Characterisation of porous and barrier layers of anodic oxides on different aluminium alloys

Electrochemical impedance spectroscopy (EIS) provides a powerful tool for obtaining detailed information on the electrochemical properties of both porous and barrier layers on different aluminium alloys. The impedance value at a given frequency can serve to calculate the electrochemical parameters of the oxide layers represented by each component of the equivalent circuit (EC) which reproduces the behaviour of the studied systems. It is thus possible, with these parameters, to analyse the effects of any factor on the sealing and ageing processes of anodic aluminium oxide layers. Electrochemical results are completed with a detailed analytical study of the oxide layers by XPS, with gravimetric determinations of the changes experienced in the anodising and sealing processes, and with microstructural characterisation of the anodic films by scanning electron microscopy (SEM).     

Electrochemical impedance spectroscopic study of barrier layer thinning in nanostructured aluminium

The electrochemical behaviour of electropolished and anodised aluminium was studied by electrochemical impedance spectroscopy (EIS). Freshly electropolished aluminium behaves as a pure capacitor exhibiting Warburg impedance at low frequencies. Storage of the electropolished aluminium, even in an air-tight bottle, results in the reconstruction of a uniform compact barrier layer. The impedance response of a stored electropolished aluminium as well as anodised aluminium after oxide removal, done by chemical etching, exhibits only a capacitive loop in the complex plane. The effect of the oxide layer thickness on the impedance data was investigated for layers formed during anodising at a cell potential of 15 or 23 V. Impedance measurements carried out over a wide range of frequencies gave useful information on the efficiency of the thinning of the barrier layer at the bottom of porous aluminium oxide layers. The rate of thinning of the barrier layer was estimated for samples anodised at different voltage.     

Electrochromic properties of porous NiO thin film as a counter electrode for NiO/WO3 complementary electrochromic window

Highly porous nickel oxide (NiO) thin films were prepared on ITO glass by chemical bath deposition (CBD) method. SEM results show that the as-deposited NiO film is constructed by many interconnected nanoflakes with a thickness of about 20 nm. The electrochromic properties of the NiO film were investigated in a nonaqueous LiClO₄–PC electrolyte by means of optical transmittance, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The NiO film exhibits a noticeable electrochromic performance with a variation of transmittance up to 38.6% at 550 nm. The CV and EIS measurements reveal that the NiO film has high electrochemical reaction activity and reversibility due to its highly porous structure. The electrochromic (EC) window based on complementary WO₃/NiO structure shows an optical modulation of 83.7% at 550 nm, much higher than that of single WO₃ film (65.5% at 550 nm). The response time of the EC widow is found to be about 1.76 s for coloration and 1.54 s for bleaching, respectively. These advantages such as large optical modulation, fast switch speed and excellent cycle durability make it attractive for a practical application.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics, adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Effect of bath concentration and curing time on the structure of non-functional thin organosilane layers on aluminium

The corrosion resistance of aluminium alloys can be improved by different surface treatments such as painting. A pre-treatment based on chromate is the current method used to increase the corrosion resistance and the adhesion of the organic layer. Silane films seem to be an interesting alternative system to replace the toxic chromates. In this paper, the characterisation of bis-1,2-(triethoxysilyl)ethane (BTSE) thin layers has been evaluated by coupling optical techniques like spectroscopic ellipsometry (SE) and infra-red spectroscopic ellipsometry (IRSE) along with electrochemical methods (electrochemical impedance spectroscopy (EIS)). This approach has been chosen to have a better understanding of the protection provided by these organosilane thin films. It will be demonstrated that the BTSE bath concentration modifies the thickness of the layers and that the curing of this thin film can also improve the barrier properties by forming a denser layer.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics,adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Polyurethane films modified by antithrombin–heparin complex to enhance endothelialization: An original impedimetric analysis

In this paper, polyurethane (PU) was deposited as a thin layer onto the surface of ITO (indium tin oxide) and was then modified with an antithrombin–heparin complex (ATH). The resulting films were characterized by ATR spectroscopy, contact angle measurements and electrochemical impedance spectroscopy (EIS). Physicochemical characterization confirmed the surface modifications.The obtained films were used as substrates for endothelial cell attachment and growth. These processes were characterized using electrochemical impedance spectroscopy (EIS). We observed that the addition of a small amount of heparin and AT additives onto the polymer surface resulted in a considerable change in the surface characteristics, and we found that PU films that were modified by the ATH complex were able to greatly enhance adhesion and proliferation of endothelial cells (ECs).     

