Clive R. Clayton

XPS and XANES Studies of Uranium Reduction by Clostridium sp.

Speciation of uranium in cultures of Clostridium sp. by X-ray absorption near-edge spectroscopy (XANES) at the National Synchrotron Light Source and by X-ray photoelectron spectroscopy (XPS) showed that U(VI) was reduced to U(IV). In addition to U(IV), a lower oxidation state of uranium, most probably U(III), was detected by XANES in the bacterial cultures. Reduction of uranium occurred only in the presence of growing or resting cells. Organic acid metabolites, the extracellular components of the culture medium, and heat-killed cells failed to reduce uranium under anaerobic conditions. The addition of uranyl acetate or uranyl nitrate (>210[mu]M U) to the culture medium retarded the growth of the bacteria as evidenced by an increase in the lag period before resumption of growth, by decreases in turbidity, and in the total production of gas and organic acid metabolites. These results show that uranium in wastes can be stabilized by the action of anaerobic bacteria. 31 refs., 2 figs., 3 tabs.

Passivity of high-nitrogen stainless alloys: the role of metal oxyanions and salt films

Abstract Surface-analytical studies of high-nitrogen austenitic stainless steels exposed to de-aerated 0.1 M HCl have revealed that nitrogen alloying additions influence the composition of salt layers and the passive film/alloy interface. It was shown that nitrogen, nickel and molybdenum additions stimulate selective dissolution of iron, resulting in a significant enrichment of chromium beneath the passive film. The build-up of a protective ferrous molybdate layer was seen to be most strongly enhanced with additions of nickel and, to a lesser extent, nitrogen. While the primary kinetic barrier to anodic dissolution of high-nitrogen stainless steels is a chromium-oxide-based passive film, it appears that a mixed nitride surface layer and an ultra-thin layer of ferrous molybdate act as secondary kinetic barriers.

Applications of ion implantation for the improvement of localized corrosion resistance of M50 bearing steel

Abstract Pitting corrosion of M50 alloy steel bearings used in turbojet engines has been found to be a severe problem. The difficulty arises when salt-spray condensates accumulate in the engine lubricants of aircraft not in regular use. Ion implantation was applied to this problem because in the early stages of this work it was shown to be able to maintain both the dimensional stability and the contact fatigue lifetime of the M50 bearings. Qualitative tests, which simulated the geometry and thermal cycle conditions leading to pitting of the M50 bearing surface, were performed using oil containing 3 ppm NaCl. Initially it was found that chromium surface alloys containing 20–25% chromium substantially reduce the level of attack. Prior to further corrosion simulation tests, potentio-kinetic studies were carried out on M50 implanted with chromium, molybdenum and titanium in order to screen both the passivating tendency of the surface alloys formed and their resistance to localized forms of corrosion. Singular additions of chromium, molybdenum and titanium were found to increase the resistance of M50 to localized breakdown significantly. The highest resistance to localized breakdown was found for a multi-implantation of chromium and molybdenum.

Development of a technique to prevent radiation damage of chromate conversion coatings during X-ray photoelectron spectroscopic analysis

Photoreduction of hydrated sodium dichromate in the presence of carbon compounds has been studied by X-ray photoelectron spectroscopy (XPS). XPS results show that sodium dichromate also undergoes thermal breakdown during analysis. The photochemical and thermal reduction appears to be prevented by cooling with liquid nitrogen and using hydrocarbon-free vacuum pumping conditions. A model for the photoreduction of dichromate has been put forward based on an earlier photoreduction model developed by this group. Using the insight gained from this work, chromium spectra obtained from XPS of chromate conversion coatings (CCCs) on AA2024-T3 aluminum alloy have for the first time been fitted with six species of chromium compounds. The Cr 2p spectra have been critically examined for photoreduction of the hexavalent species: tetravalent and trivalent decomposition products have been identified. To explain the observed photochemical breakdown in CCCs even when performed under clean pumping conditions and liquid nitrogen cooling, its has been verified that the reduction is partly due to the cyanide species present in the CCCs. Cooling with liquid nitrogen prevents the adsorbed water on the coating from reacting with Cr(IV) to form trivalent species as the water molecules are immobilized and kinetics are slowed. As the Cr(VI)/Cr(III) ratio is critical to the performance of the CCC, and its accurate determination by XPS has not formerly been possible due to the photoreduction of Cr(VI) to Cr(III), we present a reliable method by which XPS can be used in characterization of chromate conversion coatings.

Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings

High-velocity oxy-fuel (HVOF) spraying is a versatile technique that can yield high-density coatings with porosities less than 1% by optimization of the process variables. Oxidation of the coating material can be greatly reduced by inert gas shrouding during the spraying process. This work evaluates the electrochemical behavior of HVOF-sprayed coatings aimed to have better or similar corrosion behavior as stainless steel AISI 316 in acidic medium. A total of eight different chromium-containing coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C have been studied in hydrochloric acid medium. Porosity measurements, gravimetric analysis, open-circuit potential measurements, potentiodynamic polarization and optical microscopy have been performed. The electrochemical behavior of each coating has been compared with that of bulk AISI 316. The results indicate that HVOF-sprayed AISI 316 coating offers a lower corrosion resistance compared to bulk AISI 316. High nickel- and chromium-containing coatings appear to offer a corrosion resistance comparable to bulk AISI 316. However, results indicate that other alloying elements like molybdenum might be essential for obtaining higher corrosion protection.

