Corrosion behaviour of some stainless steel alloys in molten alkali carbonates (I)

In the present work it is aimed to study the corrosion behaviour of two types of stainless steel alloys (one ferritic and two austenitic) in molten Li₂CO₃- Na₂CO₃- K₂CO₃ mixture. This mixture is of interest in corrosion studies because of its low melting point (397°C) and good electrical properties. In this investigation the following techniques of measurements are used: (i) open circuit-potential, (ii) galvanic current, (iii) impedance, (iv) atomic absorption spectroscopy for the determination of the amount of metals dissolved in the melt (v) corrosion tests, carried out on the oxide scales formed during the oxidation of stainless steel alloys in carbonate melt. In this melt the electrode Ag/AgCl was used as a reference electrode. In molten carbonates, the oxide ions originate by self-dissociation according to the equilibrium CO₃ ^2– CO₂ + O^2–. The oxide ions, O^2–, and carbonate ions, CO₃ ^2–, play an important role in the oxidation process of these alloys and their passivation in the carbonate melt. As previously mentioned in references it can be assumed that the oxide scales formed on the alloy surface consist mainly of LiCrO₂ and LiFeO₂. The cathodic path of the corrosion process may be the reduction of CO₂ and/or CO₃ ^2–. The resistance of alloys against corrosion in melt increases with the increase of temperature. This may be due to the increase of concentration of O^2– and CO₂, enhancing both the anodic and cathodic reactions. The activation energy was calculated and found to be 91.496, 23.412 and 37.956 kJ/mol for the alloys 1, 2 and 3 respectively. The above mentioned techniques of measurements showed that the oxide scales of the austenitic stainless steel alloys (2, 3) are more passive and protective than of ferritic stainless steel alloy (1). This means that the resistance against corrosion, in the carbonate melts, of austenitic stainless steel alloys is higher than that of ferritic one.     

Corrosion Behavior of α-Fe and 20kh13 Steel in Contact with Oxygen-Containing Lead Melts

We study the structure and chemical composition of the reaction products formed in the process of contact of -Fe and 20Kh13 steel with oxygen-containing stagnant lead melts (600–700°C, 3000 h, C _O[Pb] (1–4) · 10^–5 wt.%). It is shown that a heterogeneous structure is formed in the interaction zone. This structure consists of an external intermediate layer (with low hardness, the same composition as the matrix, and lead accumulated on the grain boundaries) and a thin oxide film (Fe₃O₄ for -Fe and FeCr₂O₄ and Cr₂O₃ for 20Kh13 steel), which separates the intermediate layer from the internal porous suboxide layer of the matrix and blocks the process of penetration of lead into the matrix.     

Effects of surface oxide layers on crack initiation and growth of HSLA steel under cyclic loading in air and in ultrahigh vacuum

Effects of the thermally grown wustite on the fatigue crack initiation and growth in HSLA steel are evaluated as a function of oxide thickness, strain amplitude, and gaseous environment in the push-pull plastic strain control mode, with special attention being given to the early stage of microcrack initiation. Specimens with a wustite surface layer thermally grown to 0.2 and 0.6 m thicknesses show predominantly intergranular cracking at plastic strain amplitudes of 5×10^–4 and 1×10^–3 both in air and in ultrahigh vacuum (UHV), in contrast to the as-polished specimens where slip band cracking is the favoured mode. The cracking mode in the oxide layer is discussed in terms of the strain amplitude and the dislocation behaviour near the oxide/metal interface. The features of microcrack initiation in the oxide layer is not affected by the gaseous environment. Once, however, the surface oxide fractures, the rate of crack growth through the base metal is greatly reduced in UHV.     

Coating of CaTiO3 on titanium substrates by hydrothermal reactions using calcium-ethylene diamine tetra acetic acid chelate

Titanium plates were treated in [Ti(O₂)EDTA]^2-– -Ca(EDTA)^2- mixed solutions and/or Ca(EDTA)^2- solutions (where EDTA is ethylene diamine tetra acetic acid) at pH 9–13 and 150–250 °C for 0.5–12 h. The film, about 50 m thick, and consisting of mixtures of CaTiO₃ and TiO₂ was formed in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution at pH 13 and 250 °C for 6 h. The film consisted of large icosahedral and hexagonal particles, of about 10 m diameter, and small aggregated particles, of about 1 m diameter. On the other hand, the film, about 20 m thick, consisted of hexagonal plate-like CaTiO₃ particles, of about 1 m diameter, was formed in 0.01 M Ca(EDTA)^2- solution at pH 13 and 250 °C for 6 h. The thickness of both films increased with time, where the film formation rate in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution was much faster. The CaTiO₃ film formed on the surface of titanium promoted the precipitation of hydroxyapatite on the substrate by the hydrothermal reactions in Ca(EDTA)^2-–PO ₄ ^3- mixed solutions.     

