Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

The kinetics of redox reactions on some passsive metals and alloys

The polarization resistance of the [Fe(CN)₆]^4??[Fe(CN)₆]^3? redox system on passive metals, Fe, Ni, Cr, the alloys FeCr16, FeCr18Ni9, FeCr18Ni10Mo2Ti and Pt was measured in dependence on the concentration of the redox system and of OH^? ions. The order of the catalytic efficiency of the studied metals was determined from the calculated exchange currents.Electron transfer by tunneling is considered to be the controlling process in the reaction; its rate depends on the composition and thickness of the barrier film and on the surface concentration of the species from which the transfer takes place.The importance of the dependences obtained for corrosion processes is pointed out and examples of the use of these measurements are given for the monitoring of extraction corrosion of alloys, for control of artificial passive films and for the determination of the surface roughness factor of steel.     

Peculiar properties of UMB4 (M = V,Cr, Mo,W) uranium borides

ABSTRACT

UMB₄ (M?=?V, Cr, Fe, Co, Mo, W, Re, Os) materials prepared by arc-melting have UMB₄ as the main phase, with (V, Cr, Fe, Co) crystallising in the YCrB₄ structure type, while (Mo, W, Re, Os) adopt the ThMoB₄-type. For M?=?V, Cr, Mo, W the magnetic susceptibility is low and only weakly temperature dependent, but higher than α-U, which can be ascribed to a higher density of states at the Fermi level. Heat capacity results point to a very high Debye temperature for both structures, which is associated with very high lattice stiffness of the boron network. However, for W and Mo the boron vibrations are accompanied by low-energy Einstein modes coming from vibrations of the atoms inside the boron cages that could lead to superconductivity. The electronic structure of UMB₄ is influenced by a strong 5f-d hybridization and by the B-2p states, which explains the observed weak Pauli paramagnetism.     

Effects of surface structure on the electrochemical properties of Nimetal complex oxide film electrode

Effects of surface structure on the electrochemical properties of Ni—metal complex oxide film electrodes prepared by radio frequency magnetron sputtering method and by thermal decomposition method have been investigated. Rotating disc electrode technique was applied to evaluate the electron transfer rate of the redox system [Fe(CN)₆]^3?/[Fe(CN)₆]^4? in 0.1 M NaOH solution. Dynamic impedance method to detect the impedance change of electrode/electrolyte interface and X-ray photoelectron spectroscopy were also used. The NiMo complex oxide film electrode prepared by rf magnetron sputtering method was found to have good stability, good reproducibility and effective electrochemical properties in comparison with the oxide film electrodes prepared by thermal decomposition method. Then, of the electrodes prepared by thermal decomposition method, the NiMo complex oxide film and the NiRe complex oxide film were shown to have good electrochemical properties. It was found by XPS analysis that both Mo ion at a higher oxidation state and oxygen influenced from this Mo ion, which showed clear occurrence for the NiMo oxide film prepared by rf sputtering method and no occurrence by thermal decomposition method, played a very important role in the reaction.     

W6M05Cr4V2模具钢电化学抛光工艺研究

本文采用电化学方法对W6M05Cr4V2电抛光工艺进行了研究。抛光溶液采用无铬酸盐的磷酸-硫酸体系添加高分子聚乙二醇。研究结果表明，聚乙二醇能有效地形成粘膜，明显提高W6M05Cr4V2电抛光的效果。该工艺减少了环境污染、降低了溶液成本，可以使W6M05Cr4V2表面达到镜面光亮的效果。     

Trace metal distribution in fractions of solvent-refined coal by ICP-OES and implications regarding metal speciation

Separations of Amax solvent-refined coal according to its acid/base/neutral components and by selective elution from a silica column (SESC) to yield nine fractions have been carried-out. After ashing with H₂O₂-H₂SO₄, twenty metals (Mg, Al, P, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Mo, Cd, Sr, Zr, W, Hg and Ba) were analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). A majority of elements (Zn, Cd, Hg, Cr, Mo, W, and Fe) were found concentrated in the acid fraction while Co, Ni and Cu preferred the neutral and base fraction. In the step gradient SESC fractions, the greatest concentration of metal was found in fractions that had been characterized as enriched with phenols.     

