Influences of structure and composition on the photoelectrochemical behaviour of anodic films on Zr and Zr-20 at.%Ti

A photoelectrochemical investigation on anodic films of different thickness grown on sputter-deposited Zr and Zr-20 at.%Ti was carried out. The estimated optical band gap and flat band potential of thick (U_F ≥ 50 V) anodic films were related to their crystalline structure and compared with those obtained for thinner (U_F ≤ 8 V/SCE) anodic oxides having undetermined crystalline structure. The E_g values obtained by photocurrent spectroscopy were also compared with the experimental band gap estimated by other optical ex situ techniques and with the available theoretical estimates of the zirconia electronic structures in an attempt to reconcile the wide range of band gap data reported in the literature. Finally, the optical band gap and flat band potential values obtained for tetragonal Zr_0.8Ti_0.2O₂ mixed oxide were compared with that expected on the basis of a correlation between E_g and the film composition.     

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.     

The passive behaviour of AISI 316 in alkaline media and the effect of pH: A combined electrochemical and analytical study

In this paper, thin surface films were formed on AISI 316 immersed in alkaline solutions of pH ranging from 13 to 9 under open circuit potential conditions. The electrochemical behaviour of the films formed under these conditions was evaluated by different electrochemical techniques: d.c. potentiodynamic polarisation, capacitance measurements (Mott-Schottky approach) and electrochemical impedance spectroscopy. The chemical composition was studied by X-ray photoelectron spectroscopy (XPS).The results reveal a good agreement between the film composition, the electronic properties and the electrochemical behaviour.The analytical results showed that the composition of the surface film changes with the pH. The films become enriched in Cr(III) and Fe(III) species and depleted in Fe(II) and Ni species, as the pH decreases from 13 to 9. The results are consistent with the formation of a film, presenting a bilayer structure, composed of an outer oxide/hydroxide layer, enriched in iron, and an inner anhydrous layer, rich in chromium and nickel oxides.Impedance measurements reveal that the resistance of the surface film increases during immersion and that the pH influences the evolution of the charge transfer resistance and the evolution of the film capacitance.Capacitance measurements based on the Mott-Schottky approach show that the concentration of electronic defects decreases with pH and that the films formed at pH 13 present the highest density of donors and, therefore, a more conductive behaviour.     

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.     

The effect of thickness on the composition of passive films on a Ti-50Zr at% alloy

Anodic films were grown potentiodynamically in different electrolytes (pH = 1-14) on a Ti-50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS).Regardless of the anodizing conditions, passive films on the Ti-50Zr at% alloy consist of a single layer mixed oxide phase containing both TiO₂ and ZrO₂ groups. However, an enrichment of Ti within the passive film, increasing with the film thickness, is detected both by PCS and XPS. This leads to concentration profiles of Ti⁴⁺ and Zr⁴⁺ ions along the thickness, and to different electronic properties of very thin films (more insulating) with respect to thicker films (more semiconducting), as revealed by the photocurrent-potential curves.     

Thermal and electrochemical properties of cobalt containing Finemet type alloys

This paper presents results concerning the effect of cobalt substitution on bulk (thermal) and surface (electrochemical) properties of Finemet type alloys. Thermal properties of the alloys were determined by differential scanning calorimetry (DSC). The partial substitution of iron for cobalt up to 10 at.% improves the stability of the alloys. The results obtained using various electrochemical techniques show that the new component does not improve notably the electrochemical characteristics of the basic composition alloy sample in KOH solutions. On the other hand, the high corrosion rate registered in HCl solutions does not allow us to obtain quantitative data.     

Improved electrochemical performance of trivalent-chrome coating on Al 6063 alloy via urea and thiourea addition

This project aims at improving the electrochemical performance of trivalent-chrome coating through urea and thiourea addition. The electrochemical behaviors of coatings formed with different concentrations of urea and thiourea were investigated in 3.5 wt.% NaCl solution at 25 °C, using potentiodynamic polarization curves and EIS. The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea. To explain the EIS results of the coatings, a simple equivalent circuit was designed. The EIS parameters were obtained by fitting the EIS plots. The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor. The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS, respectively. The results indicated that the trivalent chromium coating was developed on Al 6063 alloy, urea and thiourea inhibitors were also deposited on the substrates, respectively. A noticeable chemical shift was also observed.     

