Dissolution Of Zn Research Articles

The anodic and cathodic dissolution of α-phase Zn-68Al in alkaline media

The mechanisms of Zn and Al dissolution of a pure α-phase Al-Zn in an alkaline electrolyte have been characterized using atomic emission spectroelectrochemistry to decouple the elementary dissolution rates. It was found that the selective dissolution of Al leads to the formation of a metallic Zn(0) surface layer. In the metallic form, this layer did not seem to affect the dissolution rate of Al. However, its oxidation led to a significant decrease in the Al dissolution rate, referred to as the negative correlation effect. These mechanisms were identified and characterized via linear sweep voltammetry and potentiostatic experiments.

Onboard experiment investigating metal leaching of fresh hydrothermal sulfide cores into seawater

We observed the initial release rate of metals from four fresh (i.e., without long time exposure to the atmosphere) hydrothermal sulfide cores into artificial seawater. The sulfide samples were collected by seafloor drilling from the Okinawa Trough by D/V Chikyu, powdered under inert gas, and immediately subjected to onboard metal-leaching experiments at different temperatures (5 °C and 20 °C), and under different redox conditions (oxic and anoxic), for 1–30 h. Zinc and Pb were preferentially released from sulfide samples containing various metals (i.e., Mn, Fe, Cu, Zn, Cd, and Pb) into seawater. Under oxic experimental conditions, Zn and Pb dissolution rates from two sulfide samples composed mainly of iron disulfide minerals (pyrite and marcasite) were higher than those from two other sulfide samples with abundant sphalerite, galena, and/or silicate minerals. Scanning electron microscopy confirmed that the high metal-releasing sample contained several galvanic couples of iron disulfide with other sulfide minerals, whereas the low metal-releasing sample contained fewer galvanic couples or were coated by a silicate mineral. The experiments overall confirmed that the galvanic effects with iron disulfide minerals greatly induce the initial release of Zn and Pb from hydrothermal sulfides into seawater, especially under warm oxic conditions.

Pseudomorphic Rhythmically Banded and Oscillatory Tetrahedrite–Tennantite Aggregates in the Darasun Gold Deposit (Eastern Transbaikalia, Russia): A Result of Coupled Dissolution–Reprecipitation Reactions

The rhythmically banded aggregates of Fe–tennantite with oscillatory zoning resulting from simultaneous variations in the concentrations of Sb and As and Zn and Fe were registered in the Darasun gold deposit (Eastern Transbaikalia). These aggregates were formed upon pseudomorphic replacement of early Zn–tetrahedrite. This process was initiated by the dissolution of Zn–tetrahedrite with the formation of an aggregate of galena, bournonite, and late tetrahedrite–tennantite. It is shown that rhythmically banded aggregates with an oscillatory zoning were formed as a result of coupled dissolution–reprecipitation reactions under the conditions that are far from the mineral–fluid chemical equilibrium and variations in the fluid composition controlled by kinetic factors.

Dihydropyridine derivatives as controllers for production of hydrogen during zinc dissolution

Two of dihydropyridine derivatives, namely, 4,6-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile (DPCN) and 4,6-dimethyl-2-oxo-1,2-dihydropyridine-3-carboxamide (DPCA) were evaluated as controllers for H2 production during the dissolution of Zn in dilute HCl. Different chemical (thermometry, gravimetry, and gasometry) and electrochemical (potentiodynamic polarization and impedance) techniques were used. SEM and optical profilometry surface investigations for some corroded and un-corroded Zn samples were carried out to describe the Zn surface during the dissolution process. The rate of H2 production was found to increase with the immersion time and temperature and was lowered by the inhibitor. The potentiodynamic data proved that DPCN and DPCA inhibit the dissolution of Zn according to a mixed-type mechanism.

Formation mechanism of zinc-doped fayalite (Fe2-xZnxSiO4) slag during copper smelting.

