Band 219, Heft 11 - 11

The stability of passive films on titanium and Ti-Al alloys (Ti-6Al-4V, Ti-6Al-7Nb implant alloys and Ti-50Al intermetallic compound) were studied in 0.5 M H 2 SO 4 electrolyte. Upon exposure to the acidic solution, fast open-circuit activation of native passive films on Ti-Al-V, Ti-Al-Nb and Ti-50Al takes place, whereas pure Ti remains in the passive state. Passivation in an acidic solution leads to a higher protectiveness of the passive films, as well as to a higher stability against open-circuit activation. Surface analytical experiments suggest that the stability of the passive films is related to the amount of oxidized Al in the film. No sites of high-Al content on the surface could be detected after active dissolution, which may indirectly indicate that selective dissolution of Al takes place out of the higher-Al containing phases.

Anodic layers grown on Al-Ta sputter deposited amorphous alloys were studied by electrochemical methods, AES, and RBS. The alloy composition ranged from Al 90 Ta 10 to Al 70 Ta 30 . It has been found that an increase of breakdown potential E np′ caused by refractory metal content extends over a wide concentration range from 0.01 M up to 1 M NaCl. AES measurements have shown that Ta remains in an oxidized state over the whole thickness of the anodic layer thus confirming the model of Ta acting as a passivity promoter rather than a dissolution moderator. RBS results clearly demonstrate that in low Ta content layers a thin alumina film is formed on the top surface of the anodic layer. For higher Ta content only a depletion in Ta is observed on the surface. The latter result differs from those obtained previously by other authors and may be caused by differences in experimental conditions and/or the alloy structure. The role of refractory metals in the passivity of Al-based amorphous alloys is discussed.

Passive layers on tin, CuSn4 and CuSn19 were prepared in 0.1 N KOH (pH 12.5) under potentiostatic control with systematic variation of the relevant parameters, such as potential and time, and examined by X-ray photoelectron spectroscopy. XPS studies of potentiostatically polarised Sn-electrodes show the linear growth of the oxide layer thickness with increasing potential. Angular resolved-XPS measurements and XPS-depth-profiles allow the development of a model of the passive layer: In the passive potential range it consists mainly of SnO 2 with a small contribution of SnO. Tin hydroxide was found only on top of the layer. Sputter profiles indicate a dominating SnO 2 part which increases with the potential and the passivating time. SnO was detected underneath a layer of SnO 2 with a thickness constant with potential. The passive behaviour of CuSn4 and CuSn19 alloys is significantly different: CuSn4 behaves almost like pure Cu: In the passive range copper oxides are the passivating components with the well known duplex structure Cu 2 O/CuO,Cu(OH) 2 . Only a small amount of tin oxide is present within the layer. On CuSn19 the formation of passive layers starts at E  = −0.3 V(SHE) similar to the situation for pure tin and increases with the potential. The outer part of the passive film consists of copper oxide. Tin oxides are accumulated in the inner part of the film up to 75 atomic percent with an analogue sequence of SnO 2 on the top of SnO. Time-dependent studies for 2 s to 90 min at E  = 0.5 V(SHE) demonstrate reaction sequence of both elements.

The aim of the present study was to investigate the early stages of protective oxide layer growth on Fe-15 at.%Al and Fe-40 at.%Al oxidized at 700 to 900 °C for 5 to 300 min in dry, synthetic air. Emphasis was put on the analysis of the oxide layer with respect to chemical and phase composition using X-ray photoelectron spectroscopy (XPS), grazing incidence XRD with synchrotron radiation and TEM investigations. The results indicate that at all conditions an external oxide layer grows and that this oxide layer consists of an outer, approximately 40 nm thin, Fe 2 O 3 -containing layer followed by a nearly pure Al 2 O 3 layer which grows continuously with increasing temperature and time. The Fe 2 O 3 content of the outer oxide layer decreases with time, temperature and Al content of the binary iron aluminide. The phase compositions of oxide layers less than 100 nm in thickness are determined, indicating the formation of hematite (α-Fe 2 O 3 ) and corundum (α-Al 2 O 3 ) on Fe-15 at.%Al at 700, 800 and 900 °C after 300 min of oxidation. Moreover, TEM analyses indicate that for both alloys at 700 °C a metastable Al 2 O 3 layer, most likely consisting of θ-Al 2 O 3 or γ-Al 2 O 3 was formed epitaxially at the iron aluminide/oxide interface. Exposure times of 300 min at 700 °C lead to the transformation of metastable Al 2 O 3 to stable α-Al 2 O 3 . However, a very thin Al 2 O 3 layer of about 5 nm thickness seems to remain at the iron aluminide/oxide interface, most probably a metastable Al 2 O 3 modification.

