Development of a microwave-assisted digestion method using ICP-OES to measure metals in gum deposits of internal combustion engines

The heterogeneous nature of gum samples obtained from the deposits of internal combustion engines is the main difficulty in accurate determination of their metal content. A microwave-assisted digestion method was implemented using factorial experimental design. The optimization of this procedure was carried out by first evaluating the effects of variables on the response (i.e., the residual carbon content or RCC). The variables of maximum heating power, heating time and nitric acid volume were studied. The RCC response was measured by inductively coupled plasma optical emission spectrometry (ICP-OES). The time of maximum heating power was essential to obtain a desired RCC. The surface response was constructed with optimal conditions presented at 6 min and heating power of 700 W. Amounts of aluminum (Al), calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) were determined in 26 gum samples by ICP-OES. Ca, Fe, Mg and Zn were found at mg g⁻¹ levels in the samples, while Al, Cu, Mn, Ni and Pb were found at mg kg⁻¹ levels. Principal component analysis (PCA) was used to establish a correlation between the gum samples and the different metal contents. Three distinct groups were separated according to the characteristics of the collected samples.     

Obtaining of high surface roughness copper deposits by electroless plating technique

The use of pyridine-2,6-dicarboxylic and 4-hydroxypyridine-2,6-dicarboxylic acids as copper(II) ligands in formaldehyde-containing alkaline electroless copper plating solutions allowed to obtain copper layers with extremely high surface roughness factor reaching approximately 120. The Cu deposits of higher surface area were formed at highly negative open-circuit (mixed) potentials; the correlation between copper electrode overpotential and roughness of the deposit was found and discussed. The copper films obtained demonstrate a high electrocatalytic activity in anodic formaldehyde oxidation process, the oxidation rate reaches 40 mA cm⁻² and exceeds considerably that for other copper surfaces.     

Study on the preparation of Mg-Li-Zn alloys by electrochemical codeposition from LiCl-KCl-MgCl2-ZnCl2 melts

Electrochemical codeposition of Mg, Li, and Zn on a molybdenum electrode in LiCl-KCl-MgCl₂-ZnCl₂ melts at 943 K to form Mg-Li-Zn alloys was investigated. Cyclic voltammograms (CVs) showed that the potential of Li metal deposition, after the addition of MgCl₂ and ZnCl₂, is more positive than the one of Li metal deposition before the addition. Chronopotentiometry measurements indicated that the codeposition of Mg, Li, and Zn occurs at current densities lower than −0.78 A cm⁻² in LiCl-KCl-MgCl₂ (8 wt.%) melts containing 1 wt.% ZnCl₂. Chronoamperograms demonstrated that the onset potential for the codeposition of Mg, Li, and Zn is −2.000 V, and the codeposition of Mg, Li, and Zn is formed when the applied potentials are more negative than −2.000 V. X-ray diffraction (XRD) indicated that Mg-Li-Zn alloys with different phases were prepared via galvanostatic electrolysis. The microstructure of typical α + β phase of Mg-Li-Zn alloy was characterized by optical microscope (OM) and scanning electron microscopy (SEM). The analysis of energy dispersive spectrometry (EDS) showed that the elements of Mg and Zn distribute homogeneously in the Mg-Li-Zn alloy. The results of inductively coupled plasma analysis showed that the chemical compositions of Mg-Li-Zn alloys are consistent with the phase structures of the XRD patterns, and that the lithium and zincum contents of Mg-Li-Zn alloys depend on the concentrations of MgCl₂ and ZnCl₂.     

Cyclic oxidation processes during anodizing of Al-Cu alloys

The influence of copper on the morphologies of porous anodic alumina has been investigated under current and voltage control using a sputtering-deposited Al-2.7 at.% Cu alloy and a commercial AA 2024-T3 aluminium alloy anodized in either sulphuric acid electrolyte or the same electrolyte but with addition of tartaric acid. The findings indicate that film development involves repeated formation of embryo cells of anodic alumina at the metal/film interface. During the initial stages of anodizing at constant voltage, cell formation is accompanied by current peaks in the current-time response. The porosity of the resultant films has a lateral aspect due to the layering of embryo cells. The thickness of individual layers is proportional to the formation voltage, with a ratio of the order 1 nm V⁻¹. The cell formation is accompanied by enrichment of copper in the alloy, incorporation of copper species into the anodic film, in low amounts relative to the alloy, and evolution of oxygen. These processes disrupt the formation of the classical pore morphology, characteristic of high purity aluminium, due to continuous formation of fresh embryo cells and re-direction of pores. The main effect of the tartaric acid addition to the sulphuric acid was to reduce the rate of anodizing of the alloys at constant voltage by about 10-20%.     

