Air-formed Oxide Film Research Articles

Electrochemical Impedance Spectroscopy (EIS) Explanation of Single Crystal Cu(100)/Cu(111) in Different Corrosion Stages.

Copper and its alloys are used widely in marine environments, and anisotropic corrosion influences the corrosion kinetics of copper. Corrosion of copper in an electrolyte containing Cl- is described as a dissolution-deposition process, which is a prolonged process. Therefore, it is laborious to clarify the corrosion anisotropy in different stages. In this paper, electrochemical impedance spectroscopy (EIS) following elapsed open circuit potential (OCP) test with 0 h (0H), 24 h (24H) and 10 days (10D) was adopted. To exclude interruptions such as grain boundary and neighbor effect, single crystal (SC) Cu(100) and Cu(111) were employed. After 10D OCP, cross-sectional slices were cut and picked up by a focused ion beam (FIB). The results showed that the deposited oxide was Cu2O and Cu(100)/Cu(111) experienced different corrosion behaviors. In general, Cu(100) showed more excellent corrosion resistance. Combined with equivalent electrical circuit (EEC) diagrams, the corrosion mechanism of Cu(100)/Cu(111) in different stages was proposed. In the initial stage, a smaller capacitive loop of Cu(111) suggested preferential adsorption of Cl- on air-formed oxide film on Cu(111). Deposited oxide and exposed bare metals also played an important role in corrosion resistance. Rectangle indentations and pyramidal structures formed on Cu(100)/Cu(111), respectively. Finally, a perfect interface on Cu(100) explained the tremendous capacitive loop and higher impedance (14,274 Ω·cm2). Moreover, defects in the oxides on Cu(111) provided channels for the penetration of electrolyte, leading to a lower impedance (9423 Ω·cm2) after 10D corrosion.

Kinetics of oxidation of metals in the air at room temperature using EDX

The kinetics of oxidation of Zn, Cu, Ni and C. steel were studied using the surface techniques, SEM, EDX and XRD, and electrochemical impedance spectroscopy (EIS) technique. EDX results showed that the variation of weight percent oxygen (Wt% O) on the surface of the four metals with the exposure time gave the same behavior, there is an increase of Wt% O with increasing the exposure time until about 2 h followed by a plateau, then the Wt% O increases again with increasing the time of exposure than 4 h. Fitting the experimental data of the four metals to the kinetic laws of oxidation indicated that the logarithmic law was fitted in short exposure (0.5 – 4 h), however, the parabolic law was fitted in long exposure (6 – 10 h). The values of each logarithmic rate constant (kl) and parabolic rate constant (kp)for the four metals were calculated. The kinetic of oxidation of the metals under study with exposure time is confirmed utilizing XRD technique. The protective action of the air-formed oxide films was investigated by EIS technique. The electrochemical impedance spectroscopy results confirmed the EDX results.

Investigation of Electrochemical Dissolution Behavior of Near-α TA15 Titanium Alloy in NaCl Solution with Low-Frequency Pulse Current

TA15 material is a typical near-α titanium alloy and widely used for the aircraft key load bearing components. Electrochemical machining (ECM) is a cost-effective method to machine difficult-to-cut TA15. Due to the high chemical reactivity of titanium, titanium alloy is prone to passivation, which increases the difficulty of ECM, especially for some common ECM methods with low-frequency pulse currents. To investigate the change of surface characteristics of titanium alloy in pulse ECM, the dissolution behavior of TA15 in NaCl solution under the low frequency pulse current was analyzed by scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). The results showed that even after removal of initial air-formed oxide film, TA15 would be passivated to form a new passive film during long pulse-off time. Under short pulse-on time, the passive film formed in pulse-off time could not be effectively removed in one pulse-on time, and the passivation effect would be accumulated, resulting in poor machined surface with humps and pits. When the pulse-on time was long enough to completely remove the newly formed passive film, TA15 was dissolved normally to obtain excellent surface with clear crystal structure.

