Breakdown Of Oxide Film Research Articles

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.

Correlations between stress-induced positive charges and time-dependent dielectric breakdown in ultrathin silicon oxide films

With attention to total energy loss of injected electrons through Fowler–Nordheim (F–N) tunneling, time-dependent dielectric breakdown (TDDB) of 5-nm-thick silicon oxide films was investigated. Metal-oxide-semiconductor (MOS) diodes with four combinations of gate electrode and substrate type were fabricated. This produced different energy losses of injected electrons at a constant F–N tunneling current stress in spite of the same oxide thickness. TDDB lifetime was strongly affected by this energy loss difference. Since other electrical changes caused by trapping of stress-induced positive charges in oxide also exhibited the similar energy loss dependence to the lifetime, TDDB for ultrathin oxide films is probably dominated by the trapping of positive charges, which are mainly generated near anode-side oxide interfaces through the surface plasmon mechanism.

Extrusion of rapidly crystallized aluminum

1. A reduction in extrusion temperature for alloy AD1 leads to an increase in its strength characteristics and a reduction in ductility. The maximum ultimate breaking strength (230 N/mm2) and yield strength (210 N/mm2) for the alloy are achieved after extrusion at 20°C with λ=32. 2. During extrusion of rapidly quenched particles of alloy AD1 a clear subgranular structure forms and there is intense breakdown of oxide film on the surface of particles with λ>9.

Electrodischarge compaction of nickel powders

The electrodischarge compaction (EDC) experiment by Davies and Al-Hassani for nickel powders has been re-analyzed, and additional EDC experiments were carried out using deliberately oxidized nickel powders to understand the mechanism of the oxide disruption and neck formation during passage of a high voltage and high density current. Detailed analysis of power-discharge time curves obtained by Davies and Al-Hassani and also by the present authors was performed together with microstructural observation of the neck interface. It is proposed that the EDC process consists of four stages: stage I, electronic breakdown of oxide films and heat accumulation at the metal- oxide interface; stage II, physical breakdown of oxide film due to sublimation of the metal under the oxide film; stage III, neck formation; stage IV, neck growth and dissipation of energy through the conductive mass.

Fretting corrosion of a high strength low alloy steel in synthetic seawater environments

Fretting in seawater between normally loaded orthogonally crossed cylinders of the high strength low alloy steel Cor-Ten A has been investigated at selected frequencies and amplitudes under controlled electrochemical conditions. Electrochemical and surface profiling techniques and electron microscopy have given a quantitative assessment of the degree and nature of the surface damage suffered by the steel fretted in aerated and nitrogen saturated seawater, and with the steel cathodically protected. When fretted in aerated seawater, Cor-Ten A exhibits a significant increase in corrosion rate compared with that measured under static contact conditions. There is a rise in the rate of corrosion during the early stages of fretting usually followed by a lower corrosion rate as fretting proceeds. The increase in rate has been associated with the breakdown of surface oxide films to expose underlying active metal, and a reduction in rate to a slowing in the growth rate of the damaged zone as fretting continues. The relative electrochemical and mechanical contributions to the amount of material lost have been estimated. In aerated seawater electro chemical dissolution processes were found to be responsible for the major proportion of the surface material removed; under cathodically protected conditions there remained a substantial degree of mechanical damage.

Use of 18O/SIMS and Electrochemical Techniques to Study the Reduction and Breakdown of Passive Oxide Films on Iron

Secondary ion mass spectrometry (SIMS) has been used to study the cathodic removal and open‐circuit breakdown of passive oxide films formed on Fe in 18O‐enriched borate buffer solution. For film removal by galvanostatic cathodic reduction, the initial stage of reduction resulted in a layer by layer thinning of the film. The rates of film removal with cathodic charge passed were determined to be 1.56 and 0.30 nm/mC cm−2 during the two distinct stages of reduction. These results are analyzed in terms of the various models for the passive film on Fe. Immersion of the passivated Fe electrodes in acidic sulfate solutions on open circuit resulted in a distinct potential arrest followed by a very rapid potential decrease. The length of this arrest is strongly dependent on the oxide film thickness and on the solution pH. 18O SIMS and electrochemical results are discussed in terms of mechanisms of reductive and chemical dissolution on open circuit.

