Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C

New Pourbaix diagrams were calculated at 25, 75 and 95 °C for the Nb-H₂O system. The species and were considered. Potentiodynamic polarization and mass loss experiments (14 days) were conducted in concentrated H₂SO₄ (20, 40 and 80 wt%) and HCl (20 and 38 wt%) solutions at 75 and 95 °C. Nb forms a metastable pentoxide (Nb₂O₅) in H₂SO₄ and HCl solutions which dissolves as . Corrosion rates decrease between the 40% and the 80% H₂SO₄ solutions. SEM micrographs show generalized pitting in the 20% and 40% H₂SO₄ solutions. Mass loss corrosion rates did not exceed 306 μm/yr. Corrosion rates estimated by Tafel extrapolation were within two orders of magnitude of those measured by mass loss and it is shown that this finding is consistent with the thickening of the oxide.     

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl₂ (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl₂ solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.     

Enhancement of corrosion resistance of Mg-9 wt.% Al-1 wt.% Zn alloy by a calcite (CaCO3) conversion hard coating

A calcite (CaCO₃) coating on Mg alloy, formed by chemical conversion treatment, was investigated. Aqueous with Ca²⁺ concentration of ∼220 ppm was employed in the chemical conversion treatment. Cross-sectional microstructures of the coated sample after 2 h of treatment revealed a two-layer coating structure. The corrosion current density (I_corr) of the coated sample was approximately two orders of magnitude lower than that of the untreated sample. Electrochemical impedance spectroscopy (EIS) and an appropriate equivalent circuit suggested that each of the layers of the two-layer coating effectively protects Mg alloy against corrosion.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Thermal stability of heavily drawn Cu-0.4 wt.%Cr-0.12 wt.%Zr-0.02 wt.%Si-0.05 wt.%Mg

Cu-0.4 wt.%Cr-0.12 wt.%Zr-0.02 wt.%Si-0.05 wt.%Mg was prepared by casting, quenching, aging and cold drawing. The microstructure was studied by electron microscope and X-ray diffraction. Vickers hardness was measured for the alloy after the annealing treatment at different temperatures covering a wide temperature range from room temperature to 700 °C. The ribbonlike structure is replaced by gross equiaxed grains. The crystal orientation is gradually approaching the full annealed specimen and the hardness difference between longitudinal and transverse directions vanishes by recovery and recrystallization. The thermal analysis was carried out and the stored energy was calculated. The release of stored energy and the reduction of resistivity are primarily due to the decrease of dislocation density. The main strengthening effect is attributed to dislocation mechanism.     

Photoelectron spectroscopy study of the inhibition of mild steel corrosion by molybdate and nitrite anions

Passive films formed on mild steel in aqueous 8.6 mM NaCl solutions (pH 8), containing either or , have been studied with X-ray photoelectron spectroscopy. For either anion these films are ∼5 nm deep, and the primary chemical state of iron is Fe³⁺. Following exposure to , the film consists of a sub-layer (∼4.1 nm) composed largely of ferric oxide/hydroxide, overlaid by Fe₂(MoO₄)₃ (∼0.6 nm). As regards , spectra are consistent with the film being closely related to γ-Fe₂O₃. Furthermore, a reduction product of , potentially N₂, is present, displaying a depth profile comparable to that of molybdate.     

Structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70−xNd_x)Sr_0.30Mn_0.70Cr_0.30O₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x = 0, 0.10 and 0.20, while for x = 0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T_C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M_S values at 5 T and 2 K are, respectively, 47.9, 47.3 and 47.5 emu/g for x = 0.10, 0.20 and 0.30, compared to 48.2 emu/g for x = 0), indicating that the Nd³⁺ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn⁴⁺–O–Cr³⁺and Cr³⁺–O–Cr³⁺ is responsible for the semi-conducting state.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Investigation on microstructure characterizations and phase compositions of rapidly solidification/powder metallurgy Mg-6 wt.% Zn-5 wt.% Ce-1.5 wt.% Ca alloy

