Air-formed Film Research Articles

The passivation behavior of sputter-deposited W-Ta alloys in 12 M HCl

A series of nanocrystalline or amorphous single solid solutions of W-Ta alloys are prepared by D.C. magnetron sputtering. The passivation behavior of these alloys is studied by immersion test, electrochemical measurements and X-ray photoelectron spectroscopy (XPS) analysis. The W-Ta alloys are passivated spontaneously and show significantly high corrosion resistance in 12 M HCl at 30 °C. Their corrosion rates are about two orders of magnitude lower than that of sputter-deposited tungsten and are lower than that of sputter-deposited tantalum. XPS analysis shows that tantalum is concentrated in both the air-formed films and the passive films formed spontaneously on the alloys after long time immersion in 12 M HCl. The surface films are composed of double oxyhydroxides of Ta 5+ and W 4+ ions. The formation of spontaneous passive film composed of a double oxyhydroxide of tungsten and tantalum ions is responsible for high corrosion resistance of the W-Ta alloys in concentrated hydrochloric acid solution.

An XPS study of passive films on corrosion-resistant CrZr alloys prepared by sputter deposition

X-ray photo-electron spectroscopy (XPS) has been used to examine spontaneously passivated films formed on sputter-deposited CrZr alloys in 6 M HCl solution open to air at 30 °C, for a better understanding of the high corrosion resistance of these alloys. The open circuit potentials of the CrZr alloys are located in the passive regions of both chromium and zirconium, and all of the CrZr alloys examined are spontaneously passivated. An increase in chromium content of the alloys enhances the cathodic activity for oxygen reduction and content of the alloys enhances the cathodic activity for oxygen reduction and decreases the anodic current density with a consequent ennoblement of the open circuit potential. XPS analysis indicates that the air-formed films on these alloys are composed of homogeneous double oxyhydroxide consisting of both chromium and zirconium ions. The cationic composition of the film is almost the same as the alloy composition although slight enrichment of zirconium occurs. The air-formed film is protective enough to lead to spontaneous passivation of these alloys. The immersion for a long period of time results in a slight deficiency in zirconium in the exterior of the passive film as a result of gradual dissolution of zirconium. At the same time, the total amount of chromium in the passive film increases. The passive film consists of double oxyhydroxide of chromium and zirconium, in which the cationic composition is almost the same as the alloy composition. The formation of the passive film consisting of double oxyhydroxide of chromium and zirconium is responsible for the higher corrosion resistance of these CrZr alloys in comparison with chromium and zirconium metals.

Spontaneously passivated films on sputter-deposited Cr-Ti alloys in 6 M HCl solution

Among sputter-deposited Cr-Ti alloys, 30–65 at% Ti alloys are amorphized. The Cr-Ti alloys with 22at% or less Ti are bcc single phase alloys. Regardless of the crystallinity, the open circuit potentials of sputter-deposited Cr-Ti alloys are located in the passive regions of both titanium and chromium in 6 M HCl solution open to air at 30 °C, and all Cr-Ti alloys are spontaneously passivated. X-ray photo-electron spectroscopy (XPS) analysis reveals that the spontaneously formed passive film as well as air-formed film is slightly rich in titanium ions mainly because of preferential oxidation of titanium. According to angle-resolved XPS measurement, no concentration gradient of Cr 3+ and Ti 4+ ions has been detected in depth of the passive film. Analysis of binding energies of core electrons of Cr 3+ and Ti 4+ cations shows that Cr 3+ and Ti 4+ cations are located very closely in the film so as to show the electronic interaction, and the film is not composed of a heterogeneous mixture of chromium and titanium oxyhydroxides but of homogeneous double oxyhydroxide consisting of Cr 3+ and Ti 4+ cations. The formation of the homogeneous double oxyhydroxide film containing both Cr 3+ and Ti 4+ cations is responsible for extremely higher corrosion resistance of the homogeneous single phase Cr-Ti alloys in 6 M HC1 open to air at 30 °C in comparison with the corrosion resistance of chromium and titanium metals.

Electrochemical and xps studies of the corrosion behavior of sputter-deposited amorphous W-Zr alloys in 6 and 12 M HCl solutions

Amorphous W-Zr alloys containing 23–76 at% zirconium have been successfully prepared by DC magnetron sputtering. The W-Zr alloys are spontaneously passive and show significantly high corrosion resistance in 6 and 12 M HCl solutions. In particular, their corrosion resistance in 12 M HCl is higher than that of the alloy constituents. The pitting resistance of zirconium is greatly improved by alloying with tungsten. XPS analysis showed that zirconium is enriched in both the air-formed films and the passive films in HCl solutions. From angular-dependent XPS measurements, the passive films on W-Zr alloys are found to be composed of the double oxyhydroxide of tungsten and zirconium ions, although there are some concentration gradients of tungsten and zirconium ions. The formation of the homogeneous double oxyhydroxide of tungsten and zirconium ions acts synergistically in improving the corrosion resistance of W-Zr alloys in HCl solutions open to air at 30°C.