Evaluation of the Protective Ability of Air-formed Oxide and Passive Films Formed on Nb-Bearing High Interstitial Stainless Steels

Evaluation of the Protective Ability of Air-formed Oxide and Passive Films Formed on Nb-Bearing High Interstitial Stainless Steels

A Blade-Abrading Method for Surface Pretreatment of Mg Alloys

A blade-abrading method was newly developed for the preparation of clean Mg alloy surface and reported for the first time in detail in this paper. The blade-abrading method includes abrading of the Mg alloy surface with a sharp blade or knife to remove the surface oxide and contaminants on the skin of Mg alloys mechanically, thereby providing very clean surface of Mg alloys. The clean surface prepared by the blade-abrading method in the air was found to be covered with thin and dense air-formed oxide films. Based on the experimental results obtained in this work, it is concluded that the newly developed blade-abrading technique is a very simple and useful pretreatment method which can provide clean and well-defined surface for the following surface treatments.

Influence of noble metals alloying additions on the corrosion behaviour of titanium in a fluoride-containing environment

Titanium alloys exhibit excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film, and some of these alloys (particularly Ti-6Al-7Nb) have been chosen for surgical and odontological implants for their resistance and biocompatibility. Treatment with fluorides (F(-)) is known to be the main method for preventing plaque formation and dental caries. Toothpastes, mouthwashes, and prophylactic gels can contain from 200 to 20,000 ppm F(-) and can affect the corrosion behaviour of titanium alloy devices present in the oral cavity. In this work, the electrochemical corrosion behaviour of Ti-1M alloys (M = Ag, Au, Pd, Pt) was assessed in artificial saliva of pH = 3.0 containing 910 ppm F(-) (0.05 M NaF) through open circuit potential, E(OC), and electrochemical impedance spectroscopy (EIS) measurements. The corrosion behaviour of the Ti-6Al-7Nb commercial alloy was also evaluated for comparison. E (OC) measurements show an active behaviour for all the titanium alloys in fluoridated acidified saliva due to the presence of significant concentrations of HF and HF(2) (-) species that dissolve the spontaneous air-formed oxide film giving rise to surface activation. However, an increase in stability of the passive oxide layer and consequently a decrease in surface activation is observed for the Ti-1M alloys. This behaviour is confirmed by EIS measurements. In fact, the Ti-6Al-7Nb alloy exhibits lower impedance values as compared with Ti-1M alloys, the highest values being measured for the Ti-1Au alloy. The experimental results show that the corrosion resistance of the studied Ti-1M alloys is similar to or better than that of Ti-6Al-7Nb alloy currently used as biomaterial, suggesting their potential for dental applications.

Native Air-Formed Oxide Film and its Effect on Magnesium Alloys Corrosion

The present work uses X-ray photoelectron spectroscopy (XPS) analysis to compare the chemical composition of native oxide films formed spontaneously on commercial pure magnesium and AZ31, AZ80 and AZ91D magnesium alloys. The study considers both the outer surface and inner regions of the films with the assistance of argon ion bombardment. Possible relationships are established between the alloy Al content and the native oxide film characteristics. The Al content is very similar in the oxide films on all three studied alloys. XPS identifies a much greater film thickness on AZ80 and AZ91D specimens than on AZ31 and pure Mg specimens, which seems to be related with the presence ofphase (Mg17Al12) on the AZ91D alloy surface and of the eutectic � -Mg/� on the AZ80 alloy surface. Considerable Ca segregation is observed (directly related with the calcium impurities content in the bulk material) towards the outer surface of the metal, where it appears in the form of calcium oxide. Direct correspondence is found between the thickness of the native oxide film formed spontaneously on the surface of magnesium and its alloys and their subsequent corrosion resistance in exposure to a 3.5 wt% NaCl solution.

Chromium nitride/Cr coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cell

Chromium nitride/Cr coating has been deposited on surface of 316L stainless steel to improve conductivity and corrosion resistance by physical vapor deposition (PVD) technology. Electrochemical behaviors of the chromium nitride/Cr coated 316L stainless steel are investigated in 0.05 M H 2SO 4 + 2 ppm F − simulating proton exchange membrane fuel cell (PEMFC) environments, and interfacial contact resistance (ICR) are measured before and after potentiostatic polarization at anodic and cathodic operation potentials for PEMFC. The chromium nitride/Cr coated 316L stainless steel exhibits improved corrosion resistance and better stability of passive film either in the simulated anodic or cathodic environment. In comparison to 316L stainless steel with air-formed oxide film, the ICR between the chromium nitride/Cr coated 316L stainless steel and carbon paper is about 30 mΩ cm 2 that is about one-third of bare 316L stainless steel at the compaction force of 150 N cm −2. Even stable passive films are formed in the simulated PEMFC environments after potentiostatic polarization, the ICR of the chromium nitride/Cr coated 316L stainless steel increases slightly in the range of measured compaction force. The excellent performance of the chromium nitride/Cr coated 316L stainless steel is attributed to inherent characters. The chromium nitride/Cr coated 316L stainless steel is a promising material using as bipolar plate for PEMFC.

