Showing papers by "DECHEMA published in 1997"

Improvement of the oxidation resistance of TiAl alloys by using the chlorine effect

Abstract: Most TiAl alloys lack from their insufficient oxidation resistance in air at temperatures above 800°C. Recently it has been found that small amounts of Cl in the alloy may significantly improve the oxidation resistance. However, the mechanisms responsible for this effect were not yet understood. In the paper oxidation experiments with TiAl alloys are reported where Cl was alloyed to the material by the P/M route or applied to the material surface either by ion implantation or by PVD. The results of the investigations revealed that the Cl-effect is due to the formation of volatile metal chloride species. It is shown that chlorine can have a beneficial effect on oxidation resistance in a certain Cl-range which is quantified by thermodynamic calculations. It is assumed that the Cl-effect offers a significant potential for improvement of the oxidation resistance of technical TiAl alloys.

The initial stages in the oxidation of TiAl

Abstract: The oxidation behaviour of titanium aluminides containing 36 wt.-% Al (Ti36Al) and 35 wt.-% Al plus 5 wt.-% Nb (Ti35Al5Nb) has been investigated by electron microscopic methods with emphasis on transmission electron microscopy (TEM). The oxidation experiments were carried out at 800 to 1000°C in laboratory air for 0.5 h to 4 h. In addition thermogravimetric measurements were made. It has been shown that the shortterm oxidation of TiAl can be divided into two stages. In stage I the preferred formation of aluminium oxide leads to an aluminium depletion of the metal subsurface zone and the subsequent formation of titanium nitrides which enhances the oxidation rate. After consumption of the depletion layer a repeated cycle of aluminium oxide formation, subsequent local depletion of the metal subsurface zone in Al and consumption of the Ti-rich metal phase by nitride formation is observed leading to linear oxidation behaviour (stage II). In the niobium containing alloy the dissolution of alumina in titania is decreased and thus the formation of aluminium oxide at the metal/oxide interface is favoured. By electron diffraction it has been found that the aluminium oxide formed at the metal/oxide interface most probably is an aluminium oxynitride Al27O39N. The aluminium depleted metal phase has been analyzed to consist of α2-Ti3Al and a new cubic phase with a composition between of α2-Ti3Al and γ-TiAl. Die Anfangsstadien bei der Oxidation von TiAl Das Oxidationsverhalten von Titanaluminiden mit einem Aluminiumgehalt von 36 Gew.% (Ti36Al) und 35 Gew.%Al, sowie 5 Gew.% Nb (Ti35Al5Nb) wurde mit Hilfe von elektronenmikroskopischen Methoden untersucht, wobei ein Schwerpunkt auf der Transmissionselektronenmikroskopie (TEM) lag. Isotherme Auslagerungsversuche wurden bei 800 bis 1000°C fur 0,5 bis 4 h an Laborluft durchgefuhrt. Zusatzlich wurden thermogravimetrische Messungen vorgenommen. Es wurde gezeigt, das die Anfangsoxidation von TiAl in zwei Stadien eingeteilt werden kann. Im Stadium I fuhrt die bevorzugte Oxidation von Aluminium zu einer Aluminiumverarmung in der Metallrandzone und der nachfolgenden Bildung von Titannitriden, mit einer erhohten Oxidationsgeschwindigkeit als Folge. Nach dem Aufbrauchen der Verarmungszone wird ein wiederholter Zyklus aus Aluminiumoxidbildung, nachfolgender lokaler Aluminiumverarmung der Metallrandzone und dem Aufbrauchen des Ti-reichen Metalls durch Nitridbildung beobachtet, der lineares Oxidationsverhalten zur Folge hat (Stadium II). In der Legierung, die Niob enthalt, wird die Auflosung des Aluminiumoxids im Titanoxid vermindert und dadurch die Bildung von Aluminiumoxid an der Metall/Oxid-Grenzflache begunstigt. Mit Hilfe der Elektronenbeugung wurde gefunden, das das Aluminiumoxid, das sich an der Metall/Oxid-Grenzflache bildet, sehr wahrscheinlich ein Aluminiumoxynitrid Al27O39N ist. Bei der an Aluminium verarmten Metallphase wurde festgestellt, das sie aus α2-TiAl und einer neuen kubischen Phase mit der Zusammensetzung zwischen α2-Ti3Al und γ-TiAl besteht.

Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

Abstract: In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidatio...

Oxidation of the intermetallics Mosi2 and TiSi2 : A comparison

Abstract: The oxidation behavior of two MoSi2 variants, one Mo-rich and one Si-rich, and TiSi2 was investigated between 1000 and 1400°C in air, oxygen and an 80/20-Ar/O2 mixture. A protective SiO2 scale develops on MoSi2 in all atmospheres in the temperature range investigated. The SiO2 modification changes around 1300°C from tridymite to cristobalite. This change in SiO2 modification seems to cause an enhanced formation of SiO2 and evaporation of MoO3. The SiO2 grows at the MoSi2-scale interface. In air a two-layer scale grows on TiSi2 between about 1000 and 1200°C with an inner inwards growing fine-grain mixture of SiO2 + TiO2 and an outer outward-growing TiO2 partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N -free atmospheres a scale of a SiO2 matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi2 in air at T > 1200°C. In oxygen the TiO2 precipitates grow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO2 compared with SiO2, these TiO2 needles act as “oxygen pipes,” causing an enhanced oxidation of TiSi2 in front of these needles. The SiO2 scale dissolves about 1–2% TiO2. This doping causes a mixed oxygenand Si transport with the consequence that the SiO2 scale on TiSi2 grows partly by oxygen transport inwards and Si transport outwards. The SiO2 modification is cristobalite over the entire temperature range investigated.

Selection of Materials for Use at Temperatures above 1500°C in Oxidizing Atmospheres

Abstract: The purpose of this work was to select materials for applications in oxidizing atmospheres at temperatures of a minimum of 1500°C. Several requirements had to be fulfilled, e.g. long-term oxidation resistance, low vapour pressure, high creep resistance and low gas permeability. Another important point was to avoid reactions and sintering in material combinations with silica forming materials. The investigation was based on a detailed literature screening, followed by compatibility tests at 1600°C in air with potential materials (Al2O3, ZrO2, CeO2, La2O3, Y2O3, HfO2, MgO·Al2O3) and Mosi2 as an example of a silica-forming material. The result of these experiments was, that only Y2O3 and HfO2 did not show severe reactions in contact with MoSi2. Since most of the materials available did not fulfill all the requirements, some new materials were investigated: CIPed MoSi2 with different additives (SiC, ZrB2) and recrystallized SiC (RSiC) with a polymer (polysiloxane) derived MoSi2-filled coating. The oxidation behaviour of these materials was evaluated by continuous thermogravimetric measurements at 1500°C over a period of 100 h in air and by detailed postexperimental investigations.

Impact of accessibility and acidity on novel molecular sieves for catalytic cracking of hydrocarbons

Abstract: The catalytic properties of extra-large pore aluminosilicate MCM-41, silicon containing VPI-5 and aluminium containing ETS-10 were investigated by MAT (Micro Activity Test) using n-hexadecane and 1,3,5-triisopropylbenzene as the model feeds and were then compared with the results obtained from Y zeolite in the H-form and a commercial FCC (Fluid Catalytic Cracking) catalyst. It could be demonstrated that not only the acidic properties of the investigated materials, but also the accessibility to the internal active sites of the investigated materials have a significant influence on activity and selectivity. It can be assumed that, by optimising structural and chemical properties, the mesoporously organised MCM-41 and related materials are suitable as active components in cracking catalysts for “deeper” cracking of high boiling hydrocarbons.

Scale formation on γ-TiAl during oxidation at 800 and 900°C in air and in He + 20% O2

Abstract: The oxide scale formation on γ-TiAl at 800 and 900°C was studied using high temperature X-ray diffraction as anin situ-method. The experiments were performed in air and in He with 20 vol.% O2. The formation of alumina in the form of α-Al2O3 and of TiO2 in the form of rutile was observed in both atmospheres and the formation of TiN was detected in air. Depending on the atmosphere the diffraction peaks of two different additional phases were detected, which do not exist in any data base nor in the Ti-Al-O phase diagram. One of them, the Z-phase, appears in He with 20 vol.% O2 and the other, the X-phase, in air. The Zphase was also found at room temperature after oxidation at 900°C in air. The growth of both phases, X and Z, starts immediately with the oxidation process and follows the parabolic rate law.