The influence of pre-immersion on the potentiostatic polarization behavior of amorphous Fe-Cr-Mo-P-C alloys in de-aerated 1 M HCl

The effect of immersion prior to polarization on the potentiostatic polarization behavior of amorphous Fe-8Cr- xMo-13P-7C ( x = 0, 2, 4 and 6 at%) alloys and the composition of the surface film produced by potentiostatic polarization at a low potential in the passive region in de-aerated 1 M HCl is investigated. Passivation of amorphous Fe-8Cr-13P-7C alloy becomes difficult with immersion prior to potentiostatic polarization and the specimen immersed for 10 min in de-aerated 1 M HCl does not passivate even after potentiostatic polarization of several hours. By contrast, the extension of pre-immersion time for molybdenum-containing alloys results in a decrease in the current density necessary for passivation. Although chromium ions accumulate rapidly in the surface film on the molybdenum-free alloy polarized immediately after immersion, the chromium concentration in the film does not increase by prolonged polarization. The chromium concentration in the surface film on the molybdenum-free alloy immersed previously for 10 min increases continuously with polarization time up to a very high value, which is much higher than that in the surface film on molybdenum-bearing alloys. Chromium is enriched in the surface film on molybdenum-bearing alloys throughout the polarization experiment of 5h. Preliminary immersion of molybdenum-bearing alloys decreases the enrichment of chromium in the passive film formed by polarization. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys by pre-immersion, and is enriched remarkably in the film after prolonged polarization.

The influences of Mo addition and air exposure on the corrosion behavior of amorphous Fe8Cr13P7C alloy in de-aerated 1 M HCl

The change in the corrosion behavior of amorphous Fe8Cr xMo13P7C ( x = 0, 2, 4, 6) alloys in deaerated 1 M HCl with molybdenum content and pretreatment has been studied. Amorphous Fe8Cr13P7C alloy shows active dissolution in de-aerated 1 M HCl. This alloy becomes corrosion-resistant with the addition of molybdenum. X-ray photo-electron spectroscopic analysis reveals that the chromium content of the films decreases continuously with immersion time despite an initial sharp increase in chromium concentration in the surface film. No enrichment of chromium is detected in the surface films on molybdenum-free and molybdenum-bearing alloys except for 6 at % molybdenum alloy after prolonged immersion. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys throughout the immersion experiment for 20 h. Molybdenum oxyhydroxide, rather than chromium oxyhydroxide, is considered to be responsible for the formation of the passive film on molybdenum-bearing alloys immersed in de-aerated 1 M HCl. The addition of 6 at% molybdenum is effective in maintaining metallic luster for prolonged immersion due to the formation of the passive film in which molybdenum and to some extent chromium are concentrated. Phosphorus is little involved in the airformed films on all alloys but accumulates by immersion in the passive films on molybdenum-containing alloys up to its bulk composition, whereas the phosphorus content of the surface film on the molybdenum-free alloy is low. Air exposure for 24 h prior to polarization or immersion raises the initial open circuit potential of specimens and retards the enrichment of molybdenum in the passive film on the 6 at% molybdenum alloy in de-aerated 1 M HCl.

The effect of air exposure on the corrosion behavior of amorphous Fe-8Cr-Mo-13P-7C alloys in 1 M HCl

Amorphous Fe-8Cr-13P-7C alloy is corroded severely in 1 M HCl open to air. The addition of more than 2 at% molybdenum is effective in promoting spontaneous passivation. According to XPS analysis, chromium accumulates continuously in the surface film on molybdenum-containing alloys throughout the immersion experiment for 20 h, whereas for the molybdenum-free alloy an initial slight increase in chromium concentration in the surface film is followed by an almost continuous decrease. Molybdenum and phosphorus are deficient in the air-formed film but become concentrated in the film with immersion time. The Mo 4+ species which is the constituent of the passive film on molybdenum metal is increased. Air exposure for 30 min prior to the polarization or immersion experiment in 1 M HCl raises the open circuit potential of specimens and retards the enrichment of chromium in the passive film on molybdenum-bearing alloys.

