The influence of sulphur on the dissolution and the passivation of a nickel-iron alloy—II. surface analysis by ESCA

Sulphur-containing (0.0082 at. %) single crystals of a Ni-25Fe alloy were anodically polarized in 0.05 M H₂SO₄ in the potential range 300–1300 mV (SHE). Sulphur is enriched on the surface by selective dissolution of the metal elements. The passive film is not formed on the sulphur-covered surface. At the lowest potential, which corresponds to the potential at which a sulphur-free alloy begins to passivate, the surface of the alloy is covered by a monolayer of adsorbed sulphur. Then the nucleation and growth of a sulphide film take place. The sulphide grows to a continuous layer 25a?A thick. The stoichiometry of the sulphide is close to Ni₂.₅Fe₀.₅S₂. This sulphide has no passivating effect, instead the anodic current increases linearly with the potential. A model for the adsorption of sulphur followed by nucleation and growth of the sulphide is suggested from the comparison of the intensities of the ESCA signals with the total amounts of sulphur measured by the radiotracer method.     

Oxide film growth on manganese in neutral aqueous solutions

Abstract

The open circuit potential of the Mn electrode is followed in aerated solutions of Cl^?, Br^?, I^?, SO?&SetFont Typeface="44";24, CrO?&SetFont Typeface="44";24, CO?&SetFont Typeface="44";23 and NO?2 with different concentrations, till steady state values are attained. In all solutions studied, the steady state potentials are approached from negative values indicating oxide film growth. The rate of oxide film thickening is determined from the linear relationship between the open circuit potential and the logarithm of immersion time, t, as evident from the relation: E?=?a+b logt, where a and b are constants. Oxide film growth is assumed to occur by ion conduction under a high field. The rate of oxide film thickening depends on anion type and concentration. The concentration of the inhibitive anions, CrO&SetFont Typeface="44";2?4, CO&SetFont Typeface="44";2?3 and NO?2 that can withstand a certain concentration of the aggressive ions, Cl^?, Br^?, and I^?, varies according to the relation: logC_inh.?=?A+n logC_agg., where A and n are constants.     

The electrochemical behaviour of leaded brass in neutral CL and SO4 media

The electrochemical behaviour of lead brass with different leaded content in neutral chloride and sulphate solutions was investigated using the EIS technique. For comparison, the behaviour of the pure components of the alloy was investigated under the same conditions. The corrosion process was found to proceed via oxygen reduction following a diffusion controlled mechanism. The Cu electrode showed a higher polarization resistance due to film formation during oxygen reduction. Zn and Pb showed markedly lower impedance values due to continuous dissolution. The two investigated brass alloys (1.8% and 3.5% Pb, respectively) showed higher impedance values indicating the passivation of the surface in the Cl⁻ or SO₄⁻ media. Brass II was found to be more stable against corrosion indicating the beneficial effect of the lead content in the alloy.At cathodic potentials, the only process is the oxygen reduction. Anodic polarization leads to selective dissolution of Zn. At more positive potentials simultaneous dissolution of the alloy components with the deposition of Cu(I) salt takes place leading to the passivation of the alloy surface. At higher potentials, film breakdown occurs producing Cu(II) compounds whose diffusion control the corrosion process. At potentials higher than − 0.1 V, pitting corrosion was observed and a transmission line type in the impedance spectra was recorded.     

The effects of yttrium ion implantation on the oxidation of nickel-chromium alloys. I. The microstructures of yttrium implanted nickel-chromium alloys

Yttrium ions of 150 keV energy were implanted into the alloys Ni-20Cr, Ni-4Cr, and into nickel. The microstructures were then characterized using transmission electron microscopy, selected area channeling patterns and back-scattered electron images. Low yttrium fluences between 1×10¹⁴ and 5× 10¹⁵ Y⁺/cm² did not alter the microstructures of Ni-20Cr. However, fluences of 1×10¹⁶, 5×10¹⁶, and 7.5×10¹⁶ caused the crystalline structures of the alloy to be replaced by an amorphous phase. Fluences of 7.5×10¹⁶ Y⁺/cm² also rendered Ni-4Cr and nickel amorphous. Self-ion implantation experiments on Ni-20Cr did not cause the amorphous phase to form. The depth distribution of elements in Ni-20Cr following yttrium ion implantation (7.5× 10¹⁶ Y⁺/cm²) was determined by Auger electron spectroscopy. This showed in addition to the added yttrium a surface depletion in nickel concentration and a simultaneous enrichment in chromium concentration. At approximately 500 Å, the chromium concentration is approximately 32 at.%. This depletion/enrichment zone extends throughout the implanted layer. Annealing the Ni-20Cr implanted with 7.5×10¹⁶ Y⁺/cm² in vacuum for one hour at 600°C resulted in the recrystallization of Ni-Cr solid solution and the formation of very fine grains of Y₂O₃. Annealing at 800°C for 5 minutes showed recrystallized Ni-Cr, Y₂O₃, and an additional phase or phases.     