The effect of surface treatments on the surface characteristics of aluminium alloy 2024-T3 before the appearance of filiform corrosion (FFC) is investigated. The nature of the surface prior to coating and initiation of FFC, with particular respect to the intermetallics is investigated in this work. The SKPFM (Scanning Kelvin Probe Force Microscopy), ToF-SIMS (Time of Flight Secondary Ion Mass Spectroscopy), XPS (X-Ray Photo Electron Spectroscopy) and SEM (Scanning Electron Microscopy) surface analysis techniques were used to characterize polished AA2024-T3 before and after etching or after etching with subsequent chromating treatments. The etching pretreatment is intended to remove surface intermetallics and increase the oxide layer thickness. In these respects, the treatment was partially successful: some, not all, of the particles were eliminated from the surface and the oxide thickness increased by about 25%. In addition, XPS depth profiling showed a copper and iron enrichment at the oxide–metal interface from this treatment. The oxide thickness is drastically increased following the application of the chromate conversion coating. Furthermore, XPS and ToF-SIMS analysis revealed that Cu and Fe were enriched on the oxide surface indicating that either Cu is complexed into the Cr-Al layer following chromating or that the chromating coating is insufficiently formed on the Cu-containing intermetallics. SKPFM analysis provided further information on the distribution of these complexes on seen by distinct points of high potential on the treated alloy surface.

Potentiostatic Pulse Testing (PPT) was investigated as a technique to assess the early stage of organic coating failure. Current transients collected from dummy cells and real coated samples were fitted to an exponential decay function to evaluate the values of equivalent circuit parameters. PPT only provided values for only some of the circuit elements, whereas Electrochemical Impedance Spectroscopy (EIS) revealed most values of the assumed circuit elements. However, it was difficult to know a priori whether to use a one- or two-time-constants model for EIS data obtained from the real coated panels. Fast Fourier Transform (FFT) analysis was used to compare the data generated in the two different domains. The impedance spectra from the Fourier transforms was over only a part of the frequency range accessible by the EIS, but the spectra from the two methods exhibited reasonably good agreement.

A novel technique is described which facilitates the investigation of photoelectrochemical processes occurring within electrically free standing organic (polymer) films pigmented with metal chalcogenide dispersions. The films are models for delaminated and partially delaminated organic coatings on metallic substrates and consist of polyvinylchloride membranes containing sub-micron dispersions of titanium dioxide used to separate the two compartments of a symmetrical electrochemical cell. Photoelectrochemical charge transport is shown to occur across asymmetrically illuminated membranes in the absence of any externally applied potential gradient. Steady state photocurrents are shown to be strongly dependent of the concentration of dissolved oxygen in the membrane contacting electrolyte. A mechanism is proposed in which the membrane photoresponse results from interparticle electronic charge percolation coupled with an imbalance in the rate of photoanodic and cathodic processes occurring on the illuminated and dark sides of the membrane. It is further argued that this rate imbalance arises from the difference in diffusion length for photogenerated electrons and holes in titanium dioxide.

Two possible mechanisms for the corrosion protection by conducting polymer coatings are discussed in this paper. The ennobling mechanism based on the assumption that the high potential of the polymer is able to keep the metal surface in the passive state and the possible capability to smear out the sites of oxygen reduction. Using a special model sample set up it could be shown that electrochemically deposited polypyrrole films can indeed protect small pin hole like defects from corrosion. In the case of immersion into chloride free solution the polypyrrole is able to maintain the defect in the passive state for a few hours while in chloride containing solution the passivitiy breaks down after few seconds. The size of the defect significantly determines the degree of possible protection. The polypyrrole showed ability to recharge itself after disconnection from the defect so it could provide a real protection under cyclic climatic conditions. In the presence of a larger defect, however, the positive effect of the conducting polymer turns into a negative one and fast delamination is observed. Experiments carried out in 18 O 2 containing atmosphere gave answer to the question concerning the oxygen reduction site. Sputter profiles measured using ToF-SIMS indicate a presence of 18 O − and 18 OH − ions created during the delamination just on the top of the polypyrrole layer. That excludes the possibility of the high conductive polymer like pure polypyrrole to smear out the sites of oxygen reduction.

Surface analytical findings of thick anodic passive layers on titanium are discussed and analysed in order to give a critical overview about possible artefacts for the system Ti/TiO 2 . In line with previous studies, the pretreatment of the surface can have an influence on the morphology of the passive film. On polycrystalline surfaces, the oxide film shows a different composition and morphology on differently oriented grains, but although the crystallographic orientation of the substrate influences the thickness and the morphology of the passive layer the defect density does not seem to depend so strongly on the substrate condition. Ion sputtering strongly alters the TiO 2 surface morphology and composition which can be directly seen in SEM images of the surface and indirectly from the change in XPS spectra throughout a sputter depth profile. The material removal is not homogeneous and sputtering leads to reduction of the surface oxide. In AES experiments the passive layer on titanium undergoes changes induced by the primary electron beam that is used for the analysis. All these effects make the Ti/Ti-oxide a challenging system particularly when employing surface analytical methods for its investigation. Recent developments like the growth of nanotube structures on titanium give the system a renewed importance.