The oxidation of lead-free Sn alloys by electrochemical reduction analysis

The oxidation of pure tin and Sn-0.7Cu, Sn-3.5Ag, Sn-1Zn, and Sn-9Zn alloys at 150°C was investigated. Both the chemical nature and the amount of oxides were characterized using electrochemical reduction analysis by measuring the electrolytic reduction potential and total transferred electrical charges. X-ray photoelectron spectroscopy was also conducted to support the results of reduction analysis. The effect of copper, silver, and zinc addition on surface oxidation of tin alloys is reported. For tin, Sn-0.7Cu, and Sn-3.5Ag, SnO grew first and then the mixture of SnO and SnO₂ was found. SnO₂ grew predominantly during long-time aging. For zinc-containing tin alloys, both ZnO and SnO₂ were formed. Zinc promotes the formation of SnO₂. For more information, contact Sungil Cho, Korea Advanced Institute of Science and Technology, Department of Materials Science and Engineering, 373-1 Kusung-dong, Yusung-gu, Daejon 305-701, Republic of Korea; +82-17-394-1026; e-mail sicho@kaist.ac.kr.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Effect of SiC on the corrosion behaviour of chromium coatings

Abstract

Cr–SiC composite coatings were plated in Cr(VI) baths containing SiC green powder. Analysis of Cr–SiC coatings has demonstrated a relatively low percentage (about 0·1–0·9 wt-%) of the particles incorporated into the Cr coating. XPS data suggest that the surface of SiC was covered with a layer of SiO₂ formed as a result of oxidation of SiC in the Cr(VI) bath. The corrosion behaviour of Cr–SiC composite coatings in an acidic sulphate solution was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy techniques. Impedance spectra were recorded at different times of exposure to the sulphate solution and the data interpreted using various equivalent circuit models. The best fit of the experimental data corresponded to a R ₁(Q ₁ R ₂)(Q ₂[R ₃ W]) circuit. The simulation of EIS data with the proposed equivalent circuit model made it possible to obtain a quantitative evaluation of some parameters reflecting the corrosion behaviour of the samples studied at the solution/electrode interface. Bode plots have shown that at the beginning of immersion the phase angle showed two peaks. It is suggested that the high frequency peak represents the dielectric characteristic of the protective oxide film on the surface of coating and the low frequency peak corresponds to the substrate/electrolyte interface via pinholes. The data obtained suggest that the change in the phase angle with frequency applied qualitatively represents the processes occurring in the coating and at the steel/solution interface via pinholes. Analysis of the data suggests that SiC particles enhance the corrosion resistance of the chromium coating as a result of the structural characteristics of the coatings obtained; namely, the size and number of pores.     

Chelant enhanced passivation of carbon steel in deoxygenated alkaline aqueous solutions

Abstract

The passivation characteristics of carbon steel surfaces at 473 K in the presence of certain chelating agents such as NTA DTP A, and HEDTA are described. The relative impacts of these chelants on the passivation process are evaluated by using base metal loss, soluble and insoluble iron concentrations in the mediums SEM studies of the topography of the surface coatings, and electrochemical investigations of the protection afforded by the oxide coating. The results have been compared with the passivation behaviour obtained under simple alkaline pH (LiOH) treatment (without chelating agents) and that previously obtained in the presence of EDTA. It is found that at 473 K, the presence of these complexing agents greatly increases the base metal loss, although a lower base metal loss is observed with LiOH. but the oxide film formed is more protective than the one formed under LiOR or EDTA treatments. The morphology of the coatings formed under complexing conditions has revealed highly developed crystallite faces on the outer surface layer. It is concluded that at 473 K passivation by LiOR is preferred to that given by the chelants.     

Photocorrosion inhibition -of n-GaAs in acidified aqueous iodide solution

Abstract

The photocorrosion of n-GaAs electrodes in aqueous solutions represents one of the most important factors limiting their application in solar energy conversion. In continuation of previous studies, the corrosion of n-GaAs has been investigated in 0·5M H₂SO₄ using a pllOtoelectrochemical cell. Illumination of the n-GaAs anode with white light of intensity 20 mW cm^-2 generated a saturation photocurrent of 27 mA cm^-2 at biasing voltages more positive than +0·45 V(SCE). This current corresponded to hydrogen discharge at the platinum counter electrode and to oxidation of the GaAs surface. When 0·01-6·0M LiI was added to the H₂SO₄ solution, 12 was formed at the n-GaAs electrode and dissolved in the iodide solution. The onset potential of the photocurrent (E_p) shifted from -0·36 to -0·73 V(SCE) as the concentration was varied from 0·01 to 6·0M I^-. Addition of I^-₃ shifted E_p to more positive values. The extent of corrosion was studied by comparing the ratios of photogenerated I₂ and H₂. In 0·01, 1·0, and 6·0M I^- solutions, I₂/H₂ was 0·38, 0·87, and ～1 respectively. These ratios imply that, at lower I^- concentrations, the holes photogenerated at the n-GaAs surface were consumed in the simultaneous oxidation of I^- and GaAs. However, in 6·0M I^-, where I₂/H₂ = 1, surface oxidation of GaAs was completely suppressed owing to total consumption of the photogenerated holes by the oxidation of I^-. The results are considered on the basis of the band model of the semiconductor/electrolyte interface.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Preparation and oxidation behaviour of electrodeposited Ni–CeO2 nanocomposite coatings

