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.     

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.     

Photocurrent spectroscopy study of passive films on hafnium and hafnium-tungsten sputtered alloys

Anodic and air-formed films on sputtered Hf and W-Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH)₂. This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials (V_f>10 V). In the case of W-Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising in different electrolytes. A PCS analysis of films grown on sputtered alloys is performed on the basis of previously proposed correlation between the band gap of anodic films and difference of electronegativity of their constituents.     

Polymeric sol–gel coatings as protective layers of aluminium alloys

Sol–gel route is an emerging technology to synthesize coatings of a wide variety of properties taylored. In this work three low temperature cured coatings has been studied to evaluate their protective properties in order to be used as protective barrier coatings for aluminium alloys with potential architectural and automotive applications. These three coatings are novel modified silane nanocomposites coatings obtained mixing two sols separately prepared: a pre-hydrolysed 3-glycidyloxypropyl trimethoxysilane (GPTS) with acidic catalyst and another obtained from tetraethylorthosilicate/methyltriethoxysilane (TEOS/MTES). Particulated coatings were obtained by addition of 25 wt.% particles of Aerosil 300 and Aerosil R972, respectively. The protective properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) which showed notably differences among them not only from the protective viewpoint but the hydrophobic nature of the coatings and the controlling corrosion mechanism in each case.     

Recent advances in photocurrent spectroscopy of passive films

The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-metal alloys gives satisfactory agreement with the theoretical expectations. The experimental results evidence the role of the alloying metal in determining the crystalline or disordered structure of passive films grown on Al,d-metal alloys.     

Electrochemical characterisation of passive films on Ti alloys under simulated biological conditions

An electrochemical characterisation of the passive film on Ti, and its alloys Ti6Al4V and Ti6Al7Nb under simulated biological conditions is described. Electrochemical impedance spectroscopy and photoelectrochemistry were employed as in-situ techniques to monitor the evolution of the passive film on exposure to simulated body fluid environments. Of particular interest were the selective interactions of phosphate and calcium ions present in simulated body fluid with the oxide films, immediately after exposure. The effects of these ions on the TiO₂-electrolyte interface could be detected already within the first hour of exposure at 37 °C and were found to be the only ions present in simulated body fluid to interact with the titanium oxide surface. Photocurrent spectra showed no change in the semi-conductive properties of the passive films on Ti and the two alloys, but large variations on the intensity of the generated photocurrent indicated the presence of adsorbed ions on the surface. Different behaviour between the three metals investigated was also observed.     

Corrosion study of surface-modified vanadium-free titanium alloys

The present work is part of an investigation aimed to improve the corrosion resistance of three vanadium-free titanium alloys of biomedical interest, Ti-6Al-7Nb, Ti-13Nb-13Zr and Ti-15Zr-4Nb, by growing on their surfaces an oxide protective layer. For this goal, different samples were oxidized in air at 750 °C for times ranging from 6 to 48 h. Thickness, morphology and composition of the oxide scales for different oxidation times were investigated by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). After equal oxidation time, the Ti-6Al-7Nb alloy exhibited a thinner, more compact and dense oxide layer than the TiNbZr alloys, indicating a slower oxidation rate. Several oxidation times were used in order to obtain oxide scales with similar properties for the three alloys. Different electrochemical techniques were then applied to evaluate the corrosion behavior of the different samples. The oxidized Ti-6Al-7Nb alloy showed the lowest corrosion current densities as well as the best pitting corrosion behavior, and is thereby considered as the best of these materials for biomedical applications.     

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.     

Oxygen reduction on anodically formed titanium dioxide

Oxygen reduction on anodically formed TiO₂ was studied in both alkaline and acidic water solutions, using dc voltammetry combined with the rotating disk technique. The possible interference with hydrogen evolution reaction was considered. By means of Levich-Koutecky plot, in alkaline media, 4e⁻ reduction path was confirmed, while with the transition to acidic media, the rising participation of the indirect, 2e⁻ reduction path was evidenced.     

