Alison J. Davenport

The Structure of the Passive Film That Forms on Iron in Aqueous Environments

In situ surface X-ray diffraction was used to identify the detailed structure of the passive film that forms on (001)- and (110)-oriented iron single crystals in a borate buffer solution at +0.4 V vs. mercurous sulfate reference electrode, a high passive potential. The passive film is a new phase: a spinel with a fully occupied oxygen lattice, octahedral site occupancy of 80 ± 10%, tetrahedral site occupancy of 66 ± 10%, and an octahedral interstitial site occupancy of 12 ± 4%. The passive film forms with an epitaxial relationship to the substrate iron; for growth on Fe(001), film(001∥Fe(001) and film[110]∥Fe[100], while for growth on Fe( 110), film(111)∥Fe(110) and film[110]∥Fe[100]. The in-plane lattice parameter for the passive film (the LAMM phase) is 8.39 ± 0.01 A for growth on both faces, and the out-of-plane lattice parameter is 8.25 ± 0.1 A [Fe(001)] and 8.42 ± 0.1 1 A [Fed 110)]. The passive film forms a nanocrystalline microstructure with numerous defects. Specifically, the grain size is 50-80 A in-plane and about 30 A out-of-plane. There is a small mosaic spread of 2.5 to 4.1° and a high density of antiphase boundaries and stacking faults. The structure of the film determined in situ was found to be identical to that found for an emersed sample, indicating that the high potential film studied here is stable on removal from the electrolyte. Some of the implications of the film structure on passivity are discussed.

Electrochemical Behavior of Cr2 O 3 / Fe2 O 3 Artificial Passive Films Studied by In Situ XANES

The electrochemical behavior of thin sputter-deposited mixed Cr 2 O 3 /Fe 2 O 3 oxide films with Cr 2 O 3 -contents of 10, 20, 50, and 90% was studied with in situ X-ray absorption near edge spectroscopy (XANES). These measurements gave information on the chemical states and dissolution rates during anodic and cathodic polarization in different electrolytes. At low Cr oxide concentrations, the films dissolve when cathodically polarized and are resistant to dissolution when polarized in the anodic direction. At high Cr 2 O 3 concentrations, dissolution occurs when the films are anodically polarized, but the films are stable against cathodic dissolution. In the intermediate Cr oxide concentration range, the oxides neither dissolve under anodic nor cathodic polarization. However, in all the cases, even when no dissolution takes place, the species show electroactivity in that the redox reactions Fe 3+ → Fe 2+ and Cr 3+ → Cr 6+ can take place under cathodic and anodic polarization, respectively. In the mixed oxides a solid-state conversion takes place in the iron oxide phase during reduction, whereas the oxidation of the chromium oxide phase converts only the outermost layer. An acidic environment accelerates both anodic and cathodic dissolution, associated with chemical dissolution of the iron oxide. The results further show that critical threshold values exist for the dissolution resistance of the oxide. These values are different for anodic and cathodic reactions and further strongly depend on the solution chemistry.

Intergranular Corrosion and Stress Corrosion Cracking of Sensitised AA5182

AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.

Synchrotron X-Ray Microtomography Study of the Role of Y in Corrosion of Magnesium Alloy WE43

Synchrotron X-ray microtomography has been used to study the distribution of Y in the aerospace magnesium alloy WE43 and its role in the morphology of corrosion attack. A three-dimensional Y map was obtained by acquiring tomographic images above and below the Y K-edge at 17 keV. Tomographic images from a heavily corroded as-cast WE43 sample showed that Y-rich regions of the matrix slow the propagation of corrosion. In situ time-dependent tomographic images of corrosion of heat-treated WE43 show that homogenization of the distribution of Y decreases the corrosion rate, but both general attack and pitting is still observed.

Do ‘passive’ medical titanium surfaces deteriorate in service in the absence of wear?

Globally, more than 1000 tonnes of titanium (Ti) is implanted into patients in the form of biomedical devices on an annual basis. Ti is perceived to be ‘biocompatible’ owing to the presence of a robust passive oxide film (approx. 4 nm thick) at the metal surface. However, surface deterioration can lead to the release of Ti ions, and particles can arise as the result of wear and/or corrosion processes. This surface deterioration can result in peri-implant inflammation, leading to the premature loss of the implanted device or the requirement for surgical revision. Soft tissues surrounding commercially pure cranial anchorage devices (bone-anchored hearing aid) were investigated using synchrotron X-ray micro-fluorescence spectroscopy and X-ray absorption near edge structure. Here, we present the first experimental evidence that minimal load-bearing Ti implants, which are not subjected to macroscopic wear processes, can release Ti debris into the surrounding soft tissue. As such debris has been shown to be pro-inflammatory, we propose that such distributions of Ti are likely to effect to the service life of the device.

In Situ XANES Study of Galvanostatic Reduction of the Passive Film on Iron

In situ XANES (X-ray absorption near-edge structure) was used to study galvanostatic reduction of the passive film on iron. In a borate buffer, the film appears to be removed in a layer-by-layer fashion by reductive dissolution during the first potential arrest. A burst of dissolution takes place at the end of the arrest for slow reduction rates. During the course of reduction, the remaining film shows a composition change from ferric oxide toward Fe{sub 3}O{sub 4}. This may indicate that the film has an inner layer of Fe{sub 3}O{sub 4}. Galvanostatic reduction of the passive film grown on iron in 0.1M KOH leads to complete reduction of the passive film to ferrous oxide or hydroxide without any detectable dissolution.

