Electrochemical methods in the study of localized corrosion attack

Electrochemical methods for determining the characteristic pitting potentials of 90Cu–10Ni alloy in slightly alkaline chloride solutions are summarized and the results of measurements carried out using potentiostatic, quasi-potentiostatic, potentiodynamic and galvanostatic techniques, complemented byex situ techniques — SEM and EDXA — are discussed. In borate buffer solution a passive state is established due to the formation of the oxide film with low ionic conductivity. However, in the presence of Cl^– ions, at potentials higher than a certain critical value, breakdown of the anodic passivity occurs, caused by field-stimulated chloride entry into the passive oxide film at singular point defects. The brightening of the pits formed after oxide film breakdown was established to be due to the conversion of the passivating oxide film to one of high ion conductivity Contaminated oxide permits the passage of the metal cation into and through it, finally leaving it at the film/solution interface where pitting can proceed. During localized attack two characteristic potentials have to be distinguished: the potential of pit nucleation,E _n, above which pit nucleation starts, and the breakdown potential,E _b, above which the growth of nucleated pits develops. An attempt is made to compare the values ofE _n andE _b obtained through different methods, and to determine the factors influencing these values in each particular method.Presented at the 11th International Corrosion Congress, Florence, Italy, April 1990     

Characterisation and behaviour of Ti/TiO2/noble metal anodes

The morphology, composition and the electrical and electrochemical behaviour of the anodic microporous layer, prepared by the galvanostatic anodisation of Ti after sparking, followed by galvanostatic deposition of Pt or Ir have been investigated. These electrodes are proposed to function as dimensionally stable anodes (DSAs). For Ti/TiO₂/Pt electrodes, Pt is deposited within some of the micropores of the oxide film. In contrast, for Ti/TiO₂/Ir, the metal is deposited preferentially on the top surface. This difference is thought to result from the position of the metal deposition potential with respect to the flat band potential of n-TiO₂. Optical imaging of both types of DSA suggests that only a few sites on the surface are responsible for electron exchange at the DSA-electrolyte interface. C-AFM measurements of Ti/TiO₂/Pt samples subjected to long-term anodic polarisation, suggest that the Ti-noble metal contact is progressively insulated by thickening of the TiO₂ barrier layer, promoting passivation of the DSA. For Ir coated anodes, catalytic activity is directly related to the presence of Ir and to the stability of the catalytic oxide layer. Under Cu electrowinning conditions, the electrochemically formed hydrated Ir oxide was found to be catalytically less stable, than the iridium oxide film subjected to a heat treatment.     

Synthesis of self-organized mixed oxide nanotubes by sonoelectrochemical anodization of Ti-8Mn alloy

Self ordered arrays of titanium manganese mixed oxide nanotubes were prepared by anodization of Ti8Mn alloy (UNS R56080) under ultrasonication in diluted ethylene glycol containing fluoride. The dimensions of the nanotubes (diameter: 20-100 nm and length: 0.5-2.0 μm) could be tuned by changing the synthesis parameters. The as-anodized nanotubes showed a stoichiometry of (Ti,Mn)O₂. Upon annealing at 500 °C in oxygen atmosphere, the nanotubes contained a mixture of anatase + rutile phases of TiO₂ and Mn₂O₃. The composition of the oxide nanotubes was influenced by the chemistry of the phases present in the alloy. More manganese content was observed in the oxide formed on the β-phase than in the oxide layer of α-phase. Anodization in the ultrasonic field increased the kinetics of nanotubular oxide formation and resulted in homogeneous ordering of the nanotubular arrays as compared to the anodization by conventional stirring in the fluoride containing ethylene glycol solution. Whereas, anodization in aqueous acidified fluoride solutions resulted in severe attack of the β-phase and did not show presence of nanotubular oxide structure.     

