Air-formed Film Research Articles

The Role of Alloying Elements on the Surface Treatment and Finishing of Aluminium

The influence of alloying elements, deliberate or otherwise, and material processing on the performance of aluminium during surface treatment and finishing are considered. Thus, with a significant focus on copper, but with consideration of other elemental additions, the behaviour of aluminium during growth of oxide at ambient temperature, etching or pickling, conversion coating and anodizing, essential processes for generation of fit-for-purpose products, is highlighted. Further, such processes generate, modify or transform the initially present air-formed alumina film. Consequently, with knowledge of the phenomena proceeding at the alloy/film and film/environment interfaces and those within anodic or other films, the possibility of controlling features of nanoscale dimensions for improved performance arises. For example, deliberate selection of alloying elements enables control of nanotextures formed at treated surfaces, and formation of compositionally and morphologically modified films as well as generation of nanoparticles with various functional properties.

On the Passivation of 349TM Stainless Steel in Simulated PEMFC Cathode Environment

Austenite stainless steel 349TM has been tested in a simulated polymer electrolyte membrane fuel cell (PEMFC) cathode environment, and a stable passive film was formed within 15 minutes of polarization. X-ray photoelectron spectroscopy (XPS) has been utilized to investigate these surface films on the steel. The surface oxide film formed in air (air-formed film) composed of iron oxides and chromium oxide, with a Fe-rich outer layer and a Cr-rich inner layer. Neither of them was a dominating species in the air-formed film. On the other hand, the passive film formed in the simulated PEMFC cathode environment was dominated by Cr-oxide. The thickness of the films was estimated to be 2.4 nm for air-formed film, and 3.0 nm for the passive film. The difference in morphology between fresh and passived steels was identified by means of atomic force microscopy (AFM).

Evolution of passivity in air exposure of an iron passive film

The evolution of passivity in air exposure of air-formed films on as-polished bare irons and electrochemically reduced bare irons was investigated by electrochemical methods. Passivity and surface structure were characterized by the recovery duration against open-circuit breakdown and the spontaneous potential, respectively. A new factor, pre-exposure duration (tpre, exposure duration prior to electrochemical treatments), was found to affect both surface structure and passivity of air-formed films. At tpre<20min, the surface structures of air-formed films underwent a distinctly four-stage transformation in air exposure while the passivity of air-formed films underwent a two-stage transformation, being drastically improved after critical exposure duration in air. As a comparison, the surface structure of the passive film formed at −0.4V for 30min exhibited two-stage dependence on air exposure time, distinctly different from that of the air-formed films, and its passivity showed two-stage dependence on air exposure time. At tpre<20min, the critical exposure duration for the passivity of air-formed films is 216.9min, more than that (139.9min) of the passive film while it is 19.1min at tpre>6h, in agreement with that (20.7min) of the passive film. These results are explained by the dependence of the passivity on the crystal structure.

Morris Cohen Graduate Student Award Address- Surface Chemistry and Corrosion Behavior of Aluminum-Copper Systems: Air Formed Films to Complex Conversion Coatings

not Available.

Nanoscale surface properties of iron treated by electrochemical and physico-chemical methods

Abstract The nanoscale surface structure of surface oxide films formed on pure iron treated by UV irradiation at 3 mW/cm 2 and electrochemical passivation at an applied potential of 800 mV vs. Ag/AgCl in a borate buffer solution have been observed with scanning probe microscopy and spectroscopic ellipsometry. UV irradiation of the air-formed film decreased the film thickness, surface roughness and surface area. UV irradiation of electrochemically passivated surfaces increased the film thickness and the terrace width of the oxide film.

Investigation of a Duplex Stainless Steel as Polymer Electrolyte Membrane Fuel Cell Bipolar Plate Material

Duplex 2205 stainless steel was investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment; stable passive films formed quickly under these conditions. The interfacial contact resistance (ICR) values for 2205 steel with the air-formed film were identical to those for 349™ and lower than those for AISI446 steel. The ICR values for the passive film formed in a simulated PEMFC anode environment were lower than those for the passive film formed in the cathode environment, but both were higher than those for air-formed film. X-ray photoelectron spectroscopy analysis revealed that the passive films were mainly while the air-formed film was composed of Fe-oxides and An estimate of the thickness of the surface films was made. © 2005 The Electrochemical Society. All rights reserved.

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.