Zinc deposited polymer coatings for copper protection

Polypyrrole (PPy) and polyaniline (PAni) coatings were electrosynthesized on copper, by using cyclic voltammetry technique. Then, these coatings were modified with the deposition of zinc particles from aqueous zinc sulphate solution. The electrodeposition of zinc was achieved at a constant potential value of −1.20 V, in the amount of ∼0.75 mg/cm². The corrosion performance of zinc modified polymer coatings were investigated in 3.5% NaCl solution; by using the electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The zinc particles improved the barrier property of polymer films, thanks to formation of voluminous zinc corrosion products within the pores of polymer coating. Also, the zinc particles provided cathodic protection to the substrate, where the polymer film played the role of conductance between zinc particles and copper.     

High throughput electrochemical screening and dissolution monitoring of Mg–Zn material libraries

A combinatorial study on Mg–Zn material libraries obtained by thermal evaporation is performed in order to investigate the effect of alloying magnesium on the electrochemical behaviour and dissolution rate of zinc in borate buffer of pH 7.4. The surface morphology of the graded samples is complex and subject to a detailed discussion, whereas the crystal composition revealed Mg, MgZn₂ and Zn exclusively.Open circuit potential measurements and potential sweeps along the graded samples are combined with downstream zinc detection and revealed several strongly non linear dependencies between electrochemical features and magnesium content. While the chemical dissolution rate of zinc by the electrolyte was found to reflect the film stoichiometry except in the regions of high surface roughness, the open circuit potential revealed a local minimum around 20 at.% magnesium accompanied by a maximum in the current plateau during the anodic sweep and a high thickness of the native oxides present prior to electrochemical experiments. All compositions showed passive like behaviour during anodic sweeps with high plateau current densities (200–350 μA cm⁻²) originating from slow but constant oxide dissolution as supported by XPS analysis of the surface before and after contact with the electrolyte.     

Spectroelectrochemical characterisation of copper salen-based polymer-modified electrodes

Electrogenerated polymers based on copper salen-type complexes were characterised electrochemically and by in situ UV-vis and ex situ EPR spectroscopy. The films, poly[Cu(salen)] and poly[Cu(saltMe)], exhibit reversible oxidative electrochemical behaviour in a wide potential range (0.0-1.5 V). Different regimes for charge transport behaviour are accessed by manipulation of film thickness and experimental time scale: thin films (surface concentration, Γ < ca. 80 nmol cm⁻²) show thin-layer/surface behaviour in the scan rate range used (0.020-2.0 V s⁻¹), whereas thicker polymers (Γ > ca. 90 nmol cm⁻²) exhibit a changeover from thin-layer to diffusion control regime at a critical scan rate that depends on polymer and film thickness: 0.15-0.20 V s⁻¹ for poly[Cu(salen)], 90 < Γ < 130 nmol cm⁻² and 0.20-0.30 V s⁻¹ for poly[Cu(saltMe)], 170 < Γ < 230 nmol cm⁻².UV-vis and EPR spectroscopies have allowed the characterisation of electronic states in the reduced and oxidised forms. The role of the copper atom during film oxidation was probed by combining UV-vis data with EPR on copolymers of the copper and nickel complexes. Data from both techniques are consistent and indicate that polymerisation and redox switching are associated with ligand-based processes. EPR of Ni-doped Cu polymers provided evidence for the non-involvement of the metal centre in polymer oxidation; like the analogous nickel polymers, copper polymers behave like delocalised π-system (‘conducting’) rather than discrete site (‘redox’) polymers.     

High corrosion resistance of magnesium coated with hydroxyapatite directly synthesized in an aqueous solution

Anticorrosion coatings are crucial for practical applications of magnesium alloys, which are used to reduce the weight of vehicles, aircraft, electronics enclosures etc. Hydroxyapatite (HAp) potentially offers high corrosion resistance and no environmental toxicity because its thermodynamic structural stability is high and it is a basic component of bone. However, direct synthesis of HAp on magnesium in aqueous solutions has been a scientific challenge because Mg ions prevent HAp crystallization. A new method of direct synthesis of HAp on magnesium was developed using a Ca chelate compound, which can maintain a sufficiently high concentration of Ca ions on the magnesium surface to overcome prevention of HAp crystallization with Mg ions. Highly crystallized HAp coatings were successfully formed on pure magnesium and AZ series alloys. Corrosion behavior of HAp-coated pure magnesium was examined by cyclic dry and wet tests with 1 g m⁻² NaCl on the surface and polarization tests in a 3.5 wt% NaCl solution. A HAp-coated pure magnesium showed no noticeable corrosion pits after the dry and wet test. HAp-coated specimens showed 10³-10⁴ times lower anodic current density than as-polished specimen in the polarization test. The results demonstrate the remarkable anticorrosion performance of HAp coatings on magnesium for the first time.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Influence of 8-aminoquinoline on the corrosion behaviour of copper in 0.1 M NaCl

