Passive Oxide Film Research Articles

Direct Growth of Aligned Multiwalled Carbon Nanotubes on Treated Stainless Steel Substrates

Growth of aligned carbon nanotubes (CNTs) on electrically conductive substrates is promising for many applications; however, the lack of complete understanding of the substrate effects on CNT growth poses a lot of technical challenges. Here, we report the direct growth of aligned multiwalled nanotubes (MWNTs) on chemically treated stainless steel (Type 304) using a chemical vapor deposition (CVD) process. A detailed X-ray photoelectron spectroscopy (XPS) analysis has been carried out for the various treated samples in order to better understand the correlation between the surface properties of the substrates and the MWNT growth. The XPS studies revealed that the CNTs prefer to grow on the enriched surface of iron oxides obtained by the chemical treatment rather than on the passive chromium oxide films present on the surface of the as-received stainless steel substrates. The density and alignment of the MWNTs could therefore be controlled by tuning the ratio of the iron oxides to chromium oxides through the chemical treatment on the stainless steel surfaces. On the basis of this method, selective growth of CNT patterns on stainless steel has also been demonstrated.

Development of nickel incorporated aluminum alloy matrix as an efficient sacrificial anode

Al–Zn + Ni matrix anodes were fabricated using Ni as activator. The Al–Zn + Ni anodes had good physico-chemical characteristics. Reinforcement by Ni resulted in improvement in grain size of the metal–metal matrix. The effect of Ni on the galvanic performance of the anodes was evaluated based on several electrochemical methods. Incorporation of Al–Zn anodes with Ni caused an increase in the galvanic performance. Breakdown of the passive oxide film was facilitated by the presence of nickel. The optimum Ni content to achieve maximum galvanic performance was 2.00 ± 0.25 mg cm−3. The developed Ni-reinforced metal–metal matrix anodes showed excellent electrochemical characteristics as efficient sacrificial anodes.

Passivation oxide film on Nd–Fe–B permanent magnets in borate buffer solution by ellipsometry

The passivation of Nd–Fe–B permanent magnet was investigated in neutral borate solution at pH 8.4. The thickness of the passive oxide film on the magnets was measured by ellipsometry and the composition was estimated by glow discharge optical emission spectroscopy (GD-OES). The passivation of the magnets takes place in the potential range between −0.2 and 1.0 V vs. Ag/AgCl/Sat. KCl. In the potential range, current density decays to the lower than 10 −6 A cm −2 after potentiostatic oxidation for 1800 s. The passive oxide film growth is assumed to be optically simulated from a model with a homogeneous film with complex refractive index, N = 2.1 − j0.086. The thickness estimated from the refractive index linearly increases with potential from 3.6 nm at −0.2 V to 7.8 nm at 1.0 V. The passive film growth follows the ionic migration model under high electric field, i.e., the Cabrera–Mott growth model. The ionic conductivity estimated from the model is about κ = 1.7 × 10 −16 Ω −1 cm −1. The passive oxide film is preferentially composed of iron oxide/hydroxide. Boron and neodymium are, respectively, concentrated at the surface of the oxide film and at the inner layer in the oxide film.

Effect of Load Deflection on Corrosion Behavior of NiTi Wire

For dental orthodontic applications, NiTi wires are used under bending conditions in the oral environment for a long period. The purpose of this study was to investigate the effect of bending stress on the corrosion of NiTi wires using potentiodynamic and potentiostatic tests in artificial saliva. The results indicated that bending stress induces a higher corrosion rate of NiTi wires in passive regions. It is suggested that the passive oxide film of specimens would be damaged under bending conditions. Auger electron spectroscopic analysis showed a lower thickness of passive films on stressed NiTi wires compared with unstressed specimens in the passive region. By scanning electron microscopy, localized corrosion was observed on stressed Sentalloy specimens after a potentiodynamic test at pH 2. In conclusion, this study indicated that bending stress changed the corrosion properties and surface characteristics of NiTi wires in a simulated intra-oral environment.

The corrosion/wear mechanisms of Ti–6Al–4V alloy for different scratching rates

Titanium and its alloys are currently in use as implant materials for orthopaedic surgery. These materials possess outstanding corrosion resistance due to a dense and passive oxide film on the surface. When these alloys are implanted in a living body, removal of surface material takes place because of mechanical wear and corrosion. Simultaneous chemical and mechanical effects also limit long-term use of such materials. The aim of the present study is to clarify the damage accumulation mechanism of Ti–6Al–4V alloys under the simultaneous reaction of corrosion and wear. For this purpose, we developed a new tribocorrosion system and then evaluated corrosion wear characteristics of the alloys with special attention focused on the effects of scratching rate on repassivation behavior. Two types of corrosion wear tests were carried out: one was the free corrosion potential measurement and the other was the potentiostatic polarization test both with simultaneous application of wear damage. In corrosion wear environment, a transition of wear type from abrasive to adhesive was observed with increase of the scratching rates. Higher scratching rate prevented regeneration of passive films on worn surfaces, resulting in generation of relatively large corrosion pits and a change in the wear mechanism. These results suggested that the scratching rate was an important factor affecting the damage accumulation process.