Abrasive wear of thick silica films prepared by vacuum evaporation

We investigate the wear behaviour of thick silica films 2–3m thick evaporated on a polymethylmethacrylate substrate in vacuum. The wear test is performed with kaolin mineral powder suspended in water. Abrasive wear is not affected by silica film hardness varying from 100 to 550 kg mm^–2, which corresponds to film preparation pressures ranging from 5×10^–3 to 5×10^–5 torr. The wear characteristics and the appearance of the wear tracks are strong evidence that abrasive wear of the deposited silica films consists of the mechanical and molecular removal of a hydrolysed silica film surface by abrasive kaolin grains. This is very similar to the case of glass polishing.     

Titanothermal reduction of oxides on iron and steel under conditions of self-evacuation

We study the process of contactless titanothermal reduction of oxides on Armco iron and Kh18N10T steel. The kinetic curves of oxide films thinning on iron and steel obtained in the work reveal two different mechanisms of these processes. Oxide reduction on iron runs at lower temperatures and the kinetic curves are characterized by a certain incubation period for which no oxide film thinning is observed. The appearance of this period can be explained by two processes that run either in the diffusion mode or in the kinetic mode. The contactless titanothermal reduction of oxide films on steel occurs at higher temperatures and oxide films are not completely reduced. Moreover, as temperature increases, its thinning becomes slower. The data of X-ray analysis indicate the presence of titanium sublimation with subsequent condensation on the steel surface. In this case, the composition of the film on the steel surface does not change.Paton Electrical Welding Institute, Ukrainian Academy of Sciences, Kiev. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 2, pp. 84–90, March–April, 1994.     

Oxide Protection of Materials in Melts of Lead and Bismuth

Corrosion and mass transfer in lead–bismuth flow circuits proceed in the range of concentrations of dissolved oxygen of 5·10^–7–10^–12 mass %. We have developed and implemented a technology of continuous oxide protection of materials against corrosion in lead–bismuth nonisothermal flow circuits with the heat carrier mass up to 70 tons, the surface area of steels up to 2000 m², and temperatures up to 650°C under conditions of control, regulation, and maintenance of the oxidation potential of a heat carrier in flow circuits.     

Effect of Nb,Mo, V contents on oxidation resistance of Ti3Al based alloys

In the present paper, the effect of the contents of Nb, Mo, V on the oxidation properties (700°C, in air) of Ti₃Al based alloys has been studied. It has been shown that the alloys were oxidized rapidly as exposed at 700°C in the air. After 100 h exposure, oxygen-affected alloy surface layer of about 10 thickness has been formed on account of the poor protection of the oxide film. An addition of (11–13%)¹ Nb enhanced the oxidation resistance. The addition of Mo and V in the Ti₃Al–Nb system alloy reduced the oxidation resistance significantly.     

Surface free energy of nickel and stainless steel at temperatures above the melting point

The surface tension (liquid-state surface free energy) of pure nickel and type 304 stainless steel was measured in a narrow temperature range above the melting point by the sessile drop method. The temperature coefficients of surface free energy in the liquid state were found to be –1.76 erg cm^–2 C^–1 for nickel and –2.48 erg cm^–2 C^–1 for stainless steel. These values are shown to be a factor of 2 larger than those previously determined for the solid surface free energies of nickel and stainless steel, but have the same sign. The latent heats of fusion of nickel and 304 stainless steel were determined by comparison of variations of solid and liquid-state surface energies with temperature and found to be 292 and 284 erg cm^–2 respectively. Measurement of the contact angle for a nickel sessile drop on thoria and a stainless steel sessile drop on alumina showed a decrease in the angle with an increase in temperature.     

Estimation of the unknown parameters in the melt-spinning process

The free-surface temperature history of the melt spinning of copper measured by Tenwick and Davies [3] is compared with those calculated using a thermokinetic model assuming different parameters. The heat-transfer coefficient, nucleation temperature and the crystal-growth kinetics were thus estimated for the melt spinning of copper at a wheel speed of 35 ms^–1 as follows: heat-transfer coefficient during liquid cooling stage HL=1.0 × 10⁷ W m^–2K^–1, heat-transfer coefficient after solidification finished HS=1.0 × 10⁵ W m^–2K^–1, heat-transfer coefficient during solidificationH= 1.0 x 10⁷- 1.2 x 10¹¹ (t-t _n) (W m^–2K^–1), the nucleation temperatureT _n 1233 K and the crystal-growth kinetic lawV=4.0 × 10^–3 T^1.1 (ms^–1).     