The corrosion behaviour of copper in aerated 0.1 M NaCl solution in presence of 8-aminoquinoline (8-AQ), using open circuit potential (OCP) measurements, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) measurements and atomic force microscopy (AFM), was studied. The measurements revealed that the effect of 8-AQ is dependent on its concentration. For concentrations up to 10⁻³ M, the organic compound displaces the corrosion potential following no trend and also reduces the anodic current. In contrast, for concentrations higher than 10⁻³, 8-AQ reduces markedly both, the anodic and cathodic currents and consequently, the corrosion current density of copper. After 9 days of exposure in chloride solution, containing the organic compound, potentiodynamic polarization analyses showed a significant reduction in the anodic response and a less significant reduction in the cathodic response, which is associated with a film formed at the copper surface of about 10 μm in thickness and visually observed by a colour change of the copper surface.In order to elucidate the most likely interaction between the 8-AQ molecule and the different molecular structures probably present on copper surfaces in chloride solutions, some results obtained from theoretical calculations are presented. The following molecular structures were considered: CuCl molecule, CuCl₂⁻ complex, and little copper clusters defect representation built as five atoms on C_4v symmetry. Thus, based on the geometric, energetic, frontier orbital, and Total Electronic Density analysis done for the optimized states found for the systems investigated, we suggest that the most probable interaction of 8-AQ proceeds above CuCl units and free copper sites.     

Oxide film growth on Al-In alloys in a borate buffer solution in conditions of galvanostatic anodising

Anodic oxide films have been formed galvanostatically on Al-In alloys (containing up to 0.074% In) in a borate buffer solution (pH 7.8) at different current densities (20-100 μA cm⁻²). The mechanism, kinetics of growth and properties of formed oxide films have been investigated. The study of charge curves suggests that the growth of oxide films on Al-In alloys occurs by an activation-controlled ionic conduction under the influence of the high electric field through the oxide film according to an exponential law, like on valve metals. The following parameters have been calculated: the constants of the exponential law, ionic conductivity through the film, the effective activation distance for ion movement and the corresponding field strength. The values for the field strength, of the order of magnitude of 10⁶ V cm⁻¹, justify the application of the high field migration mechanism. Properties of anodic oxide films have been determined by means of electrochemical impedance spectroscopy; the resistance and thickness of the oxide film have been found to increase with the increase in the indium content in the alloy and with increased anodic current density. It has been established that the current efficiency in oxide films formation on Al-In alloys is lower than 100%: the increase of the indium content in the alloy, as well as the increase in anodic current density, increases the value of current efficiency.     

Electrochemical investigations of the oxidation-reduction of furfural in aqueous medium: Application to electrosynthesis

The present study concerns the electrochemical properties of furfural in aqueous medium on noble (Au and Pt) and non-noble (Pb, Cu and Ni) metal electrodes. The anodic and cathodic reactions are investigated by cyclic voltammetry on Au, Pt and Ni electrodes and during prolonged electrolyses on Pt, Pb and Cu in order to find the optimum conditions for a paired electrosynthesis. Anodic reactions are controlled by diffusion in the range of the stability of the solvent (water). Beside these limits, the gas evolution competes with the conversion of furfural. The best conditions for preparative electrooxidation (Ni anode, 0.5 M NaOH, j=0.8 mA cm⁻¹) gave furoic acid in a 80% yield and furfuryl alcohol was obtained by electroreduction in a 55% yield on Cu cathodes at pH 10 and 30 mA cm⁻².     

Hydrometallurgical process for the recycling of copper using anodic oxidation of cuprous ammine complexes and flow-through electrolysis

Flow-through electrolysis for copper electrowinning from cuprous ammine complex was studied in order to develop a hydrometallurgical copper recycling process using an ammoniacal chloride solution, focusing on the anodic oxidation of cuprous to cupric ammine complexes. The current efficiency of this anodic oxidation was 96% at a current density of 200 A m⁻² under a batch condition. In a flow-through electrolysis using a sub-liter cell and a carbon felt anode, the anodic current efficiency increased with the flow rate and was typically higher than 97%. This tendency was explained by the backward flow of the cupric ammine complex, which was formed on the anode, through the diaphragm. The anodic overpotential was lower than 0.3 V even at an apparent current density of 1500 A m⁻². A similar current efficiency and overpotential were also achieved in a liter scale cell, which indicates the scale flexibility of this electrolysis. The power consumption requirements for copper electrowinning in this cell were 460 and 770 kWh t⁻¹ at the current densities of 250 and 500 A m⁻², respectively, which were much lower than that of the conventional copper electrowinning despite the longer interpolar distance.     