The interactions between Fe2SiO4 and ZnO play an essential role in the recovery of zinc from copper slag. The dissolution and substitution mechanism of ZnO in fayalite were investigated by using TG-DSC, XRD, PPMS DynaCool, XPS, Mossbauer and SEM-EDS analyses and compared with MS calculation results. The results indicate that the dissolution and substitution are actually processes of the penetrating dissolution of Zn(II) ions that can be divided into three steps: 1) ZnO dissociates into Zn1-yO and Zn(II) ions; 2) Zn(II) penetrates the gap of the octahedron outer layer to substitute Fe(II) sites in the internal structure of SiOFe(II) (M2) to form (Fe2-x, Znx)SiO4; 3) Fe(II) is forced to migrate to the surface of (Fe2-x, Znx)SiO4 to form (Zn1-y, Fe(II)y)O. These findings can be derived the occurrence state and distribution of zinc in copper slag theoretically.

Interactions between elemental components during the dealloying of Cu-Zn alloys

The elemental dissolution of Cu-Zn alloys was investigated as a function of Zn content ranging from 0 to 45 wt%. Atomic emission spectroelectrochemistry (AESEC) was utilized to directly monitor Cu2+ and Zn2+ release and oxide growth as function of time during potentiodynamic experiments. It was determined that Cu dissolution undergoes a simultaneous mechanism of Cu2O formation and Cu2+ release. The addition of Zn in Cu-Zn alloy does not measurably change the dissolution mechanism of Cu2+ and the rate of aqueous Cu2+ was only dependent on the potential. Zn dissolution was however blocked by the formation of a Cu(0) film which shifted the Zn dissolution in the anodic direction.

A Comparative Assessment on the Effect of Different Supplemental Iron Sources on the Bio-dissolution of Zn, Pb, Cd, and As from a High-grade Zn–Pb Ore

The present study investigates the influence of ferrous iron (as FeSO4) and ferric iron (as Fe2 (SO4)3), and pyrite (FeS2) on the ability of bacterial leaching of a high-grade sulfide Zn–Pb ore. In this regard, shake flask experiments were carried out at 5% (w/v) pulp density of the ore sample (having 40.7% Zn and 12.4% Pb initial metal content) using a consortium of mesophilic iron and sulfur-oxidizing acidophiles. A concentration of 0.04 mol/L of ferric iron in the leaching media was found to be optimum for zinc extraction without affecting growth of the microorganisms. Under this concentration, the dissolution of Zn, Pb, Cd, and As was found to be 57%, 0.2%, 0.03%, and 9.9% in 25 days. Using ferrous iron in the media, 0.16 mol/L of Fe2+ was found to be the optimum concentration for efficient bacterial growth and metal dissolution (54.6% Zn, 0.08% Pb, 0.03% Cd, and 10.2% As) from the sample in 25 days. On the other hand, using pyrite as the source of energy for bacterial growth, an initial 12-day lag period was observed when compared to the effect of ferrous iron in the media. Under the optimum concentration (test with 0.24 mol/L iron in the form of pyrite), the dissolution of Zn, Pb, Cd, and As was found to be 39.8%, 0.1%, 0.03%, and 10% in 25 days. The surface chemistry analysis indicated formation of a sulfur layer over the particle surface that hindered reagent diffusion and affected metal recovery through bioleaching.