The passive behavior of 304 stainless steel in PCH 2O and DMEH 2O mixtures

The passivation and breakdown behavior of 304 stainless steel in propylene carbonate (PC) or dimethoxyethane (DME) mixtures with water and containing 0.5 M LiAsF 6 were studied. The air-formed film of 304 stainless steel provides a good passive film that resists breakdown. No pitting was observed in either PCH 2O-0.5M LiAsF 6 mixtures or mixed DMEH 2O-0.5M LiAsF 6 solutions if the polarizations were conducted below the oxidation potential [1800mV ( sce)] of the hexafluoroarsenate electrolyte. It is apparent that even small amounts of water were sufficient to stabilize the passive film on stainless steel through a probable oxide-oxyhydroxide formation. Pitting of the 304 stainless steel was observed in both mixed organic-water solutions at potentials above the oxidation potential of AsF 6 − because of the lack of passivating mechanisms such as salt film formation, electropolymerization, or oxide-oxyhydroxide formation.

The corrosion behavior of sputter-deposited amorphous Mo-Zr alloys in 12 M HCl

Amorphous Mo-Zr alloys were prepared in a wide composition range of 40–80 at% zirconium by a magnetron sputtering method. The sputter-deposited Mo-Zr alloys exhibit lower corrosion rates than those of molybdenum and zirconium in 12 M HCl solution open to air at 30 °C. The corrosion resistance of Mo-Zr alloys increases with increasing alloy zirconium content, but molybdenum and zirconium improve the corrosion resistance synergistically. Mo-Zr alloys containing 60 at% or more zirconium exhibit a passive region in a wide potential range without transpassive dissolution of molybdenum. The presence of zirconium suppresses both the cathodic reaction for hydrogen evolution and the anodic reduction. The air-formed film is composed of a double oxyhydroxide of zirconium and molybdenum. Spontaneously passivated films on the alloys containing less than and more than 40 at% of zirconium are composed of molybdenum-enriched and zirconium-enriched oxyhydroxides, respectively. They seem to consist of a bilayer structure; the outer zirconium and inner molybdenum oxyhydroxides. Weak points in each layer in the bilayered passive film seem to be protected mutually by another layer. This appears to be responsible for the high corrosion resistance of the Mo-Zr alloys in comparison with alloy constituents.

The passive behavior of 304 stainless steel in dimethoxyethane solutions

304 stainless steel, an alloy that is more corrosion resistant in aqueous solutions than carbon steel, was examined to assess the processes involved in forming a passive film in an aprotic, nonaqueous solvent such as dimethoxyethane (DME). Electrochemical tests show that the passive ranges of 304 stainless steel in “dry” DME solutions containing LiAsF 6, LiClO 4 and TBAP were similar. Breakdown of the passive state occurred above 1700–1800 mV ( sce). The high anodic potentials were due to the expected superior nature of the air-formed film on 304 stainless steel. Removing the surface oxide film on 304 stainless steel resulted in rapid repassivation in the DME/0.5 M LiAsF 6 solutions at potentials below breakdown. Post-test examinations did not clearly define the passivating mechanism in DME/0.5 M LiAsF 6 solutions but, in DME/saturated LiAsF 6, evidence indicated repassivation around 1800–2000 mV ( sce) by salt film formation. Small water additions to DME/0.5 LiAsF 6 solutions caused slight surface activation of the stainless steel, but no premature breakdown of the passive film. Small acid additions or removal of water below 50–100 ppm triggered surface pitting of the 304 stainless steel above the breakdown potential in DME/0.5 M LiAsF 6 solutions. A critical amount of water appears to be required to passivate the stainless steel surface and neutralize acids produced during electrolyte oxidation.

Effect of phosphorus on the passivation behavior of amorphous Fe8Cr13P7C alloy in 9M H 2SO 4 solution

The beneficial effect of phosphorus on the corrosion and passivation behavior of amorphous Fe8Cr13P7C alloy was studied in 9M H 2SO 4 solution at 30 °C. The corrosion loss of the amorphous Fe8Cr13P7C alloy in an initial period of immersion was relatively high but was scarcely changed afterwards. The corrosion potential was initially in the active region of this alloy but was shifted positively up to the passive region within 30 min. The air-formed film was enriched in chromium but was not protective and dissolved quickly in the initial stage of corrosion. The concentration of chromic ions in the surface film increased with the immersion time. Elemental phosphorus was accumulated on the alloy surface during the initial dissolution. The formation of the elemental phosphorus layer made the open circuit potential more positive as a result of the acceleration of the cathodic hydrogen evolution and by the suppression of the anodic dissolution current. This led to spontaneous passivation as a result of the formation of the chromium-enriched passive film.

