Fernando Rizzo

Multistep structural transition of hydrogen trititanate nanotubes into TiO2-B nanotubes : a comparison study between nanostructured and bulk materials

H-trititanate nanotubes obtained by alkali hydrothermal treatment of TiO(2) followed by proton exchange were compared to their bulk H(2)Ti(3)O(7) counterpart with respect to their thermally induced structural transformation paths. As-synthesized and heat-treated samples were characterized by XRD, TEM/SAED, DSC and spectroscopy techniques, indicating that H(2)Ti(3)O(7) nanotubes showed the same sequence of structural transformations as their bulk counterpart obtained by conventional solid state reaction. Nanostructured H(2)Ti(3)O(7) converts into TiO(2)(B) via multistep transformation without losing its nanotubular morphology. The transformation occurs between 120 and 400 degrees C through topotactic mechanisms with the intermediate formation of nanostructured H(2)Ti(6)O(13) and H(2)Ti(12)O(25), which are more condensed layered titanates eventually rearranging to TiO(2)(B). Our results suggest that the intermediate tunnel structure H(2)Ti(12)O(25) is the final layered intermediate phase, on which TiO(2)(B) nucleates and grows. The conversion of nanostructured TiO(2)(B) into anatase is completed at a much lower temperature than its bulk counterpart and is accompanied by loss of the nanotubular morphology.

The corrosion of Co-Nb alloys in reducing/oxidizing-sulfidizing gases at 600–800°C

Abstract The corrosion of the two pure metals and of two alloys containing 15 and 30 wt% Nb has been studied at 600–800°C in H2-H2S-CO2 gas mixtures providing 10−8 atm S2 at all temperatures and 10−24 atm O2 at 600°C and 10−20 atm O2 at 700 and 800°C. The corrosion kinetics were rather complex, being sometimes parabolic and in other cases nearly linear. Under a constant temperature the addition of niobium generally reduced the corrosion rate, except at 700°C when pure cobalt corroded more slowly than the two alloys. The corrosion rates for the same material decreased with an increase in temperature under the same sulfur pressure. Except at 800°C under 10−8 atm S2, which is below the dissociation pressure of cobalt sulfide, the scales presented an outer layer of pure cobalt sulfide and an inner layer of complex composition containing a mixture of double sulfide, niobium oxide and in some cases of unreacted metallic cobalt particles. The addition of niobium was generally beneficial, the effect increasing with its concentration in the alloy, but the corrosion rates of the alloys were still much higher than that of pure niobium, mainly as a result of the lack of formation of a protective layer of niobium sulfide. The corrosion behavior is examined with special reference to the consequences of the low solubility of niobium in cobalt and to the relation between the microstructure of the alloys and the scales.

Effect of Chromium Content on the Oxidation Behaviour of High-Speed Steels under Dry and Moist Air Environments

Development and use of high-speed steels for manufacturing the roll outer shell of hot strip mills represented a major recent technological advancement in the hot rolling field. However, it was observed that the oxidation behavior of these steels was different from that of conventional rolls. The high-speed steel oxidation rate is about four times higher than the alloys previously used. Furthermore, the rolling conditions are quite aggressive. Contact of the roll with the hot strip, air and water of the cooling system is expected to increase the oxidation of the roll surface in this wet atmosphere. Therefore, it is necessary to study their oxidation behavior in order to achieve the full potential of high-speed steel rolls for hot strip mills. In the present work, the oxidation behavior of three high-speed steels with differences in chromium content was studied. Corrosion tests were carried out in a thermobalance under dry and moist (12.5 % H2O) atmospheres at 765oC for 240 minutes. The corroded samples were examined by X-ray diffraction, scanning electron microscopy and energy dispersive micro-analysis. It was found that the variation of chromium content of the high-speed steels studied was sufficient to influence the oxidation behaviour. Samples with high chromium contents presented smallest final mass gain. The presence of humidity had a significant effect on the oxidation behavior.

A study of the formation of intermetallic phases in the FeZn system using an image plate detection system

Abstract The high temperature formation of intermetallic phases in the Fe Zn system has been studied by means of synchrotron radiation X-ray diffraction, using an image plate detector. In order to record the time-resolved diffraction pattern we have developed a novel method of simultaneous scanning of the image plate during the exposure. At least three intermetallic phases could thus be identified during the alloying process. The intial results show that the method can be used to obtain quantitative information on the phase composition in such systems, and that there are definite advantages in comparison with commonly used position sensitive detectors.

Estudo comparativo de eletrodos comerciais para soldagem subaquática molhada

With the objective to supply subsidies to select ferritic electrodes for underwater wet welding, some weld metal characteristics of commercial electrodes were determined. The welding trials were carried out at 0.5m fresh water depth in an aquarium, using a gravity device for welding. The study aimed also to obtain data to help to elect the consumables of best performance to be tested in higher depths. The following characterizations were performed: microstructural analysis, quantification of inclusions, weld metal and inclusions chemical composition, mechanical properties, diffusible hydrogen and weldability evaluation. The obtained results helped to classify the tested consumables in two groups with quite different characteristics. The first group is composed of one oxidizing type electrode and the second one is composed of four rutile type electrodes. Regarding to hydrogen in the weld metal, the obtained results show that the oxidizing electrode is able to produce welds with considerable low diffusible hydrogen content. As a consequence, smaller potential risk of cold cracking is expected when using this electrode. Meantime, regarding to the arc stability and other operational indicators, the oxidizing electrode presented inferior performance. Considering mechanical properties, the rutile electrodes presented the best results. The properties differences among the electrodes tested are shown and discussed in the present work.

Nax−yHyTi2−xFexO4·nH2O nanosheets with lepidocrocite-like layered structure synthesized by hydrothermal treatment of ilmenite sand

Nax−yHyTi2−xFexO4·nH2O nanosheets with lepidocrocite-like layered structure were produced through alkaline hydrothermal treatment at very low temperatures (130°C) from ilmenite sand. The crystal structure, morphology and optical properties were investigated by X-Ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive spectroscopy and UV-Vis spectroscopy. The product shows leaf-like nanosheet morphology with thickness <30 nm and lengths <1 µm. Three lepidocrocite-like titanates (Imm2 space group) with similar a and c lattice parameters but different interlayer distances (b/2) were identified. This appears to be the first preparation of lepidocrocite-like layered nanosheets by a simple, energy efficient (low temperature) and low cost (starting from mineral sand) procedure.

Corrosion resistance of a steel under an oxidizing atmosphere in a fluid catalytic cracking regenerator

In the present work, the corrosion resistance of an ASTM A 387 G11 steel was evaluated under two conditions: an oxidizing atmosphere in a fluid catalytic cracking regenerator of a petroleum processing unit and a simulated atmosphere in the laboratory, at temperatures of 650 °C and 700 °C. The characterization of the phases present in the oxidized layer was carried out by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) with X-ray energy dispersive analysis (EDS). Severe corrosion was observed after exposure to both the real and simulated conditions, with formation of several iron oxides (Fe2O3, Fe3O4 and FeO) in the product scale layer, as well as a slight inner oxidation and sulfidation of chromium in the substrate. Internal nitridation of the silicon and the manganese was observed only in the real condition, probably related to the long-term exposure inside the regenerator.