Comparison of Voltammetric Responses of Toluene and Xylenes at Iron(III)-Doped, Bismuth(V)-Doped, and Undoped β-Lead Dioxide Film Electrodes in 0.50 M H 2 SO 4

Voltammetric activities are compared for the designated compounds in 0.50 M H 2 SO 4 at the specified film electrodes configured as rotated disks. The effective number of electrons (n eff equiv mol -1 1 ) calculated from the Koutecky-Levich plot is 3.1 ± ± 0.2 for toluene oxidation at the Fe-PbO 2 electrode, compared to 2.0 0 ± 0.05 and 4.1 ± 0.3 at the Bi-PbO 2 and PbO 2 electrodes, respectively. Gas chromatography--mass spectrometry data confirm that the primary product of toluene oxidation is benzyl alcohol (2 equiv mol -1 ) at the Fe-PbO 2 electrode with production of small amounts of henzaldehyde (4 equiv mol 1 ) and benzoic acid (6 equiv mol -1 ). X-ray photoelectron spectroscopy data indicate the ratio of Fe:Pb is ca. 1: 100 (atom:atom) in the Fe-PbO 2 films. By comparison, a Bi:Pb ratio of 33:100 (atom:atom) is easily attained in Bi-PbO 2 films. Nevertheless, the apparent heterogeneous rate constant (k app , cm s -1 ) for toluene oxidation is larger at the Fe-PbO 2 electrode (6.5 ± 0.06 × 10 -3 ) compared to the Bi-PbO 2 electrode (2.0 ± 0.05 × 10 -3 ). X-ray absorption near-edge structure data confirm speculation that iron exists in the 3+ oxidation state with octahedral coordination by O atoms in Fe-PbO 2 films. The large activity of the Fe-PbO 2 electrode is attributed to the benefit of adsorption of aromatic molecules at Fe(III) sites.

Magnetic Field Effects on the Corrosion of Artificial Pit Electrodes and Pits in Thin Films

The effect of magnetic fields on localized corrosion was explored for one-dimensional artificial pits that are used to simulate pitting corrosion, and for two-dimensional pits in thin metal films. It was found that the pitting dissolution of ferromagnetic electrodes was significantly affected by the presence of a magnetic field. The presence of magnetic fields can lead to local stirring in the vicinity of artificial pits in ferromagnetic metals (iron, cobalt, or nickel) and pits in FeCo thin films, through the effects of the high field gradient force and the Lorentz force, causing an increase in the diffusion-limited current when the field is applied parallel to the dissolving surface. When the field is applied perpendicular to the dissolving surface, the field gradient force draws paramagnetic corrosion products toward the metal surface, protecting the metal from dissolution. The field gradient is, however, absent for paramagnetic stainless steel artificial pits, and the Lorentz force itself (in a field of 0.35 T) is insufficient to cause stirring within the pit cavities. Fields that are changing show a considerably bigger effect than static fields.

Lymphoid aggregates that resemble tertiary lymphoid organs define a specific pathological subset in metal-on-metal hip replacements.

Aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) has been used to describe the histological lesion associated with metal-on-metal (M-M) bearings. We tested the hypothesis that the lymphoid aggregates, associated with ALVAL lesions resemble tertiary lymphoid organs (TLOs). Histopathological changes were examined in the periprosthetic tissue of 62 M-M hip replacements requiring revision surgery, with particular emphasis on the characteristics and pattern of the lymphocytic infiltrate. Immunofluorescence and immunohistochemistry were used to study the classical features of TLOs in cases where large organized lymphoid follicles were present. Synchrotron X-ray fluorescence (XRF) measurements were undertaken to detect localisation of implant derived ions/particles within the samples. Based on type of lymphocytic infiltrates, three different categories were recognised; diffuse aggregates (51%), T cell aggregates (20%), and organised lymphoid aggregates (29%). Further investigation of tissues with organised lymphoid aggregates showed that these tissues recapitulate many of the features of TLOs with T cells and B cells organised into discrete areas, the presence of follicular dendritic cells, acquisition of high endothelial venule like phenotype by blood vessels, expression of lymphoid chemokines and the presence of plasma cells. Co-localisation of implant-derived metals with lymphoid aggregates was observed. These findings suggest that in addition to the well described general foreign body reaction mediated by macrophages and a T cell mediated type IV hypersensitivity response, an under-recognized immunological reaction to metal wear debris involving B cells and the formation of tertiary lymphoid organs occurs in a distinct subset of patients with M-M implants.

Electrochemical Behavior of the Active Surface Layer on Rolled Aluminum Alloy Sheet

The highly deformed, micrograined layer on the outermost surface of a rolled Al-Fe-Si-Mn model alloy was electrochemically characterized. The thickness of this deformed surface layer in a 1.0 mm thick sheet was approximately 1 μm. Polarization curves in 5% NaCl solution at pH 3.0 and 11.5 were obtained at different depths from the surface using controlled sputtering in a glow discharge optical emission spectrometer for sample preparation. Both the anodic and the cathodic reactivity of the deformed surface layer were significantly higher than that of the bulk. Consistent with this, image analysis of scanning electron microscopy backscattered images revealed an increased number of fine intermetallic particles in the surface layer as compared with the bulk of the material. The corrosion morphology of the outermost surface was characterized by a high density of fine pits, while fewer and larger pits were observed in the bulk. The results highlight the importance of heavily deformed surface layers in controlling corrosion behavior of rolled aluminum products.