Sealing ZnO nanorods for deeply rechargeable high-energy aqueous battery anodes

Rechargeable Zn-based batteries are a safe alternative to Li-ion for compatibility with aqueous electrolyte. Also, theoretical volumetric energy density of Zn-based batteries (e.g. Zn-air) is ~85% of lithium-sulfur battery. However, the rechargeability and specific capacity of Zn anodes are limited by passivation and dissolution. Here we report a ZnO@TiNxOy core/shell nanorod structure for deeply rechargeable Zn anodes. The small diameter (<500 nm) of ZnO prevents passivation and allows full utilization of active materials, while the thin and conformal TiNxOy coating mitigates Zn dissolution in alkaline electrolyte, mechanically maintains the nanostructure, and delivers electron to nanorods. As a result, the ZnO@TiNxOy core/shell nanorod anode achieves superior specific capacity and cycle life compared with bulk Zn foil and uncoated ZnO nanorod anodes. The discharge capacity of this anode is twice as large as that of the uncoated ZnO nanorod anode. Remarkably, our ZnO@TiNxOy nanorod anode achieves a much higher specific discharge capacity of 508 mA h/g(Zn) than that of many previously reported zinc anodes. It can deeply cycle >640 times (64 days) in a beaker cell and deliver excellent long-term electrochemical performance (more than 7500 cycles) when cycled under start-stop conditions. The nanoscale design principles reported here is an important step towards practical deeply rechargeable Zn anodes, and can potentially be applied to overcome intrinsic limitations of other battery materials that involve soluble intermediates or insulating discharge products.

Low-Cd tomato cultivars (Solanum lycopersicum L.) screened in non-saline soils also accumulated low Cd, Zn, and Cu in heavy metal-polluted saline soils.

Many reclaimed tidal flat soils feature high salinity and heavy metal (HM) accumulation. Consumption of vegetables cultivated in this type of cropland may cause health risks. Low-Cd tomato cultivars (Solanum lycopersicum L.) were identified in non-saline soil in our previous studies (Tan et al. 2014). However, further research should determine whether these low-Cd cultivars will maintain in the repeatability and stability in saline soil and whether they have low accumulation abilities for accompanying metals (such as Zn and Cu). A soil-pot trial was implemented to measure Cd, Zn, and Cu concentrations in low- and high-Cd cultivars of both common and cherry-type tomatoes grown on HM-polluted reclaimed tidal flat saline soil. Then, cultivar differences in dissolution of Cd, Zn, and Cu in soil and their uptake and redistribution in plants were analyzed. Results showed that the cherry type accumulated more Cd, Zn, and Cu than the common type. Low-Cd cultivars of both types in saline soil accumulated low concentrations of Cd, Zn, and Cu in fruits. Low HM accumulation in fruits is partly attributed to a low root/shoot (R/S) biomass ratio. Low amounts of soil HMs were dissolved because of the low level of rhizosphere organic compounds, which possibly decreased HM uptake by the roots. Low-Cd cultivars of both tomato types had a higher ability to retain HMs in the roots than their high-Cd cultivars. These findings may provide a scientific guidance for the safe cultivation of HM-polluted saline soils.

Dealloying kinetics and mechanism of porosity evolution in mechanically alloyed Ag25Zn75 powder particles

The mechanism and kinetics of dealloying of mechanically alloyed Ag25Zn75 powder particles was investigated at four different temperatures under free corrosion conditions in freely aerated 2 N HCl. The activation energies of 30.65 ± 2.08 kJ/mole and 41.51 ± 3.30 kJ/mole, corresponding to the dissolution of Zn and pore coarsening, respectively, were obtained. The pore formation is controlled by Zn dissolution from the alloy and coarsening is controlled by diffusion of Ag atoms on the alloy/electrolyte interface. Ag rearrangement was found to be the rate controlling phenomenon for the overall dealloying.

Combined DFT and Differential Electrochemical Mass Spectrometry Investigation of the Effect of Dopants in Secondary Zinc-Air Batteries.

Zinc-air batteries offer the potential of low-cost energy storage with high specific energy, but at present secondary Zn-air batteries suffer from poor cyclability. To develop economically viable secondary Zn-air batteries, several properties need to be improved: choking of the cathode, catalyzing the oxygen evolution and reduction reactions, limiting dendrite formation and suppressing the hydrogen evolution reaction (HER). Understanding and alleviating HER at the negative electrode in a secondary Zn-air battery is a substantial challenge, for which it is necessary to combine computational and experimental research. Here, we combine differential electrochemical mass spectrometry (DEMS) and density functional theory (DFT) calculations to investigate the fundamental role and stability when cycling in the presence of selected beneficial additives, that is, In and Bi, and Ag as a potentially unfavorable additive. We show that both In and Bi have the desired property for a secondary battery, that is, upon recharging they will remain on the surface, thereby retaining the beneficial effects on Zn dissolution and suppression of HER. This is confirmed by DEMS, where it is observed that In reduces HER and Bi affects the discharge potential beneficially compared to a battery without additives. Using a simple procedure based on adsorption energies calculated with DFT, it is found that Ag suppresses OH adsorption, but, unlike In and Bi, it does not hinder HER. Finally, it is shown that mixing In and Bi is beneficial compared to the additives by themselves as it improves the electrochemical performance and cyclic stability of the secondary Zn-air battery.