Morphology effect on the electrochromic and electrochemical performances of NiO thin films

NiO thin films on ITO substrate were prepared by chemical bath deposition (CBD) and sol–gel method, respectively. The microstructure and morphology of the NiO films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Both the films have polycrystalline cubic NiO, but have distinct morphology. The CBD NiO thin film with a highly porous structure exhibited a noticeable electrochromic performance. The variation of transmittance was high up to 82% at 550 nm and the coloration efficiency (CE) was calculated to be 42 cm² C⁻¹. The sol–gel NiO thin film with a smoothly compact structure presented 35% and 28 cm² C⁻¹ at 550 nm, respectively. The electrochemical properties of both the NiO thin films were investigated in 1 M KOH electrolyte by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The CV and EIS measurements revealed that the CBD NiO thin film had better electrochemical reversibility, higher reactivity and reaction kinetics due to its highly porous structure.     

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.     

The growth kinetics and properties of potentiodynamically formed thin oxide films on aluminium in citric acid solutions

The growth kinetics and properties of potentiodynamically formed thin oxide films on Al were investigated in 0.05 M citric acid solutions of different pH (5, 6 and 7) by means of potentiodynamic polarization and a.c. electrochemical impedance spectroscopy (EIS) measurements. Al showed passive behaviour within the pH range that was examined. The potentiodynamic growth of the oxide film on Al takes place due to ionic conductivity under the influence of the high electric field. Characteristic kinetic oxide film growth parameters such as the high-field growth exponential law constants (A and B), ionic conductivity through the oxide film, field strength and half barrier width have been calculated. Impedance measurements were used to determine the parameters related to the characteristic sizes and properties of oxide film. The capacitive response of the impedance spectrum was related to the thickness and dielectric properties of the barrier oxide film. The oxide film resistance values were very high, indicating that the oxide films formed under potentiodynamic conditions are highly uniform in thickness and very resistant. The anodic behaviour of Al in the citric solutions under potentiodynamic conditions were characterized by the rapid growth of the oxide film which diminished the influence of relaxation processes on the growth kinetics and structural characteristics of the aluminium/anodic oxide film/electrolyte system.     

Electrochemical investigation of oxide films formed on nickel alloys 182, 600 and 52 in high temperature water

Nickel-based alloys 182, 600 and 52 were exposed to simulated Pressurized Water Reactor (PWR) primary water (1000 ppm B, 2 ppm Li, O₂ <10 ppb, 325 °C) under different dissolved hydrogen (DH) conditions [0, 2, 25 and 50 cm³ H₂(STP) kg⁻¹] for times up to 1 month in a recirculating autoclave. The influence of exposure time and DH on oxide films formed on the alloys was evaluated by means of electrochemical tests; electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S). The in situ EIS was performed every day, allowing the monitoring of the oxide layer formation and change. M–S was performed at room temperature after the full exposure time. The results showed that the maximum in the defect concentrations obtained via M–S analysis and the maximum crack growth rate are at the same DH content, thus relating electrochemical testing to stress corrosion cracking observations. A conceptual separation between the electrochemical behavior of (1) the oxide layer (visible in the higher frequencies of EIS) and (2) the oxide layer – solution interface (visible in the lower frequencies of EIS) was able to explain the effect of hydrogen on the low-frequency EIS impedance results.     

EIS investigation of passive film formation on mild steel in oxalic acid solution

The properties of oxide films on mild steel formed anodically in aqueous solution containing oxalic acid were investigated by means of electrochemical impedance spectroscopy (EIS). The morphology of the passive layers at different stages of film formation was investigated using scanning electron microscopy (SEM). Passivation of mild steel in oxalic acid solution is a multi-stage process that occurs before passive layer breakdown. The influence of potential on the electrochemical behavior of the passive layer was also investigated. Depending on the passive layer potential, the EIS spectra obtained exhibited a one- or two-time constant system. This suggests the formation of either a one layer or two layer oxide film on the mild steel surface, depending on the passivation potential.     

Characterization of conducting poly(3‐methylthiophene) films prepared under sono‐electrochemical conditions

Poly(3‐methlthiophene) films were prepared under “silent” and “sono‐electrochemical” potentiostatic (SEP) conditions. A three‐electrode one‐compartment sono‐cell was used with a working platinum disc electrode. The sono‐electrochemically formed polymer films were deposited with different working electrode‐to‐horn distances. The composition, electrochemical, spectroscopic, and morphological characteristics of the resulting polymer films were determined. Elemental analysis, FTIR‐spectra, and X‐ray photoelectron spectroscopy (XPS) data proved that the polymer films prepared under SEP conditions have predominant α‐α′‐couplings between the 3MT units, and the aromatic ring integrity is maintained in the film. Scanning electron microscopy showed that those films are more compact and less porous compared to the films prepared under silent conditions. The use of sono‐irradiation during electropolymerization enhanced the diffusion of the monomer units towards the electrode surface and resulted in relatively less doped polymers with less conductivity. Electrochemical impedance spectroscopy (EIS) data for films prepared under silent and SEP conditions were collected in a monomer‐free solution. The results show that the impedance of SEP films is relatively higher than those prepared under silent conditions, and a combination of charge transfer kinetics with diffusion‐controlled conduction mechanism within the films. The diffusion was found to be a function of the porosity of the film. Conductivity measurements are in good agreement with EIS, elemental analysis, and XPS data. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2416–2425, 2006     