Rapidly solidification/powder metallurgy (RS/PM) Mg-6Zn-5Ce-1.5Ca (wt.%) alloy in form of rods has been prepared by hot extrusion with RS powders, produced via atomizing the alloy melt and subsequent splat-quenching on the water-cooled copper twin-rollers. Microstructure characterizations and phase compositions of the alloy have been investigated. The results showed that the RS/PM Mg-6Zn-5Ce-1.5Ca alloy was the characteristics of very fine grains with the size ranging from 200 to 650 nm and was composed of α-Mg, Mg_xZn_yCe_z phase with a few Ca (about 1 at.%) shortened as the Mg_xZn_yCe_z-(Ca) phase, and a small quantity of Mg₅₁Zn₂₀ phases. The melting point of the Mg_xZn_yCe_z-(Ca) phase in the alloy was a little higher than that of the Mg_xZn_yCe_z phase in RS/PM Mg-6Zn-5Ce alloy, which may be due to the dissolving of Ca in it. Moreover, the atomic percentage ratio of Zn to Ce in the Mg_xZn_yCe_z-(Ca) phase around the grain boundary was close to 1.5.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Microstructure characterization of as-fabricated and 475 °C annealed U-7 wt.% Mo dispersion fuel in Al-Si alloy matrix

High-density uranium (U) alloys with an increased concentration of U are being examined for the development of research and test reactors with low enriched metallic fuels. The U-Mo fuel alloy dispersed in Al-Si alloy has attracted particular interest for this application. This paper reports our detailed characterization results of as-fabricated and annealed (475 °C for 4 h) U-Mo dispersion fuels in Al-Si matrix with a Si concentration of 2 and 5 wt.%, named as “As2Si”, “As5Si”, “An2Si”, “An5Si” accordingly. Techniques employed for the characterization include scanning electron microscopy and transmission electron microscopy with specimen prepared by focused ion beam in situ lift-out. Fuel plates with Al-5 wt.% Si matrix consistently yielded thicker interaction layers developed between U-Mo particles and Al-Si matrix, than those with Al-2 wt.% Si matrix, given the same processing parameters. A single layer of interaction zone was observed in as-fabricated samples (i.e., “As2Si”, “As5Si”), and this layer mainly consisted of U₃Si₃Al₂ phase. The annealed samples contained a two-layered interaction zone, with a Si-rich layer near the U-Mo side, and an Al-rich layer near the Al-Si matrix side. The U₃Si₅ appeared as the main phase in the Si-rich layer in “An2Si” sample, while both U₃Si₅ and U₃Si₃Al₂ were identified in sample “An5Si”. The Al-rich layer in sample “An2Si” was amorphous, whereas that in sample “An5Si” mostly consisted of crystalline U(Al,Si)₃, along with a small fraction of U(Al,Si)₄ and U₆Mo₄Al₄₃ phases. The influence of Si on the diffusion and reaction in the development of interaction layers in U(Mo)/Al(Si) is discussed in the light of growth-controlling mechanisms and irradiation performance.     

Strong correlation between structural, magnetic and transport properties of non-stoichiometric Sr2FexMo2−xO6 (0.8 ≤ x ≤ 1.5) double perovskites

Sr₂Fe_xMo_2−xO₆ (x = 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5 wt.%) (SFMO) double perovskite oxides of different compositions have been prepared by sol-gel method. These materials were subjected to X-ray diffraction and found that crystal structure changes from tetragonal to cubic around x = 1.2 wt.%. Lattice parameters and unit cell volume have been calculated using X-ray diffraction data. Magnetization studies have been carried out using Vibrating Sample Magnetometer ranging from −15 kOe to +15 kOe and saturation magnetization (M_s) has been determined. Electrical resistivity and magnetoresistance studies have been carried out in the magnetic field range of −40 kOe to +40 kOe keeping the temperature constant at 5, 150 and 300 K using standard four-probe method. Resistivity studies have also been carried out in the temperature ranging from 5 to 300 K keeping the magnetic field constant at 0, 10, 20 and 40 kOe. Maximum degree of Fe/Mo ordering (η_max) of SFMO has been calculated and compared with magnetic and transport properties. It has been found that there is a strong correlation between 3 parameters η_max, M_s and MR (%), i.e. all of them show a maximum at x = 1.0 wt.% and decreases as x deviates from 1.0 in SFMO. It has been also found that there is a different resistivity behavior between x ≤ 1.2 wt.% and x > 1.2 wt.% samples of SFMO. Semiconductor metal transition temperature was found to be maximum at x = 1.0 wt.%.