Chromium electroplating from trivalent chromium baths as an environmentally friendly alternative to hazardous hexavalent chromium baths: comparative study on advantages and disadvantages

In this communication, we present a generalization of our investigations concerning current state in the development of trivalent chromium baths as an environmentally friendly alternative to hazardous electroplating baths containing extremely toxic hexavalent chromium compounds. The main technological properties of sulfate trivalent chromium baths containing carbamide and formic acid as organic additives are described and compared to those typical of common Cr(VI) plating bath. It is shown that thick nanocrystalline chromium–carbon deposits may be obtained from the Cr(III) electrolytes under study, with some physicochemical and service properties of such coatings exceeding those of “usual” chromium deposits. The proposed trivalent chromium baths are distinguished by their high current efficiency and electrodeposition rate. The covering power of the Cr(III) baths is poorer than in case of hexavalent chromium baths; it may be improved by applying basic chromium sulfate (chrome tanning agent) instead of chromium sulfate as a source of Cr(III) ions in solution. There is no need to utilize toxic lead anodes in Cr(III) deposition processes. It is noted that a grave shortcoming of trivalent baths is their low conductivity in comparison with the hexavalent ones.     

Ti基DSA阳极在三价铬硫酸盐镀液中的电化学行为

为了获得满足三价铬硫酸盐镀液镀铬要求的阳极,采用热分解法制备了具有Pt-Ir-Ta-Sn氧化物涂层的Ti基DSA阳极,并研究了3种Ti基DSA阳极在三价铬硫酸盐镀液中的电化学行为.结果表明:具有Pt-Ir-Ta-Sn氧化物涂层的Ti基DSA阳极表面结构致密、稳定性好;在三价铬硫酸盐镀液中的析氧电位较低、对阳极析氧反应具有较好的催化活性;不发生Cr3 被氧化成Cr6 的反应;金属Ta-Sn氧化物的引入可以增加Ti基DSA阳极的活性表面面积,提高催化活性和增强其稳定性.     

Electrodeposition of chromium coatings from sulfate–carbamide electrolytes based on Cr(III) compounds

We propose an electrolyte based on chromium sulfate (1 mole/liter Cr(III)) and containing both formic acid and carbamide (urea). This electrolyte enables one to get Cr coatings with a thickness of several micrometers. It is shown that the current yield and deposition rate increase as the current density and pH value increase and temperature decreases. We select the optimal conditions of electrolysis under which bright high-quality chromium deposits are obtained. In this case, the deposition rate of the metal varies from 0.5 to 1.5 μm/min. It is shown that the optimal concentration of both formic acid and carbamide is equal to 0.5 mole/liter. The necessity of using certain surface-active substances to prevent the formation of pitting on the surface of the deposit is demonstrated. Moreover, it is discovered that the microhardness of Cr deposits attains its highest values (950–980kg/mm²) for currents with densities of 30–35 A⋅dm⁻² and decreases as the pH value and temperature increase. Electrolysis is realized by using titanium–manganese-dioxide anodes and, hence, it is not necessary to separate the cathodic and anodic spaces.     

Zinc recovery and waste sludge minimization from chromium passivation baths

This work reports the feasibility of applying emulsion pertraction technology (EPT) aiming at zinc recovery and waste minimization in the zinc electroplating processes that include Cr (III) passivation. The assessment consists of firstly the lifetime extension of the passivation baths by selective removal of the tramp ions zinc and iron, and secondly, the recovery of zinc for further reuse. Spent passivation baths from a local industry were tested, being the major metallic content: Cr³⁺ 9000 mg L⁻¹, Zn²⁺ 12,000 mg L⁻¹, Fe³⁺ 100 mg L⁻¹. Working in a Liqui-Cel hollow fiber membrane contactor and using the extractant bis(2,4,4-trimethylpentyl) phosphinic acid, reduction of zinc and iron concentrations below 60 mg L⁻¹ and 2 mg L⁻¹, respectively were obtained, while trivalent chromium, the active metal that generates the passivation layer, was retained in the baths. Zinc was selectively transferred to an acidic stripping phase that in the experimental time reached a concentration of 157,000 mg L⁻¹. Zinc recovery by electrowinning from the acidic stripping phase without any pretreatment of the electrolyte solution provided a purity of 98.5%, matching the lower commercial zinc grade. As a result of the extension of the life time of the passivation bath, significant environmental advantages are derived such as minimization of the volume of hazardous wastes and savings in the consumption of raw materials.     