Ozone-oxygen gas enhanced passivation of pure iron

Pure iron samples were passivated in borate buffer solution and were exposed to ozone-oxygen gas to study the possibility of improving the surface nanostructure such as atomically flat terrace structure. The treated samples were analyzed with spectroscopic ellipsometry and scanning tunneling microscopy, and the test of corrosion resistance was also carried out. The thickness of the oxide film increased by passivation at 800 mV (Ag/AgCl) and increased by ozone-oxygen gas exposure, but the oxidation film thickness decreased in air due to the reconstruction after rapid growth of oxides by passivation. The reconstruction of the oxide film was estimated by the change of the film thickness and compositions which were analyzed by spectroscopic ellipsometry using several layer models of Fe(OH) 3, γ-Fe 2O 3 and Fe 3O 4 and so on. The widest terrace width of the oxidation film was obtained on large particles based on the reconstruction after the combination treatment of passivation at 800 mV as well as subsequent ozone-oxygen gas treatment. The terrace width of the oxide film after passivation and ozone-oxygen gas treatment was three times larger than that of air-formed oxide film. The terrace width with atomic scale flatness was correlated with the corrosion resistance except for the increase in oxide film thickness.

The influence of initial surface conditions on field crystallization of anodic aluminum oxide films determined by synchrotron X‐ray diffraction

AbstractX‐ray diffraction measurements were performed using synchrotron radiation at the SPring‐8 facility and electrochemical techniques to investigate the effect of polishing methods and storage conditions on the crystal structure of air‐formed oxide films and anodic oxide films formed on highly pure aluminum. Storage in an N2 environment hinders local film breakdown during anodizing, and it was established that the X‐ray diffraction measurements showed the presence of a γ‐Al2O3 in the anodic oxide film formed on mechanically polished (MP) specimens. Formation of γ‐Al2O3 during anodizing was inhibited by electropolishing because of the removal of the work‐hardened layer that was formed on the MP by electro‐polishing. The X‐ray diffraction results do not show clear differences in the influence of the polishing method on the crystal structure of air formed oxide film. This is due to the very fast oxidation rate of the air‐formed oxide film and very long storage times for the X‐ray measurements. The anodic oxide film formed on aluminum, which has a very flat surface, shows color and the color depended on grain orientation. The electrochemical impedance of the MP specimen is slightly lower than that of the mechanically and then electrochemically polished specimen at the middle frequency range. This impedance difference may be due to formation of γ‐Al2O3 in the amorphous anodic oxide film and the thickness of the film. Copyright © 2010 John Wiley & Sons, Ltd.

Electrochemical Properties and Contact Resistance of Titanium in PEFC Bipolar Plate Environment

In order to assess the applicability of titanium as a separator material for polymer electrolyte fuel cells (PEFCs), the electrochemical surface characteristics of commercially available pure titanium were examined in a simulated PEFC environment. The results of the electrochemical tests showed that the anode and cathode operating potentials of PEFC, -0.36V and 0.64V (vs.Ag/AgCl(3.33M KCl)), lay in the cathodic and anodic polarization regions for Ti, respectively. Under the PEFC cathode condition, a barrier-type film with high charge transfer resistance and high interfacial contact resistance grew on the specimen. Under the PEFC anode condition, an air-formed oxide film dissolved and an oxidation film with low charge transfer resistance and low interfacial contact resistance grew.

