In situ electrochemical Scanning Kelvin Probe Blister-Test studies of the de-adhesion kinetics at polymer/zinc oxide/zinc interfaces

Fundamental investigations of the polymer/zinc oxide/zinc interface corrosion stability were performed in situ by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister-Test (HR-SKP-BT) under controlled atmospheric conditions. A hole under an adhesive layer film served as electrolyte reservoir to initiate cathodic de-adhesion processes. Then a combinatorial approach was undertaken to simultaneously study the influence of electrolyte pressure at constant defect polarisation and of relative atmospheric humidity on the de-adhesion rate. The time resolved blister growth and the propagation of the three phase boundary polymer/oxide covered zinc/interfacial electrolyte layer could be detected. It could be proven that the oxygen reduction induced electrochemical damage of the interface precedes the subsequent mechanical de-adhesion process. By variation of the relative atmospheric humidity the water concentration within the bulk adhesive and its interphase adjacent to the metal substrate could be adjusted. These processes were further analysed by peel-tests and in situ Attenuated-Total-Reflection Infrared Spectroscopy (ATR-IR) studies of water diffusion. A decrease of the interphasial water concentration led to a deceleration of the de-adhesion kinetics for constant defect conditions and to smaller interfacial ion transport rates. This could be assigned to an inhibition of the electron transfer reactions at the front of de-adhesion and an increased adhesion force between polymer film and oxide covered metal preventing the formation of an extended interfacial electrolyte layer.     

In situ wetting of aluminium during the growth of porous alumina by anodic oxidation

The growth of nanoporous alumina at an aluminium surface which is not completely covered by an electrolyte has been investigated. At one point the electrolyte forms three interfaces: oxide/electrolyte, metal/electrolyte, electrolyte/air. At this three-interfaces-point a self-wetting induced oxide growth occurs. The wetting increases up to 7 mm from the electrolyte contact edge. At this final stage of wetting an equilibrium between the wetting induced flow (bottom-up) and the gravity-driven flow (top-down) is reached. The thickness of the nonporous layer decreases with the distance from the starting point.     

The anodic oxidation of valve metals—II. The influence of the anodizing conditions on the transport processes during the anodic oxidation of zirconium

The transport numbers for metal and oxygen in anodic ZrO₂ have been measured as a function of the oxide film thickness, growth rate, electrolyte composition, metal surface finish and electric field using ²²²Rn as an inert marker. The field in the oxide has also been determined as a function of current density and electrolyte composition.

The metal transport number apparently diminishes with increasing oxide thickness but this is shown to arise from the formation of a hydrated layer on the oxide surface which leads to greater energy loss of the -particles and apparently greater burying. However, this effect only has significance for thin films. For thick films the measured transport numbers are independent of thickness but show increases from very low values as the current density increases and with changes in the anodizing electrolyte in the sequence: sodium hydroxide; ammonium borate; sodium sulphate. The maximum metal transport number observed was 0.22 for 0.5 M sodium sulphate at 50 mA current density. The field in the oxide also increased with increase in current density and with electrolyte in the same sequence.     

Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.     

Anodic chlorine/nitrogen co-doping of reduced graphene oxide films at room temperature

Room temperature simultaneous doping of reduced graphene oxide films with oxygen, nitrogen and chlorine was performed through anodic polarization in a neutral nitrogen-deaerated KCl electrolyte. The thermodynamic electrochemical windows of water, dissolved nitrogen and chlorine anions were analyzed on the basis of the Pourbaix diagram. Anode polarization demonstrated that the nitrogen, water and chlorine anions can be oxidized at an applied potential of 1.7 V vs. NHE. The oxidative products, i.e. oxygen, nitrate anion and hypochlorous acid, can react with the reduced graphene oxide surface. X-ray photoelectron spectroscopy proved the chlorine–nitrogen co-doping of the treated film, along with an increase of oxygen groups. Surface structure evolution was also confirmed by Raman and Fourier-transform infrared spectroscopies. The anodic doping can be hindered by covering the reduced graphene oxide surface with sulfate anions or forming stable carbon–nitrogen bonds. Incorporation of oxygen, nitrogen and chlorine also helps to enhance the supercapacitance of the doped film.     