Effect of the compatibility on dielectric performance and breakdown strength of poly(vinylidene fluoride)/low‐density polyethylene blends

Immiscible polymer blends with high dielectric constant (ε) and improved breakdown strength (E_b) performance were obtained by composing poly(vinylidene fluoride) (PVDF) with low‐density polyethylene (LDPE) or the LDPE grafted with maleic anhydride (LDPE‐g‐MAH) through melt‐blending way. The dielectric properties of these blends were emphasized for considering the compatibility effect on the energy storage application. Interface morphology, co‐continuity behavior, and grafted ratio were simultaneously investigated to detect the compatibility enhancement after introducing MAH. Results showed that the MAH positively improved the dielectric properties. Both the measured E_b of PVDF/LDPE and PVDF/LDPE‐g‐MAH blends showed a minimum value at v^PVDF = 50 vol % because of the worst compatibility; meanwhile, higher E_b of PVDF/LDPE‐g‐MAH than that of PVDF/LDPE blend was observed owing to the better compatibility. For considering the effect interface morphology on the dielectric performance, layer‐structure films composing with pure PVDF and LDPE layers were further constructed and studied. It was revealed that the layered structure could be treated as a helpful way to improve ε and E_b for immiscible polymer blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42507.     

Structure and enhanced antimicrobial activity of mechanically activated nano TiO2

Titanium dioxide is a photocatalyst, known not only for its ability to oxidize organic contaminants, but also for its antimicrobial properties. In this article, significant enhancement of the antimicrobial activity of TiO₂ (up to 32 times) was demonstrated after its activation by ball milling. The antimicrobial activity was analyzed for one fungal and 13 bacterial ATCC strains using the microdilution method and recording the minimum inhibitory concentration (MIC) values. In order to further investigate the correlation between the mechanical activation of TiO₂ and its antimicrobial activity, the structure, morphology and phase composition of the material were studied by means of Electron Microscopy, X-ray diffraction and nitrogen adsorption-desorption measurements. UV-Vis diffuse reflectance spectra were recorded and the Kubelka-Munk function was applied to convert reflectance into the equivalent band gap energy (E_g) and, consequently, to investigate changes in the E_g value. X-ray photoelectron spectroscopy was used to analyze the influence of mechanical activation on the Ti 2p and O 1s spectra. The presented results are expected to enable the development of more sustainable and effective advanced TiO₂-based materials with antimicrobial properties that could be used in numerous green technology applications.     

A mechanical and modelling study of magnetron sputtered cerium-titanium oxide film coatings on Si (100)

Ce/Ti mixed metal oxide thin films have well known optoelectrical properties amongst several other physio-chemical properties. Changes in the structural and mechanical properties of magnetron sputtered Ce/Ti oxide thin films on Si (100) wafers with different Ce:Ti ratios are investigated experimentally and by modelling. X-ray Photoemission Spectroscopy (XPS) and X-ray diffraction (XRD) confirm the primary phases as trigonal Ce₂O₃ and rutile form of TiO₂ with SiO₂ present in all prepared materials. FESEM imaging delivers information based on the variation of grain size, the mixed Ce/Ti oxides providing much smaller grain sizes in the thin film/substrate composite. Nanoindentation analysis concludes that the pure cerium oxide film has the highest hardness value (20.1?GPa), while the addition of excess titanium oxide decreases the hardness of the film coatings. High temperature in-situ XRD (up to 1000?°C) results indicate high thermal phase stability for all materials studied. The film with Ce:Ti?=?68%:32% has a new additional minor oxide phase above 800?°C. Contact angle experiments suggest that the chemical composition of the surface is insignificant affecting the water contact angle. Results show a narrow band of 87.7–95.7° contact angle. The finite element modelling (FEM) modelling of Ce/Ti thin film coatings based on Si(100); Si(110); silica and steel substrates shows a variation in stress concentration.     