Electrochemical Studies of AC/DC Anodized Mg Alloy in NaCl Solution

The corrosion behavior of a Mg-based alloy, WE43, ac/dc anodized using a waveform involving a final period of current decay in an alkaline silicate solution, has been investigated using electrochemical impedance spectroscopy during exposure to a 0.86 M NaCl solution. The change of the oxide film morphology with immersion time was also examined using scanning electron microscopy. Anodization of the WE43 alloy significantly improves its corrosion resistance and greatly increases the time to pitting in the NaCl solution, which for air-formed films is in the range 2-3 h. By fitting the impedance data to a two-time-constant equivalent circuit and by tracking the open-circuit potential, it is demonstrated that film hydration initially decreases the corrosion resistance, followed by an increase in the resistance due to the gradual conversion of MgO to which leads to partial blocking of the film pores. During this time period, the underlying barrier film is slowly thinned and/or penetrated by chloride ions, consistent with its increasing capacitance and decreasing resistance, ultimately leading to a loss in corrosion resistance. Overall, both the porous and barrier oxide layers contribute to the corrosion protection of WE43, and the higher the voltage and the longer the time of current decay in the latter stages of anodization, the lower the alloy corrosion susceptibility. © 2004 The Electrochemical Society. All rights reserved.

チタンの大気酸化皮膜の模擬ベントナイト接触水中における変質

チタンの大気酸化皮膜の模擬ベントナイト接触水中における変質

Photocurrent spectroscopy study of passive films on hafnium and hafnium–tungsten sputtered alloys

Anodic and air-formed films on sputtered Hf and W–Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH) 2. This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials ( V f>10 V). In the case of W–Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising in different electrolytes. A PCS analysis of films grown on sputtered alloys is performed on the basis of previously proposed correlation between the band gap of anodic films and difference of electronegativity of their constituents.

Interaction of carboxylic acids with the oxyhydroxide surface of aluminium: poly(acrylic acid), acetic acid and propionic acid on pseudoboehmite

High sensitivity and spectral resolution XPS, polarisation-modulation reflection FTIR and static SIMS have been used in this investigation of the interfacial chemistry between poly(acrylic acid) (PAA) and an aluminium oxyhydroxide surface. The pseudoboehmite oxyhydroxide surface is used as a model for the chemistry found at the surface of the air-formed film after ambient exposure of aluminium metal. Conversion of carboxylic acids to monodentate carboxylate species at the PAA–AlOOH interface has been identified using reflection absorption FTIR. By virtue of the very low concentration of surface carbon on hydrothermally formed AlOOH, quantification of the interfacial carboxylate concentration was achieved by curve fitting the C1s core level. Through analysis of a range of PAA overlayer thicknesses, a limit to interfacial carboxylate formation with the surface was identified. To determine the reason for this limit, monoacids that chemisorb to the AlOOH surface exclusively through carboxylate bonds were analysed. It is proposed that steric and/or conformational hindrance, associated with the polymer backbone, prevent reaction of all carboxylic acid groups in PAA with surface hydroxyl functionalities to form carboxylate complexes. The implications of this finding for the application of PAA as an adhesion promotion pretreatment are considered.

An Alternative Explanation for the Apparent Passivation of Molybdenum in 1 mol/L Hydrochloric Acid

A key question arising from studies on the corrosion behavior of molybdenum has been whether the passivation of molybdenum originates from a three-dimensional oxide film or from chemisorbed oxygen. In the present paper an alternative explanation for the apparent passivation behavior of molybdenum is given based on XPS measurements in combination with electrochemical techniques. Below the transpassive potential region, anodic current is obtained only if the sample has been cathodically activated and not after activation by ion etching. The experiments also show that the commonly used cathodic activation is insufficient to reduce the air-formed film. Thus, for such samples the anodic current in the apparent passive region could be due to oxidation of low valent oxides on the surface. However, a transient anodic current is observed also for ion-etched samples that have been subjected to cathodic polarization. In addition, impedance measurements show an unexpectedly large capacitance in the apparent passive region, indicating some kind of adsorption equilibrium at the metal-solution interface. This leads to the suggestion that hydrogen formed in the reduction process can diffuse into the molybdenum metal and cause a limiting oxidation of hydrogen upon positive polarization. Thus, in the apparent passive region the current is mainly due to oxidation of hydrogen and an extension of the transpassive corrosion yields a very low corrosion current for molybdenum. This is in agreement with the XPS experiments, where no more than a monolayer of oxidized molybdenum can be detected in the apparent passive region. © 2001 The Electrochemical Society. All rights reserved.