The ability of a surface charge approach to describe barrier film growth on tungsten in acidic solutions

A renewed version of a surface charge approach to describe the impedance response of anodic film growth on passive metals in acidic solutions is presented. It is based on the chemistry of the Point Defect Model, the fact that oxygen vacancies are the main charge carriers in a range of anodic oxides and the suggestion of a constant field strength in the bulk of the barrier layer. Generalized transport equations valid for any field strength are employed to the oxygen and metal vacancy motion and the two limiting cases of high and low fields are discussed. A negative surface charge due to accumulation of metal vacancies near the film/solution interface accelerates the oxygen vacancy transport, thus explaining the pseudo-inductive behaviour of the metal/film/electrolyte system under small amplitude ac perturbation. Using the W/WO₃/electrolyte system as a model, the basic parameters characterizing film growth are determined on the basis of the potential dependence of ac impedance spectra.     

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10-30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l⁻¹ KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.     

Untersuchung über die anionenadsorption mit hilfe markierter ionen

From previous work, the adsorption of anions is regarded as an essential factor for the different corrosion behaviour of metals in solutions containing different anions. Adsorption is measured by means of ³⁶Cl⁻, ⁸²Br⁻, ¹⁸ F⁻, ³⁶ClO₄⁻, ³⁵O₄²⁻, H³⁵S⁻ and ¹⁴CN⁻ on Pt, Ni and Fe in the form of sheets and evaporated films. Besides the determination of the adsorption after dipping into the solution, a method has been developed for the measurement of adsorption in contact with the solution and for the determination of its kinetics. The method can also be applied to O₂-free metal surfaces produced under vacuum. In this case, however, very rapid adsorption is observed, whereas normally saturation is reached only after many hours. It is concluded that, in general, exchange between oxygen on the metal and the anion takes place rather than simple adsorption.

The distribution of the anions adsorbed on the metal surface has been studied by autoradiography; adsorption takes place preferentially at the grain boundaries and increases when the crystal size decreases.

These results confirm the interpretation of passivation as a competition between various processes: metal dissolution, coverage by a passivating oxide film, and displacement of oxygen by anions.     

Transport processes of hydrated ions at polymer/oxide/metal interfaces: Part 1. Transport at interfaces of polymer coated oxide covered iron and zinc substrates

Localised electrochemical and spectroscopic techniques were jointly applied for the evaluation of hydrated ion transport processes along polymer/oxide/metal interfaces. In situ Scanning Kelvin Probe (SKP) studies of the local interfacial potentials of organically coated oxide covered zinc and iron substrates were performed in humid nitrogen atmospheres. They were supported by ex situ small spot X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis of the interfacial ion distribution. Based on the experimental results it is concluded that also in atmospheres of strongly reduced oxygen partial pressure at which no corrosive delamination takes place, a negatively charged layer of adsorbed hydroxide ions determines ion transport processes along interfaces between polymer films and oxide covered metals. No ion transport was observed for zinc substrates while hydrated cations were selectively transported along the polymer/iron interface. The reduction of oxygen molecules on the highly reactive iron oxide surface is assumed to be responsible for the generation of adsorbed interfacial hydroxide ions. On the other hand such oxygen reduction induced hydroxide formation in humid nitrogen atmospheres with strongly reduced oxygen partial pressure does not seem to take place on oxide covered zinc. The variation of free volumes at the polymer/substrate interface did not lead to a principal change of this phenomenon.     

Measurement of the adhesion strength of metal oxide and metal nitride thin films sputtered onto glass by the direct pull-off method

We have measured the adhesion strengths of metal oxide and metal nitride thin films reactively sputtered onto glass substrates using a specially devised direct pull-off test. For double-layer coatings such as metal nitride (CrN_x, TiN_x)/metal oxide (Al₂O₃, SnO₂, Ta₂O₅, TiO₂, ZnO, ZrO₂)/glass, separation usually took place at the nitride/oxide interface. The adhesion strength at the interface was found to depend on the strength of chemical bonding in the films concerned: for the same nitride top layer, the adhesion strength increased as the strength of the metal-oxygen (M-O) bond in the oxide underlayer decreased. X-ray photoelectron spectroscopy (XPS) measurements showed that a mixed layer was created at the nitride/oxide interface and that the adhesion strength at the interface increased with increasing thickness of the mixed layer. For single-layer coatings such as metal nitride (CrN_x, TaN_x, TiN_x, ZrN_x)/glass, the adhesion strength of the film to the glass substrate was found to increase with increasing strength of the M-O bond between the metal atom (M) in the nitride film and an oxygen atom (O). These adhesion behaviors could be explained by adhesion models based on chemical bonds at the interfaces.     

Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.     

Development and application of a holistic model for the steady state growth of porous anodic alumina films

A holistic model was developed and applied to anodic alumina films galvanostatically grown in sulphuric acid solution at different anodising conditions thus characterised by different structural characteristics. The O²⁻ and Al³⁺ species transport numbers near the metal|oxide interface were determined that depended on both temperature and current density. The rate of film thickness growth was found to be proportional to the O²⁻ anionic current through the barrier layer near the metal|oxide interface. The results introduced a new growth mechanism theory embracing the rarefaction of barrier layer oxide lattice towards the metal|oxide interface. The oxide density near the metal|oxide is closely independent of anodising conditions and is related to the transformation of Al lattice to a transient oxide lattice about 37% rarer than that of γ-Al₂O₃ that is further suitably transformed to denser, amorphous or nanocrystalline material as this oxide is shifted to the oxide|electrolyte interface and becomes the pore wall material. This gradual lattice density variability can explain many peculiar properties of anodic alumina films.     

Role of metal ion impurities in generation of oxygen gas within anodic alumina

The generation of oxygen gas within an amorphous anodic alumina film is reported. The film was formed by anodizing aluminum, which was first electropolished and then chemically polished in CrO₃-H₃PO₄ solution, in sodium tungstate electrolyte. The procedure results in incorporation of mobile Cr³⁺ species, from the chemical polishing film, and mobile W⁶⁺ species, from the electrolyte, into the amorphous structure. The tungsten species are present in the outer 27% of the film thickness, while Cr⁶⁺ species occupy a thin layer within the tungsten-containing region. Above the Cr³⁺ containing layer, a band develops that contains oxygen bubbles of a few nanometres size. The oxygen is generated by oxidation of O²⁻ ions of the alumina. A mechanism of oxygen generation within the alumina is proposed based on the electronic band structure of the oxide, modified by the Cr³⁺ and W⁶⁺ species, and on the ionic transport processes during oxide growth.     

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.     

A model for galvanostatic anodising of Al in alkaline solutions

In this paper, the modelling of anodic oxide film growth on aluminium in alkaline solutions under galvanostatic conditions is discussed. The material balance of dissolved and adsorbed hydroxyl anion species at the substrate-electrolyte interface is considered, taking into account electrochemical processes of oxide film formation and dissolution via field-assisted and chemical mechanisms associated with evolution of aluminate anions. A differential equation describing the kinetics of oxide film growth under the influence of the above processes is derived and numerically solved. Simulation of the voltage behaviour during oxide film growth, depending on the main electrolysis conditions (such as current density, electrolyte concentration and agitation rate), is performed. Possible reasons for observed discrepancies with experimental data are discussed.     

Characterisation of hydrophobic carbon nanofiber-silica composite film electrodes for redox liquid immobilisation

Carbon (50-150 nm diameter) nanofibers were embedded into easy to prepare thin films of a hydrophobic sol-gel material and cast onto tin-doped indium oxide substrate electrodes. They promote electron transport and allow efficient electrochemical reactions at solid|liquid and at liquid|liquid interfaces. In order to prevent aggregation of carbon nanofibers silica nanoparticles of 7 nm diameter were added into the sol-gel mixture as a “surfactant” and homogeneous high surface area films were obtained. Scanning electron microscopy reveals the presence of carbon nanofibers at the electrode surface. The results of voltammetric experiments performed in redox probe—ferrocenedimethanol solution in aqueous electrolyte solution indicate that in the absence of organic phase, incomplete wetting within the hydrophobic film of carbon nanofibers can cause hemispherical diffusion regime typical for ultramicroelectrode like behaviour.The hydrophobic film electrode was modified with two types of redox liquids: pure tert-butylferrocene or dissolved in 2-nitrophenyloctylether as a water-insoluble solvent and immersed in aqueous electrolyte solution. With a nanomole deposit of pure redox liquid, stable voltammetric responses are obtained. The presence of carbon nanofibers embedded in the mesoporous matrix substantially increases the efficiency of the electrode process and stability under voltammetric conditions. Also well-defined response for diluted redox liquids is obtained. From measurements in a range of different aqueous electrolyte media a gradual transition from anion transfer dominated to cation transfer dominated processes is inferred depending on the hydrophilicity of the transferring anion or cation.     