Bias voltage dependence properties of cadmium oxide films deposited by d.c. reactive magnetron sputtering

Cadmium oxide films were grown on glass substrates using d.c. reactive magnetron sputtering technique by sputtering from a metallic cadmium target in an oxygen partial pressure of 1×10^–3 mbar under various substrate bias voltages. The substrate bias voltage significantly influences the crystallographic structure of the deposited films. The influence of substrate bias voltage on the electrical and optical properties of the films was systematically studied. The films formed at a substrate temperature of 473 K and bias voltage of –80 V showed an electrical resistivity of 1×10^–3 cm, optical transmittance of 86%, optical band gap of 2.47 eV and a figure of merit of 7×10^{–3 –1}.     

Precision submillimeter wave laser reflection measurements of high-temperature superconductors

Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 . Assuming anf ² frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10^–5 at 10 GHz. Capability for 10^–7 sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm.     

Effects of lithium addition and wheel velocity on the microstructure of aluminium-lithium alloy ribbons

The effects of Li addition and wheel velocity on the microstructure of as-cast AI-Li ribbons were studied by optical metallography, scanning and transmission electron microscopy and by X-ray diffraction. Li addition has a marked effect on the ribbon solidification mechanism. Ribbons spun at 12.5 ms^–1 and which contained up to 1.79 wt% Li experienced a cooling rate of about 10⁴ Ks^–1 and solidified under the melt jet with a growth rate greater than 0.3 m s^–1. However, when Li content exceeded about 3 wt%, the ribbons were undercooled (cooling rate 10⁶ Ks^–1) and formed far from the melt puddle effect. Under these conditions, unusual secondary featureless zones occurred through the ribbon thickness and the detected phases corresponded to those expected to be formed under equilibrium conditions. Nevertheless, (AI₃Li) phase did not occur in ribbons containing up to 1.79 wt % Li or in the featureless zones of the most Li-rich ribbons (Li >about 3 wt%). Changing of the wheel velocity from 12.5 to 22.2 or 35.7 ms^–1 did not affect the ribbon formation mechanism, but it favoured the precipitation and decreased slightly the grain size.     

Influence of a lead melt on plastic deformation of high-alloyed heat-resistant steels

The effect of liquid-metal embrittlement in lead melt is typical of Kh12MVSFBR ferrite-martensitic steel and is observed in the temperature range of 300 – 500°C at a deformation rate of 8 · 10^–4 sec^–1. The maximum decrease in the plasticity of tempered specimens occurs at a temperature of 450°C. A decrease in the amount of -ferrite in the composition (martensite hardening without high-temperature annealing) results in a substantial decrease in plasticity and in broadening of the temperature range of embrittlement. 08Kh16N11S3MB austenitic steel is not affected by liquid metal embrittlement in the investigated temperature range for the deformation rates under consideration.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 71–76, July – August, 1994.     

Application of diamond films from CO-H2 plasma to tool blade coating

Diamond films were coated on WC-Co alloy tool blades using a microwave plasma of CO-H₂ mixed gas. Diamond film prepared at a CO concentration of 10% had good properties: Vickers hardness % 8500 kg mm^–2 and adhesion force 1.7 kg mm^–2. No peel-off of the diamond film was observed after a cutting test of an Al-Si alloy rod at a cutting speed of 450 m min^–1 for 0.5 h. It is thought that the high quality of the film originated from the selective etching of cobalt from the tool blade surface by CO-H₂ plasma.     

Effect of thermal annealing on transparent conductive LaNiO3 thin film prepared by an aqueous method

Dense, crack-free and uniform lanthanum nickel oxide (LNO) thin films were prepared by an aqueous method on various substrates, such as single crystal silicon, microcrystalline glass ceramic (GC) and amorphous glass. The effects of various thermal annealing temperatures on the microstructure, interface and electrical properties of the LNO films were investigated by XRD and SEM with the EDX and a four-probe method, respectively. It was found that with the increase in thermal annealing temperature, the LNO film on Si substrates displayed a structure change from pseudocubic to rhombohedral and was accompanied by the appearance of a NiO impure phase, while the LNO film on a GC substrate diffused into the substrate. In these cases, the film resistivity was increased. As a result, a LNO thin film with a resistivity of 2–3 × 10^–5 · m was achieved by thermally annealing at 750–800°C for 1 hour in air. The measurement of the surface resistance under different temperatures shows that the LNO film possesses better high temperature stability. Its transmittance spectrum was also observed.     