Electrochemical and surface properties of aluminium in citric acid solutions

The electrochemical and surface properties of aluminium in 0.05 M citric acid solutions of pH 2–8 were studied by means of open circuit potential (OCP), potentiodynamic polarization and potentiostatic current–time transient measurements. The OCP reached a steady-state value very slowly, probably due to the slow rate of detachment of surface complexes into the solution. Aluminium exhibits passive behaviour in citric acid solutions of pH 4–8. Tafel slopes b _c were characteristic for hydrogen evolution on aluminium covered by an oxide film. The corrosion kinetic parameters E _corr, i _corr and b _a suggest that surface processes are involved in the dissolution kinetics, especially in the pH range 3–6. Current–time transient measurements confirm that, in citric acid solutions of pH 3–6, the dissolution is controlled by surface processes, that is, by the rate of detachment of surface complexes, while in solutions of pH 2, 7 and 8 dissolution is under mass-transport control. The addition of fluoride ions to citric acid changes the controlling steps of the dissolution process. Citrate and fluoride ions compete for adsorption sites at the oxide surface, and adsorption of these ions is a competitive and reversible adsorption.     

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.     

Electrochemical behavior of current collectors for lithium batteries in non-aqueous alkyl carbonate solution and surface analysis by ToF-SIMS

Several metals (Cu, Fe, Al, Ti, and Cr) as current collector for lithium-ion battery were investigated to understand their electrochemical behavior and passivation process in a non-aqueous alkyl carbonate solution containing LiPF₆ salt. From cyclic voltammetric study, it was found that Cu and Fe metals were dissolved into the electrolyte below 4 V vs. Li/Li⁺. Alternatively, Al and Ti were stable up to 5 V vs. Li/Li⁺. Their scratched surfaces at 5 V vs. Li/Li⁺ were polarized in a transient mode and it was found that the surfaces were passivated during the polarization test. Formed passive film was composed of two hybrid layers: outer layer by metal (Al and Ti) fluoride and inner by metal oxide, as confirmed by time-of-flight secondary ion mass spectroscopy. Presence of HF in the electrolyte was indispensible to form the metal fluoride layer on the oxide layer. The outer fluoride layer would protect the inner oxide layer and metal substrate from HF attack, bringing about satisfactory corrosion resistance under lithium-ion battery environment.     

Electrochemical behavior of Ti-20Cr-X alloys in artificial saliva containing fluoride

In this study, electrochemical investigation was carried out on commercially pure titanium (c.p. Ti) and as-cast Ti-20Cr-X (X = Nb, Mo, Fe, and Cr) in fluoride solutions containing artificial saliva. Open-circuit potential (OCP) and potentiodynamic polarization measurement were used to characterize the electrochemical behavior, and X-ray photoelectron spectroscopy (XPS) was used to characterize the composition of the passive films on the samples. The OCP results indicated that all specimens presented spontaneous passivation. XPS results showed that pre-test, a passive film consisting of TiO₂ and Cr₂O₃ formed on the surface of the Ti-20Cr-X alloys, and Na₂TiF₆ formed on the surface of all specimens after the polarization test in artificial saliva with 0.5 wt% NaF. Although the Ti-20Cr-X alloys exhibited a Na₂TiF₆ film on their surface, their corrosion rate was still lower than that of c.p. Ti. In particular, Ti-20Cr-Mo alloy exhibited the lowest steady state current density compared to c.p. Ti and the other Ti-20Cr-X alloys. All these results suggest that the Ti-20Cr-X alloys hold promise for application as dental materials in a natural oral environment where NaF may be present.     

Surface analysis of the electropolishing layer on Si(111) in ammonium fluoride solution

The influence of potential and pH on the thickness and composition of the anodic films formed on n-Si(111) in dilute ammonium fluoride solutions is investigated by photoelectron spectroscopy. A combined electrochemistry/surface analysis system with very low contamination levels is used for the investigation of the oxidic layers. XPS data show that the film thicknesses increase with decreasing pH reaching 25 Å at pH 3. A potential dependence measured at lower coverage at a pH of 4.9 reveals changes in thickness and composition by increasing the potential from flatband to +5 V. Fluorine is incorporated into these films in substantial amounts which otherwise consist predominantly of silicon oxide. Silicon fluoride and silicon oxyfluoride are found. We define a current transmissivity, T_c, of the layers which shows strong variations with potential and pH. For a pH of 3, T_c is about 5×10² larger than for pH 4.9. The potential dependence shows a variation of T_c by a factor of three and an increase for thicknesses exceeding 5 Å. These observations are explained by a structural transition from a high density pseudomorphic interfacial layer to a more open structure at larger thicknesses (up to 25 Å). The analysis of the UP-spectra shows only little changes of the electron affinity χ with coverage. We obtain χ4.15±0.1 eV. A distinct band bending is not detected. From HeII spectra, we determine the valence band offset ΔE_v between Si and the oxidic layer to be 5.2 eV.     