Microstructures and Mechanical Properties of Electron Beam-Welded Titanium-Steel Joints with Vanadium,Nickel, Copper and Silver Filler Metals

Electron beam welding experiments of titanium alloy to stainless steel with V, Ni, Cu and Ag filler metals were carried out. The interfacial microstructures of the joints were examined by optical microscopy, scanning electron microscopy, and x-ray diffraction analysis. Mechanical properties of the joints were evaluated according to tensile strength and microhardness. The results showed that all the filler metals were helpful to restrain the Ti-Fe intermetallics formed in the Ti/Fe joint. The welds with different filler metals were all characterized by solid solution and interfacial intermetallics. And the type of solid solution and interfacial intermetallics were depended on the metallurgical reactions between the filler metals and base metals. The interfacial intermetallics were Fe₂Ti + Ni₃Ti + NiTi₂, TiFe, Ti₂Ag, and Cu₂Ti + CuTi + CuTi₂ in the joints welded with Ni, V, Ag, and Cu filler metals, respectively. The tensile strengths of the joints were primarily determined by the hardness of the interfacial intermetallics. The highest tensile strength was obtained in the joint welded with silver filler metal, which is about 310 MPa.     

Interface reaction between copper and molten tin–lead solders

The formation and growth of Cu–Sn intermetallic film at the interface between molten Sn–Pb solders and Cu were studied at different temperature and exposure times. The η-phase (Cu₆Sn₅) was observed to form at all conditions except at the lowest Sn level of 27 wt% and at the two highest temperatures of 290 and 310°C. The ε-phase (Cu₃Sn) was then obtained. At high Sn contents and short times, a cellular film with a rugged interface was obtained which evolved into a compact film with a scalloped interface as the Sn content decreased and exposure time increased. The ε-phase film always formed with a relatively planar interface. The intermetallic grains showed preferred crystallographic orientations. Thickness measurements showed that the net growth rate depends not only on diffusion through the film but also on the film dissolution. Precipitation of the η-phase whiskers was detected during cooling, particularly when the temperature was significantly high.     

Kinetics of passive layer formation on Fe 20 Cr in comparison with Fe

The passivation of Fe 20 Cr alloys was investigated in borate buffer, pH 8.4, and compared with corresponding data for Fe. After the reduction of electropolished electrodes at ?0,7 V potentiostatic pulse measurements were carried out in dependence on the polarization potential ε_P and polarization time τ_P. During the polarization, the current density i(t), the charge q(t) and the capacity C(t) were measured. Subsequent potentiodynamic reduction curves yielded the cathodic reduction charge. The kinetic data of passivation show the exponential influence of ε_P and the film thickness d_Ox on the oxide growth. Hence, the characteristics of the high field mechanism of passivation are observed which are similar to oxide films on pure metals. The agreement with the direct or inverse logarithmic law of the oxide growth, however, is poor. Various diagrams which test both laws show unexpected deviations for thick layers. Taking into account these results and those of the surface analysis, which show initial layers IL enriched with chromium at low ε_P, mixed passive layers PL at intermediate ε_P, and chromium depletion of the transpassive layer TPL at high ε_P, the results can be explained by the processes I and II shown in Fig. 2. During anodisation at low potentials I yields a slow transformation of IL into a mixed oxide PL of several nm thickness. This first oxide growth I is much slower than on passive iron. At high potentials (ε_P > 1.3 V/HESS, process II) the Cr dissolution yields the Cr-depleted film TPL, which behaves similarly to passive iron in the transpassive region. In the cathodic process the preferential reduction of Fe-oxides dominates. Hence Cr-oxides are enriched again in the reduced layer RL which differs from IL. The electronic properties of the passive layer PL are similar to those of passive iron. At low energies, however, additional surface states, which are created by chromium, change the behaviour at potentials ε_P > 1.3 V/HESS.     