Electrochemical behavior of current collectors for lithium batteries in non-aqueous alkyl carbonate solution and surface analysis by ToF-SIMS

Several metals (Cu, Fe, Al, Ti, and Cr) as current collector for lithium-ion battery were investigated to understand their electrochemical behavior and passivation process in a non-aqueous alkyl carbonate solution containing LiPF₆ salt. From cyclic voltammetric study, it was found that Cu and Fe metals were dissolved into the electrolyte below 4 V vs. Li/Li⁺. Alternatively, Al and Ti were stable up to 5 V vs. Li/Li⁺. Their scratched surfaces at 5 V vs. Li/Li⁺ were polarized in a transient mode and it was found that the surfaces were passivated during the polarization test. Formed passive film was composed of two hybrid layers: outer layer by metal (Al and Ti) fluoride and inner by metal oxide, as confirmed by time-of-flight secondary ion mass spectroscopy. Presence of HF in the electrolyte was indispensible to form the metal fluoride layer on the oxide layer. The outer fluoride layer would protect the inner oxide layer and metal substrate from HF attack, bringing about satisfactory corrosion resistance under lithium-ion battery environment.     

Electrochemical nucleation and growth of copper on chromium-plated electrodes

Nucleation and growth of copper electrodeposited on chromium plated electrodes in copper sulfate electrolytes were examined, focusing on the influence of prior Cr plating conditions on the nucleation density and growth kinetics of the copper electrodeposits. The Cr-plated electrodes were made by electrodeposition of Cr on copper sheets for 2 to 60 s at 0.1 A cm^–2 in CrO₃ 350 g L^–1 + H₂SO₄ 3.5 g L^–1. Copper was then electrodeposited onto the Cr-plated electrode under potentiostatic conditions. Copper initially nucleated and grew according to a three-dimensional diffusion controlled progressive nucleation process, and later according to an instantaneous nucleation process. The period during which copper nucleation is controlled by the diffusion controlled progressive nucleation process decreases with increasing Cr plating time. The nucleation density of copper was extremely high on the 2 s Cr-plated electrode, producing an extremely fine and uniform electrodeposit. However, on the 4 s Cr-plated electrode, the nucleation density of copper significantly reduced to one hundredth of that on the 2 s Cr-plated electrode, and then decreased slightly with increasing Cr plating time (thickness of Cr layer). These results appear to be associated with the IR drop across the Cr layer, including the surface Cr oxide/hydroxide film (termed the cathode film), which significantly reduces the driving force for the electrodeposition of copper under potentiostatic plating conditions.     

Elastic and electronic properties of XB2 (X=V,Nb, Ta,Cr, Mo,and W) with AlB2 structure from first principles calculations

The crystal structure, electronic properties, mechanical properties, and anisotropy of XB₂ (X=V, Nb, Ta, Cr, Mo, and W) were calculated by first principles calculations based on density functional theory (DFT) with the generalized gradient approximation (GGA). The results are in good agreement with available theoretical and experimental values. The calculated cohesive energy and formation enthalpy indicate that they are thermodynamically stable structures. The elastic constants satisfy all of the mechanical stability criteria. The mechanical moduli were predicted by the Voigt–Reuss–Hill approximation. The mechanical anisotropy was indicated by the surface constructions of Young's moduli, and the results show that anisotropy of WB₂ is stronger than others. The electronic structure indicates that the bonding behaviors of XB₂ (X=V, Nb, Ta, Cr, Mo, and W) are the combinations of covalent and metallic bonds. The hardness of the borides was also evaluated, and the result reveals that TaB₂ is the hardest compound among them.     

The anodic properties of FeCr alloys in organic solvents

The influence of the water concentration on the passivation of FeCr alloys in sulfuric acid—organic solvents solutions has been investigated with the aid of potentiodynamic curves of the anodic polarisation. The dependence of the passivation potential or the break potential from the chromium concentration in the alloys and the water concentration in the solution has been determined. At low water concentrations the passivation potential decreases with increasing water content. This fact can be accounted for by the strengthening of the electrolyte structure and the resulting decrease of the solvolytic capabilities of solvents. In this range of water concentrations the salt—hydroxid type of the film is formed on the surface of the alloy. The increasing concentration of chromium in the alloy facilitates the formation of the films. At high water concentrations, when water molecules are used for the solvation of cations, only the alloys containing more than 18 per cent of Cr can form a stable anodic film below the passivation potential of iron. The passivation potential for the alloys which contain lower amounts of chromium has been found to increase as the water concentration increases. This is due to the fact that Fe⁺² compounds from the anodic layer are exhibiting higher solubility in the solutions.     