Censored Weibull statistics in the dielectric breakdown of thin oxide films

Self-healing dielectric breakdown in thin oxide films has been shown to obey Weibull statistics after allowance has been made for both left and right censoring that is intrinsic to the measuring technique. Two overlapping distributions have been found under constant DC voltage stress testing but under ramp stress conditions these were not always observed. Nevertheless it is shown that the two types of test gave consistency in their analyses. Two particular features of the results reported are that the number of potential events, in some cases, was an exponential function of the applied field and that under ramp test conditions the magnitude of the applied potential at the breakdown estimator time appears to be constant and independent of the sample thickness. The implications of these observations are discussed.

ChemInform Abstract: Formation and Breakdown of Passive Oxide Films on Nickel in Halide Solutions.

AbstractTo study the influence of different halide ions on the passivation of nickel, passive oxide films are formed on Ni in pH 3.0, 0.15 N Na2SO4, and then subjected to potentiostatic polarization in 0.08 M C1′, Br′, or F′ solutions.

A phenomenological approach to the mechanical breakdown of anodic oxide films on zirconium

A phenomenological theory of the mechanical breakdown of films growing on valve metals during galvanostatic oxidation is presented and discussed in detail for ZrO 2 anodic films. It is shown that the mechanical breakdown voltage, V mb, can be linearly related to the logarithm of the anodizing current density both in the case of constant and variable critical thickness, L c, at which the breakdown occurs. It is also shown that the A mb and B mb parameters of the relationship: V mb = A mb + B mb log i are strictly related to the kinetic parameters of growth of the films in the different solutions. The expressions of A mb and B mb parameters are derived for films grown in the presence as well as in the absence of space-charge effects. A quantitative check of the equations obtained is presented for different ZrO 2/electrolyte systems.

Potential of shock-wave methods for preparing and compacting rapidly quenched materials

Interest in rapidly quenched metal alloys (amorphous, fine- and microcrystalline) is connected with a number of their unique properties: low magentic losses; high-strength characteristics; corrosion and radiation resistance, etc. Shock-wave loading methods make it possible not only to compact amorphous materials but, according to the authors, may possibly be used for the rapid quenching process itself. These types of application for shock-wave deformation are due to several features discussed in this paper: high heating rate; nonuniform warm-up of porous materials; high cooling rate of compacts; breakdown of oxide films at particle surface, and dependence of component temperature in porous mixtures in material size fraction and properties. Criteria for obtaining strong compacts and the basic properties of materials obtained by dynamic compaction are also discussed. The features of dynamic loading for porous materials make shock-wave methods promising, both from the point of view of compacting rapidly quenched alloys, and from the point of view of carrying out the quenching process itself.

Formation and Breakdown of Passive Oxide Films on Nickel in Halide Solutions

The influence of Cl−, Br−, and F− on the passivation of nickel in pH 3.0, has been studied. The presence of Cl− and Br− only temporarily delays the establishment of passivity; in contrast, F− has little short‐term (< 1s) influence, but a considerable long‐term effect with the anodic current falling to only ∼ 1 mA‐cm−2. To study the influence of the different halide ions, passive oxide films were formed on nickel in , and then subjected to potentiostatic polarization in 0.08M Cl−, Br−, or F− solutions. In Cl− and Br− solutions, the anodic current remains very low but increases abruptly with pit initiation. The breakdown results in F− solution are completely different, with the measured anodic current increasing gradually with time of polarization; the rate of current increase is at first very low, but then increases with time and eventually slows as passivity breakdown approaches completion. While the kinetics of passivity breakdown depend on both the prior passivation treatment and the conditions existing in the F− solution, the results generally give S‐shaped curves, suggesting that the breakdown process is one of nucleation and lateral growth of “holes” in the passive oxide film. This mechanism of film breakdown in F− is confirmed by replica electron microscopy of the surface at various stages in the breakdown process, with local attack initiating at selected sites and then gradually spreading over the entire surface. The difference in behavior between F− and Cl− and Br− is discussed in terms of the adsorption characteristics of the ions.

