The influence of grain orientation on oxygen generation during anodizing of chemically polished titanium at 20 mA cm
−
2
in 0.026 M Na
3PO
4 and 0.02 M Na
4P
2O
7 is investigated in the pre-sparking ...region using scanning electron, atomic force and transmission electron microscopies. The findings reveal preferential oxygen generation in the anodic film on titanium grains that tend to a basal-like orientation. The resultant oxygen-filled voids that develop within the film increase in size and coalesce with progress of anodizing. Their presence leads to formation of blisters and the eventual film rupture due to release of the high-pressure gas. In contrast, relatively few voids, and hence smoother film, develop on grains of prism-like orientation. The differing rates of oxygen generation are attributed to the grain orientation dependence of the amorphous-to-crystalline transition of anodic titania, with rutile being identified at sites of oxygen-filled voids in films formed in 0.02 M Na
4P
2O
7. There may also be small contributions to oxygen generation from influences of substrate impurities incorporated into the film. The grain orientation also affected the rate of chemical polishing of the titanium in HF/HNO
3, which was faster on basal-like planes.
In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn underpotential deposition (UPD) on Ni surface from 0.2 M HClO4 solution containing 10−3 M Sn2+ with relation to the ...inhibition effect of Sn on aqueous corrosion of Ni. The periodical emersion method under potentiostatic polarization, using the surface-roughened Ni plate (surface roughness Sr = 78.3) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Ni. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn Kα1fluorescence line. The Sn K-edge absorption near-edge structure (XANES) in the Sn-UPD potential region has revealed that the Sn-UPD layer on Ni is oxygenated. The extended X-ray absorption fine structure (EXAFS) analysis was performed with a two shell fit of the nearest neighbor Sn-Ni and Sn-O interactions, assuming that the uppermost Ni surface exposed to the solution is mainly oriented to the (111) plane. The results have indicated that Sn atoms are substituted like a surface alloy at face-center-cubic sites in the first Ni layer and further bonded with oxygen atoms. The strong inhibition effect of Sn on aqueous corrosion of Ni is ascribed to the bond between Sn and O atoms in addition to the bond between Sn and Ni atoms.
The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide ...film of 12
μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5
V for 3.6
ks in 0.6
mol
dm
−3 K
2HPO
4
+
0.2
mol
dm
−3 K
3PO
4/glycerol electrolyte containing only 0.04% water at 433
K. The growth efficiency is reduced by increasing the formation voltage to 20
V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20
V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10
V, during the initial anodizing at a constant current density of 250
A
m
−2. The pore diameter is not significantly dependent upon the formation voltage, being ∼10
nm.
Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible ...electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm
−3
ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO
2
phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m
−2
, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO
2
phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.
Spark anodizing of titanium, Ti–6Al–4V and Ti–15V–3Al–3Cr–3Sn in alkaline aluminate electrolyte produces highly crystalline anodic films consisting mainly of Al
2TiO
5 with α- and γ-Al
2O
3 as minor ...oxide phases, irrespective of substrate composition. However, the apparent efficiency for film formation decreases in the following order: Ti–6Al–4V, titanium and Ti–15V–3Al–3Cr–3Sn. A large amount of aluminium species are incorporated from the electrolyte, probably by plasma-chemical reaction, and become distributed throughout the film thickness. This distribution indicates that the electrolyte penetrates near to the film/substrate interface through the discharge channels. Thus, the outwardly migrating aluminium ions under a high electric field can be present even in the inner part of the anodic films. Voids are developed at the film/substrate interface, particularly on the vanadium-containing alloys, reducing the adhesion of the anodic film to the substrate.
Spark anodizing of a bcc solid solution Ti–15% V–3% Al–3% Cr–3% Sn alloy has been performed in an alkaline electrolyte containing aluminate and phosphate using dc-biased ac anodizing to form a ...wear-resistant coating on the alloy. The coating consists mainly of Al
2TiO
5, with rutile and γ-Al
2O
3 being present as minor oxide phases. Depth profiles of the coating, examined by glow discharge optical emission spectroscopy, have revealed that aluminium species, highly enriched in the coating, distribute uniformly in the coating, while phosphorus species, incorporated from the electrolyte, are located mainly in the inner part of the coating near the coating/alloy interface. The location of the phosphorus species should be associated with the porous nature of the coating, allowing access of the electrolyte directly to the inner parts of the coating. The porosity of the coating is reduced by anodizing to high voltages. The marked improvement of the wear resistance by the coating has been demonstrated from a pin-on-disc wear test.
Highly corrosion-resistant, ∼65-nm-thick, layer-by-layer aluminosilicate coatings have been prepared by multiple spin casting on Type 430 stainless steel. These coatings have been characterized by ...field emission scanning electron microscopy, transmission electron microscopy, conductive atomic force microscopy, and micro-electrochemical measurements using a microcapillary cell. The coatings annealed at 400°C are non-uniform and contain fine iron oxide nodules, which are formed in high densities on the {111} grain surface of the steel. The iron oxide nodules arise from the outward diffusion of the oxidized iron from the substrate through the coating. The coatings annealed at 400°C are more insulating compared with those before annealing; however, the nodule sites are less insulating owing to the development of more conductive iron oxide channels in the coatings. A microcapillary cell study reveals that the coated specimens prepared from diluted precursor solutions by a layer-by-layer process exhibit higher pitting potential in 3.5wt% NaCl solution compared with those prepared through a single-layer process. Moreover, the coated specimen obtained from the layer-by-layer process exhibits similarly high pitting potential even at the flaw sites in the coating; in contrast, the pitting potential in the flaw-containing regions of the coated specimen obtained from a single-layer process shifts towards the less noble direction. The layer-by-layer coating is also effective in suppressing the corrosion of the scratched region of the coated specimens, owing possibly to the excellent adhesion between the coating and the substrate.
The AC etch process effectively expands the surface area of aluminium foil as compared with DC etch process. This study explored the effect of cathodic half-cycle. The cathodic half-cycle enhanced ...passivation of pits developed during the preceding anodic half-cycle, making pit nucleation random. Anodic pulse without cathodic half-cycle produced hollows, due to preferential pit nucleation on pits produced in the preceding anodic half-cycle. The open circuit before the cathodic half-cycle does not largely influence the etch factor, but etch film formation is largely suppressed. The precipitation of aluminium hydroxide may not have a crucial role in porous layer formation.
A photoelectrochemical investigation on thin (⩽13
nm) mixed oxides grown on sputter-deposited Ti–Zr alloys of different composition by air exposure and by anodizing (formation voltage,
U
F
=
4
V/SCE) ...was carried out. The experimental results showed that the optical band gap,
E
g
opt
, increases with increasing Zr content in both air formed and anodic films. Such behaviour is in agreement with the theoretical expectation based on the correlation between the band gap values of oxides and the difference of electronegativity of their constituents. The flat band potential of the mixed oxides was found to be almost independent on the Ti/Zr ratio into the film and more anodic with respect to those estimated for oxide grown on pure Zr. The semiconducting or insulating character of the investigated films was strongly influenced by the forming conditions and the alloy composition.
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