三价铬硫酸盐体系快速电沉积可行性探讨

三价铬镀铬是替代六价铬镀铬理想的清洁生产工艺.研究了电流密度、电镀时间和不同基体金属对三价铬硫酸盐镀液中快速镀铬的影响.结果表明:采用三价铬硫酸盐镀液在铜、镍和低碳铜基体上进行快速镀铬都可得到表面连续致密、结构为非晶态的铬镀层,镀速在铜基体上比在镍和低碳钢基体上快很多;电流密度10 A/dm2下电镀10 s,铜基体上可得到0.40μm以上的铬镀层,平均镀速可达2.50μm/min;镍和低碳钢基体上只能得到0.10μm的铬镀层,平均镀速为0.50 μm/min;快速镀铬的电流效率与电流密度有关,电流密度为10～12 A/dm2时可达25.0%以上;三价铬硫酸盐镀液长时间连续快速镀铬时镀液体积明显减少、pH值降低.     

常温高效硫酸盐三价铬电镀工艺

六价铬电镀工艺严重污染环境,有望被三价铬电镀取代.三价铬电镀分为氯化物体系和硫酸盐体系,其中硫酸盐体系因具有阳极无有害气体析出、不腐蚀设备等优点,近年来发展迅速.通过大量试验开发出常温甲酸-草酸体系硫酸盐工艺,并借助小槽挂镀、Tafel曲线、EIS曲线和CASS试验等方法对镀液和镀层性能进行了分析.结果表明,在优化工艺条件下,该工艺镀层沉积速率高达0.18μm/min以上,分散能力93%,覆盖能力96%,耐蚀性好,是一种性能优良、高效的三价铬电镀工艺.     

三价铬电镀研究进展

为了取代电镀工业中严重污染环境的六价格电镀工艺，实现清洁生产，改善生态环境，人们开发了许多多代替六价铬电镀的工艺，其中三价铬电镀已经取得了长足的进展，在阳极，镀槽溶液成分和工艺条件等方面进行了多方面的改进，人们已经实现了可以长期稳定使用的三价铬电镀工艺。     

Growth and characterization of electrosynthesised zinc oxide thin films

Zinc oxide (ZnO) films have been electrodeposited from an aqueous solution containing 0.1 M zinc nitrate as the electrolyte with pH around 5±0.1. The deposition was carried out by galvanostatic reduction with an applied cathodic current density in the range between 5 and 20 mA cm⁻². The influence of bath composition on the preparation of ZnO films is studied. The effects of zinc nitrate concentration and cathodic current density on the deposition rate of ZnO films were also studied. An optimum current density of 10 mA cm⁻² is identified for the growth of ZnO film with improved crystallinity and optical transmittance. The crystalline structure of the deposits studied by X-ray diffraction reveals the possibility of growing hexagonal ZnO films under suitable electrochemical conditions. The surface morphological studies by scanning electron micrographs revealed the presence of nodular appearance for films deposited at 800 °C bath temperatures.     

Single and binary chromium(VI) and Remazol Black B biosorption properties of Phormidium sp.

Wastewaters of textile and leather dying industries may contain significant quantities of chromium(VI) ions besides anionic and water-soluble dyes. Moreover the temperature of these wastewaters may be a controlling parameter affecting the biosorption efficiency. In this study biosorption of chromium(VI) and Remazol Black B reactive dye by dried Phormidium sp., a thermophilic cyanobacterium, was studied as a function of initial chromium(VI) concentration and temperature in no dye and 100 mg l⁻¹ dye-containing media at an initial pH value of 2.0 at which the biomass exhibited the maximum chromium(VI) and dye uptakes. The decrease of both metal and dye uptakes with temperature indicated that the uptakes were exothermic in nature. Equilibrium uptake of chromium(VI) enhanced considerably with both chromium(VI) and 100 mg l⁻¹ dye concentrations. Moreover the presence of chromium(VI) also increased the uptake of dye. At 25 °C, 22.8 mg g⁻¹ chromium(VI) and 91.3 mg g⁻¹ dye were sorbed by the biomass in binary 100 mg l⁻¹ chromium(VI) and 100 mg l⁻¹ dye-containing medium. The Langmuir was the best suitable adsorption model for describing the biosorption of chromium(VI) individually and in dye-containing medium. The pseudo-second-order kinetic model described both the chromium(VI) and dye biosorptions kinetics accurately.     