Reactive intermediate in the alkali-carbonate-catalysed gasification of coal char

Based on results from a variety of experimental measurements, a detailed mechanism is postulated for the action of the inorganic catalyst in char gasification. In this mechanism, a catalyst such as potassium carbonate in contact with char undergoes a chemical and physical transformation to form a molten potassium oxide film that covers the char surface. This film serves as an oxygen transfer medium between the gaseous reactant (H₂O or CO₂) and the char. At the catalyst/char interface, an oxidation-reduction reaction occurs and the anions in the catalyst react with the oxidized char to form a phenolate-type functional group that subsequently splits out CO. The anions are replenished by reaction between the oxidizing gas (H₂O or CO₂) and the oxide at the gas/catalyst interface. Net transport of oxygen from gas to char occurs by diffusion of the species in the molten catalyst film.     

The history of the Point Defect Model for the passive state: A brief review of film growth aspects

A review is presented of the history of the Point Defect Model (PDM) for the growth and breakdown of passive films that form on the surfaces of reactive metals in contact with corrosive, condensed phase environments. The PDM has passed through three generations, with each successive generation addressing issues that have arisen from experiment or theory. Thus, the first generation model (PDM-I), developed in the early 1980s, assumed that the passive film was a single defective oxide layer that contained cation vacancies and oxygen vacancies that were generated and annihilated at the metal/film and film/solution interfaces, as inspired by the work by Wagner on high temperature oxidation. As with gas-phase systems, the film was assumed not to dissolve. However, it soon became evident that this model could not account for the properties of the passive state on metals in contact with aqueous environments and, accordingly a Generation II model (PDM-II) was developed to address these issues. PDM-II incorporated the bi-layer structure of the film comprising a defective oxide or hydride barrier layer adjacent to the metal and an outer layer that forms by precipitation of material from reaction of transmitted cations with species in the environment (including water, CO₃²⁻, HS⁻, etc.), introduced metal interstitials to the suite of defects, recognized barrier layer dissolution, and recognized the need to classify reactions as to whether they are lattice conservative or non-conservative, but assumed that control of the passive current resided in the barrier layer alone. PDM-II has enjoyed considerable success and the author knows of no instance where it has been demonstrated to be at odds with experiment when confluence between experiment and theory has been demonstrated. A Generation III model (PDM-III) has been recently developed to extend the theory to those cases (e.g., the valve metals) where the outer layer is so resistive that it controls the impedance of the interface and hence the corrosion rate. A fourth generation model that will describe passivity on alloys is now under development. The experimental evidence upon which each generation is based is reviewed.     

Anion incorporation and its effect on the dielectric constant and growth rate of zirconium oxides

Mechanically polished zirconium electrodes were potentiodynamically polarized in phosphate buffer solutions of various pH values and in 0.5 lvl NaOH. The results show that the shape of the I-E curves is independent of the solution pH. At relatively low scan rates, oxygen gas evolution was observed. The oxide film thickness was calculated from the values of the charge consumed in the anodic process assuming 100% current efficiency for oxide formation below oxygen evolution (lower values for the current efficiency are assumed for potentials above oxygen evolution). Capacitance measurements, together with the calculated oxide thickness, were used to estimate values for the dielectric constant of the oxide. Two different values of the dielectric constant were obtained for the oxides formed in the range of potential below and above oxygen evolution. Also, higher dielectric constant values were obtained with increasing solution pH. Anion incorporation was assumed to increase the conductivity of the oxide films and, hence, decrease the dielectric constant. A two-layer structure is proposed for the anodically formed oxide on zirconium in aqueous solutions; an anion-free layer near the metal and an outer layer containing the incorporated anions.     

铸铝合金微弧氧化工艺研究

对铸铝合金在水玻璃混合体系中的微弧氧化工艺进行了详细研究。结果表明,电解液中的水玻璃成分对于铸铝合金形成微弧氧化膜层具有重要的作用,钨酸钠与EDTA二钠配合使用可以提高膜层的硬度,在水玻璃,钨酸钠、EDTA二钠等组成的混合液体系中,采用适当的工艺条件,如,电流密度为40A/dm^2。配合强力搅拌,可以在铸铝合金表面制得硬度超过800HV,性能优异的氧化物陶瓷膜。