Surface modification of passive iron by alkyl-phosphonic acid layers

Phosphonate layer formation on passive iron surface has been investigated by electrochemical, conversion electron Mössbauer spectroscopy (CEMS) and atomic force microscopy (AFM) techniques. Electrochemical methods revealed that the prepassivation of iron surface results in stabilization of the phosphonate layer exhibiting favorable corrosion resistance. The rate of anodic dissolution is continuously decreasing due to the time-dependent formation of a protective phosphonate layer.The large R_ct values of 1-20 MΩ cm² indicate rather high blocking effect of metal dissolution by the phosphonate layer. The phosphonate layer formation has been also followed by the decrease of capacitance. CEMS investigations were carried out to evaluate the differences in the composition of the passive layer as a result of the phosphonate treatment. Mössbauer spectroscopy indicated the presence of iron phosphonate. Changes in morphology due to the phosphonate layer formation have been observed.     

Characterization of corroding aluminium alloys with electrochemical noise and electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) and spontaneous voltage fluctuation measurements were made on three orthogonal faces of 7039-T64 and 2519 aluminium alloy specimens during 10 day exposure periods into aerated 3% NaCl solutions. The spontaneous voltage fluctuations generated by the specimens were analysed with two techniques designed to reveal the stochastic character of these signals and their fractal dimension to establish practical correlation between noise measurements and the degree of pitting of the corroding aluminium specimens. Attempts were made to correlate the results of these analyses with parameters measurable by microscopic examination of the specimens or calculated from EIS measurements. During this study it was found that the slope of the voltage fluctuations and the depression angle of EIS results were both good indicators of the pit density observable on the corroded aluminium specimens.     

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.     

An investigation of films formed anodically on some metal silicon alloys

Anodically formed surface films on on, Ni and Co alloyed with Si were investigated. X-ray and infrared spectroscopy were used to characterise the species formed at the surface. Talysurf and optical microscopy were used to measure the film thickness and the manner in which this varied with potential, time, alloy composition.The films are found not to conform to normal rules of passivation, and an explanation for the manner in which this class of compound can operate as an anode is advanced invoking metal orthosilicates as electrocatalysts and conductors in the surface film.     

Oxidative dissolution and anion-assisted solubilisation in the transpassive state of nickel-chromium alloys

The transpassive state of pure Ni, Cr and two Ni-Cr alloys (10 and 20 wt.% Cr) in 14.8 M H₃PO₄ is studied by voltammetric, rotating ring-disc electrode and impedance spectroscopic measurements. The results indicate that the rate of transpassive dissolution of the alloys in a major part of the transpassive range is higher than the rate of this process for both pure metals. A kinetic model retaining the essential features of the corresponding models for the pure metals proposed in the literature has been found to reproduce both the steady-state current versus potential curves and the ac impedance spectra in the transpassive range. The parameters of the model are determined by a fitting procedure and the relevance of their values for the relative importance of two possible pathways in the transpassive state—oxidative dissolution and anion-assisted solubilisation—are discussed.     

Effect of oxidisable substrates on the photoelectrocatalytic properties of thermally grown and particulate TiO<Subscript>2</Subscript> layers

The photoelectrochemical properties of titanium dioxide layers, prepared by thermal oxidation of titanium at 500–750 °C, were compared with those of layers of particulate (Degussa) P25, especially for oxidation of oxalic acid. The thermally formed oxide layers had rutile structures with a particle size of about 100 nm. Values of incident photon-to-current conversion efficiencies increased with rutile layer thickness and reached a maximum at about 1 μm. Photocurrents for particulate TiO₂ layers were about one order lower than those for thermal layers, due to the poor contact among individual particles, resulting in high electric resistance of the whole layer. The presence of oxalic acid had no effect on the photocurrent of thermal TiO₂ layers, while in the case of porous particulate layers, the photocurrent increased strongly, due to oxalate adsorption and subsequent enhanced oxidation rate with photogenerated holes. For oxalic acid concentrations ≤10⁻³ M, the photocurrent decayed due to mass transfer limitations, resulting in oxalate depletion in the porous particulate layer.     

