters for the HKF model (Helgeson–Kirkham–Flowers) for hydroxo and hydroxofluoride niobium complexes. F– ion is the most important factor governing the concentration of dissolved Nb: neutral hydroxo complex Nb(OH)5(aq) is formed at a low HF concentration, whereas an increase in HF results in an increase in the first Nb(OH)4F(aq) and second Nb(OH)3F2(aq) fluoride complexes. The Nb(OH)5F– oxofluoride anion determines oxide solubility in alkali F-bearing fluids. Neutralization of acidic fluoride solution can be the main factor leading to niobium deposition.

Anodic processes in the chemical and electrochemical etching of Si crystals in acid-fluoride solutions: Pore formation mechanism

The interaction of heavily doped p- and n-type Si crystals with hydrofluoric acid in the dark with and without contact with metals having greatly differing work functions (Ag and Pd) is studied. The dependences of the dissolution rates of Si crystals in HF solutions that contain oxidizing agents with different redox potentials (FeCl3, V2O5 and CrO3) on the type and level of silicon doping are determined. Analysis of the experimental data suggests that valence-band holes in silicon are not directly involved in the anodic reactions of silicon oxidation and dissolution and their generation in crystals does not limit the rate of these processes. It is also shown that the character and rate of the chemical process leading to silicon dissolution in HF-containing electrolytes are determined by the interfacial potential attained at the semiconductor–electrolyte interface. The mechanism of electrochemical pore formation in silicon crystals is discussed in terms of selfconsistent cooperative reactions of nucleophilic substitution between chemisorbed fluorine anions and coordination- saturated silicon atoms in the crystal subsurface layer. A specific feature of these reactions for silicon crystals is that vacant nonbonding d 2 sp 3 orbitals of Si atoms, associated with sixfold degenerate states corresponding to the Δ valley of the conduction band, are involved in the formation of intermediate complexes. According to the suggested model, the pore-formation process spontaneously develops in local regions of the interface under the action of the interfacial potential in the adsorption layer and occurs as a result of the detachment of (SiF2) n polymer chains from the crystal. Just this process leads to the preferential propagation of pores along the crystallographic directions. The thermodynamic aspects of pore nucleation and the effect of the potential drop across the interface, conduction type, and free-carrier concentration in the crystal on the pore size and structure are discussed. The concepts developed in the study can consistently account for experimental facts characterizing the etching of silicon crystals with various electrical parameters under various conditions providing the anodic polarization of crystals in HF-containing solutions.

The Potential of Self-assembling Peptides for Enhancement of In Vitro Remineralisation of White Spot Lesions as Measured by Quantitative Laser Fluorescence.

To test the remineralisation potential of a single application of self-assembling peptides or acidic fluoride solution using quantitative light-induced fluorescence (QLF) in vitro. Bovine enamel disks were prepared, and white spot lesions were created on one half of the disk with an acidic buffer solution. After demineralisation, disks were allocated into three groups of 11 specimens each. Group A served as a control group and received no treatment. Group B had a single application of fluoride, and group C was treated once with self-assembling peptides. All disks were embedded in a plastic mold (diameter 15 mm, height 9 mm) with an a-silicone, and remineralisation was initiated using a pH-cycling protocol for five days. Four experimental regions on each disk were measured prior to the start of the study (T0), after demineralisation (T1) and after the remineralisation process (T2) using QLF. After demineralisation, all areas showed a distinct loss of fluorescence, with no statistically significant difference between the groups (ΔF from -69.3 to -10.2). After remineralisation, samples of group B (treated with fluoride) showed a statistically significant fluorescence increase (ΔF from T1 to T2 15.2 ± 7.3) indicating remineralisation, whereas the samples of control group A and group C (treated with self-assembling peptides) showed no significant changes in ΔF of 1.1 ± 1.9 and 2.5 ± 1.9, respectively. Application of self-assembling peptides on demineralised bovine enamel did not lead to increased fluorescence using QLF, indicating either lack of remineralisation or irregular crystals. Increased fluorescence using QLF indicated mineral gain following a single application of a highly concentrated fluoride.

Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching

A silicon surface with distinctive structures is fabricated by ultrashort pulsed-laser ablation and chemical etching with acidic fluoride solutions. The surface consists of micro/nanostructures that result in the quasi-superhydrophobicity of the silicon surface. By fine tuning a key process parameter (i.e., pulsed laser power), surfaces with different wettability are fabricated. The morphology and composition of the surfaces are characterized by scanning electron microscopy, which reveals nanopores. The contact angle of water on these surfaces was measured and found to be as high as 150° at optimized parameters. This work presents a novel process of fabricating a silicon-based quasi-superhydrophobic porous surface.

Hydrogen embrittlement behavior of Ni-Ti shape memory alloy with different microstructures in acidic fluoride solution

It is important to investigate the mechanism for the hydrogen embrittlement of Ni-Ti alloys in acidic fluoride solutions to improve the reliability and safety of these alloys as dental devices. Therefore, the hydrogen embrittlement behavior of Ni-Ti shape memory alloy immersed in acidic fluoride solution was investigated with a focus on the constituent phase in the microstructure of the alloy in this study. Three microstructures with different phases (parent single phase, mixture of parent and martensite phases, and martensite single phase) were prepared by tensile loading and unloading. The specimens were immersed separately in 50 mL of 0.2 % acidulated phosphate fluoride (APF) solution with pH 5.0 at room temperature (25 °C) for various periods. After immersion for 2 h, the tensile strengths of all the specimens were not significantly changed with respect to those of the non-immersed specimens. After immersion for 4 h, the tensile strengths of all the specimens immersed were decreased with respect to those of the non-immersed specimens, and the tensile strength of the martensite single phase specimen (C) was higher than that of parent single phase specimen (A) and the parent/martensite mixed phase specimen (B). After immersion for 6 h, the tensile strengths of all the specimens were decreased with respect to those of the specimens immersed for 4 h, and the tensile strength of specimen B was lower than that of specimens A and C. The susceptibility to hydrogen embrittlement of the Ni-Ti shape memory alloy with a microstructure including the parent phase tends to be high when the degree of corrosion is not significantly different for the alloy microstructure. Moreover, the effect of corrosion on the tensile strength of Ni-Ti shape memory alloy is significant when the microstructure includes the martensite phase. Hence, the significant degradation of tensile strength observed for specimen B was probably caused by a synergistic effect of hydrogen absorption and corrosion.

Effect of aging at room temperature on hydrogen embrittlement behavior of Ni-Ti superelastic alloy immersed in acidic fluoride solution

It is important to investigate the mechanism for the hydrogen embrittlement of Ni-Ti superelastic alloy in acidic fluoride solutions so that the reliability and safety of the alloy as a dental device can be improved. The purpose of the present study is to investigate the effect of aging at room temperature on the hydrogen embrittlement behavior of Ni-Ti superelastic alloy immersed in acidic fluoride solution. Specimens were immersed separately in 50 mL of 0.2 % acidulated phosphate fluoride (APF) solution with pH 5.0 at room temperature for 16 h. The hydrogen-charged specimens were aged for various periods at room temperature in air to adjust the hydrogen distribution. The specimen that was tested immediately after immersion in 0.2 % APF solution fractured near the critical stress for martensitic transformation without martensitic transformation. The tensile strength of the immersed specimen was partially recovered by aging at room temperature for 24 h. In addition, the tensile strength of the specimen immersed in 0.2 % APF solution was completely recovered by aging at room temperature for 240 h. After aging at room temperature for 24 h, the tensile strength of the immersed specimen was partially recovered. In addition, the tensile strength of the specimen immersed in 0.2 % APF solution was completely recovered by aging at room temperature for 240 h. These results indicate that the mechanism for the hydrogen embrittlement of Ni-Ti superelastic alloys aged at room temperature after immersion in 0.2 % APF solution is dependent on the aging time.