An X‐Ray Photoelectron Spectroscopic Study of the Passive Film Formed on Pure Mo and MoSi2 in 4M HCl

Electrochemical techniques and variable angle x-ray photoelectron spectroscopy (XPS) were used to study the passivation of Mo, and plasma-sprayed and hot-pressed MoSi 2 in a pitting environment (4M HCl). A thin (12 A), insoluble Mo oxychloride salt layer formed on the pure Mo electrode following 1 h of passivation at 0 mV vs. saturated calomel electrode. Both MoSi 2 samples in 4M HCl displayed a significantly higher passive current density as well as a much higher breakdown potential, indicating a thicker but perhaps less dense passive layer. XPS of the film formed on the thermally sprayed sample revealed a 34 A thick SiO 2 film incorporating hexavalent Mo for an overall stoichiometry of Mo 0.05 SiO 2 . The film formed on the hot-pressed disilicide had primarily the same stoichiometry with slight variation due to the presence of pores

Evaluation of the effect of surface roughness on thin film thickness measurements using variable angle XPS

Abstract The effects of specimen surface roughness of XPS peak intensities can be experimentally observed by using the variable angle X-ray photoelectron spectroscopy (VAXPS) technique. Surface topography can be qualitatively revealed by calculating the film thickness of ultra-thin overlayer/substrate systems over a wide range of take-off angles. Results are presented for two systems, Al 2 O 3 /Al and SiO 2 /Si. Critical angles corresponding to differing levels of surface roughness are defined to ensure accurate quantitative analysis.

Interactions of the Components of Chromate Conversion Coating with the Constituents of Aluminum Alloy AA2024-T3

Electrochemical techniques were employed to study the nature of individual interactions between the chemicals constituting a widely used commercial chromate conversion-coating treatment and the main constituents of AA2024-T3 alloy, namely, Al (99.999%), Al 2 Cu, Al-Al 2 Cu galvanic couple, and AA2024-T3 alloy. The samples were pretreated using chromate, ferricyanide, fluoride, tetrafluoroborate, and hexafluorozirconate, prior to electrochemical corrosion tests. The results were compared with untreated (control) samples. For the first time, the chemical interaction of aluminum with hexafluorozirconate has been studied. Maximum activation was observed in the case of hexafluorozirconate pretreatment, which also decreased the interfacial tension and increased surface wetting. The electrochemistry of the control and pretreated (except for hexafluorozirconate pretreatment) AA2024-T3 and Al 2 Cu were found to be similar. The cathodic electron transfer reaction rate for oxygen reduction was found to be enhanced by the presence of copper in the systems. The enhancement was proportional to the copper content. The cathodic reaction on all systems was inhibited by chromate. Aluminum was observed to undergo cathodic corrosion, leading to an enrichment of the surface copper content and subsequent enhancement of the cathodic electron transfer reaction.

On the question of nitrate formation by N-containing austenitic stainless steels

Abstract Since nitrates and nitrites are powerful corrosion inhibitors, the possibility that alloyed nitrogen may be capable, at suitable anodic potentials, of oxidizing to either of these species was considered. While no spectral evidence has been found of NO 3 − formation by N-containing stainless steels, it was decided to clarify this issue further by considering the stability of NO 3 − ions on the surface of pure Cr, polarized over a wide range of potentials from −930 to 1300 mV with respect to a saturated calomel electrode (SCE) in 0.1 M HCl + 0.5 M NaNO 3 (pH 1.1). Using X-ray photoelectron spectroscopy analysis, it was observed that reduction of NO 3 − occurred at all potentials studied, contrary to conventional interpretations using the Pourbaix diagram for the N 2 -H 2 O system. Specifically, nitride was found to be formed on Cr under conditions leading to the exposure of the metal surface to the electrolyte. This resulted in strong evidence that nitrogen alloyed with austenitic stainless steels does not oxidize to NO 3 − . A mixed surface nitride was formed on the surfaces of four types of austenitic stainless steel by reducing NO 3 − ions at the cathodic potential −700 mV (SCE) in 0.1 M HCl + 0.5 M NaNO 3 solution. This nitride exhibited an identical X-ray photoelectron spectrum with that formed by N-containing austenitic stainless steels. The anodically inhibitive role of the surface nitride formed by the reduction of NO 3 − ions was also found to be identical with the role of N alloy additions to austenitic stainless steels. This work contributes new insights into the role of alloyed N in the corrosion behavior of austenitic stainless steels and also offers a new model for understanding the mode of operation of nitrates as corrosion inhibitors in ferrous alloys.

Identification of Mo(V) commonly observed in passive films formed on stainless steels

There has been a longstanding debate and controversy in the literature with regard to the determination by XPS of the exact oxidation state of a molybdenum species commonly observed in the passive films of stainless steel. In this paper we have compared both the XPS and ESR analyses of molybdenum oxyhydroxides previously postulated to contain Mo(V) as well as Mo(IV) and Mo(VI). The compositions of these compounds closely resemble those found in the passive films of Mo-bearing stainless steel. It has been shown that the same Mo species exhibiting an XPS Mo 3d s/2 spectrum binding energy value of 230.8 eV is observed to exhibit an ESR spectra having a g factor of 1.921, which corresponds to Mo(V).