An XPS study of the corrosion behavior of sputter-deposited amorphous Cr-Nb and Cr-Ta alloys in 12 M HCl solution

Sputter-deposited amorphous Cr-Nb and Cr-Ta alloys possessing higher corrosion resistance than that of alloy constituents were spontaneously passivated in 12 M HCl solution. Their open circuit potentials were higher than those of alloy constituents in 12 M HCl solution, and the passive current densities of amorphous Cr-Nb and Cr-Ta alloys were lower than those of niobium and tantalum metals, respectively, and decreased with increasing chromium concentration. XPS analysis revealed that air-formed films and passive films on the amorphous Cr-Nb and Cr-Ta alloys were composed of double oxyhydroxides of chromium and niobium or tantalum. The higher corrosion resistance of the amorphous Cr-Nb and Cr-Ta alloys in comparison with their constituent elements is attributed not only to the uniformity and homogeneity of the amorphous alloys but also to the formation of the double oxyhydroxide film of chromium and niobium or tantalum, both of which act synergistically in improving the corrosion resistance, partly because the double oxyhydroxide films possess high activity for cathodic oxygen reduction with a consequent ennoblement of the open circuit potential.

The corrosion behavior of sputter-deposited amorphous chromium-zirconium alloys in 6 M HCl solution

Amorphous binary CrZr alloys containing 11–66 at% Zr were successfully prepared by a magnetron sputtering method. Amorphous CrZr alloys thus prepared were spontaneously passive in 6 M HCl solution. Their corrosion rates were four orders of magnitude lower than chromium metal and one-third of zirconium metal and decreased with alloy chromium content. XPS analysis revealed that airformed films and passive films on the amorphous CrZr alloys were composed of double oxyhydroxides of chromium and zirconium. This appears to be responsible for the high corrosion resistance of the amorphous CrZr alloys in comparison with their constituent elements; the formation of chromium-containing passive film is more protective than the film on zirconium metal, despite the fact that chromium metal dissolves actively in 6 M HCl solution.

The corrosion behavior of sputter-deposited amorphous titanium-chromium alloys in 1 M and 6 M HCl solutions

The preparation of binary Ti-Cr alloys was achieved by magnetron sputtering using a titanium target with small chromium disks on it. The alloys containing 37–73 at% Cr were amorphous. Amorphous Ti-Cr alloys were spontaneously passive showing significantly lower corrosion rates in comparison with those of titanium and chromium metals in 1 M and 6 M HCl solutions. XPS analysis revealed that air-formed films and passive films on the Ti-Cr alloys were composed of a chromium-titanium oxyhydroxide rather than a mixture of chromium and titanium oxyhydroxides. The passive chromium-titanium oxyhydroxide film was more protective, stable and resistant against depassivation in comparison with passive films on chromium and titanium.

The interaction of chromate species with aluminium supporting air-formed and anodic films—II. Impedance studies

The impedance behaviour of aluminium, supporting either an air-formed or an anodic film, has been monitored during immersion in chromate/dichromate solution. The impedance was obtained over a wide range of frequency (10 kHz–10 mHz) with particular attention being paid to the low frequency region (1 Hz–10 mHz). At high frequencies ( > 10 Hz) the impedance response is from the film dielectric; any changes with time are associated generally with the penetration and chemically induced disaggregation of the alumina film by Cr 6+ species. The low frequency behaviour largely reflects local processes proceeding at flaws in the films. Specifically, the impedance behaviour at 0.1 Hz has been examined to assess reaction rates during the immersion period and differences in the rates are revealed for aluminium supporting air-formed films and anodic alumina films. The variation in the reaction rates evident during the immersion period is attributed to the type and geometry of persistent flaws in the respective specimens and the related effect of thinning of the anodic alumina film during immersion in the chromate solution. It is evident that Cr 6+ species are reactive to alumina films, with chromate species encouraging healing of anodic sites on aluminium supporting an air-formed film and blocking of cathodic sites on aluminium supporting an anodic alumina film.

The corrosion of aluminium in halogenated hydrocarbon solvents

The behaviour of aluminium in 1,1,1-trichloroethane has been examined to determine conditions for the initiation of so-called bleeding corrosion and reasons for continued activity. Aided by the use of supporting electrolyte to enhance solution conductivity, it is evident that little reaction between aluminium and the solvent proceeds unless the air-formed film is mechanically damaged. However, mechanical damage per se is insufficient to allow continued reaction; thus, sufficiently high anodic potentials are also required in polarization studies. In natural immersion studies, the onset of bleeding corrosion is associated with a marked anodic excursion in potential, with dramatic net cathodic currents developed for specimens that were originally anodically polarized. Mechanisms for such behaviour suggest the requirement to sustain sufficiently large regions of bare metal, where solvent reduction outstrips repassivation in the inevitable presence of water in the solvent.