Effects of Corrosion-Resistant Elements on the Corrosion Resistance of Amorphous Bulk Ni-Cr-Mo-Ta-Nb-4P Alloys in Concentrated Hydrochloric Acids

Rod-shaped amorphous bulk Ni-Cr-Mo-Ta-Nb-P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Most of them were spontaneously passive and immune to corrosion in 6 and 12 M HCl. Corrosion resistance was improved remarkably when both chromium and molybdenum were added to the alloy with sufficient tantalum contents. Spontaneous passivation was due to presence of stable air-formed films, in which chromium was particularly concentrated in addition to enrichment of tantalum. The angle resolved XPS revealed that chromium and molybdenum were rich in the inner part of the film. The high corrosion resistance was interpreted in terms of the high stability of the air-formed oxide film consisting of the outer double oxide of chromium and tantalum with a high stability in concentrated hydrochloric acids and the inner molybdenum oxide acting as the diffusion barrier.

Study on behavior of plasma nitrided 316L in PEMFC working conditions

Stainless steel bipolar plates for the polymer electrolyte membrane fuel cell (PEMFC) offer many advantages over conventional machined graphite and graphite-composites. However, the interfacial ohmic loss between the metallic bipolar plate and membrane electrode assembly due to corrosion decreases the overall power output of PEMFC. A lower temperature (at 370 °C) plasma nitriding was applied to modify the surface of stainless steel 316L bipolar plates. The results of electrochemical measurements show that corrosion resistance of the plasma nitrided 316L is improved in simulated PEMFC anode/cathode environments purged with H 2/air at 70 °C. The surface conductivity of the nitrided layer is better than that of the air-formed oxide film. The interfacial contact resistance (ICR) between the passive film and carbon paper increases very little after potentiostatic polarization for 4 h, which indicates potential for good stability of this material in highly corrosive fuel cell environments.

Corrosion prevention of iron with novel organic inhibitor of hydroxamic acid and UV irradiation

Corrosion prevention by self-assembled monolayers (SAM) of monomer and polymer inhibitor on iron covered with air-formed oxide films was investigated by cyclic voltammetry in borate buffer solution. Anti-corrosion efficiency of the SAM-coated Fe electrodes depends on UV irradiation duration on Fe electrodes prior to coating and inhibitor concentration to form SAM. The 1-h UV-irradiated Fe electrodes coated with SAM exhibits the most effective corrosion resistance despite the anti-corrosion efficiency of air-formed films on Fe was linearly increased with UV irradiation. The addition of monomer in polymer solution improves the stability and corrosion resistance of SAM.

The activation of aluminium by mercury ions in non-aggressive media

The presence of Hg at concentration less than 300 ppm in Al base alloys causes their passivation breakdown. On alloys used as sacrificial anodes, it causes a major lowering (>0.3 V) in their operational potential in chloride media. Mercury as trace constituent in the natural gas stream causes severe damage to cryogenic heat exchangers. The present paper presents evidences of the mechanism by which mercury produces its pronounced effect in aqueous non-aggressive media. The work was carried out using pure (99.99%) aluminium and mercury (II) acetate solutions of different concentrations and pH. Open circuit potential–time responses were obtained. The surface effects were followed by means of scanning microscopy and EDAX/X-Ray analysis. The results demonstrate that immediately after immersion, the initial air-formed oxide film underwent a dynamic crack–healing process at flaws in the film, possible associated to grain boundaries. The subsequent healing process, if any, depends on the media composition. Thus, in this special case, Hg 2+ ions can be directly reduced on the bare aluminium, reaching a true metallic contact, and initiating surface diffusion. This enables the formation of an amalgam. Aluminium atoms diffuse through the liquid mercury and undergo oxidation at the amalgam/electrolyte interface. This process is responsible for the oxide detachment (by undermining) and the attack morphology (i.e., wide cavities). The presence of aggressive anions is not needed to initiate activation.

Electrochemical characterization of the passive films formed on niobium surfaces in H2SO4 solutions

The electrochemical formation and characteristics of passive films on niobium surfaces in aqueous H2SO4 solutions were studied using open circuit potential and cyclic voltammetry. In the potential region between -1.0 and 1.2 V(NHE), the cyclic voltammetry data showed that the active/passive transition involves slow metal dissolution followed by the formation of semiconducting passive oxide films. The possible electrochemical reactions and the change of the oxidation steps of some niobium oxides occurring in the passive film during the polarization are proposed. A strong influence of the natural air-formed oxide film on the chemical composition of the passive film was shown. This influence makes chemical structure of thin passive films more complicated than that of thick anodic films. It is shown that the passive films consists of more or less stable oxides, such as NbO, NbO2 and Nb2O5. The Raman spectra revealed that the thin passive films were amorphous, while the films formed at higher voltages consist, primarily, of well-crystallized Nb2O5.