Formation of Dy-Fe alloy films by molten salt electrochemical process

The electrochemical formation of Dy-Fe alloy films was investigated in a molten LiCl-KCl-DyCl₃ (0.50 mol%) system at 773 K. The deposition potential of Dy metal was 0.47 V (vs. Li⁺/Li) at a Mo electrode. Repetition of the potential sweep treatment at a fresh Fe electrode was effective in increasing the rate of formation of Dy-Fe alloys. Using an Fe electrode activated by repetition of the potential sweep treatment, a DyFe₂ film was formed by potentiostatic electrolysis at 0.55 V. However, the alloy film was thin and not adhesive. An adhesive DyFe₂ film was formed at the activated Fe electrode by potentiostatic anodic electrolysis at 0.55 V after cathodically electrodepositing Dy meal at 0.40 V. By using a similar procedure, Dy₆Fe₂₃ was formed at 0.68 V. The equilibrium potential for (2/11)Dy₆Fe₂₃+Dy(III)+3e⁻?(23/11)DyFe₂ was estimated as 0.62 V.     

Investigation of a transparent chromate (III) passive film on electroless Ni–P coating by XPS and electrochemical methods

A transparent passive film on electroless nickel phosphorus coating (ENPC) was obtained in a chromate (Cr³⁺) bath. An ENPC sample passivated in a Cr⁶⁺-containing bath was used for comparison. The corrosion properties were tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). SEM, XPS, and EDX were employed to analyze the chemical composition and surface morphology of the films. SEM results indicate that the passive film is too thin to be observed by SEM. The potentiodynamic polarization tests show that the corrosion currents of the two passivated samples are only about 1/25 of the un-passivated coating. The XPS analysis illustrate that the Cr⁶⁺-treated film comprises Cr, Ni and O, while the Cr³⁺-treated film is made up of Cr, C, O, Ni, P and N. High-resolution XPS analysis show that, both in Cr⁶⁺-treated and Cr³⁺-treated films, element Cr is only in the form of trivalent compounds, no hexavalent chromium existing. By XPSpeak analyzing, trivalent chromium compounds on the two treated surfaces were fitted as Cr(OH)₃ and Cr₂O₃. However, for the Cr³⁺ and Cr⁶⁺ passivation routes, the Cr(OH)₃ and Cr₂O₃ contents in passive films are widely divergent.     

Photoelectrochemical analysis on the passive film formed on Fe-20Cr in pH 8.5 buffer solution

The electronic properties of passive film formed on Fe-20Cr stainless steel in pH 8.5 buffer solution were examined by the photocurrent measurements of the film. The passive film on Fe-20Cr exhibited an n-type semiconductor, which was confirmed by anodic photocurrent. The photocurrent spectrum for the passive films formed on Fe-20Cr was almost same in shape to that for the passive film on Fe except for the large difference in photocurrent intensity, which demonstrated that the passive film on Fe-20Cr is composed of Cr-substituted γ-Fe₂O₃ involving the d-d and p-d electron transitions. However, the large difference in photocurrent intensity for passive film between Fe and Fe-20Cr was due presumably to the fact that Cr³⁺ ions in passive film act as effective recombination sites of electron-hole pairs. The abrupt increase in the photocurrent intensity for the passive film formed on Fe-20Cr at a transpassive potential is due presumably to the decrease in the recombination site resulting from the reduction of Cr³⁺ content accompanying the oxidation of Cr³⁺ to Cr⁶⁺ in transpassive region.     

Electrochemical behavior of Ti-20Cr-X alloys in artificial saliva containing fluoride

In this study, electrochemical investigation was carried out on commercially pure titanium (c.p. Ti) and as-cast Ti-20Cr-X (X = Nb, Mo, Fe, and Cr) in fluoride solutions containing artificial saliva. Open-circuit potential (OCP) and potentiodynamic polarization measurement were used to characterize the electrochemical behavior, and X-ray photoelectron spectroscopy (XPS) was used to characterize the composition of the passive films on the samples. The OCP results indicated that all specimens presented spontaneous passivation. XPS results showed that pre-test, a passive film consisting of TiO₂ and Cr₂O₃ formed on the surface of the Ti-20Cr-X alloys, and Na₂TiF₆ formed on the surface of all specimens after the polarization test in artificial saliva with 0.5 wt% NaF. Although the Ti-20Cr-X alloys exhibited a Na₂TiF₆ film on their surface, their corrosion rate was still lower than that of c.p. Ti. In particular, Ti-20Cr-Mo alloy exhibited the lowest steady state current density compared to c.p. Ti and the other Ti-20Cr-X alloys. All these results suggest that the Ti-20Cr-X alloys hold promise for application as dental materials in a natural oral environment where NaF may be present.     