The Electrical Discharge Compaction of Powder: Mechanics and Material Structure

The response of metal powders contained in columnar form to high voltage electrical discharge between its ends is described in terms of the electrical, physical, morphological, and compositional characteristics of the powder. The process is viewed as one involving oxide film breakdown, interparticle neck formation, and consolidation. The mechanics, structures, and industrial applications of the process are discussed.

Sem Analysis of Clad-Ceramic Coatings after Hot Corrosion Testing

Scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) have been used to study protective coatings on metals. Recently much interest has been generated in regard to the suitability of clad materials for application in a dynamic hot corrosive gas system. This interest stems not only from a standpoint of corrosion reduction (durability) but also from a standpoint of economics (reduction of consumption of scarce resources). Attention, therefore, is now being given to a class of materials that are arc plasma sprayed (APS) on metals. The APS technique produces a strong bond between the substrate material and the coating. This is due to oxide film break-down of the ASP material and the metal substrate at extremely high temperatures.

Electrical breakdown of aluminum oxide films flanked by metallic electrodes

Breakdown characteristics of anodically grown aluminum oxide films were determined in the range of temperature 4.2–354 K, of thickness 47–187 nm, on change of polarity and for gold and aluminum electrodes. Results could be consistently interpreted with the theory of breakdown due to impact ionization by one type of carrier, presented in the prior paper. The theory ascribes breakdown events to the complex interaction of impact ionization, electron injection, hole trapping, and drift and electron trapping. The effect of trapping was found to be of primary importance, since changes in trapping properties can greatly alter the breakdown properties of aluminum oxide. The wide range of breakdown observations from 3 to 13 MV/cm was explained with changes in trapped electron charge and possibly with change in impact ionization processes, trap to band at low fields, but band to band transitions at high fields. Anomalous breakdown characteristics demonstrate that a trapped electron charge can produce protection from breakdown and that trapped electron densities can be in the low 1018/cm3 up to fields of 10 MV/cm.

A.C. electrical breakdown in thin magnesium oxide films

A systematic study of a.c. breakdown in magnesium oxide films of thickness 40–200 Å fabricated into capacitors is reported. It is found that the breakdown strength is a power function of the thickness d, varying as d −0.23, as predicted by the theory of Forlani and Minnaja based on the ionization avalanche mechanism.

Breakdown of Oxide Films on Steel Exposed to Chloride Solutions

Abstract Breakdown of the thin air-formed oxide film is the first step in the corrosion of steel exposed to aqueous chloride environments. We studied oxide-film breakdown by observing changes, with time, of appearance, weight loss, and corrosion potential while varying the three components of the system: environment, substrate, and film. Environment was varied by changes in chloride and hydrogen peroxide concentrations. Low carbon steel, stainless steel, aluminum, and titanium substrates were studied. Various types of crystalline oxide film were applied to steel substrates by RF sputtering. The results of this work demonstrated that the chemical and electronic properties of the oxide film on steel are inconsequential in determining whether film-breakdown will occur. We interpret these results in terms of a film-breakdown mechanism in which substrate dissolution begins at film defects and spreads, thus causing removal of the film by undercutting. According to this model, the ability of the substrate to spontaneously reform a stable oxide and repair the film determines resistance to film-breakdown.