Fungal bioleaching of WPCBs using Aspergillus niger: Observation, optimization and kinetics

In this study, Aspergillus niger (A. niger) as an environmentally friendly agent for fungal bioleaching of waste printed circuit boards (WPCBs) was employed. D-optimal response surface methodology (RSM) was utilized for optimization of the bioleaching parameters including bioleaching method (one step, two step and spent medium) and pulp densities (0.5 g L−1 to 20 g L−1) to maximize the recovery of Zn, Ni and Cu from WPCBs. According to the high performance liquid chromatography analysis, citric, oxalic, malic and gluconic acids were the most abundant organic acids produced by A.niger in 21 days experiments. Maximum recoveries of 98.57% of Zn, 43.95% of Ni and 64.03% of Cu were achieved based on acidolysis and complexolysis dissolution mechanisms of organic acids. Based on the kinetic studies, the rate controlling mechanism for Zn dissolution at one step approach was found to be diffusion through liquid film, while it was found to be mixed control for both two step and spent medium. Furthermore, rate of Cu dissolution which is controlled by diffusion in one step and two step approaches, detected to be controlled by chemical reaction at spent medium. It was shown that for Ni, the rate is controlled by chemical reaction for all the methods studied. Eventually, it was understood that A. niger is capable of leaching 100% of Zn, 80.39% of Ni and 85.88% of Cu in 30 days.

Dissolution Induced Self-Selective Zn- and Ru-Doped TiO2 Structure for Electrochemical Generation of KClO3

We demonstrate an electrochemical approach to prepare a highly active and stable (Zn, Ru)-doped TiO2 (Ru0.26Ti0.73Zn0.01Ox) for electrochemical generation of KClO3. The essential ingredients of this approach consists of (1) co-electrodeposition of Ru, Ti and Zn as metallic alloy and then annealing it to oxide (2) dissolution under electrochemically oxidative acidic environment of Ru and Zn-rich surface clusters having high selectivity towards oxygen evolution reaction (OER) leaving Ti-enriched structure that is more selective to chlorine evolution reaction (CER) and (3) using this electrocatalyst for KClO3 production under near-neutral pH conditions. Dissolution of Ru- and Zn enriched surfaces result in porous structure with higher electrochemical surface area and roughness. Furthermore, this electrochemical dissolution of (Zn, Ru)-rich surface results in electrocatalytic structure that is self-selective towards KClO3 formation with higher specific activity. The co-electrodeposition of Zn aids (1) the formation of roughened and porous surface structure through surface dissolution of Zn- or ZnO clusters and (2) the enhancement of conductivity of electrocatalyst through formation of oxygen vacancies. The obtained structure has been found to show higher specific activity, activity and stability towards KClO3 production in comparison to untreated (Zn, Ru)-doped TiO2 or Ru-doped TiO2 or RuO2.