An X-ray photo-electron spectroscopic investigation of the air-formed film on copper

The composition and the structure of the film formed on copper by exposure to ambient air for 24 h were investigated by X-ray photo-electron spectroscopy (XPS). This characterization of the native oxide on copper provides a basis for subsequent exposures in mixed flowing gas systems. Using angle-resolved XPS, it was observed that only the univalent, diamagnetic Cu + species was present in the film. The film was modelled as a two-layer structure with an inner layer of Cu 2O and an outer layer comprising adventitious hydrocarbon and bound hydroxyl and water, with determined thicknesses of approximately 1.6 and 0.8 nm, respectively.

The morphology, structure and mechanism of growth of chemical conversion coatings on aluminium

The morphology and structure of the chemical conversion coating developed on aluminium in an acid chromate/fluoride solution have been examined by transmission electron microscopy of stripped films and ultramicrotomed sections. The use of annealed, high purity aluminium specimens which had been electropolished and subsequently etched prior to the conversion coating treatment allowed a clear identification of the anodic and cathodic sites for the coating growth process. Thus, for the first time, it has been found that the morphology of the coating is strongly dependent on the crystallographic orientation of the substrate which, in turn, appears to control the preferential deposition of the hydrated chromium oxide at grain boundaries or cellular boundaries. Such boundaries predominantly contain the presumed flaw sites due to impurity segregation in the substrate. Based on these observations, the mechanism of conversion coating growth has been developed. Deposition of hydrated chromium oxide is observed at cathodic sites, associated with metal ridges developed on the aluminium surface as a result of the initial pretreatment. The anodic sites lie between the metal ridges, where the initial air-formed film is chemically dissolved in the fluoride-containing electrolyte. Dissolution of aluminium proceeds through the chemically thinned film, revealing scalloped regions between the regions of cathodically developed chromium oxide.

Etch pitting and stress-corrosion cracking of copper

We present evidence that an elemental metal can undergo stress-corrosion cracking by an anodic dissolution process under nominally film-free or active dissolution conditions. Oriented copper single crystal samples tested at high strain rates in non-tarnishing cupric ammonia solutions undergo severe stress-corrosion cracking. The occurrence of stress-corrosion cracking in these samples is correlated with the development of uniform microscopic porosity which develops owing to the spectacular effect that dynamic straining has on the etch pitting process. The porous surface morphology and the cracking obtains when there is a certain balance between the rate of corrosive attack which is manifested as etch pitting and the strain rate. At too low a strain rate the relatively high lateral velocity of etch pit walls prevents distinguishable etch pits of sub-micron size from forming. At too high a strain rate failure occurs by normal ductile processes. Based upon a simple model, we develop general quantitative arguments by consideration of the statistics of etch pit nucleation which demonstrate how the width of the distribution describing nucleation times could be a sensitive function of strain rate. In the model we assume etch pit nucleation to result from thermally activated localized desorption or breakdown of the air-formed film in the neighborhood of a dislocation core, however, other possible scenarios relating to issues controlling nucleation are discussed. Our analysis demonstrates how dynamic straining collapses the width of the distribution of etch pit nucleation times which results in the development of a homogeneous microporous morphology leading to cleavage like stress-corrosion cracking.

The corrosion behavior of amorphous Fe-Cr-Mo-P-C and Fe-Cr-W-P-C alloys in 6 M HCl solution

Polarization measurement shows that the additions of molybdenum and tungsten to amorphous Fe-8Cr-13P-7C and Fe-15Cr-13P-7C alloys significantly decrease the active dissolution current and passivation potential in 6 M HCl solution. XPS and electrochemical investigations reveal that molybdenum and tungsten prevent dissolution of chromium from the air-formed films during potentiostatic passivation, although small amounts of iron dissolve from the air-formed films and alloys. The tungsten addition is more effective than the molybdenum addition. This difference seems to result from the fact that molybdenum dissolves actively at low potentials in the active region of the alloys, while tungsten does not dissolve at potentials lower than 100 mV(SCE). When an excess amount of molybdenum or tungsten is added, the passivity becomes unstable and the passive film is less enriched in chromic ions, because of transpassive dissolution of molybdenum and tungsten.