Structural and complex impedance spectroscopic studies of Mg-substituted CoFe2O4

In this work, we present the effect of Mg substitution on the structural and impedance spectroscopic characteristics of Co_1−xMg_xFe₂O₄ (x=0.0, 0.3, 0.6, 0.9 and 1.0) samples, prepared by sol-gel auto-combustion method. As-burnt and fluffy powder samples were grinded and subsequently calcined at 600 °C for 6 h to change single phase cubic spinel structure, as confirmed by X-ray diffraction. The data obtained from diffraction was also utilized to perform Rietveld's refinement which provided detailed information on the structural changes occurred during the substitution. Frequency and temperature dependent electrical, and impedance studies were performed using an impedance analyzer in a wide frequency and temperature range. The behavior of characteristic dielectric parameters has been investigated using Maxwell-Wagner's model and Koop's theory. The impedance plot was used to define electro-active regions of the prepared samples, useful in modelling an equivalent circuit for each region. Frequency dependent Nyquist plot revealed the effect of grain, grain boundaries and electrode effect. Samples exhibited distinct grain and grain boundary contributions to the conductivity. Temperature dependent electrical characterization revealed that samples had negative temperature coefficient of resistance. The binding energies at different temperatures have also been estimated.     

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.     

On nonaqueous electrochemical behavior of titanium and Ti compounds

In this study, we examined the electrochemical behavior of titanium and Ti⁴⁺ compounds in THF solutions. Tetra butyl ammonium chloride (TBACl) was used as supporting electrolyte in order to increase the ionic conductivity of the solutions. Electrodeposition of pure titanium could not be obtained. A variety of analytical techniques have been used in conjunction with electrochemical methods in order to analyze the reduction process of Ti⁴⁺. These included Raman and UV-vis spectroscopy, ICP and CHNS elemental analyses. Ti⁴⁺ is being reduced to Ti³⁺ in TiCl₄/THF/TBACl solutions.In addition we show that metallic titanium can be electrochemically dissolved from an organo-metallic electrolyte solution comprising EtAlCl₂ and LiCl in THF. The product is Ti⁴⁺. While LiCl is insoluble in THF it reacts with EtAlCl₂ to form ionic species. Hence, these solutions possessed reasonable ionic conductivity.We could not obtain electroactive Ti⁴⁺ with TiBr₄ or TiI₄ as starting materials, in similar solutions.     

On the composition and structure of thin layers of titanium oxide on platinum surfaces

This work reviews the available data about the structure and composition of thin layers of titanium oxide on the surface of platinum. These systems can be prepared by vapor deposition and subsequent oxidation of Ti on a massive Pt substrate or by oxidation at low pressure of the Pt₃Ti alloy. For both types of substrate, the XPS data show that two distinct Ti states in the oxide layer can be detected. The structure and the composition of the oxide layer varies depending on the substrate composition and on the conditions of preparation. On pure Pt, the preferential formation of a defective TiO_2–x oxide was observed. On Pt₃Ti, the oxide film a layer was formed of TiO of thickness of the order of a single atomic layer and of crystallites of nearly stoichiometry TiO₂.     

Semiconductive behavior of passive films formed on pure Cr and Fe-Cr alloys in sulfuric acid solution

Photoelectrochemical current response and capacitance were analyzed for passive films formed on pure Cr and Fe-Cr alloys in sulfuric acid solution. The photocurrent was plotted as a photoelectrochemical action spectrum that could be separated into two components, which were mainly derived from Cr₂O₃ (E_g=3.6-3.7 eV) and Cr(OH)₃ (E_g=2.5 eV). The band gap energy, E_g, of each component was almost constant for various applied potentials and polarization periods. The photoelectrochemical response showed negative photocurrent for most potentials in the passive region, which means that the photocurrent apparently exhibited p-type semiconductor behavior. On the other hand, Mott-Schottky plots for pure Cr exhibited positive slope, which means that passive films on Cr behave as n-type semiconductors. Furthermore, the passive films on Fe-18Cr alloy exhibited both positive and negative slopes, which means that passive films on Fe-18Cr in sulfuric acid solution have both p- and n-type semiconductive properties. These apparently conflicting results can be rationally explained, if we assume that passive films on Cr and Fe-Cr alloys formed in the sulfuric acid solution are composed of an outer hydroxide layer with an n-type semiconductor property and of a p-type inner oxide layer.