In-Depth Study of Zn Electrode Passivation in Alkaline Solutions for Zn Batteries

Recently, increasing demand for energy storage options has rekindled work on Zn based alkaline batteries, particularly Zn-Air battery systems. Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, Zn is abundant in the earth’s crust and aqueous alkaline electrolytes are cheap and easy to work with, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues associated with Zn electrode passivation need to be addressed that lead to a loss of performance during cycling. During the oxidation of Zn to discharge the battery, a passive layer of ZnO is formed on the electrode. This reduces the discharge current possible. Additionally, passivation is linked to electrode morphology changes between cycles, producing inconsistent performance and a reduction in capacity of the battery. Here we describe the current knowledge about Zn passivation and recent work done to better understand Zn passivation in alkaline solutions. Zn passivation is not well understood. There are multiple mechanism of passivation proposed and studies often have presented different results and conclusions despite using the same experimental conditions. Recently there have been calls in the literature for new in-depth studies that clear up the confusion present in the understanding of Zn passivation and dissolution processes in general[1, 2]. The Electrochemical Quartz Crystal Microbalance (EQCM) is a technique which can observe mass changes on an electrode at the order of 15 nanograms. The change in mass observed can be correlated to the charge passed at a given point during an experiment to give a value of mass change per mole of electron passed. This can then be used to determine the equivalent mass of the species being added to or removed to or from the electrode during an electrochemical reaction. EQCM experiments in KOH solutions of various concentrations and saturated to different levels with Zn have been conducted to observe the process by which the passivation layer forms on the surface and the process by which it is removed. Cyclic voltammogram (CV) experiments show that peaks associated with passivation removal give values of 8 to 16 grams per mole electron which corresponds well with the transition of ZnO or Zn(OH)2 to Zn metal on the surface. The transition of the passive layer to active Zn metal on the surface may contribute to electrode shape change. During the anodic dissolution part of the CV sweep, initially values near 32 grams per mole of electron are observed in the EQCM plots, corresponding to direct removal of Zn in a 2-electron reaction. The mass to charge ratios subsequently transition to values near 49 grams per mole electron which would indicate that ZnO or Zn(OH)2 is being formed and removed from the surface. This suggests that the formation of the passivating Zn species is potential dependent and is not solely precipitation of a film on the electrode surface. Linear sweep voltammetry and chronoamperometry experiments will be discussed as well as the impacts of additives to the solution on the electrochemistry of the electrode. Additionally, rotating disc electrode experiments were conducted using a Zn electrode in KOH solutions of various molarities saturated with Zn to understand the mass transport dependence of the passivation behavior. Changes in the location and height of peaks associated with passivation with rotation speed and sweep rate show a mass transport dependence on when passivation occurs but not on the removal of the passivation layer. Dissolution steps were conducted using Zn plate electrodes with different electrolyte concentrations and saturations of Zn at various potentials to observe major influencing factors in the passivation layer formation. These showed that passivation is heavily dependent on the amount of Zn in the solution as well as the potential applied. We conclude that the formation of the passive layer may depend on the ‘escape’ of the ZnO and Zn(OH)­2 species formed during dissolution from the electrode surface. This will be discussed in the context of the various proposed passivation mechanisms and will lead to some thoughts about improved electrode design. Acknowledgement We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). Bockelmann, M., et al., Electrochemical characterization and mathematical modeling of zinc passivation in alkaline solutions: A review. Electrochimica Acta, 2017Mainar, A., et al., Alkaline aqueous electrolytes for secondary zinc-air batteries: an overview. International Journal of Energy Research, 2016 Figure 1

Dissolution Induced Self-Selective Zn- and Ru-Doped TiO2 Structure for Electrochemical Generation of KClO3

An electrochemical approach towards synthesis of highly active, stable and surface selective towards chlorine evolution reaction (CER) of (Zn, Ru)-doped TiO2 (Ru0.26Ti0.73Zn0.01Ox) for the purpose of electrochemical generation of KClO3 is demonstrated. The essential ingredients of this approach consists of (1) co-electrodeposition of Ru, Ti and Zn as metallic alloy and subsequent annealing to oxide (2) dissolution under electrochemically oxidative acidic environment of Ru and Zn-rich surface clusters having high selectivity towards oxygen evolution reaction (OER) leaving Ti-enriched structure that is more selective to CER and (3) using this electrocatalyst for KClO3 production under near-neutral pH conditions. Dissolution of Ru- and Zn enriched surfaces result in higher porosity and surface roughness. Furthermore, this electrochemical dissolution of (Zn, Ru)-rich surface results in electrocatalytic structure that is self-selective towards KClO3 formation with higher specific activity.