Prevention of passive film breakdown on iron in a borate buffer solution containing chloride ion by coverage with a self-assembled monolayer of hexadecanoate ion

Breakdown of a passive film on iron in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻ was suppressed by coverage of the passive film surface with a self-assembled monolayer (SAM) of hexadecanoate ion C₁₅H₃₁CO₂⁻ (C₁₆A⁻). The pitting potential of an iron electrode previously passivated in the borate buffer at 0.50 V/SCE increased by treatment in an aqueous solution of sodium hexadecanoate for many hours, indicating protection of the passive film from breakdown caused by an attack on defects of the film with Cl⁻. No breakdown occurred over the potential range of the passive region by coverage with the SAM of C₁₆A⁻ in some cases. Structures of the passive film and the monolayer were characterized by X-ray photoelectron and Fourier transform infrared reflection spectroscopies and contact angle measurement with a drop of water.     

The influence of sulphur on the dissolution and the passivation of a nickel-iron alloy—I. electrochemical and radiotracer measurements

The role of sulphur in the dissolution and the passivation of a Ni-25Fe alloy was investigated. The concentrations of sulphur on the surface were quantitatively measured by the radiotracer technique³⁵S. The influence of the surface structure was accounted by using single crystal electrodes of well defined crystallographic orientation. Sulphur was pre-adsorbed in H₂S-H₂ under pressure and temperature conditions leading to the formation of a complete monolayer of adsorbed sulphur. The presence of the monolayer of adsorbed sulphur enhances the dissolution rate and delays the passivation of the alloy. The passivation can take place only when the sulphur coverage is lowered by partial desorption. When sulphur is in solid solution in the alloy, with a content of 0.008 at. % the passivation is totally precluded. This effect is due to sulphur enrichment on the surface by selective dissolution of the metal elements. The sulphur concentration on the surface increases with the potential. A stationary state is finally obtained, with a sulphur coverage of 8–10 layers. This thin sulphide film has no protective effect, in contrast with the oxide film formed on the sulphur-free alloy.     

A study of the film formation kinetics on zinc in different acidic corrosion inhibitor solutions by quartz crystal microbalance

Chromates conversion coatings provide very effective corrosion protection for many metals. However, the high toxicity of chromate leads to an increasing interest in using non-toxic alternatives such as molybdates, silicates, rare earth metal ions and etc. In this work, quartz crystal microbalance (QCM) was applied as an in-situ technique to follow the film formation process on zinc (plated on gold) in acidic solutions containing an inorganic inhibitor, i.e. potassium chromate, sodium silicate, sodium molybdate or cerium nitrate. Using an equation derived in this work, the interfacial mass change during the film formation process under different conditions was calculated, indicating three different film formation mechanisms. In the presence of K₂CrO₄ or Na₂SiO₃, the film growth follows a mix-parabolic law, showing a process controlled by both ion diffusion and surface reaction. The apparent kinetic equations are 0.4t = −17.4 + 20Δm_f + (Δm_f)² and 0.1t = 19.0 + 8.4Δm_f + 10(Δm_f)² respectively (t and Δm are in seconds and μg/cm²). In solutions containing Na₂MoO₄, a logarithmic law of Δm_f = −24.7 + 6.6 ln t was observed. Changing the inhibitor to Ce(NO₃)₃, the film growth was found to obey an asymptote law that could be fit into the equation of Δm_f = 55.1(1 − exp(−2.6 × 10⁻³t)).     

Hydrogenation reaction of Mg-Based alloys fabricated by rapid solidification

Mg-23.5wt%Ni-xwt%Cu (x=2.5, 5 and 7.5) alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The Mg-23.5Ni-5Cu alloy had crystalline Mg and Mg₂Ni phases. Mg-23.5Ni-5Cu had an effective hydrogen capacity of near 5 wt%. The activated Mg-23.5Ni-5Cu alloy absorbed 4.50 and 4.84 wt%H at 573K under 12 bar H₂ for 10 and 60 min, respectively, and desorbed 3. 21 and 4.81 wt%H at 573K under 1.0 bar H₂ for 10 and 30 min, respectively. The activated Mg-23.5Ni-5Cu alloy showed a quite high hydriding rate like Mg-10Fe₂O₃, and higher dehydriding rates than the activated Mg-xFe₂O_3?yNi. This likely resulted because the melting before melt spinning process has led to the homogeneous distribution of Ni and Cu in the melted Mg, and the Mg-23.5Ni-5Cu alloy has a larger amount of the Mg₂Ni phase than the Mg-xFe₂O_3?yNi alloy.     