Physicochemical Mechanism for the Continuous Reaction of γ‐Al2O3‐Modified Aluminum Powder with Water

Previous experiments showed that γ‐Al2O3‐modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH− ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH− ions react with Al and release H2. Because of the limited H‐soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H2 molecules accumulate and form small H2 gas bubbles at the Al:Al2O3 interface. When the reaction equilibrium pressure in H2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as‐received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H2 bubbles decrease as well.

Passivation and corrosion behaviours of cobalt and cobalt–chromium–molybdenum alloy

Passivation and corrosion behaviour of the cobalt and cobalt-base alloy Co30Cr6Mo was studied in a simulated physiological solution containing chloride and bicarbonate ions and with pH of 6.8. The oxido-reduction processes included solid state transformations occurring at the cobalt/electrolyte interface are interpreted using theories of surface electrochemistry. The dissolution of cobalt is significantly suppressed by alloying it with chromium and molybdenum, since the alloy exhibited “chromium like” passivity. The structural and protective properties of passive oxide films formed spontaneously at the open circuit potential or during the anodic polarization were studied using electrochemical impedance spectroscopy in the wide frequency range.

Insights into Grain Structures and Their Reactivity on Grade-2 Ti Alloy Surfaces by Scanning Electrochemical Microscopy

The corrosion resistance of titanium and its alloys is a desirable property for many applications in materials science. In this report, the properties of the passive oxide film (TiO2) were examined by scanning electrochemical microscopy (SECM). A homemade closed-loop scanning electrochemical microscope was used to study these properties at the micrometer scale using ferrocenemethanol (Fc) as the redox mediator. The experimental results showed that the passive properties of a well-polished ASTM Grade-2 titanium (Ti-2) sample differed from region to region as indicated by the different current responses of the ultramicroelectrode (UME) on the SECM images. Selected spots with high feedback tip current were examined more closely by additional scanning. It was concluded that active spots with high positive feedback current were indicative of reactive structures located along crystalline grain boundaries, especially at the triple points. This observation is confirmed by comparing SECM results with optical micrographs reported in the literature. Previous metallographic investigations show these are the sites where the impurity iron content of the alloy congregates. This is confirmed by our experiments on scanning electron microscopy and energy dispersive X-ray analysis. For the first time, grain microstructure maps have been constructed on the SECM images, based on the above active sites. The properties of TiO2 films on Ti-2 were also assessed at various applied potentials. The reactivity of the oxide-covered Ti-2 surface increased and more active spots appeared when the Ti-2 bias potential was made more negative. This was attributed to oxide reduction (Ti(IV) to Ti(III)) and the onset of the absorption of hydrogen. The potential at which this change occurred varied at different sites on the scale of the grain structure (20−100 μm). These results demonstrate that SECM is a noninvasive analytical methodology that can provide insights into the structures and reactivities of Ti-2.

The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of PMo 12 O 40 3 - (PMo 12) and HPO 4 2 - anions for stabilization of the passive oxide film at the metal–polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo 12. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy–PMo 12/PPy–DHNDS < PPy–PMo 12/PPy–NDS < PPy–PMo 12/AnqDS < PPy–PMo 12/PPy–DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.

EIS characterisation of Ti anodic oxide porous films formed using modulated potential

Titanium, due to its excellent corrosion resistance, good mechanical properties and biocompatibility has been employed in many fields, such as in biomedical and dental implants. A passive oxide film formed on titanium surface is responsible for its high corrosion resistance. In the present study, oxide films were grown on titanium surface employing an anodising procedure using modulated potential, in a hydrofluoric acid solution. Electrochemical impedance spectroscopy was used to study the electrochemical behaviour of anodised Ti surface in a NaCl solution. The dependence of oxide film characteristics on forming conditions was analysed employing equivalent circuits. Samples were observed by scanning electron microscopy, in Jeol-JSM-5900LV microscope of the LME/LNLS, Campinas. It was observed that final anodising potential has an influence on the porous layer film morphology and that the anodising procedure with the chosen solution produces a double layer porous anodic film.