Non-covalent modification of graphene sheets in PEDOT composite materials by ionic liquids

An ionic liquid (IL) supported composite of poly(3,4-ethylene dioxythiophene) (PEDOT) and graphene oxide (GO) is presented. GO was dispersed in ILs and electropolymerization carried out after loading of EDOT to the dried dispersion. The content of GO was optimized to obtain high electrical conductivity of the composite material. The IL acts as the dispersant for GO and as dopant in the synthesis of PEDOT leading to films with a highly porous structure indicated from the scanning electron microscopy (SEM) images. Subsequently, GO was reduced electrochemically by cyclic voltammetry to obtain PEDOT/rGO composite films. The successful formation of composite materials was confirmed using Raman and X-ray photoelectron spectroscopy (XPS) techniques. XPS was also used to verify removal of oxygen-containing functional groups upon electrochemical reduction of the composite films. The electrochemical properties of PEDOT, PEDOT/GO and PEDOT/rGO were studied using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The results show that electrochemical reduction clearly increases the capacitance of the composite and furthermore the cycling stability. Such an increase could be obtained if >20 cycles, extending to highly negative potentials (−2.0 V), was used during the electroreduction of incorporated GO. Owing to the high porosity, favorable electrochemical properties and cycling stability these hybrid materials shows great potential towards supercapacitor applications.     

Synthesis and Study of a Polypyrrole–Aluminum Oxide Nanocomposite Film on an Aluminum Surface

Using the method of electrochemical anodization, aluminum oxide porous films are obtained in a sulfuric acid solution. The morphology of the aluminum oxide surface is studied by the method of scanning electron microscopy. The high-quality elemental analysis of the initial and oxidized Al films is performed using the method of electron microprobe analysis. A protective composite polypyrrole-aluminum oxide film is produced on the aluminum surface on top of a porous aluminum oxide film in the galvanostatic oxidation mode by the electrochemical synthesis method. The properties of the polypyrrole–aluminum oxide composite film are studied by the methods of voltammetry, as well as impedance and FTIR spectroscopy.     

Development of layer-by-layer assembled thin coatings on aluminium alloy AA2024-T3 for high resolution studies of local corrosion processes

The aim of this study is to develop nanometer-thin epoxy-based films on aluminium alloy AA2024-T3 as a model coating system for high resolution corrosion studies. Spin coating was used for the layer-by-layer (LbL) deposition of poly-(ethylenimine) (PEI) and poly([o-cresyl glycidyl ether]-co-formaldehyde) (CNER) bilayers. The film chemistry and the cross-linking process were characterized by means of Fourier-transform infrared spectroscopy (FTIR). Ellipsometric data confirmed the linear increase of film thickness. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results indicate the improvement of the film barrier properties with increasing film thickness. Mapping of the topography and the volta potential was performed by means of scanning Kelvin probe force microscopy (SKPFM). The results indicate the presence of a homogeneous film structure, while the intermetallic phases can still be identified below the coating. The SKPFM analysis confirmed that the model films are suitable for investigation of corrosion processes at the coating/metal interface.     

Impedance spectroscopy on porous materials: A general model and application to graphite electrodes of lithium-ion batteries

Electrochemical impedance spectroscopy (EIS) was applied to porous negative graphite electrodes for lithium-ion batteries in the EC:DMC, 1 M LiPF₆ electrolyte. The effect of porosity on the electrode response time was studied and a theoretical model was developed, based on free path of the current lines between subsequent reaction sites. The effect of porosity on the electrode response is evidenced by the impedance spectra in which the high frequency capacitive semicircle is distorted. Fresh electrodes (before the formation of the solid electrolyte interphase, SEI) and cycled electrodes have different shapes of the impedance spectra indicating a change of processes at the surface. In particular, the shape of the spectrum for a fresh electrode can be related to an adsorption process. Impedance spectra of fresh electrodes were fitted using a simple model that considers porosity and the assumed electrochemical processes, giving good agreement between model and data. A correlation was found between adsorption sites and irreversible charge capacity in the first cycle.     

Solid-phase electrochemical reduction of graphene oxide films in alkaline solution

Graphene oxide (GO) film was evaporated onto graphite and used as an electrode to produce electrochemically reduced graphene oxide (ERGO) films by electrochemical reduction in 6 M KOH solution through voltammetric cycling. Fourier transformed infrared and Raman spectroscopy confirmed the presence of ERGO. Electrochemical impedance spectroscopy characterization of ERGO and GO films in ferrocyanide/ferricyanide redox couple with 0.1 M KCl supporting electrolyte gave results that are in accordance with previous reports. Based on the EIS results, ERGO shows higher capacitance and lower charge transfer resistance compared to GO.