Electrochemical formation and characterization of porous titania (TiO2) films on Ti

Galvanostatically and potentiostatically formed surface oxide films on titanium in H₂O₂ free and H₂O₂ containing H₂SO₄ solutions were investigated. Conventional electrochemical techniques, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy, were used. In the absence of H₂O₂, the impedance response indicated a stable thin oxide film which depends on the mode of anodization of the metal. However, in the presence of H₂O₂ the film characteristics were changed. A significant decrease in the corrosion resistance of the surface film was recorded. The film characteristics were also found to be affected by the mode of oxide film growth and polarization time. The EIS results and the impedance data fitting to equivalent circuit models have shown that the oxide film consists of two layers. The electrochemical characteristics of the anodic films formed under different conditions have been discussed.     

Performance of self-healing epoxy with microencapsulated healing agent and shape memory alloy wires

We report the first measurements of a self-healing polymer that combines a microencapsulated liquid healing agent and shape memory alloy (SMA) wires. When a propagating crack ruptures the embedded microcapsules, the liquid healing agent is automatically released into the crack where it contacts a solid catalyst embedded in the matrix. The SMA wires are then activated to close the crack during the healing period. We show that dramatically improved healing performance is obtained by the activation of embedded SMA wires. We conclude that improved healing is due to a reduction of crack volume as a result of pulling the crack faces closed, and more complete polymerization of the healing agent due to the heat produced by the activated SMA wires.     

Mechanical and electrochemical behavior of nanocrystalline surface of 304 stainless steel

This paper reports our recent studies on nanocrystalline surface layer of 304 stainless steel (304SS) produced using a sandblasting and annealing process. The grain size of the sandblasted surface layer was less than 20 nm. Mechanical and electrochemical properties of the nanocrystalline surface and its passive film were investigated using nano/micro/-indentation, micro-scratch, scanning Kelvin probe (SKP), potentiodynamic scanning and electrochemical scratch techniques. It was demonstrated that the nanocrystalline surface was markedly superior to that of original 304SS with enhanced passive film. The polarization, electrochemical scratch and SKP measurements indicated that the nanocrystalline surface had higher resistance to corrosion, greater capability of repassivation and higher chemistry stability. All results demonstrated that the nanocrystallization surface did not only enhance the mechanical properties of the surface layer and its passive film, but also benefited the passivation capability of the steel with improved corrosion resistance.     

Evaluation of cure temperature effects in cataphoretic automotive primers by electrochemical techniques

The temperature dependences of dc electrical conductivity, σ, and Seebeck coefficient, S, for six recently synthesized quaternary salts of bipyridine and indolizine pyridine (GAL compounds) in thin films, have been investigated. The thin-film samples (d = 0.06–0.60 μm) were deposited by spin-coating using dimethylformamide solutions, onto glass.

XRD was used for structure investigations, while AFM technique, corroborated to optical microscopy, was used for the examination of surface morphology of samples.

The present compounds behave as typical n-type polycrystalline semiconductors.

The activation energy of electrical conduction, ΔE, ranged between 1.61 and 1.73 eV, while the ratio of charge carrier mobilities, b, laid in the range 1.08–1.14.

By studying optical absorption direct band gaps ranged between 3.78 and 4.00 eV have been found.

Some correlations between semiconducting parameters and molecular structure of the compounds were established. The model based on band gap representation can be conveniently used for the explanation of electronic transport in investigated compounds.

The investigated compounds are also suitable for applications in thermistor manufacture.     

Effect of alloy composition and surface pretreatment on the durability of adhesive-bonded aluminium alloy joints

Boeing wedge test data which indicate the influences of six surface pretreatments and four alloy compositions on the durability of adhesively bonded joints are reported. X-ray photoelectron spectroscopy data which indicate the effects of the same surface pretreatments and alloy compositions on the chemical compositions of the adherend surfaces are summarized. Possible correlations between the durability and X PS data are discussed. Three different methods of modifying the chromic acid anodize (Def 151) treatment of clad alloy adherends, in order to improve durability, are described and discussed.     