Etching effect of acidic fluorides on human tooth enamel in vitro

ObjectiveThis in vitro study aimed to examine the etching effect of acidic fluoride solutions on enamel. Materials and methods24 human teeth divided into 48 enamel-specimens were partly isolated with impression material. Specimens were exposed for 10min to 20ml of the following solutions: 1.6% TiF4, 3.9% SnF2, 0.2% HF and 1.8% citric acid (CA). The isolation was removed and 24 specimens analysed by profilometry (Δheight: exposed/isolated enamel surfaces, surface roughness parameters). For the remaining 24 specimens [Ca2+] in the test solutions was analysed by atomic absorption spectroscopy. ResultsMedian Δheights (μm) after exposure were: TiF4 0.07, SnF2 −0.03, HF −0.14 and CA −5.92. TiF4-exposed surfaces showed both deposits and etched areas and exhibited statistically significant different surface roughness parameters compared to the HF- and SnF2-exposed surfaces. Median [Ca2+] values (ppm): TiF4 1.88, SnF2 0.11, HF 0.10 and CA 2.17. ConclusionAt the [F] tested in this study it can be concluded that SnF2- and HF solutions had negligible erosive effects on enamel. TiF4 solution resulted in an incomplete surface deposition associated with calcium dissolution suggesting that TiF4 applied as solution may not be advisable.

Effect of acidic fluoride solution on the corrosion resistance of ZrTi alloys for dental implant application

The electrochemical behaviour of Zr5Ti, Zr25Ti, and Zr45Ti, with and without surface modification were monitored in acidic artificial saliva (pH=3) containing NaF concentrations 0.2, 0.5, and 1wt.%, simulating the fluoride concentrations in dental rinses. A passive behaviour for thermally oxidized ZrTi alloys was found using EIS, and XPS data show that the protective oxide film contains both TiO2 and ZrO2, though titanium contents in the outer layer bigger than those in the base alloy result from thermal oxidation. High corrosion resistance to acidic fluoridated environments of ZrTi alloys treated using thermal oxidation in air at 500°C.

Development and validation of an in vitro model for measurements of cervical root dentine permeability

ObjectivesThe aim of this series of studies was the development and validation of a new model for evaluation of dentinal hypersensitivity (DH) therapies.Materials and methodsRoots from extracted human teeth were sealed with a flowable composite. In the cervical area, a 3-mm-wide circular window was ground through the seal 1 mm deep into dentine. The pulp lumen was connected to a reservoir of artificial dentinal fluid (ADF) containing protein, mineral salts and methylene blue. At increased pulpal pressure, the ADF released through the said window was collected in containers each with 20 ml of physiologic saline for a consecutive series of 30-min intervals and ADF concentration (absorption) was determined photometrically. The model was verified by three experiments. In experiment 1, the lower limit of quantification (LLoQ, coefficient of variation = 20 % and difference of 5 standard deviations (SD) from blank) of ADF in physiologic saline was determined by measuring the absorption of 15 dilutions of ADF in physiologic saline (containing 0.625 ng to 12.5 μg methylene blue/ml) photometrically for ten times. In experiment 2, long-term linearity of ADF perfusion/outflow was investigated using 11 specimens. The ADF released through the window was collected in the said containers separately for each consecutive interval of 30 min for up to 240 min. Absorption was determined and analysed by linear regression over time. In experiment 3, perfusion before (2×) and after single treatment according to the following three groups was measured: BisGMA-based sealant (Seal&Protect®), an acidic fluoride solution (elmex fluid®) and control (no treatment).ResultsIn experiment 1, the LLoQ was 0.005 μg methylene blue/ml. In experiment 2, permeability was different within the specimens and decreased highly linearly with time, allowing the prediction of future values. In experiment 3, Seal&Protect® completely occluded dentinal tubules. elmex fluid® increased tubular permeability by about 30 % compared to control.ConclusionsA model comprising the use of artificial dentinal fluid was developed and validated allowing screening of therapeutic agents for the treatment of DH through reliable measurement of permeability of cervical root dentine.Clinical relevanceThe described in vitro model allows evaluation of potential agents for the treatment of DH at the clinically relevant cervical region of human teeth.