Rapid electroplating of photocatalytically highly active TiO2-Zn nanocomposite films on steel

Nanocomposite films consisting of TiO₂ and Zn with thickness of 10–15 m (TiO₂-Zn) have been electrodeposited on steel plates by rapid plating from a ZnSO₄-based bath (I _d > 10 A dm^–2). Upon addition of NH₄NO₃ to the bath (<0.3 g L^–1), the uptake of TiO₂ in the film significantly increased. Glow discharge optical emission spectrometry clarified that TiO₂ particles were incorporated throughout the film and the loaded amount increased near the surface. The first-order rate constant (k/h^–1) for gas-phase CH₃CHO oxidation was employed as an indicator of the photocatalytic activity. The k value for the TiO₂-Zn film prepared at I _d = 12 A dm^–2 (0.20 h^–1) was comparable to that for the sample from a ZnCl₂-based bath at I _d = 4 A dm^–2 (0.27 h^–1). X-ray diffraction measurements indicated that a TiO₂-ZnO nanocomposite layer was generated on the surface by the heat treatment in air at 673 K for 6 h. Consequently, the photocatalytic activity was further improved (k = 0.29 h^–1); this effect was explained in terms of the synergy of TiO₂ and ZnO in photocatalysis.     

Effects of thermal annealing of thin Au film on Fe40Ni38Mo4B18 in ultrahigh vacuum (UHV)

Thin films (20 nm) of Au were vapour-deposited on melt-spun amorphous ribbon specimens of the alloy Fe₄₀Ni₃₈Mo₄B₁₈ at room temperature. The specimens were subsequently annealed in UHV (10^–8 mbar) at 723 and 823 K in order to observe any dispersion of Au as nanoparticles in the alloy matrix. The motivation for these investigations was derived from similar experiments carried out earlier in nitrogen and in low vacuum conditions, wherein a model based on segregation and oxidation of matrix elements was proposed in order to explain the observed dispersion of Au in the alloy matrix. The present investigations in UHV were carried out as a critical test of this model. However, XPS investigations carried out on these specimens in UHV did not show any dispersion of Au particles after annealing at these temperatures. Further examination of annealed specimen surfaces by SEM and AFM revealed the formation of Au-rich islands on the surface. Native oxide film underneath the Au film and crystallization of the alloy during thermal annealing do not seem to have any effect on depth profiles of Au. These results, when compared with those obtained after annealing the specimens in nitrogen and in low vacuum conditions (10^–1–10^–3 mbar), are suggestive of the crucial role of the annealing atmosphere during thermal annealing.     

Interaction of 18Cr-10Ni stainless steel with liquid aluminium

The dissolution of an 18Cr-10Ni stainless steel in liquid aluminium at 700 to 850 ° C was found by the rotating disc technique to be non-selective and diffusion controlled. Experimentally determined values of the parameters characterizing the dissolution run are presented. In the case of saturated aluminium melts two intermetallic layers were found to form between the steel and the melt material at 700 °C. The compact layer adjacent to the steel surface is probably a solid solution based upon the Fe₂Al₅ compound. Its thickness,x, tends with increasing time to the limiting valuex _max = 10m. The porous layer adjacent to the melt material is probably a solid solution based upon the FeAl₃ compound. After a certain period of non-linear growth its thickness,Y, increases with time,t, according to the linear law:Y = 1 × 10^–8 t + 6 × 10^–6 m. The time dependence of the total thickness of both layers is well described in terms of the paralinear kinetics. In the case of undersaturated aluminium melts the formation of a single-phase intermetallic layer, 3 to 11 m thick, was observed at 700 ° C for 100 to 600 sec. The steel-to-aluminium transition joints with good mechanical properties were made by interaction of a solid steel material with liquid aluminium.     

Ba2HoNbO6: A new perovskite ceramic substrate for superconducting YBa2Cu3O7−δ thick film (T c (0) = 92 K)

A complex perovskite oxide Ba₂HoNbO₆ has been developed as a non-reacting substrate for YBa₂Cu₃O_7– superconducting film with lattice constant a = 8.3905 Å. The dielectric constant (30) and loss factor value (5 × 10^–3) of the material are in the range suitable for its use as substrate for microwave applications. A YBa₂Cu₃O_7–delta; superconducting thick film dip coated on Ba₂HoNbO₆ substrate gave a T _c(0) of 92 K and current density of 1.2 × 10⁴ A cm^–2.