Characterisation of a laboratory electrochemical ozonation system and its application in advanced oxidation processes

An electrochemical reactor for oxygen/ozone production was developed using perforated planar electrodes. An electroformed -PbO₂ coating, deposited on a platinised titanium substrate, was employed as anode while the cathode was a platinised titanium substrate. The electrodes were pressed against a solid polymer electrolyte to minimise ohmic drop and avoid mixing of the gaseous products (H₂ and O₂/O₃). Electrochemical ozone production (EOP) was investigated as function of current density, temperature and electrolyte composition. Electrochemical characterisation demonstrated ozone current efficiency, Φ_EOP, ozone production rate (g h⁻¹), , and grams of O₃ per total energy demand (g h⁻¹ W⁻¹), increase on decreasing electrolyte temperature and increasing current density. The best reactor performance for EOP was obtained with the base electrolyte (H₂SO₄ 3.0 mol dm⁻³) containing 0.03 mol dm⁻³ KPF₆. Degradation of reactive dyes used in the textile industry (Reactive Yellow 143 and Reactive Blue 264) with electrochemically-generated ozone was investigated in alkaline medium as function of ozone load (mg h⁻¹) and ozonation time. This investigation revealed ozonation presents very good efficiency for both solution decolouration and total organic carbon (TOC) removal.     

Synthesis and electrochemical evaluation of polypyrrole coatings electrodeposited onto AA-2024 alloy

Polypyrrole coating was successfully deposited on anodized 2024 unclad aluminum alloy showing that the presence of the anodic film is the key factor to ensure better adhesion to the polymer coating. Corrosion resistance of the conductive polymer layers grown for three and five cycles were evaluated in a 3 wt.% NaCl solution using polarization curves and impedance spectroscopy. It was found that the thermally treated polymer coating with higher thickness (five cycles) exhibited the best corrosion performance. This best coating shifts the corrosion potential towards nobler values, about 650 mV, and the exchange current density decreases two orders of magnitude regarding the anodic layer. The partial reduction of the structure of the polymer promoted by the thermal treatment plays a key role in the corrosion resistance of the coating allowing to the thermally treated polymer layer to act as a physical barrier against corrosion.     

New strategy for impedance measurement of surface oxidation characteristic in ceramic fiber by controlling aspect ratio

Silicon carbide fibers (SiC_f) are promising materials for high-temperature applications, because of their excellent thermal and mechanical properties. Due to the inevitable oxidation of SiC_f used at high-temperature, the surface and interfacial properties of SiC_f can be changed. Impedance analysis is necessary to confirm the useful information for various properties. However, since single SiC_f has a long length and a very small cross-section area, its capacitance is so small that impedance analysis is almost impossible. In this study, an in-situ analysis tool that can estimate the impedance of SiC_f to confirm the resistance elements is proposed using bundled fiber specimens. It is confirmed that it is possible to measure the impedance of the SiC_f under high-temperature and oxidation atmospheres by controlling the aspect ratio of the specimen. This tool provides a new pathway that can be applied in various systems by investigating the electrical properties according to the oxidation behavior.     

On origin of resistive and capacitive contributions to impedance of memory states in Cu/TiO2/Pt RRAM devices by impedance spectroscopy

The resistive switching phenomenon is attributed to local formation and rupture of conductive filaments (CF), however some aspects of the switching mechanism still need wider consensus. The origin of resistance contribution to low resistance state (LRS) and high resistance state (HRS) is although well known, the origin of capacitive contribution to the memory state of resistive random access memory device (RRAM) is ambiguous. Here we report impedance spectroscopic studies carried on Cu/TiO₂/Pt devices to address the origin of resistive and capacitive contribution, where the effect of DC bias voltage, reset stop voltage and set compliance current on the impedance of RRAM memory states were investigated in detail. The studies revealed that the capacitive contribution was dominated by the surrounding TiO₂ bulk and contrary to existing knowledge, gap formed owing to ruptured CF has negligible role. These studies also reaffirmed the origin of the resistive contribution to the HRS and LRS state, which was dictated by the ruptured gap and connected CF, respectively. Results of this study may be significant towards improving the switching time and operating frequency performance of futuristic memory, RF switch and neuromorphic computing applications of RRAM devices.     