Effects of acidic sodium fluoride-treated, commercially pure titanium on periodontal pathogens and rat bone marrow cells

The purpose of the present study was to clarify the potentiality of acidic fluoride solution in treating peri-implantitis. We examined bactericidal activity of fluoride solution against periodontal pathogens; and evaluated the effects of fluoride on titanium, and the effects on cell proliferation and differentiation of rat bone marrow cells on the fluoride-treated titanium. Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis were used to determine minimal inhibitory concentration (MIC) and shorttime exposure. The cells were seeded on the titanium surface with or without fluoride treatment. Then, cellular proliferation, differentiation and mineral deposition were analyzed. The MIC values for A. actinomycetemcomitans and P. gingivalis were 225 and 900 ppm F(-), respectively. In short-time exposure test, both bacterial strains exhibited a significant decrease in a concentrationdependent manner. Cell proliferation and mineral deposition were significantly increased on the fluoride-treated surface. Within the limitation of this study, acidic sodium fluoride solution has the potentiality in treating peri-implantitis.

Corrosion and hydrogen absorption of commercially pure zirconium in acid fluoride solutions

The corrosion and hydrogen absorption of commercially pure zirconium have been investigated in acidulated phosphate fluoride (APF) solutions. Upon immersion in 2.0% APF solution of pH 5.0 at 25°C, a granular corrosion product (Na3ZrF7) deposits over the entire side surface of the specimen, thereby inhibiting further corrosion. In 0.2% APF solution, marked corrosion is observed from the early stage of immersion; no deposition of the corrosion product is observed by scanning electron microscopy. A substantial amount of hydrogen absorption is confirmed in both APF solutions by hydrogen thermal desorption analysis. The amount of absorbed hydrogen of the specimen immersed in the 2.0% APF solution is smaller than that in the 0.2% APF solution in the early stage of immersion. The hydrogen absorption behavior is not always consistent with the corrosion behavior. Hydrogen thermal desorption occurs in the temperature range of 300–700°C for the specimen without the corrosion product. Under the same immersion conditions, the amount of absorbed hydrogen in commercially pure zirconium is smaller than that in commercially pure titanium as reported previously. The present results suggest that commercially pure zirconium, compared with commercially pure titanium, is highly resistant to hydrogen absorption, although corrosion occurs in fluoride solutions.

Dinuclear oxofluorometallates as a new structural type of d 0 transition metal oxofluoride compound

Five isomorphous d(0) transition metal oxofluoride compounds A(3)[M(2)O(x)F(11-x)]·(AF)(0.333) (A = K, Rb, NH(4); M = Nb, Mo, W; x = 2, 4) have been synthesized from acid fluoride solutions, and their crystal structures have been determined by single-crystal X-ray diffraction. The basic structural building units are dinuclear M(2)X(11) (dimers) formed from NbOF(5) or Mo(W)O(2)F(4) octahedra connected by the fluorine bridging atom. In the Nb(2)O(2)F(9) dimer, the O atoms occupy apical corners. In the M(2)O(4)F(7) (M = Mo, W) dimers two O atoms are also apically placed, whereas the other two O atoms are statistically disordered in equatorial planes. The arrangement of dimers is so that the hexagonal tunnels containing `free' fluoride ions are formed. During the irradiation process the orthorhombic structure of K(3)Nb(2)O(2)F(9)·(KF)(0.333) transforms into a pseudo-trigonal one with a = 23.15 Å, which is the [101] diagonal of the orthorhombic unit cell. The other four trigonal crystals are merohedral twins.