Ni–CeO₂ nanocomposite coatings with different CeO₂ contents were prepared by codeposition of Ni and CeO₂ nanoparticles with an average particle size of 7 nm onto pure Ni surfaces from a nickel sulfate. The CeO₂ nanoparticles were dispersed in the electrodeposited nanocrystalline Ni grains (with a size range of 10–30 nm). The isothermal oxidation behaviours of Ni–CeO₂ nanocomposite coatings with two different CeO₂ particles contents and the electrodeposited pure Ni coating were comparatively investigated in order to elucidate the effect of CeO₂ at different temperatures and also CeO₂ contents on the oxidation behaviour of Ni–CeO₂ nanocomposite coatings. The results show that the as-codeposited Ni–CeO₂ nanocomposite coatings have a superior oxidation resistance compared with the electrodeposited pure Ni coating at 800 °C due to the codeposited CeO₂ nanoparticles blocking the outward diffusion of nickel along the grain boundaries. However, the effects of CeO₂ particles on the oxidation resistance significantly decrease at 1050 °C and 1150 °C due to the outward-volume diffusion of nickel controlling the oxidation growth mechanism, and the content of CeO₂ has little influence on the oxidation.     

Electrochemical behaviour of lead alloys as anodes in zinc electrowinning

Abstract

The electrochemical and corrosion properties of the ternary and quaternary lead alloys Pb–0·18Ag–0·012Co, Pb–0·2Ag–0·06Sn–0·03Co, and Pb–0·2Ag–0·12Sn–0·06Co (wt-%) have been investigated. The formation of oxide layers on the surface of these alloys was traced by cyclic voltammetric methods and the composition of the anodic oxide layers was determined by X-ray and SEM analyses. The cobalt inclusions reduce the anodic polarisation and improve the corrosion resistance of the alloy. The Pb–0·2Ag–0·12Sn–0·06Co alloy displays electrochemical and corrosion properties similar to those of Pb–1Ag and can be used as a substitute for the latter as a material for anodes in the electrowinning of zinc from sulphate electrolytes.     

Characteristics of nickel sulphide reduction in Ni2+ free background and Watts nickel plating electrolytes

Abstract

The electrochemical behaviour and composition of nickel sulphide coatings deposited on glassy carbon (GC) electrode by the successive ionic layer adsorption and reaction (SILAR) method using a nickel(II) ammonia complex and Na₂S solutions have been studied in Ni²⁺ free background, Watts nickel plating and 0·05 M H₂SO₄ electrolytes by the cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) methods. Analysis of XPS data suggests that the coating is a mixture of two sulphides where NiS dominates. In the Ni²⁺ free background solution at first the electrochemical reduction of the sulphur rich nickel sulphide to NiS occurs. After that the NiS to metallic Ni is reduced in the potential range of H₂ evolution. During the cathodic reduction of the coating in Watts nickel plating electrolyte the sulphur is reduced in the potential range from 0·0 to ?0·4 V, while at the potential values <?0·5 V the sulphur rich nickel sulphide and NiS reduction in the H₂ evolution range and Ni plating occur.     

Gas phase synthesis of SnOx nanoparticles and characterization

Tin oxide nanoparticles with an average size ranging from 4 to 80 nm were synthesized by oxidation of tin vapor in a low vacuum (10–40 mbar) reactor containing an Ar/O₂ gas mixture. The effect of oxygen and argon partial pressure on the phase formation, size characteristics and morphology of the particles was studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD) method. Electron spectroscopy chemical analysis (ESCA) was used to study the state of the particle surfaces. It was shown that with increasing the oxygen partial pressure, coarser SnO_x particles were synthesized. The ESCA shift for the tin 3d_5/2 line was 2 eV and the separation between this line and the oxygen 1s line was 44 eV. The oxygen 1s line was narrow and symmetric without the OH tail, indicating clean surface. The O/Sn ratios were estimated by using the areas of the tin and oxygen lines, and it was found that the oxidized tin vapor consisted of both SnO and SnO₂. At the oxygen concentration of 50 vol% and the total pressure of 10 mbar, the surface composition was estimated to be SnO_1.2. The particles were transformed to tetragonal SnO₂ after heat treatment at 350 °C for 24 h in air.     