Polyaniline for corrosion prevention of mild steel coupled with copper

Polyaniline emeraldine base/epoxy resin (EB/ER) coating was investigated for corrosion protection of mild steel coupled with copper in 3.5% NaCl solution. EB/ER coating with 5–10 wt% EB had long-term corrosion resistance on both uncoupled steel and copper due to the passivation effect of EB on the metal surfaces. During the 150 immersion days, the impedance at 0.1 Hz for the coating increased in the first 1–40 days and subsequently remained constant above 10 9 Ω cm 2, whereas that for pure ER coating fell below 10 6 Ω cm 2 after only 30 or 40 days. Immersion tests on coated steel–copper galvanic couple showed that EB/ER coating offered 100 times more protection than ER coating against steel dissolution and coating delamination on copper, which was mainly attributed to the passive metal oxide films formed by EB blocking both the anodic and cathodic reactions. Salt spray tests showed that 100 μm EB/ER coating protected steel–copper couple for at least 2000 h.

Theory of Passive Film Stability

Some of the physico-electrochemical properties of passive oxide films that form on reactive metal and alloy surfaces, and which protect the underlying metals from reaction with corrosive environments, are reviewed within the context of the Point Defect Model (PDM). This model yields analytical expressions for the steady state current and film thickness and for the transients in these properties that can be used to predict deterministically the accumulation of general corrosion damage to metal surfaces, provided that the evolutionary path to the future state is continuous and can be specified. The conditions under which passivity may exist are defined in terms of Phase Space Analysis (PSA) of the PDM and it is found that passivity is invariably a meta-stable phenomenon. PSA leads to the development of Kinetic Stability Diagrams (KSDs) as alternatives to the equilibrium thermodynamic Pourbaix diagrams that are now used to describe the conditions under which passivity is observed.

Dissolution Characteristics of Iron and Stainless Steels in Chloride under Electrochemical Machining Conditions

Ultrasound has been used to produce time-resolved gap data during electrochemical machining (ECM), enabling calculation of the time-dependent dissolution valency and surface overpotential as a function of ECM dissolution current . Measurement of the - dissolution characteristics of iron and the stainless-steels Jethete (J) and SS410 in chloride electrolyte under the relatively high workpiece feed, high current density, high electrolyte flow conditions applicable to industrial ECM has shown that, in each case, the ECM voltage is initially predominantly applied across the workpiece surface, consistent with the presence of a passivating surface oxide film. This voltage then falls, consistent with a marked reduction in passivation. Reproducible, near-ohmic - behavior is then observed, independent of applied ECM voltage, initial gap, feed, and electrolyte concentration, suggesting that the fundamental - dissolution characteristics of the workpieces are being measured. For Fe, J, and SS410, relatively low values are observed, consistent with active dissolution along with mean near-ohmic resistances of , consistent with the presence of a resistive metal salt film on the workpiece surface. As well as giving insight into the dissolution process occurring under ECM conditions, these data provide a parameter base for the accurate modeling of the ECM process.

Enhancement of corrosion resistance of a biomedical grade 316LVM stainless steel by potentiodynamic cyclic polarization

The paper discusses the results on the use of a simple cyclic linear potentiodynamic polarization technique as a method of improving corrosion properties of passive oxide films formed on a biomedical-grade 316LVM stainless steel surface in phosphate buffer. The results demonstrate that the modification of 316LVM surface by cyclic potentiodynamic polarization between the potential of hydrogen and oxygen evolution results in the formation of a passive film that offers significantly increased corrosion resistance (both pitting and general) when compared to the naturally grown passive film. The effect of number of cycles and anodic potential limit on the resulting corrosion properties is discussed. The capacitance analysis demonstrates that the major difference between the electrochemically formed and naturally grown passive film is in the type of semiconductivity in the potential region where pitting on the unmodified surface occurs. The XPS analysis shows that this is due to the presence of Cr(VI)-species in the electrochemically formed passive film, which contribute to the increased density of metal vacancies, and thus to the increased pitting corrosion resistance of the passive film.

Oxidation kinetics of commercially pure titanium

The aim of this work was to perform thermal characterization of commercially pure titanium in dry air to determine its oxidation kinetics and the structure of the oxide. The oxidation kinetics were determined thermogravimetrically under isothermal conditions in the temperature range 300 to 750 ºC for 48 hours and the structure of the oxides was determined by differential thermal analyses and X-ray diffraction in the temperature range room temperature - 1000ºC. The oxidation rate of titanium increased with increase in temperature. It was high in the initial stages of oxidation and then decreased rapidly with time, especially up to 600 ºC. The kinetic laws varied between inverse logarithmic at the lower temperatures (300 and 400 ºC) and parabolic at the higher temperatures (650, 700 and 750 ºC). Evidences from X-ray diffraction and differential thermal analyses data revealed that the passive oxide film formed at room temperature crystallized into anatase at about 276 ºC. The crystallized oxide formed in the range 276 - 457 ºC consisted of anatase, in the range 457 - 718 ºC consisted of anatase and rutile sublayers, and at temperatures beyond 718 ºC consisted of a layer of pure rutile. Scanning electron microscopy observations reveled that the oxidized surfaces were crack-free and the surface roughness increased steadily with oxidation temperature.