GDOES depth profiling analysis of the air-formed oxide film on a sputter-deposited Type 304 stainless steel

An air-formed film, ∼2 nm thick, on sputter-deposited Type 304 stainless steel has been analysed by glow discharge optical emission spectroscopy (GDOES) depth profiling. The distributions of iron, chromium, nickel and hydrogen in the film and alloy substrate have been revealed, with excellent depth resolution. The film consists of two distinct layers: an outer iron oxide layer and an inner oxide layer highly enriched with chromium oxide. Further, a thin nickel-enriched layer is evident in the alloy immediately beneath the oxide film. The implications of these findings for the structure and mechanism of growth of the film are discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Characterization of the oxide/hydroxide surface of aluminium using x-ray photoelectron spectroscopy: a procedure for curve fitting the O 1s core level

The performance of coated and bonded aluminium relies heavily upon its surface chemistry and hence characterization of the aluminium surface is important. A method to quantify the hydroxide concentration at aluminium oxide/hydroxide surfaces by curve fitting the O 1s peak is developed and tested in this paper. Pseudoboehmite, AlO(OH), is formed at the surface of aluminium after immersion in boiling water. The surface of this material was used to determine the binding energy of the unresolved O 1s component peaks that were referenced to the binding energy of the Al 2p oxide component. In vacuo heating resulted in changes in the elemental and functional composition that were consistent with dehydration of the pseudoboehmite. It is proposed that the resultant film comprises γ-alumina with residual hydroxide groups. The O 1s curve-fitting method was applied to air-formed films with known atmospheric exposure histories before and after heating in vacuo. The change in both the elemental composition and functional stoichiometry of the films upon heating was consistent with significant but incomplete dehydration. The probable surface phases are determined from the functional and elemental composition. Copyright © 2000 John Wiley & Sons, Ltd.

Dissolution and passivation of iron in acetonitrile and acetonitrile–water mixtures

The dissolution and passivation of iron in neutral acetonitrile–water mixtures have been studied under potentiodynamic and potentiostatic conditions. In “dry” acetonitrile (water content <0.02% or 200 ppm), passivation, due to an air-formed film, is maintained for a wide range of potentials up to 0.50 V, well above the corrosion potential E corr≈−0.4 V. Transpassive dissolution at higher potentials is normally controlled by interfacial reactions rather than diffusion in the solution. Addition of a relatively small amount of water (0.5% or ∼0.28 M) to acetonitrile initiates active dissolution on iron surfaces damaged by previous transpassive dissolution. At a level of 2% water (∼1.1 M) active dissolution is initiated on undamaged surfaces and proceeds relatively slowly under control by interfacial reactions. Transpassive dissolution at similar potentials meanwhile becomes diffusion-controlled. As water content is increased further (e.g., to 6% or ∼3.3 M) the electrochemistry of iron becomes progressively similar to that in aqueous solutions, with both active dissolution and passivation being enhanced. Passivation is promoted by addition of hydrogen peroxide as a passivator, but the dynamic balance between the two processes can, as expected, be shifted to dissolution by increasing solution acidity. The similarities and differences in acetonitrile, water and their mixtures are discussed in terms of the relative reactivity and concentration of the two solvents. The profound effects of water on the kinetics of iron dissolution and passivation are attributed to the dominant reactivity associated with acidic hydrogen. Based on the potentiostatic and potentiodynamic features, the mechanism of phase change during dissolution is described in terms of a model of a shifting oxide film formed through two-dimensional nucleation and growth.

Physico-chemical characterisation of native air-formed oxide films on Al-Mg alloys at low temperature. Influence of water

Natural oxide films on two cold-rolled Al-Mg alloys were studied using several techniques (XPS, SIMS, ToF-SIMS, GDOS, SEM, TEM, Wetting measurements). The aim was to characterise and compare the oxide layers formed on the two materials in order to better understand the influence of the thermomechanical history. The experimental tools used allowed the determination of the influence of the annealing process on the nature and structure of the oxides. The hydration and Bronsted behaviour were also investigated. The thickness of the layer, the amount of magnesium oxide and its dissolving resistance in water are strongly modified by the annealing. The oxide film formed on the annealed material is twice as thick as the one on the non-reheated material. The layer on the annealed material contains more magnesium oxide and contains the crystalline forms:β-Al2O3 and bayerite (Al(OH)3). On the contrary, the oxide film formed on the as-rolled material seems to be amorphous. The magnesium oxide is less soluble in water in the annealed film than in the non-reheated one. Furthermore, aluminium and magnesium oxides were found to be hydroxylated on both alloys, and the layer surfaces to behave like a Bronsted meaning base.