Mass transport control of localised corrosion processes: in situ local probing and modelling

ABSTRACTIn this paper, the importance of mass transport in localised corrosion process is demonstrated using two examples describing the corrosion behaviour of two types of cut-edge electrodes regarding, (i) the sacrificial dissolution of a sacrificial Zn coating on steel (galvanised steel) and (ii) the release of inhibitors from a primer on an aluminium alloy (AA2024). In the two cases, in situ pH imaging with an ion-selective microprobe is used to confirm, (i) the role of corrosion products on the galvanic current profiles and (ii) the competition between the inhibition and the triggering of the local corrosion activity around intermetallic particles. It is illustrated how the experimental results can be supported by a 2D mass transport modelling.This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).

Process optimization and leaching kinetics of zinc and manganese metals from zinc-carbon and alkaline spent batteries using citric acid reagent

Zn-Carbon and Alkaline spent batteries contains heavy metals, such as zinc and manganese, which can causes environmental problem if not handled properly. Usually the recovery of these metals were done by leaching method using strong acid, but the use of strong acids as leaching reagents can be harmful to the environment. This paper concerns the recovery of Zn and Mn metals from Zn-C and alkaline spent batteries with leaching method using citric acid as the environmental friendly leaching reagent. The leaching conditions using citric acid were optimized and the leaching kinetics of Zn and Mn in citric acid solution was investigated. The leaching of 89.62% Zn and 63.26% Mn was achieved with 1.5 M citric acid, 90°C temperature, and 90 minutes stirring time. Kinetics data for the dissolution of Zn showed the best fit to chemical control shrinking core model, while the diffusion controlled model was suitable for the dissolution of Mn kinetics data. The activation energy of 6.12 and 1.73 kcal/mol was acquired for the leaching of Zn and Mn in the temperature range 60°C-90°C.

Isolation and characterization of potential Zn solubilizing bacteria from soil and its effects on soil Zn release rates, soil available Zn and plant Zn content

In this study, experiments were designed to isolate, characterize and evaluate an array of bacteria isolates for their Zn solubilization potential. Out of the six promising Zn solubilizing bacteria (ZnSB), ZnSB2 (B. megaterium, KY687496) was found to be the most potential strain owing to its enhanced Zn solubilization in vitro. In the quantitative study, the net Zn solubilized by ZnSB2 was significantly higher than those solubilized by the other ZnSB at all days of sampling. Similar effects of ZnSB2 was observed in the soil per se, wherein the rate of release of available Zn by ZnSB2 was markedly higher at all days of incubation (25.6%–40.7% of added Zn), with a peak on the 8th day. Such enhanced rates of Zn release by ZnSB2 were attributed to marked decrease in pH owing to enhanced gluconic acid production. In fact, gluconic acid production by ZnSB2 was 1884.7 ± 413.4 μg mL−1, which was 35.3–69.7% greater than the other shortlisted ZnSB isolates. Further evaluation of ZnSB2 was done in the green house using turmeric as the test crop. ZnSB2 was applied either alone or in combination with chemical Zn (75% and 100% of recommended Zn). The results revealed that soil available Zn level in the treatment with 75% Zn + ZnSB2 (12.69 ± 2.96 mg kg−1) was on par with the level in the treatment with 100% Zn (12.74 ± 2.63 mg kg−1) at 120 days after planting, while at harvest the treatment with 75% Zn + ZnSB2 maintained 65.0% higher available Zn levels than 100% Zn. The positive effect of ZnSB2 was also manifested on rhizome yield, which was at par in the treatments with ZnSB2 + 75% Zn (154.2 g ± 36.0 pot−1) and 100% Zn (177.2 ± 36.7 g pot−1). Besides, the Zn concentration in the rhizome was significantly higher (P < 0.05) in the treatment with ZnSB2 + 75% Zn (40.5 ± 3.5 mg kg−1), which was at par with 100% Zn, but was greater by 98.5% compared to control. The study indicated that ZnSB2 strain was a potential candidate for enhanced Zn dissolution in soil, which would allow reduced inorganic Zn application rates. Nonetheless, in vitro interaction studies (dual culture) suggested that this strain was seriously lacking in disease suppressing traits. But its compatibility with several plant growth promoting rhizobacteria enhanced the possibility of co-inoculation or applying ZnSB2 in a consortium mode especially in condition wherein both soil Zn solubilization and disease suppression becomes imperative.