Leaching characterisation and geochemical modelling of minor and trace elements released from recycled concrete aggregates

The pH dependent release of Cd, Cr, Cu, Mn, Mo, Ni, Pb, V and Zn from different recycled concrete aggregate samples was determined. Geochemical speciation modelling was applied on the concentrations of Cu, Cr, Mo and Ni in the leachates in order to predict the measured concentrations and the specific release mechanisms. The model was able to reproduce the characteristic pH dependent release patterns for these elements and reasonable to sometimes excellent matches between the predicted and measured concentrations were achieved. Binding of Mo and Cr as oxyanions (MoO₄²⁻ and CrO₄²⁻) to ettringite was modelled with fair agreement for Cr only. For Cu and Ni, the predicted and measured concentrations agreed well for the partly carbonated sample at high alkaline pH (11–13). The importance of complexation to humic substances was also shown in samples derived from construction debris.     

Aspects of electrochemical behaviour of carbon steel in different Bayer plant solutions

Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 °C, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous film. The pore resistance model describes the properties of the oxide films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe_3–x Al _x O₄. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)₃ ^– ion rather than HFeO₂ ^– ion. Moreover, the Al(OH)₄ ^– ion was considered instead of AlO₂ ^– ion in the oxidation process.     

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H₂O₂ of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ～ 2, 1 M H₂O₂, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H₂O₂. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm²) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.     

Lattice stability,mechanical and thermal properties of a new class of multicomponent (Fe,Mo, W)6C η carbides with different atomic site configurations

Transition metal carbides are candidates for high-temperature structural ceramics because of their high melting point, high hardness, and high strength. However, one challenge is overcoming their high intrinsic brittleness. In this study, we investigated a new class of (Fe, Mo, W)₆C carbides, which have three Wyckoff positions for metallic atoms (16d, 32e, and 48f) and one Wyckoff position for carbon (16c). These different Wyckoff positions provide a great opportunity to optimize the mechanical properties by the partial replacement of atoms at each Wyckoff position to obtain high-entropy carbides. The current results show that the phonon spectra have no imaginary frequency when Fe occupies the 16d or 32e positions, but a soft mode is observed when Fe occupies 48f. (Fe, Mo, W)₆C η carbides have a higher fracture toughness compared with those of M₃C and MC carbides owing to their low carbon content (14.3 at.%). The mechanical anisotropy of (Fe, Mo, W)₆C is weak, which is beneficial for increasing the damage tolerance. The thermal expansion coefficients of the (Fe, Mo, W)₆C η carbides are predicted to be approximately (8.5–9.5) × 10^?6 K^?1 at 1400 K.     

Effect of alternating voltage passivation on the corrosion resistance of duplex stainless steel

Potentiodynamic polarization and E _corr versus t curves were obtained, together with electrochemical impedance spectroscopy (EIS) measurements, in order to understand the effects of alternating voltage (AV) passivation on the corrosion resistance of duplex stainless steel (DSS). SEM, EDS and XPS were employed to further investigate the influence of AV passivation on the properties of the passive film. The results of the electrochemical measurements showed that AV passivation significantly improved the corrosion resistance of DSS. SEM images indicated that the surface exhibited a unique morphology after AV passivation treatment, and XPS results suggested that AV passivation greatly increased the thickness of the passive film. Furthermore, significant chromium enrichment and a higher ratio of Fe³⁺/Fe²⁺ were observed in the passive film after AV passivation. Mott–Schottky results confirmed that AV passivation had a strong influence on the semiconducting properties of the passive film.     

Surface layer formation on Sn anode: ATR FTIR spectroscopic characterization

Surface layer formed on Sn thin film electrode in 1 M LiPF₆/EC:DMC electrolyte was characterized using ex situ FTIR spectroscopy with the attenuated total reflection technique. IR spectral analyses showed that the immersion of Sn film in the electrolyte resulted in a chemical interfacial reaction leading to the passivation of Sn surface with primarily PF-containing inorganic surface species and small amount of organics. When constant current cycling was conducted with lithium cells with Sn film electrode at 0.1-1.0 V vs. Li/Li⁺, the interfacial reaction between Sn and electrolyte appeared significantly intensified that the features of PF-containing species became enhanced and new IR features of organic species (e.g. alkyl carbonate/carboxylate metal salts and ester functionalities) were observed. The surface layer continued to form with cycling, partly due to non-effective surface passivation as well as particle pulverization accompanied by enlargement of active surface area. Comparative IR spectral analyses indicated that the interfacial reaction between Sn and PF₆⁻ anion played a leading role in forming the surface layer, which is different from lithiated graphite that had mainly organic surface species. The data contribute to a better understanding of the interfacial processes occurring on Sn-based anode materials in lithium-ion batteries.