Physicochemical modeling of interactions in the passive oxide film of an alloy. Thermodynamic model of a regular solution of oxide “molecules”

A physicochemical model of interactions between the components of a mixed oxide, which is equilibrium to the corrosive solution, is proposed as the first step to a theory of reciprocal effect of the components of a passive layer. Proceeding from the kinetic-electrostatic model of the passive metal developed earlier with the participation of the author and the classical theory of regular solutions and setting the quantitative relations between the concentrations of components in the alloy and in the solution, the model proposed forms a basis for modeling the dissolution kinetics of the components of a passive alloy. Some methodological aspects of describing the above interactions and correspondingly modifying the theory of regular solid solutions are discussed. The work was financially supported by the Russian Foundation for Basic Research, projects nos. 96-03-33978 and 99-03-32478a.     

Corrosion regularities of passive alloy in terms of a physicochemical model of mixed passivating oxide as a regular solid solution of molecules

On the basis of the thermodynamic model of interaction between components (which treats a mixed oxide as a regular solution of oxide “molecules”) and the transition state theory, a quantitative model for the steady-state dissolution of a passive metal is proposed. The thermodynamic properties of the oxide and the dissolution kinetics of the passive alloy are found to correlate in a certain manner. In particular, partially modified physical concepts of the theory of regular solutions, such as an asolvotropic composition and the possibility of existence of metastable states, can also be used for describing the kinetics of the oxide dissolution. This approach enables one to understand the physical nature of those states of a system, the corrosion resistance of which is extremal, namely minimal or maximal. A scheme for comparing the results of this theory with experimental data is proposed. The work was financially supported by the Russian Foundation for Basic Research (project no. 99-03-32478a).     

In situ studies of the transition from solution to solid film of poly(octylthiophene)

The drying of thick solutions of the conjugated polymer poly(octylthiophene) (POT) was followed in situ by grazing incidence diffraction of synchrotron X-rays from the solution surface, with the solvent evaporation rate being controlled by a flow of dry nitrogen gas. It was found that a solid, partially crystalline film is formed on the upper surface while the interior is still fluid. The initially formed film is swollen and of restricted correlation length, but the swelling disappears gradually on further drying. A considerable fraction (>20%) of the polymer within the effective beam volume near the surface is transformed from the disordered to the ordered state only during the removal of the last 5% of the solvent.     

Correlation between the corrosion resistance and the semiconducting properties of the oxide film formed on AZ91D alloy after solution treatment

The corrosion resistance and semiconducting properties of the oxide film formed on the AZ91D alloy were evaluated. The alloy was tested in the as-cast condition and after a solution annealing treatment. Electrochemical impedance spectroscopy measurements and potentiodynamic polarization curves were obtained in a H₃BO₃ (0.05 M) + Na₂B₄O₇⋅10H₂O (0.075 M) solution with pH = 9.2 at room temperature. The semiconducting properties of the oxide film were evaluated using Mott–Schottky plots. The corrosion resistance of the AZ91D was reduced after the solution treatment while the semiconducting properties of the passive films were little affected.     

Decomposition of a supersaturated solid solution in the Mg-3.3 wt % Yb alloy

Kinetics of decomposition of the supersaturated solid solution in the Mg-3.3 wt % Yb alloy has been investigated during aging in a temperature range of 150–225°C using electron microscopy, X-ray diffraction, and hardness measurements. Mechanisms of the decomposition of the supersaturated solid solution and the nature of phases precipitating at different stages of aging, namely, in the underaged, maximum-hardness, and overaged states, have been established. The character of structural components responsible for the hardness changes during aging process has been established.     

Kinetics of the solution of the oxide film and the strength of welded joints

Metal Science and Heat Treatment -     

Reaction behavior between the oxide film of LY12 aluminum alloy and the flux

In this paper, the brazing mechanism of LY12 aluminum alloy at middle range temperature was presented. The CsF-AlF3 non-corrosive flux was utilized to remove the complex oxide film on the surface of LY12 aluminum alloy. The resultsrevealed that the oxide film was removed by the improved CsF-AlF3 flux accompanied with the occurrence of reaction as well as dissolution and the compounds CsF played an important role to remove the oxide film. Actually, the high activity of flux, say,the ability to remove the oxide film, was due to the presence of the compounds, such as NH4F,NH4AlF4 and composite molten salt. The production of HF was the key issue to accelerate the reaction and enhance to eliminate the oxide film by dissolution.It was found that the rare earth element La at small percentage was not enriched at the interface. Moreover, the rare earth fluoride enhanced the dissolution behavior.     