Vacuum Carbothermal Reduction for Treating Tin Anode Slime

In this work, a process of vacuum carbothermal reduction was proposed for treating tin anode slime containing antimony and lead. During vacuum carbothermal reduction, the antimony and lead were selectively removed simultaneously by reducing and decomposing the less volatile mixed oxide of lead and antimony into the more volatile Sb₂O₃ and PbO. Then the tin was enriched in the distilland and primarily present as SnO₂. Crude tin was obtained via vacuum reduction of the residual SnO₂. The results showed that 92.85% by weight of antimony and 99.58% by weight of lead could be removed at 850°C for 60 min with 4 wt.% of reductant and air flow rate at 400 mL/min corresponding to the residual gas pressure of 40 Pa–150 Pa. Under these conditions, an evaporation ratio of 52.7% was achieved. Crude tin with a tin content of 94.22 wt.% was obtained at temperature of 900°C, reduction time of 60 min, reductant dosage of 12.5 wt.%, and a residual gas pressure of 40 Pa–400 Pa. Correspondingly, the direct recovery of tin was 94.35%.     

钛基材上电化学沉积羟基磷灰石

通过电沉积法在经过阳极氧化的钛基材表面沉积磷酸钙盐涂层,再经碱热处理使磷酸钙涂层转变为羟基磷灰石涂层。扫描电镜(SEM)观察了阳极氧化后生成的TiO2纳米管的微观结构,以及生成的羟基磷灰石的形貌。X射线衍射仪(XRD)分析了涂层的相组成,同时测定了涂层与基体的结合强度。试验结果表明:电沉积涂层CaHPO4·2H2O经碱处理后转变为羟基磷灰石;电沉积添加双氧水与钛基材经过阳极氧化后使得涂层与基体结合强度有所提高。模拟体液浸泡试验表明涂层具有良好的生物活性。     

Development and tribological characterisation of nanostructured Zn-Ni and Zn-Co coatings: a comparative study

Zn-Ni and Zn-Co alloy coatings were electrodeposited on mild steel from sulphate-based baths. The morphology, microstructure, microhardness and tribological behaviours of the coatings have been studied and discussed. While the Zn-5wt-% Co layers presented a nanocrystalline simple nodular structure (45?±?5?nm), the Zn-14wt-% Ni showed a particular structure called cauliflower morphology (30?±?7?nm). The X-ray diffraction analysis showed that each of the electrodeposits was formed from zinc solid solution with a uniform zinc-cobalt intermetallic phase γ₂ (CoZn₁₃) for Zn-5wt-% Co alloy. However, a single γ-phase (intermetallic compound Ni₅Zn₂₁) was presented for the Zn-14wt-% Ni alloys. The Zn-14wt-% Ni films were found to be harder and rougher than the Zn-5wt-% Co layers. Plastic deformation and oxide layers production were the main wear mechanisms for the investigated coatings. The Zn-14wt-% Ni coatings were found to have the best wear resistance due to their microhardness and particular structure.     

Study on factors affecting microstructure of tin coatings cemented on zinc. Part 1

Abstract

The process of tin cementation on zinc resulting in the formation of fine grained tin coatings was studied. The influence of the experimental conditions such as Sn(II) concentration, duration of the reaction, solution temperature and pH on the microstructure of the cemented tin films was investigated. The dependence between the experimental conditions and the grain sizes of the deposited tin films was established using the method of the design of experiment. The diminution of Sn(II) concentration, the decrease in temperature and the increase in solution pH were found to promote the deposition of fine grained tin coatings with the grain sizes 0·3–1 μm. The fine granulation of tin films was shown to be insufficient for obtaining coatings suitable for the use as protective and/or intermediate layers on zinc surface. The evolution of hydrogen was found to be the most negative factor deteriorating the quality of coatings owing to the formation of pores.     

Electrochemical corrosion behaviour and surface morphology of electrodeposited zinc,zinc–cobalt and their composite coatings

Abstract

The electrochemical behaviour as well as the surface morphology characteristics of electrodeposited Zn and Zn-Co (≈3 wt-%) alloy coatings on low carbon steel samples, with and without included nanoparticles of organic co-polymers in their structure, are discussed. Their corrosion resistance and protective properties were studied in a model corrosion medium, 5 wt-% NaCl, using electrochemical methods such as polarisation resistance (R _p) measurements and potentiodynamic (PD) polarisation curves. Scanning electron microscopy (SEM) was performed to examine the surface morphology changes of the samples before and after the corrosion treatment. Some conclusions are drawn about the influence and importance of the organic nanoparticles on the corrosion behaviour of the coatings presented.     