Low-melting-point titanium-base brazing alloys—part 1: Characteristics of two-, three-, and four-component filler metals

The melting point, microstructure, phase, and electrochemical behavior of Ti-21Ni-15Cu alloy, together with two-, three-, and four-component low-melting-point titanium-base brazing alloys, are presented in this paper. Five filler metals were selected for the study, in which melting points were measured by differential thermal analysis, phases identified by x-ray diffractometry, and corrosion behaviors tested by potentiodynamic polarization. The experimental results show that the three-component Ti-15Cu-15Ni and the newly developed Ti-21Ni-14Cu alloys exhibit the combination of lower melting point and superior corrosion resistance compared to the two-and four-component titanium alloys, 316L stainless steel, and a Co-Cr-Mo alloy in Hank’s solution at 37 °C. On a short time basis, the presence of Ti₂Ni and Ti₂Cu intermetallics in the Ti-15Cu-15Ni and Ti-21Ni-14Cu alloys should not be preferentially dissolved in galvanic corrosion with respect to the dissimilar Ti-6Al-4V alloy.     

Mott–Schottky analysis of passive films on Cu containing Fe–20Cr–xCu (x = 0, 4) alloys

Effect of copper on the defect density of Fe–20Cr–xCu (x?=?0, 4) stainless steel alloys was investigated in deaerated pH 8·5 borate buffer solution at room temperature using Mott–Schottky analysis. Mott–Schottky analysis revealed that the addition of copper increased the acceptor density (N_A, V_Cr^?3), i.e. decreased the Cr⁺³ content of the passive film. Also the donor densities, shallow donor (N_D1, V_O⁺²) and deep donor (N_D2, V_Cr⁺⁶), of the passive films formed were increased. XPS analysis confirmed the decrease in Cr content and enrichment of copper in the passive film of Cu containing alloys, which ultimately dictated their lower corrosion resistance, i.e. decreased film protectiveness and stability.     

Effect of Benzotriazole on the dissolution of copper single crystal planes in dilute sulphuric acid

The kinetics of the dissolution of copper single crystal planes in aerated 0·1N H₂SO₄ containing various concentrations (10⁻⁶−10⁻²M) of Benzotriazole have been studied. The dissolution rates which were controlled by surface reaction, were a function of the temperature, crystallographic orientation and the concentration of Benzotriazole. The stabilities of the crystal planes were in the order (100) > (110) > (111). At 7·5 × 10⁻³M Benzotriazole, Cu-Benzotriazole film appeared on the surface, bringing mechanical passivity. Benzotriazole acted as cathodic inhibitor at low concentrations and anodic inhibitor at high concentrations. The corrosion potentials of the crystal planes were in the order (100) > (110) > (111) at all concentrations of benzotriazole.     

The kinetics of phosphate film formation on the type MA 21 magnesium-lithium alloys

Kinetics of phosphating MA 21 magnesium-lithium alloys in a zinc-nitrate-phosphate solution (0.25 M Zn²⁺+0.34M PO ₄ ³⁻ +0.17M NO ₃ ⁻ +0.02M F⁻, pH = 2.7) at 25°C is studied with various surface preparation procedures. The effect of the alloy surface pretreatment in titanium-containing activating solutions on the electrochemical process rate and the initial stage of the phosphate film crystallization is revealed. Peculiarities of the phosphate film growth during different stages of the magnesium-lithium alloy phosphating are elucidated. Combining the activating pretreatment with zinc-phosphate bath allows accelerated applying finegrain thin phosphate conversion layers with enhanced protective effect on magnesium-lithium alloys. Original Russian Text ? L.A. Isaicheva, N.M. Trepak, L.K. Il’ina, A.L. L’vov, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 4, pp. 425–429.     