Hydrogen storage properties of Mg(Al) solid solution alloy doped with LaF3 by ball milling

LaF₃ was doped to the Mg(Al) solid solution alloy for enhancing the hydrogen absorption and desorption by ball milling. XRD was used to analyze the phases of the samples and the phase transition induced by hydrogenation and dehydrogenation. The microstructure and phase distribution were investigated by SEM and STEM. The hydrogen storage properties were measured by Sieverts method. For Mg_0.93Al_0.07-5wt.%LaF₃ nanocomposite, the hydrogen storage kinetic properties were significantly improved by reducing the hydriding and dehydriding activation energies to 65 and 78 kJ/mol, respectively, and the dehydriding enthalpy was calculated to be 69.7 kJ/mol. The improved hydrogen storage properties were mainly attributed to the catalytic effects of the in situ formed nanostructure Al₁₁La₃ and MgF₂ together with the dissolving of Al in Mg lattice.     

Role of the interface oxide film/alloy composition and stability of stainless steels

X‐ray photoelectron spectroscopy (XPS), a powerful method for investigating composition and thickness of thin films in the nanometer range, has been used to characterize the surface of two stainless steels with different bulk composition, DIN 1.4301 (18%Cr 8%Ni) and DIN 1.4529 (20%Cr 25%Ni 6%Mo) after mechanical polishing and after potentiostatic passivation in a wide range of potentials in sulfate solutions at different pHs. The XPS results confirm that the passive film thickness increases with applied potential, its composition is enriched in chromium oxy‐hydroxide after immersion in acidic solutions and polarization at low potentials whereas oxidized nickel is depleted. Special emphasis is given in this paper to the composition of the metal phase underneath the passive film. It is shown that the composition of the interface is strongly enriched in nickel for both stainless steels being 28 ± 2 wt% Ni versus a value of 8% in the bulk and 45 ± 2 wt% versus 25 wt%Ni in the bulk, respectively. The composition of the interface for a given alloy remains unchanged in a wide range of conditions of passive film formation such as pH, applied potential, passivation time. These results are discussed with respect to the influence of the interface composition on the corrosion rate of alloys in the passive state and on their pitting resistance.     

The effects of gaseous pretreatment and oxide sintering on oxidation of Ti-4.32 wt.%Nb alloy

Oxidation kinetics in either air or oxygen of prenitrided Ti-4.32 wt.% Nb alloy are investigated in the temperature range of 900–1200°C. Based on X-ray and electron microprobe analyses, thermogravimetric measurements, microhardness data, and sintering experiments, a quantitative oxidation model is developed to explain the gas-metal reactions. Temperature cycling experiments, in the temperature range of 900–1200°C and for a 12-hr duration, are performed in an attempt to reduce the oxidation rates of the alloy in air or oxygen. The oxidation resistance of nitrogen pretreated alloy is comparable to that in air and it is considerably higher than in oxygen alone.Presently on academic leave at Max-Planck-Institut für Metallforschung, Institut für Werkstoffwissenschaften, Stuttgart, Bundesrepublik Deutschland.     

缓冲溶液和分散剂对共沉淀法制备纳米级ITO粉体的影响

用化学共沉淀法制备ITO粉体。制备过程中,分别将NH_4Cl-NH_3H_2O、(NH_4)_2SO_4-NH_3H_2O和NH_4AC溶液作为缓冲溶液,保持反应过程中pH值基本不变。采用可溶性淀粉、PVP、十二烷基磺酸钠作为分散剂。借助XRD、TEM、BET及四探针电阻仪,研究缓冲溶液和分散剂对制得粉体的物相、形貌、分散性及导电性能的影响。结果表明:采用缓冲溶液制备出的粉体为单相In_2O_3粉体。采用NH_4AC作为缓冲溶液,用淀粉作为分散剂,当pH值为6~7时,所制粉体的粒径在10 nm左右,颗粒为立方体形,粒度均匀,分散性能好,电阻率相对较低。     

Strain-rate effects in the environmentally assisted fracture of a commercial high-strength aluminium alloy (7049)

Specimens of this high-strength aluminium alloy, cut parallel to the short transverse direction, have been tested in tension at slow strain-rates in the T651 condition using various environments: vacuum, dry air, laboratory air and sea-water, with and without prior exposure to sea-water. Accurate measurements of the reduction in area at fracture reveal a reversible pre-exposure embrittlement due to hydrogen absorption that is observed at slow strain-rates, the critical strain-rate decreasing in the less aggressive environments. Recovery from pre-exposure embrittlement only occurs in this material when specimens are subsequently strained in an inert environment, otherwise the pre-exposure effects and the embrittling effect of the test environment are additive.Fractographic examination indicates that absorption of hydrogen leads first to transgranular and then to intergranular brittle failure and the evidence is consistent with the hypothesis that the transition occurs at lower local hydrogen concentrations (i.e. shorter pre-exposure) as the strain-rate is decreased.Stress corrosion tests carried out in tap-water under potentiostatic control reveal that a cathodic and an anodic region of embrittlement both become extended as the strain-rate is decreased. However, fractography and the observed effect of over-aging both tend to confirm that the major embrittling process at the free corrosion potential is hydrogen embrittlement.The results are discussed with reference to earlier results reported for a similar alloy (7075) and apparent discrepancies explained. In particular, the aggressiveness of laboratory air, frequently employed as a reference environment, is emphasized.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