Effect of Chloride Ion on the Localized Breakdown of Nickel Oxide Films

The effect of chloride ion on the passivation of nickel is studied and a mechanism for pit initiation on electrodes anodized in Cl−‐containing solutions is proposed. Although Cl− does not directly influence the nickel dissolution reaction, , it does interfere with formation and thereby retards passivation. This is manifested by an increase in the anodic passivation charge upon stepping the potential into the passive region. At a constant anodic potential, both the extent of retardation of oxide formation and the probability for pitting increase with increasing [Cl−]. Under conditions where pitting will occur, the anodic current falls substantially during the first 30–60 sec (after the potential step) but then begins to increase as pitting occurs. The original low bulk concentration of Cl− (≤0.1M) was not considered sufficient to cause pitting and this suggests a localized enrichment in Cl− during the 30–60 sec current decay. The oxide film is viewed as undergoing continuous breakdown and repair at localized areas and when sufficient Cl− has accumulated in some areas, repair of the oxide is not possible. For a constant bulk [Cl−], the pitting probability increases sharply with increasing potential of anodization; however, the corresponding extent of oxide retardation actually decreases. This increased pitting probability is explained in terms of the higher rate of nickel dissolution (from oxide‐free areas where the film has broken down) at the higher anodic potential being able to effectively concentrate Cl− in localized areas of the surface. The presence of Cl− ion in the anodizing solution does not result in any change in average oxide film thickness or substantial Cl−incorporation within the film.

Luminescence, breakdown and colouring of anodic oxide films on aluminium

El and Pl, breakdown and self-colouring phenomena of anodic oxide films on aluminium, which have been found to be closely related with each other, were reviewed according to newly proposed classifications. Thus the hitherto unsolved problems which had caused confusion in complete understanding of the art are almost solved. Firstly galvanostatic anodic behaviours of Al were classified. Secondly it was shown that el is caused either by (1) selected impurities (activators) (Mn, Eu etc) in Al, (2) excited carboxylate ions in the film formed in aliphatic carboxylic acids, or (3) by, ‘flaws’ in the case of el in inorganic electrolytes, which is not the el in the true sense. Thirdly pl is to be seen in aliphatic carboxylic acid films only and the luminescent centres are the same with those of el in the acid. Aromatic acid films show neither el nor pl since the acid can not enter into the films. Further, reference was made to one type of solid-type led (Al—Al 2O 3-SnO 2) constructed. Fourthly, breakdowns have been classified into: (1) electrochemical breakdown including porous film formation, carboxylate breakdown (pitting) and chloride breakdown, and (2) ‘electric breakdown during anodizing’ or ‘electrolytic breakdown’ which is a complicated mixture of electrochemical and electric breakdown at/or through oxide—electrolyte interface, and which includes ‘burning’ (luminescent ‘flame’ propagation process) and sparking (scintillation and arcing). Finally self-colouring of ‘pure’ Al (except colouring by alloying elements or impurities included) has been classified into: (1) colouring by decomposition or cyclisation of excited carboxylate ions in the film (post- el) higher electric field attained across the film. Thick, coloured films show no more el nor pl. (2) colouring by existence of Al 0 particles in the interstitial positions of the film by high electric field assisted oxide growth in the case of the so-called ‘hard anodizing’, locally grown coloured spots in inorganic acid, eg sulphuric acid, and of so-called integral colour anodizing in sulphonated aromatic dibasic acid (or higher aliphatic carboxylic acid) plus a small amount of sulphuric acid, typically K alcolor, D uranodic 300 or P ermalux processes for architectural use. These films show neither el nor pl.

Breakdown of Oxide Films on Nickel

Oxide films have been formed on nickel by either electropolishing in or anodizing at different potentials and times in pH 2.8 . The 6–8Å film of on electropolished nickel breaks down rapidly on open circuit at pH 2.8 with a poorly defined potential arrest structure. In contrast, several distinct potential arrests are observed during open‐circuit breakdown of the oxide films on anodized nickel, the number of arrests and their duration being dependent on the condition of film formation and indicative of film stability. The results are interpreted in terms of the anodic formation of a nickel oxide film ≈ 12Å thick with a small area (<5%) covered by defective oxide. Breakdown begins at the defect sites by chemical dissolution of the oxide. Oxide undermining is responsible for removal of most of the oxide and starts when the underlying nickel metal at the defect sites has been exposed. Large variations in oxide film stability are associated with very small changes in the over‐all character of the film since the small coverage with defective oxide plays the major role in film breakdown.

Breakdown in point-contacted silicon oxide films

Breakdown in point-contacted silicon oxide films