Determination of the acid-base properties of metal oxide films and of polymers by contact angle measurements

The surface isoelectric point for native air-formed oxide films on various metals has been determined by measurement of contact angles at the hexadecane/ aqueous solution interface as a function of the pH of the aqueous phase. Application of Young's equation, the Gibbs equation, and surface equilibria conditions for hydroxylated oxide films leads to a mathematical expression which shows that the contact angle goes though a maximum at the isoelectric point of the oxide. The experimentally determined values for the oxide films on aluminum, chromium, and tantalum are within one to three pH units of the reported isoelectric points for the corresponding bulk oxide powders. The surface isoelectric point of an oxide-covered Ta-implanted A1 surface lies between that of Al2O3 and Ta2O5. The acid-base properties of various polymers, including a commercially available pressure-sensitive adhesive, were determined by measuring the contributions γS + and γS - to the solid surface free energy using the contact angle approach of van Oss and Good. Adhesion measurements for a pressure-sensitive adhesive having a positive surface charge show that the peel strength is greatest when the metal substrate has a surface oxide film of basic character.

XPS and electrochemical studies on the corrosion behaviour of sputter-deposited amorphous Mn-Nb alloys in a neutral chloride solution

Amorphous Mn-Nb alloys are prepared by D.C. magnetron sputter deposition method in a wide composition range. The effects of niobium addition on the corrosion behaviour of manganese are evaluated in a borate buffer solution of pH 8.4 containing 0.1 M NaCl by using weight-loss measurement, electrochemical and XPS techniques. A clear improvement of the corrosion resistance of these alloys compared with electro-deposited manganese is found. The alloys containing 24 at.% or more Nb are spontaneously passive. Their corrosion rates are about three to four orders of magnitude lower than that of manganese. The alloys containing 40 at.% or more Nb show almost the same corrosion rate as that of niobium. XPS analysis reveals that niobium is concentrated in air-formed films, and the films formed after open circuit immersion are further enriched in niobium. The improvement of the corrosion resistance by alloying with niobium is explained in terms of the formation of niobium-rich passive films composed of double oxyhydroxides of Mn [2]and Nb [5]ions.

Electrochemical and XPS studies of the passivation behavior of sputter-deposited Cr–Ta alloys in 12 M HCl

The sputter-deposited Cr–Ta alloys show extremely high corrosion resistance in 12 M HCl. The open circuit potentials of the Cr–Ta alloys are located in the passive regions of both chromium and tantalum, and all Cr–Ta alloys are spontaneously passivated. XPS analysis indicates that the composition and thickness of the air-formed film are the same as those of spontaneously passivated film, and the composition of the passive films becomes constant after prolonged immersion. The film consists of a double oxyhydroxide of chromium and tantalum cations. The formation of the homogeneous double oxyhydroxide film consisting of both chromium and tantalum cations by air oxidation is responsible for the extremely high corrosion resistance of the Cr–Ta alloys in comparison with the corrosion resistance of chromium and tantalum metals.

An XPS study of passive films on sputter-deposited Cr-Nb alloys in 12 M HCl solution

The corrosion behavior of sputter-deposited Cr-Nb alloys in 12 M HCl solution was investigated by electrochemical and XPS methods. All of the Cr-Nb alloys are spontaneously passivated in 12 M HCl solution open to air at 30 °C. The open circuit potentials for chromium-rich alloys containing 42 at% or less niobium are located in the passive region of chromium and niobium, but those for niobium-rich alloys containing 52at% or more niobium are located in the active region of chromium. XPS analysis reveals that the passive film consists of a double oxyhydroxide of chromium and niobium. The formation of the passive film consisting of double oxyhydroxide of chromium and niobium is responsible for the high corrosion resistance of the Cr-Nb alloys. The passive films formed on the chromium-rich alloys are slightly rich in chromium, and are not appreciably different from the air-formed films before immersion. Angle-resolved XPS analysis indicates no concentration gradient of Cr 3+ and Nb 5+ ions in depth of the passive films on the chromium-rich alloys. On the other hand, the air-formed film on the niobium-rich alloys is not stable in 12 M HCl and converted to the passive film in which niobium is slightly enriched.