The effects of vacuum annealing on the film properties of titanium boride (TiBx) grown on (100)Si substrate

In order to investigate the effects of vacuum annealing on the properties of titanium boride films (TiBx) on a (100)Si substrate, TiBx/Si samples were prepared by the co-evaporation process and then annealed in the temperature range of 300≈1000°C. The interfacial reaction of TiBx/Si systems and the thermal stability of non-stoichiometric TiBx films (0≤B/Ti≤2.5) were investigated by means of sheet resistance, x-ray diffraction, transmission electron microscopy, x-ray photo-electron spectroscopy, and stress measurement. For TiBx samples with a ratio of B/Ti≥2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with the ratio of B/Ti<2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide (self-aligned silicide) process. The sheet resistance and the film stress in the Ti/Si and TiB_x/Si systems are explained well by the solid phase reactions.     

固溶温度对TC4钛合金微观组织和动态拉伸力学性能的影响

为揭示固溶温度（850、920、960℃）对TC4钛合金微观组织和动态拉伸力学性能的影响,采用XRD、SEM和EBSD方法对材料晶体结构、微观组织和晶粒取向等特征进行分析,选取分离式霍普金森拉杆（SHTB）实验装置进行了材料的动态拉伸力学性能测试,构建了Johnson-Cook(J-C)本构模型,开展了动态拉伸断口形貌分析。结果表明：随固溶温度的升高,材料中α/α′含量升高,初生α相含量降低,针状α′含量升高,晶粒尺寸减小且择优取向强度增大;TC4钛合金具有明显的应变率强化效应,随固溶温度的升高,材料屈服强度和维氏硬度逐渐增大,断裂延伸率降低;动态拉伸断口整体表现为韧性断裂,随固溶温度升高,材料塑性降低,在固溶温度960℃时,试样韧性断裂特征不显著。本研究结果可为TC4钛合金力学性能调控及抗冲击设计提供方法和数据支撑。     

The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates

We investigate the effects of film microstructures such as crystallinity and surface roughness on cracking of TCO coated on polymer substrates. Indium tin oxide (ITO), which is the most extensively used TCO for electrodes in flexible electronics, is sputter-deposited on polyethylene terephthalate (PET). Varying thickness (t_f) and deposition condition, amorphous ITO (a-ITO) with different surface roughness (R_rms) and a partially crystallized ITO with similar R_rms to a-ITO of the same t_f are coated on PET. Cracking in ITO under tension is analyzed by uniaxial tension test. The mode of cracking and the fracture strength are related to the crystallinity and R_rms of ITO analyzed by high resolution transmission electron microscopy (HR TEM) and atomic force microscopy (AFM), respectively. Our experimental results reveal that the strength of ITO is significantly improved with crystallinity. The important effect of R_rms on cracking is also clearly revealed. In a-ITO, fracture strength is decreased almost linearly with R_rms that increases with t_f. In addition, cracks are always initiated and propagated along the valleys between protrusions on the surface regardless of crystallinity. Based on our experimental results, a low temperature deposition condition to obtain ITO with a high level of functional and mechanical properties is proposed.     

The effects of pH and polyethylene glycol on the Cr(III) solution chemistry and electrodeposition of chromium

The effects of pH and the organic additive, polyethylene glycol (PEG), on the trivalent chromium electroplating were investigated in view of the solution stability, electrochemical reactions and the properties of deposited layer. It was sought out that the concentration of chromic complexes in the chromic bath with formic acid was closely dependent on pH. The chromic complex formation in the bath, examined by theoretical calculation and UV–vis absorption measurement, was not affected by PEG addition. However, adsorbed PEG molecules on the cathode surface caused the hydrogen evolution rate to be retarded and the deposited coating morphology smooth, small cracks and nodular structure. The variations of solution acidity and organic additive concentration affected the complex formation, the cathodic reaction and the physical property of the deposit.     

Improvement in the self-healing ability of a protective film consisting of hydrated cerium(III) oxide and sodium phosphate layers on zinc

A self-healing protective film prepared on a zinc electrode by previous treatment in a Ce(NO₃)₃ solution and coverage with a Na₃PO₄ layer was improved by the addition of Zn(NO₃)₂. The protective and self-healing abilities of the films against corrosion of zinc in an aerated 0.5 M NaCl solution were examined by polarization measurements and observation of pit-like anodic dissolution feature (plf) and pit formation after the electrode was scratched with a knife-edge crosswise and immersed in the solution for many hours. The film prepared on the pretreated electrode coated with 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O was significantly protective and self-healing against zinc corrosion on the scratched electrode. The protective efficiency of the film was more than 96% in the region of the immersion time between 72 and 360 h. No plf was observed at the scratches after immersion for 120 h. Results of Fourier transform infrared reflection and X-ray photoelectron spectroscopies revealed that the film was composed of Na₃PO₄, Zn₃(PO₄)₂ and Ce₂O₃ layers.