Continuous monitoring of aluminum corrosion process in deaerated water

A device was developed for the continuous measurement of the open circuit corrosion rate of aluminum in deaerated water from hydrogen pressure, resulted from the cathodic reaction. Corrosion rate of aluminum coated by the passive oxide film was examined with respect to the corrosion phenomena using this device. From the results, it was found that the corrosion rate was dependent not only on the deterioration of the film, but also on the formation of pits and of corrosion products on the film. Furthermore, the deterioration of the film was independent on the pH of the water. The pits were most probably formed by the evolution of hydrogen gas at the cathode underneath the film followed by the formation of Al ( OH ) 4 - .

Electrochemical behavior of Ti and Ti6Al4V in aqueous solutions of citric acid containing halides

b * This paper reports on an investigation of the electrochemical behavior of Ti grade 2 and Ti6Al4V alloy in aqueous citric acid solutions with pH 2.0 containing halide ions. Voltammetric studies of Ti and the alloy in citric acid, with and without chloride ions, indicate that the Ti and Ti alloy presented a passive behavior in the test solutions used. Pitting was observed at 3.0 and 2.5 V/SCE for Ti and Ti6Al4V, respectively, when bromide ions were added to the solution. In solutions containing fluoride ions, dissolution of the film occurred at potentials close to - 1.0 V/SCE in both electrodes. The iodide ions oxidized on the passive oxide film at potentials close to 1.0 V/SCE. EIS results of the materials in citric acid solutions containing chloride ions revealed that the film's resistance increased as the applied potential rose from 0 to 1.0 V. In bromide-containing solutions, breakdown of the film was confirmed at potentials above 2.0 V/SCE in both electrodes. These results suggest film reformation for Ti and the alloy in solutions containing fluoride at potentials within the passive region.

Corrosion protection of steel by bi-layered polypyrrole doped with molybdophosphate and naphthalenedisulfonate anions

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPy coating consisted of the inner layer doped with PMo 12 O 40 3 - (PMo 12) and HPO 4 2 - ions and the outer layer doped with naphthalenedisulfonate (NDS) ions. PMo 12 ions doped in the inner layer make the passive oxide film on steel stabilized and the outer layer doped with large organic ions of NDS restricts decomposition and release of PMo 12 ions in the inner layer. Corrosion tests were made in 3.5% NaCl aqueous solution for the steels covered by the single PPy–PMo 12, single PPy–NDS, and bi-layered PPy–PMo 12/PPy–NDS coatings. The single PPy–PMo 12 and PPy–NDS coatings maintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating kept the steel in passive state and prevented corrosion of the steels for the longer time period.

Ellipsometry of passive oxide films on nickel in acidic sulfate solution

Nickel passive film has been studied in acidic sulfate solutions at pH 2.3 and 3.3 by ellipsometry. During anodic passivation followed by cathodic reduction, the roughness increases with dissolution of nickel, being indicated by gradual decrease of reflectance. However, the ellipsometric parameters, Ψ (arctan of relative amplitude ratio) and Δ (relative retardation of phase), are relatively insensitive to the roughness increase. From the change of Ψ and Δ, δ Ψ and δ Δ, during the anodic passivation and reduction, thickness of the passive oxide film was estimated with assumption of refractive index of n f = 2.3 of the film. The thickness estimated is a range between 1.4 and 1.7 nm in the passive potential region from 0.8 to 1.4 V vs. RHE, having a tendency of thickening with increase of potential. Cathodic reduction at constant potential induces a change of the oxide film to an oxide film with lower refractive index of n f = 1.7, accompanied by thickening of the film about 30% more in the initial stage of reduction for 30 s. The gradual decrease of thickness takes place for the oxide with the lower refractive index in the latter stage. The potential change from the passive region to cathodic hydrogen evolution region may initially cause hydration of the passive oxide of NiO, i.e., NiO + H 2O = Ni(OH) 2, and during the latter stage of reduction, the hydrated nickel oxide gradually dissolves.

Improvement of pitting corrosion resistance of a biomedical grade 316LVM stainless steel by electrochemical modification of the passive film semiconducting properties

In the present paper we show that a significant improvement in pitting corrosion resistance of a biomedical grade stainless steel, 316LVM, can be achieved by the formation of a surface passive oxide film under cyclic potentiodyanamic polarization (passivation) conditions. A complete absence of pitting in physiological simulating solutions was demonstrated, while the electrochemically formed passive film maintained its very high pitting resistance even at higher chloride concentrations in the bulk solution. The improvement in the film’s pitting corrosion resistance was correlated with its semiconducting properties.