Photo-electrochemical investigation of anodic oxide films on cast Ti-Mo alloys. I. Anodic behaviour and effect of alloy composition

The anodic behaviour of cast Ti-Mo alloys, having different Mo contents (6-20 wt.%), was investigated in acidic and neutral aerated aqueous solutions. All sample showed a valve-metal behaviour, owing to formation and thickening of barrier-type anodic oxides displaying interference colours. Growth kinetics of passive films is influenced by both anodizing electrolyte and composition of the starting alloy. This last parameter was found to change also the solid-state properties of the films, explored by photoelectrochemical and impedance spectroscopy experiments. Thicker films (U_f = 8 V/MSE) grown on alloys richer in Mo showed more resistive character and a photocurrent sign inversion under negative bias, that revealed an insulating character, whereas corresponding films grown on alloys with lower Mo content, as well as thinner films, behaved as n-type semiconductors. Results are discussed in terms of formation of a mixed Ti-Mo oxide phase.     

Characterisation of the surface of wool and hair using microscopical and fluorescence probe techniques

Fluorescence probing methods, in combination with light microscopy, were used to image and characterise the hydrophobic surfaces of wool and human hair fibres. When applied from an aqueous dispersion, uncharged lipophilic fluorescent probes displayed a selective affinity for the boundaries of the cuticle cells on the fibre surface. Adsorption generally occurred on the surface of the lower scale emerging from each cell junction. An indication of the fibre surface polarity was given by the fluorescence fine structure of adsorbed pyrene molecules. Treatment of hair with the lipid stain Nile Red produced an unusual diamondshaped pattern (each segment approximately 40 μm in length and 10 μm in width) in areas where the cuticle had been damaged. Cuticle cells isolated from hair fibres showed distinct geometrical features on their surfaces when examined using differential interference contrast microscopy. The results are interpreted to indicate the presence of a residue of lipid-staining material adhering to the cuticle cells of hair, the residue possibly being derived from adhesive material produced at the areas of contact between presumptive cuticle cells during the early stages of cuticle shape differentiation.     

Palladium and tantalum aluminide coatings for high-temperature oxidation resistance of titanium alloy IMI 834

The present article explains the efforts made in developing new protective coatings based on palladium, tantalum, and aluminum with considerably improved oxidation resistance for effective protection of titanium alloy IMI 834. Systematic characterization was carried out on as-prepared as well as oxidized coatings and these results are presented. The performance of new coatings was evaluated by generating weight-gain data as a function of time followed by detailed characterization in order to confirm the ability of the coatings to prevent oxidation and alpha-case formation. The results showed that tantalum aluminide and simple aluminide coatings exhibit improved oxidation resistance when compared to palladium aluminide. Finally, the advantages of developed new coatings and the necessity of their use in modern gas turbine engines that allow the alloy to be used safely at high temperatures, which in turn would enhance the efficiency of gas-turbine engine-compressor sections, will be stressed.     

Evolution of non-stoichiometric iron sulfide film formed by electrochemical oxidation of carbon steel in alkaline sour environment

Non-stoichiometric iron sulfide films (Fe_xS_y) were formed electrochemically on a 1018 carbon steel/1 M (NH₄)₂S, 500 ppm CN⁻ interface, using cyclic chronoamperometry for different time intervals. The films showed great stability in medium typical of the catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN⁻ as NaCN, pH 8.8). Characterization of the films by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show two different behaviors depending on the growth time. For films grown at times <15 min, oxidation of the film was continuous, while oxidation ability for the film formed at times >15 min was lower. Film characteristics were more clearly defined by EIS experiments, as the Nyquist diagrams show depressive loops with high, real impedance values (Zr>1 kΩ cm²) for films grown at longer times. Structural characterization by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) correlated electrochemical behavior with topographical changes and chemical composition of the films formed. The abundance of sulfur and pyrrhotite is evident in the samples grown for increasing times, and is likely due to electrochemical/chemical oxidation of iron sulfides during film growth. The sulfur-rich layer is responsible for the passive character of these films. The equivalent electrical circuit describing the EIS spectra for films formed over longer times has fewer elements than that used to model EIS spectra for films grown at shorter times. In particular, diffusion of atomic hydrogen is not apparent in sulfur-enriched films, and the charge transfer is carried out at the metal/film interface with values that are insensitive to film thickness and chemical nature.