Electrochemical properties of Ti/SnO2-Sb2O5 electrodes prepared by the spray pyrolysis technique

Cyclic voltammetry for the ferri-ferrocyanide redox couple has been used to study the Ti/SnO₂-Sb₂O₅ electrode prepared by spray-pyrolysis under different conditions. The obtained results shows that increasing the preparation temperature and the duration of coating deposition results in a decrease of i _p and an increase of E _p for the ferri-ferrocyanide couple. This deviation from reversibility has been attributed to the formation of a titanium oxide layer at the Ti/coating interface. Concerning oxygen evolution at the Ti/SnO₂-Sb₂O₅ anodes, a mechanism is proposed in which water is discharged at the anode forming hydroxyl radicals which are further oxidized to form dioxygen. Finally, a generalized mechanism for oxygen evolution at oxide electrodes has been proposed.     

Pitting corrosion of polycrystalline annealed copper in alkaline sodium perchlorate solutions containing benzotriazole

The pitting corrosion of copper in alkaline solutions in the presence of benzotriazole (BTA) was investigated. The presence of BTA shifts the breakdown (E _b) and the repassivating (E _p) potentials positively with respect to the blanks. However, the shift ofE _p becomes smaller than that ofE _b, particularly at pH9 and 11. Pitting corrosion involves the formation of crystallographic pits. The kinetics of the process fits a nucleation and growth mechanism involving instantaneous nucleation and 3D growth under charge transfer control. The spatial distribution of pits indicates that there is no marked influence of a pit on the nucleation and growth of other pits.     

Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et₄NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current-time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br⁻ anions. The pitting potential (E_b) decreased with the increase of solution temperature and Br⁻ concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current-time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.     

Enantioselective formation of an α, β-epoxy alcohol by reaction of methyl 13(S)-hydroperoxy-9(Z), 11(E)-octadecadienoate with titanium isopropoxide

Methyl 11(R), 12(R)-epoxy-13(S)-hydroxy-9(Z)-octadecenoate (threo isomer) was generated from linoleic acid by the sequential action of an enzyme and two chemical reagents. Linoleic acid was treated with lipoxygenase to yield its corresponding hydroperoxide [13(S)-hydroperoxy-9(Z), 11(E)-octadecadienoic acid]. After methylation with CH₂N₂, the hydroperoxide was treated with titanium (IV) isopropoxide [Ti(O-i-Pr)₄] at 5°C for 1 h. The products were separated by normal-phase high-performance liquid chromatography and characterized with gas chromatography-mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Approximately 30% of the product was methyl 13(S)-hydroxy-9(Z), 11(E)-octadecadienoate. Over 60% of the isolated product was methyl 11(R), 12(R)-epoxy-13(S)-hydroxy-9(Z)-octadecenoate. After quenching Ti(O-i-Pr)₄ with water, the spent catalyst could be removed from the fatty products by partitioning between CH₂Cl₂ and water. These results demonstrate that Ti(O-i-Pr)₄ selectively promotes the formation of an α-epoxide with the threo configuration. It was critically important to start with dry methyl 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate because the presence of small amounts of water in the reaction medium resulted in the complete hydrolysis of epoxy alcohol to trihydroxy products.     