Effect of magnesium on the electrochemical behavior of lithium anode in LiOH aqueous solution used for lithium-water battery

The effect of adding magnesium to lithium on lithium electrochemical behavior in 4 mol L⁻¹ LiOH is studied using electrochemical techniques. The results show that the hydrogen evolution rate is decreased with increasing Mg content. Through theoretical analysis and X-ray (XRD) investigation, MgH₂ and Mg(OH)₂ are created on the surface film of lithium-magnesium alloys after discharge. The porosity of the lithium surface film is decreased by MgH₂, Mg(OH)₂ combined with LiOH and LiOH·H₂O and the hydrogen evolution rate is decreased effectively. Magnesium addition to lithium reduces the extent of hydrogen evolution by alteration of the hydride-hydroxide layer also reduces the extent of anodic dissolution without a significant change in the system efficiency.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Effect of metals on silver electrodeposition: Application to the detection of cisplatin

In the present work, the influence of several metals (Co, Ru, Pd, Os, Pt, Cu, Pb), deposited on a carbon paste electrode, towards silver electrodeposition was tested. First, adequate conditions for the electrodeposition of metals on the electrode were found. Then, the cyclic voltammograms registered (silver deposition curves and analytical signals) showed that Co, Cu, Pt and Pd were able to accelerate silver electrodeposition. Finally, a valid methodology for the detection of cisplatin was established. It is based on the deposition of silver on a Pt (from cisplatin) modified electrode and the analytical signal corresponds to the anodic stripping of the deposited silver. A limit of detection of 3.2 × 10⁻⁹ mol dm⁻³ (1 ng cm⁻³) cisplatin was obtained.     

Nucleation of Sn and Sn-Cu alloys on Pt during electrodeposition from Sn-citrate and Sn-Cu-citrate solutions

The initial stages of Sn and Sn-Cu electrodeposition from Sn-citrate and Sn-Cu-citrate solutions on Pt were studied using both current-controlled and potential-controlled electrochemical techniques. For both Sn-citrate and Sn-Cu-citrate solutions, when the current density is controlled to lower than 15 mA/cm², potentials remain almost constant which is appropriate to plate dense and uniform films. When the current density is controlled to between 25 and 35 mA/cm², potentials drop quickly initially, followed by a gradual increase to a constant value. When current density is controlled to higher than 50 mA/cm², potential oscillation happens, and significant hydrogen evolution prevents the formation of dense and continuous Sn and Sn-Cu films. A constant transition time constant indicates a diffusion-controlled process. The diffusion coefficient calculated from the Sand equation is about 3.8 × 10⁻⁶ cm²/s for the Sn-citrate solution and 4.1 × 10⁻⁶ cm²/s for the Sn-Cu-citrate solution. The morphology of both Sn and Sn-Cu deposits plated under different potentials was examined by atomic force microscopy (AFM) and the distribution of each element were analyzed using Auger imaging. Analysis of both the electrochemical results at −0.72, −1.1 and −1.5 V and AFM images for both Sn and Sn-Cu deposits at −1.1 and −1.5 V suggested progressive nucleation controlled by diffusion for both Sn and Sn-Cu electrodeposition. Tin reacted with Pt to form PtSn₄, and co-deposited with Cu to form Cu₆Sn₅ during nucleation, with more Sn forming at higher applied potentials.     

Electrically conducting polypyrrole-fuller's earth nanocomposites: Their preparation and characterisation

Cu(II)-exchanged fuller's earth was prepared by ion-exchanging Ca²⁺ ions which are present within the interlayer of fuller's earth with Cu(II)ions by the solution-phase ion-exchange process. Pyrrole was introduced into Cu(II)-exchanged fuller's earth to spontaneously polymerize to within the interlayer to result in a nanocomposite of Cu(I)-polypyrrole-fuller's earth where both Cu(I) and polypyrrole occupy within the interlayer spaces of fuller's earth. The nanomaterial [Cu(I)-PPY-FE] has been fully characterized with X-ray diffraction studies, FTIR spectroscopy, DC polarisation test with both blocking stainless steel and non-blocking copper electrodes. The material is found to be a mixed conductor whose ionic mobility is 1.5 times faster than electronic mobility. DC polarisation studies also clearly revealed that the mobile ionic species in this material to be cuprous ions. AC impedance studies have been carried out with blocking stainless steel electrodes at different applied potentials. The necessary theoretical background to explain AC impedance results is also provided and the results obtained agree very well with the corresponding data obtained by other mutually independent methods. The electronic conductivities are around 3.0 × 10⁻⁴ S cm⁻¹ and the ionic conductivities are around 9.0 × 10⁻³ S cm⁻¹. The material may find applications in semi-fuel cells such as air-metal batteries.