Characterization of in situ mechanically worked PVD Mg–Ti alloys

Mg–Ti alloys with up to 46 wt% Ti in solid solution were produced by physical vapour deposition (PVD) with in situ mechanical working. All the alloys exhibited compositional inhomogeneity, columnar microstructures and typical PVD defects. The thermal stability of the solid solutions decreased with increasing Ti content, with the most stable solid solution breaking up above 566 K. The in situ mechanical working reduced porosity by closing pores and flattened surface asperities. The oxide film on the surface of the Mg–8 wt% Ti alloy was Ti-free. The air formed oxide film on the alloys with a Ti content above 8 wt% was identiffied as a mixture of MgO and TiO2 covered by Mg(OH)2 and hydromagnesite at the outermost surface. Magnesium oxide and titanium oxide were also formed at the columnar boundaries that were not affected by the in situ mechanical working. The oxide chemistry was studied using Auger parameter analysis.

Effect of Ultraviolet Light Irradiation on Structure and Electrochemical Properties of Iron Surface

The effect of ultraviolet light (UV) irradiation (254 nm, 0.8 mW/cm2) on air-formed oxide films and passivated films on iron was investigated by electrochemical methods and scanning tunneling microscopy (STM), in particular with respect to surface micro/nanostructures and the surface protective property. An as-deposited film appeared uniformly flat after UV irradiation for 2–4 h, which is associated with a decrease in current density. UV irradiation for 1–4 h assisted N-dodecylhydroxamic acid (DHA) molecules to strongly bond to the air-formed oxide film. UV irradiation for 1 h led to the formation of a flat terrace of atomic resolution on a surface passivated at 800 mV for 15 min. However, it was difficult to observe a terrace wider than 3 nm on the passive film irradiated for 4 h.

Cathodic reduction of the duplex oxide films formed on copper in air with high relative humidity at 60 °C

The cathodic reduction of duplex air-formed oxide film on copper was performed at a constant current density of i c = −50 μA cm −2 in deaerated 0.1 M KCl solution to investigate the sequence of cathodic reduction of each oxide layer and its mechanism. The single-phase thick CuO film on copper was also cathodically reduced at i c = −50 μA cm −2 or −2.5 mA cm −2. The surface characterizations of the air-formed oxide film and single-phase CuO film before cathodic reduction and after partial or complete cathodic reduction were performed by XPS and X-ray diffraction, respectively. The two plateau regions appeared in the potential vs. time curve during cathodic reduction of the duplex air-formed oxide film on copper, while one plateau region was observed in the potential–time curve during cathodic reduction of the single-phase CuO film on copper. The potential in the first plateau region for the air-formed film coincided with that in the plateau region for the CuO film. The results of XPS and X-ray diffraction suggested that in the first plateau region, the outer CuO layer is directly reduced to metallic Cu, while in the second plateau region, the inner Cu 2O layer is reduced to metallic Cu.

Ellipsometric study of anodic oxide films formed on niobium surfaces

Anodic oxide films formed potentiostatically on niobium surfaces, from open circuit potential (OCP) to 10 V, were studied by performing in situ and ex situ ellipsometric measurements. The kinetics of the film thickness growth in 1 M H2SO4 and complex indices of refraction of these films were determined. A strong influence of the surface preparation conditions on the complex refractive indices of the metal substrate and anodic oxide films was shown. By steady-state measurements at OCP, a small thickening of the natural air-formed oxide film with chemical composition Nb2O5 in 1 M H2SO4 solution was detected. With cathodic pre-treatment, only partial reduction and small thinning of the natural air-formed oxide film was possible. The thicknesses of the natural air-formed oxide films on fine mechanically polished and electropolished Nb surfaces were determined. The build up of the natural air-formed oxide film, at ex situ conditions, on the already formed anodic oxide films was confirmed. It was shown that electropolishing gives more similar optical surface properties to the bare metal than the fine mechanical polishing.