Synthesis and characterization of chemically deposited nickel substituted CdSe thin film

The technologically important Cd_0.5Ni_0.5Se thin film has been developed by solution growth technique on non-conducting glass substrate in tartarate bath containing Cd⁺², Ni⁺² ions and sodium selenosulphate in an aqueous alkaline medium at room temperature. Various preparative conditions of the thin films are outlined. The films were characterized by X-ray diffraction, scanning electron microscope, optical absorption and electrical measurements. The X-ray diffraction study indicates that the film is polycrystalline in nature with hexagonal phase. Scanning electron micrograph shows that the film is homogeneous with well-defined grains. The films have high optical absorption coefficient. Thermoelectric power measurement shows p-type conduction mechanism.     

Spontaneous partitioning of the Ni + C60 thin film grown at RT

We report on the pattern formation in the thin film of the Ni + C₆₀ mixture deposited on the MgO(0 0 1) substrate at room temperature (RT). Using magnetic force microscopy a periodic array of the magnetic domains has been revealed. The domains reflect hidden partitioning of the Ni + C₆₀ film (i.e., separated Ni- and C₆₀-rich zones) that has not been observed by other applied methods. The effect indicates that even at RT, spontaneous separation of the Ni and C₆₀ phases may set in during the growth of the hybrid film. Thermal annealing (for 1 h at 500 °C) leads to dramatic rearrangement of the Ni + C₆₀ structure which (consequently) also results in the loss of the magnetic domain system. This phenomenon points out the thermodynamic instability of the as-prepared hybrid film that can (at elevated temperatures) trigger the process of the Ni and C₆₀ phase separation.     

The initial stage of pitting corrosion on coated steels investigated by photon rupture in chloride containing solutions

A photon rupture method, film removal by a focused pulse of pulsed Nd-YAG laser beam irradiation, has been developed to enable oxide film stripping at extremely high rates without contamination from the film removal tools. In the present study, Zn-55mass%Al alloy and Al-9mass%Si alloy-coated steel specimens covered with protective nitrocellulose film were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in 0.5 kmol m⁻³ H₃BO₃-0.05 kmol m⁻³ Na₂B₄O₇ (pH = 7.4) with 0.01 kmol m⁻³ of chloride ions to investigate the initial stage of localized corrosion. At low potentials, oxide films on both coated alloys were reformed after the nitrocellulose films were removed by this method. The oxide film formation kinetics follows an inverse logarithmic law, in agreement with Cabrera-Mott theory. However, at high potentials, localized corrosion producing corrosion products occurs at the area where nitrocellulose film was removed. Nevertheless, when the applied potential is less noble, the dissolution current of the Zn-55mass%Al-coated steel samples is higher than that of Al-9mass%Si-coated samples.     

X-ray absorption spectroscopy of electrochemically oxidized Cu exposed to Na2S

Copper surfaces have been investigated by X-ray absorption spectroscopy after electrochemical oxidation and subsequent exposure to sulfide solution. Oxide surface layers on bulk copper surfaces were electrochemically grown in an aqueous NaOH solution at two different potentials and the resulting chemical composition was investigated using X-ray absorption spectroscopy. At both potentials the resulting surfaces consisted largely of Cu₂O. At the more strongly oxidizing potential, an admixture of Cu²⁺-containing phases – mostly Cu(OH)₂ – was detected. Sulfide exposure of both surfaces was found to completely reduce the surface from Cu²⁺ to Cu¹⁺ and resulted in the formation of Cu₂S with an admixture of Cu₂O.     

中间层对高体积分数SiCp/Al复合材料真空钎焊的影响

采用铜箔、Al-Si-Mg及Al-Si-Mg/Cu/Al-Si-Mg(简称ACA)3种不同中间层对高体积分数45%SiC_p/Al复合材料进行真空钎焊连接研究.通过SEM,EDS及XRD等方法对钎缝的微观结构及界面组织进行了分析,研究了中间层种类对钎焊接头微观结构、界面组织以及连接强度的影响,阐明了不同中间层钎焊连接45%SiC_p/Al复合材料的界面形成过程及接头断裂机制.结果表明,ACA中间层兼具了铜和Al-Si-Mg钎料的优点,可降低钎料的液相线,增加其流动性,通过Cu原子优先在铝合金基体与其氧化膜的界面处扩散发生共晶反应,增强钎料的去膜作用,从而实现高体积分数45%SiC_p/Al复合材料的高质量连接.