Microstructure and tribological property of nanocrystalline Co–W alloy coating produced by dual-pulse electrodeposition

The nanocrystalline Co–W alloy coatings were produced by dual-pulse electrodeposition from aqueous bath with cobalt sulfate and sodium tungstate (Na₂WO₄). Influence of the current density and Na₂WO₄ concentration in bath on the microstructure, morphology and hardness of the Co–W alloy coatings were investigated using an X-ray diffraction, a scanning electronic microscope and a Vickers hardness tester, respectively. In addition, the friction and wear properties of the Co–W alloy coating electrodeposited under different condition were evaluated with a ball-on-disk UMT-3MT tribometer. The correlation between the electrodeposition condition, the microstructure and alloy composition, and the hardness and tribological properties of the deposited Co–W alloy coatings were discussed in detail. The results showed that the microhardness of the deposited Co–W alloy coating was significantly affected by its average grain size, W content and crystal orientation. Smaller grain size, higher W content and strong hcp (1 0 0) orientation favor the improvement of the hardness for Co–W alloy coatings. The deposited Co–W alloy coating could obtain the maximum microhardness over 1000 kgf mm⁻² by careful control of the electrodeposition conditions. The tribological properties of the electrodeposited Co–W alloy coating were greatly affected by its grain size, microhardness, surface morphologies and composition, and could be significantly improved by optimizing the electrodeposition condition.     

Effect of annealing on the characteristics of Au/Cr bilayer films grown on glass

Au layers with thickness of about 110 nm were sputter-deposited on unheated glass substrates coated with a Cr layer about 20 nm thick. The chamber was evacuated to a pressure of 2 Pa and then sputtering was carried out at Ar pressure of 4 Pa. The Au/Cr bilayer films were annealed in a vacuum of 5×10⁻⁴ Pa at 170°C, 180°C, 200°C and 250°C for from 5 to 120 min, respectively. Atomic force microscopy was used to observe the structural characteristic of the bilayer films. Auger electron spectroscopy was used to analyze the composition inside the Au layers. The sheet resistance of the films was measured using the four-point probe technique. The grain size of the bilayer film gradually increases with an increase in annealing temperature while its average surface roughness ranging from 4.5 to 6.8 nm does not show any systematic change with annealing temperature and time. No impurities such as carbon, nitrogen and oxygen are detected inside all of the Au layers. When the annealing temperature reaches 200°C and the annealing time exceeds 30 min, chromium atoms markedly diffuse into the Au layer. Furthermore, for the bilayer films annealed at 250°C, chromium atoms have markedly diffused into the Au layer even for annealing time of 5 min. Regardless of the increase in grain size of the Au layer, the diffusion of chromium atoms into the Au layer causes an increase in the resistivity of the bilayer film.     

Electrochemical preparation of Fe-Mn alloy film in organic bath

Fe-Mn alloy films have been prepared by electrodeposition in an organic bath containing FeCl₂ + MnCl₂ in dimethyl formamide. The electroreduction of Mn(II) was irreversible and the diffusion coefficient of Mn(II) was calculated to be 8.0 × 10^− 11 m² s^− 1 at 298 K. An amorphous film of Fe-Mn was obtained by potentiostatic electrolysis. The Mn content varied from 4.8 at.% to 72.3 at.% with increase in the applied cathodic potential. Scanning electron microscope investigation showed that the deposited film was homogeneous and consisted of spherical particles. Nano-sized pores were observed in the surface of these particles. After heat treatment at 773 K, large crystal grains formed and X-ray diffraction patterns indicate that solid solution of Mn in γ-Fe occurred. The alloying temperature of the Fe-Mn film was determined to be 1013 K using differential thermal analysis.     

三价铬镀液电沉积装饰性铬-磷合金层的特性

为了进一步提高三价铬镀层的性能,在硫酸盐三价铬镀液中加入次磷酸钠,电沉积出了装饰性铬-磷合金层(磷含量为9%～16%).研究了磷含量对镀液和镀层性能的影响,对比了电沉积铬镀层、三价铬-磷合金镀层和六价铬镀层的耐蚀性能.结果表明:铬-磷合金镀层为非晶态结构,具有良好的外观和耐蚀性能,能够很好地满足防护装饰性要求.     

三价铬镀铬阳极的研究

三价铬镀铬在实际应用中最大的问题是工艺的稳定性差,而阳极是影响硫酸盐三价铬镀液稳定性的关键.阐述了硫酸盐三价铬镀铬阳极的制备方法及工艺条件,分别采用电化学方法、扫描电镜、赫尔槽试验测定了阳极的电化学性能、外观形貌及电镀性能;用强化寿命试验测定了阳极的强化寿命时间.在此基础上测定了自制专用阳极在硫酸盐三价铬镀液中所得镀层的外观形貌、结合力、孔隙率及耐蚀性等性能.结果表明:所制备的阳极使用寿命长,电镀及镀层性能好.     