Ni-P/Zn-Ni compositionally modulated multilayer coatings – Part 2: Corrosion and protection mechanisms

The Ni-P/Zn-Ni compositionally modulated multilayer coatings CMMCs were electrodeposited from a single bath by switching the deposition current density. The corrosion resistance of the deposits was studied and compared with that of monolayers of Ni-P and Zn-Ni alloys via Tafel polarization, EIS and salt spray tests. Characterization of corrosion products by means of EDS and XRD revealed more details from the corrosion mechanism of the monolayers and multilayers. The corrosion current density of Ni-P/Zn-Ni CMMCs were around one tenth of Zn-Ni monolayer. The CMMC with incomplete layers performed lower polarization resistance and higher corrosion current density compared to the CMMC with complete layers. The electrical circuit that was proposed for modeling the corrosion process based on the EIS spectrum, proved that layering reduces the porosity and consequently improves the barrier properties. Although, layering of Zn-Ni layers with Ni-P deposits increased the time to red rust in salt spray test, the time for white rust formation decreased. The corrosion mechanism of both Zn-Ni and Ni-P (containing small amount of Zn) was preferential dissolution of Zn and the corrosion products were comprised of mainly Zn hydroxychloride and Zn hydroxycarbonate. Also, Ni and P did not take part in the corrosion products. Based on the electrochemical character of the layers and the morphology of the corroded surface, the corrosion mechanism of multilayers was discussed.

Leaching of yttrium, europium and accompanying elements from phosphor coatings

The aim of this work was to study and describe leaching behaviour of Y, Eu and accompanying elements (Zn, Cd) from the phosphors of waste cathode ray tube (CRT) screens in low concentrated sulphuric acid solutions under both non-oxidizing and oxidizing conditions (H2O2). Thermodynamic study showed that non-oxidizing acid leaching leads to elemental sulphur and hydrogen generation followed by formation of H2S(g). In the presence of oxidant in the leaching system SO2(g) or H2SO4 formation is possible. PbSO4 was formed during leaching independently from the presence of the oxidant. Y, Eu and Zn dissolution without using H2O2 in 0.4M H2SO4 at 80°C attained 6, 9 and 8% respectively. The appropriate content of H2O2 in leaching solution was determined as 3.9v/v %. The highest leaching efficiency of Y and Eu (around 90–95% after 20min) was achieved by using the leaching solution (400ml 0.4M H2SO4, 60ml 30v/v % H2O2) at 80°C and L/S ratio 23. Under these conditions Zn extraction was relatively low (20%) in comparison with Cd extraction (80%). The study also reveals that high leaching efficiency of both Y and Eu (around 90%) was achieved with even more diluted leaching solution (400ml 0.05M H2SO4, 60ml of 30v/v % H2O2) at 60°C, L/S ratio 23 after 20min of leaching. Zn and Cd extractions reached 20% and 55% respectively. Activation energy values calculated for Y and Eu leaching in the solution (400ml 0.4M H2SO4 with 60ml of 30v/v % H2O2) refer to diffusion mechanism in the range 40–80°C (Ea=14.85kJ/mol and 16.47kJ/mol respectively). In contrast, Ea values for Y and Eu leaching in the solution (400ml 1M H2SO4, 60ml of 30v/v % H2O2) suggest a mechanism change at the temperature around 40°C. The leaching of Y as well as Eu up to 40°C is controlled by diffusion and at higher temperatures by chemical reaction.