Study on corrosion behaviour of nanocrystalline and amorphous Co–P electrodeposits

Abstract

In this study, corrosion properties of nanocrystalline and amorphous Co–P coatings prepared using dc electrodeposition were investigated. The morphology of amorphous coatings was smoother and brighter than nanocrystalline coatings with 'cauliflower' morphology. Tafel polarisation tests of the coatings in 0·1M H₂SO₄ solution revealed that the amorphous coating had better corrosion resistance than the nanocrystalline one. Corrosion resistance of both coatings decreased after annealing. On the nanocrystalline coating, corrosion attacks were localised around the nodules on the surface while a more uniform type of attack was observed on amorphous coatings. Neither nanocrystalline nor amorphous coatings showed passivation in 0·1M H₂SO₄ solution, but both of them showed an active–passive behaviour in 10%NaOH solution due to the formation of Co(OH)₂ as fine hexagonal shape products which acted as a passive layer. The passive current density of the amorphous coating was higher than that of the nanocrystalline one, but it was decreased markedly by annealing. However, annealing had no significant effect on the passivation behaviour of the nanocrystalline coating.     

Ecologically green conversion coating for zinc–cobalt alloy

Abstract

A phytic acid based conversion coating has been designed for Zn–Co alloys. The morphology of this coating was studied by SEM and showed that immersion of coatings for 15 min is smooth and compact. The composition of conversion coatings was analysed by X-ray photoelectron spectroscopy. The analysis data showed that the coating is composed of Zn, C, P, N and O. The presence of Co in the coating was not established. Corrosion protection provided by this coating was evaluated by electrochemical measurements (polarisation curves, electrochemical impedance spectroscopy) in a 0·5M NaCl solution. Electrochemical corrosion measurement showed that the corrosion resistance of the Zn–Co alloy has been improved by the conversion treatment with phytic acid.     

Characterisation of anodic layers on Cu-10Sn bronze (RDE) in aerated NaCl solution

The anodic surfaces formed on Cu-10Sn (wt.%) alloy (α-bronze) are investigated in aerated 0.1 M aqueous chloride solution, using electrochemical reduction and characterisation methods such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the whole anodic domain, investigations performed on a bronze rotating disk electrode (RDE) reveal the systematic formation of a uniform oxidation layer. It is evidenced that the chemical composition of the layer varies with the applied anodic potential, but also that the latter always exhibits a poorly crystallised (probably nanocrystalline) hydrated and hydroxylated nature. Close to E_oc, the compounds are mainly (hydroxide) oxides of tin and copper, incorporating very low amounts of chlorides. At intermediate oxidation potentials corresponding to the active-passive transition, the first oxidation peak corresponds to the formation of hydrated tin oxyhydroxide chloride species which transforms in a more stable one - probably related to the Sn(II) → Sn(IV) oxidation. At higher anodic potential, on the current plateau, the layer contains hydrated tin (IV) oxyhydroxide and copper chloride (mainly CuCl). However, XRD and XPS results reveal that the barrier layer has a complex nature, including unidentified products and different spatially distributed charged surface zones. The corrosion mechanism involves an internal oxidation of the alloy linked to a preferential dissolution of copper, namely a decuprification. A decuprification factor f_Cu is defined and calculated. Both f_Cu and the layer thickness increase with the applied potential. We show unambiguously that the tin compounds remain in the corrosion layer, acting as stabilizing species. It is suggested that the tin species promote the formation of a network as for tin oxide xerogel, through which copper ions and anions migrate. Both the layer microstructure and the decuprification factor (f_Cu) are in agreement with those found in Type I patina of ancient bronzes.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Annual Contents

Abstract

The electrochemical reduction of Bi₂S₃ films deposited using the successive ionic layer adsorption and reaction method in a Bi(III) solution with a sulphidation agent Na₂S was investigated in Ni²⁺ free background and Ni²⁺ containing electrolytes at different pH. This investigation was carried out by means of cyclic voltammetry, electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy. It has been established that during the reduction of a bismuth sulphide film in the Ni²⁺ free background solution the electrode mass decreases due to Bi₂S₃ film reduction to metallic Bi and the transfer of sulphide ions into the bulk of the solution. At pH=3, the Bi₂S₃ film reduces under the potential region from –0·2 to –0·4 V, while at pH=4 and 5 from –0·4 to –0·6 V. The processes occurring during the reduction of Bi₂S₃ film in the Ni²⁺ containing electrolyte depend on the electroreduction medium. At pH=3, the reduction of Bi₂S₃ film to metallic Bi and Ni²⁺ ions occurs simultaneously under the potential region of the cathodic current peak (–0·2 to –0·4 V). When pH is 4 or 5, the increase in electrode mass is caused by the incorporation of Ni²⁺ ions into the bismuth sulphide film with their partial reduction to the metallic or close to metallic state with the occurrence of structural changes in the film.