Improving the adhesive,mechanical, tribological properties and corrosion resistance of reactive sputtered tantalum oxide coating on Ti6Al4V alloy via introducing multiple interlayers

Tantalum oxide film has become an investigation focus for surface modification materials in the biomedical field owing to its outstanding biocompatibility, anti-corrosion, and anti-wear performances. However, tantalum oxide films exhibit poor adhesion because of the mismatch between the properties of the film and the substrate. In this study, a novel multilayer tantalum oxide coating of Ta_mO_n/Ta_mO_n-TiO₂/TiO₂/Ti (code M-Ta_mO_n) was deposited on Ti6Al4V by magnetron sputtering with Ta_mO_n single-layer coating as control. The purpose of this work is to evaluate the influence of the introduced Ta_mO_n-TiO₂/TiO₂/Ti multi-interlayer on the microstructure, adhesive, mechanical, and anti-corrosion properties of reactive sputtered tantalum oxide coatings. The outcomes show that the Ta_mO_n-TiO₂/TiO₂/Ti intermediate layer improves the bonding strength between the Ta_mO_n layer and Ti6Al4V matrix from 17.83 N to over 50 N and enables the Ta_mO_n coating to have an increased H/E and H³/E² ratio, decreased friction coefficient and wear rate, raised potential, and reduced corrosion current density. The improved properties of the multilayer system are attributed to the positive effects of the inserted multiple interlayers in reducing the residual stress in the coating, coupling the mechanical performance between the layer and the substrate, blocking the continuous growth of penetrating defects in a film with columnar structure. These experimental results provide a workable route for improving the properties of the tantalum oxide coating on Ti6Al4V alloy for medical applications.     

High breakdown strength and low loss of polystyrene-block-poly(methyl methacrylate)/Poly(vinylidene fluoride) composites for energy storage application

Poly(vinylidene fluoride) (PVDF) composite films were prepared by introducing polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) into PVDF matrix. Uniform dispersion and good compatibility of PS-b-PMMA in matrix were observed, which was helpful for high breakdown strength (E_b). The composite film with 9 wt% PS-b-PMMA showed the maximum E_b of 522 kV/mm and the high discharged energy density (U_e) of 10.1 J/cm³, which were 1.7 times and 2.6 times higher than pure PVDF, respectively. Besides, a charged-discharged efficiency (η) of 88% was much higher than pure PVDF at 300 kV/mm, which was beneficial to energy storage.     

Electrochemical characterization of two different framework Ti(IV) species in Ti/Beta zeolites in contact with solvents

Five Ti/Beta zeolites prepared by the conventional hydroxide procedure or by using fluoride as mineralizer have been studied by cyclic voltammetry and pulse voltammetry. Three peak couples (AA′, BB′ and CC′) were recorded. The AA′ peak was due to Ti atoms leached to the solution. Peak BB′ is relatively insensitive to the electrolyte salt, while the reduction potential of CC′ decreases progressively from perchlorate or nitrate to chloride and bromide. This suggests the presence of two framework Ti populations, one less prone to expand its coordination number (proposed to be Ti(OSi≡)₄) and other Ti atoms that coordinate with ligands within the short timescales of the electrochemical measurements (either (≡SiO)₃TiOH or (≡SiO)₂Ti(OH)₂ titanols). This revised version was published online in August 2006 with corrections to the Cover Date.     

Electrocatalytic properties of ternary oxide mixtures of composition Ru0.3Ti(0.7−x)CexO2: oxygen evolution from acidic solution

Mixed oxide electrodes of general composition Ru_0.3Ti_(0.7–x)Ce _x O₂ were prepared by thermal decomposition of the respective chlorides using Ti as a support. x was varied between 0 and 70 mol %. Oxygen evolution was used as a model reaction to investigate the dependence of the electrocatalytic properties on oxide composition. Kinetics was studied by quasistationary current-potential curves and reaction order determination. A minimum Tafel slope of about 30 mV has been found in the 10–30% CeO₂ composition range. On the basis of a zero reaction order at constant overpotential, a reaction mechanism has been proposed accounting for the composition dependence of the Tafel slope. It has been concluded that the replacement of TiO₂ with CeO₂ brings about an increase in the electrocatalytic activity for oxygen evolution, while the layer becomes more prone to mechanical erosion.