Toward a carbon nanotube anode gas-filled radiation detector

A prototype gas-filled proportional counter (PC) based on micro-scale tungsten wire and carbon fiber, and nano-scale carbon nanotube (CNT) anodes was built and tested with a ⁹⁰Sr source. Tungsten anodes of 500 μm down to 4 μm diameter were used to observe the gradual decrease in operating voltage for the proportional region with a decreasing anode diameter. The 40 nm diameter CNTs anodes ranged in length from 35 to 105 μm. The absolute detection efficiency was measured at ∼10⁻⁶%. An electrostatic computer model was used to predict the resulting electric field associated with a single CNT in the coaxial configuration. For a single anode coaxial design the model predicted that the electric field was insufficient for secondary ionizations which contributed to a low amplitude signal and that the small volume of the ionization region resulted in the low absolute detection efficiency. To overcome the problems of low absolute detection efficiency and operational issues with the single anode, CNT arrays were investigated. Electrostatic modeling of 100 nm×40 μm long CNTs in an array with a 50 μm pitch conducted for a parallel plate configuration indicated that each anode functioned independently.     

三价铬镀液中配体的作用

三价铬镀铬液中的配体对镀液的稳定性和沉积速度影响极大,从三价铬镀液的化学和电化学特性分析入手,介绍了镀液中所加配体(如羧酸、羟基羧酸、氨基羧酸及其盐)的作用:(1)与三价铬离子形成活性配位离子加快电沉积速度;(2)抑制Cr3 的羟桥化反应;(3)可以掩蔽杂质金属离子,减少杂质金属离子对镀层质量的干扰,使电镀能持续进行;(4)可以稳定镀液.特别指出,只有选用能形成活性配位离子的配体,使电镀能持续进行,才能获得性能良好的厚铬镀层.     

Physico-chemical properties of chromium sesquisulphide

The range of stoichiometry and defect structure of chromium sesquisulphide, Cr₂S₃, have been determined for a range of temperature (1123 to 1273 K) and sulphur pressure (1.06×10^–1 to 1.77×10⁴ N m^–2) employing chemical analysis, marker and thermoelectric power measurement techniques. It has been shown that the upper limit of stoichiometry corresponds to CrS_1.54, while the lower limit extends to at least CrS_1.30. The conditions (temperature and sulphur pressure) under which the compound is stable at compositions intermediate to these values have been determined for S/Cr ratios at intervals of 0.02.It has been established that Cr₂S₃ is ann-type metal-excess compound within these composition limits, containing unassociated trivalent chromium interstitials and/or trivalent chromium interstitials associated with one quasi-free electron, as the predominant mobile defect species.     

Photoelectronic processes in transition-element doped n-type TiO2 electrodes

Photoelectronic properties of n-TiO₂ anodes doped with first row transition elements and giving an absorption in the visible region, have been investigated. The photocurrent has been measured vs wavelength for a given electrode potential and vs time at different anode potentials. Significant photocurrents resulting from visible light excitation are observed for n-type TiO₂ anodes doped with large amounts of trivalent chromium. When other transition elements (V³⁺ or Mn²⁺) are considered, such a long wavelength photoresponse was not observed. Co²⁺ and Ni²⁺ ions could not be introduced into n-TiO₂ anodes because of the reduction into Co or Ni metal. The experimental data have been discussed on the basis of a model involving hole tunneling from Cr⁴⁺ ions to the valence band.     

Purification of spent chromium bath by membrane electrolysis

The present study deals with the purification of spent chromium bath contaminated by trivalent chromium, iron and aluminum. The ionic transfer of Fe(III) depends on the presence of chloride ions on the pH while aluminum transfer is not affected by chromium(III) chloride. Five different commercial cation-exchange membranes were used. Nafion and PC-SK membranes showed the best results for trivalent iron and aluminum transfer.     

Study on preparation of NiOOH by a new catalytic electrolysis method

The present paper reports a new catalytic electrolysis method to prepare NiOOH. KMnO₄ is proposed as a catalyst to play the role of electron-transfer medium in the electrolysis preparation of NiOOH for the first time. Through the self-redox reaction of KMnO₄, the highly efficient electron-transfer process between the electrolyte and the electrode of the spherical Ni(OH)₂ is realized, thus resulting in a high electrolytic efficiency and short electrolysis time. The mechanism of catalytic electrolysis is preliminarily discussed. The experimental results show that the electrode prepared with the NiOOH powders by catalytic electrolysis offers a discharge capacity of 267 mAh g⁻¹ at a current density of 120 mA g⁻¹ and exhibits good cycling performance.