The nature of the acid sites in mesoporous MCM-41 molecular sieves

Abstract: The objective of this presentation is to give a characterization of aluminosilicate MCM-41, in particular with respect to acidity and the nature of the acid sites 29 Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting that the pore walls are amorphous This fact could also imply that the types of acid sites in these materials are different from those generally found in zeolitic materials The obtained results from 1 H MAS NMR and combined FTIR and temperature-programmed ammonia desorption (TPAD) investigations indicate that the washed, template-free MCM-41 and its protonic form behave as a solid-state acid with a broad acid strength distribution With regard to the presence of bridging hydroxyl groups in the investigated MCM-41 materials, the findings of these two methods are not consistent Therefore, it is assumed that the broad absorption between 3650–3400 cm −1 in the FTIR spectra is connected with hydrogen-bonded vicinal silanol pairs The results of the catalytic testing suggest that the weak Bronsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites are formed

An indirect electrochemical process for the removal of NOx from industrial waste gases

Abstract: The development of a new electrochemical process for the absorption of NOx from industrial waste gases is described. Conversion of NO was performed by an indirect outer cell process using dithionite as the redox mediator and Fe(II)EDTA as the complexing agent. The absorption process, involving complex formation with Fe(II)EDTA in the absence and presence of dithionite, was investigated using a packed bubble column. In semibatch experiments the dependence of different system parameters on the degree of NO conversion was studied: concentration of dithionite and Fe(II)EDTA, gas flow rate, oxygen content, and temperature. The reaction products in the gas phase and in the liquid phase were analyzed using gas chromatography and ion chromatography, respectively. This analysis proved that, instead of N2 or N2O, ammonia and amidosulfonic acid are the main reaction products in the solution. The kinetics and the conditions for the regeneration of dithionite by electrochemical reduction of sulfite were studied on RDE and by experiments in a divided cell. For the dithionite production a current efficiency of about 80% was found in 0.2 m Na2SO4 at pH 5.6. By combining the chemical NO absorption in a gas/liquid contactor with the electrochemical regeneration of dithionite in a divided plate and frame cell a degree of NO conversion better than 90% under continuous operation can be obtained.

Controlling the enantioselective hydrogenation of ethyl pyruvate using zeolites as catalyst support

Abstract: 5 wt-% Pt/carrier catalysts (carrier materials: NaY, mordenite, erionite and NaX as well as γ-Al 2 O 3 as a microporous, non-zeolitic support) combined with (-)cinchonidine as the chiral auxiliary were used successfully for the enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate. With acetic acid as the solvent, the catalysts Pt/NaY, Pt/mordenite and Pt/erionite produced high enantiomeric excesses (86 to 90% ee), whereas Pt/NaX (77% ee) and Pt/γ-Al 2 O 3 (67% ee) gave comparatively poor optical yields. With regard to the k 0 values, which were obtained from kinetic data of the hydrogenation related to the mass of the catalysts only, a correlation between enantioselectivity and catalytic activity was not discernible. This changed significantly, if referring to the k'value related to the mass of the catalyst and its specific Pt surface area (derived from volumetric CO chemisorption measurements). Then, the catalysts producing the highest enantiomeric excesses also resulted in the highest catalytic activities. Therefore, catalytic activity and enantioselectivity essentially depend on the specific Pt surface area of the catalyst. This means that Pt particles of appropriate geometry and size are necessary to generate high enantioselectivity and catalytic activity. Zeolites Y, mordenite and erionite turned out to be appropriate templates for the creation of such Pt particles during catalyst preparation. This was confirmed by temperature-programmed CO desorption measurements. Zeolite X was partially destroyed by acid solution during catalyst preparation, which was proved by DTA and XRD measurements. Therefore, zeolite X, as well as γ-Al 2 O 3 , could not act as a template for the creation of appropriate Pt particles.

Corrosion Characteristics of Chromated Zinc in Sodium Chloride Solutions

Abstract: The corrosion behavior of zinc with a chromate conversion coating containing ∼ 0.02 g/m2 Cr(VI) was investigated. X-ray photoelectron spectroscopy (XPS) showed Cr(VI) was located in layers near the surface of the coating. Corrosion current densities and Cr(VI) content in the coating decreased with immersion time. An electrochemical reduction of Cr(VI) to Cr(III) parallel to oxygen (O2) reduction in the first stages of immersion was reviewed. Inhibition of the corrosion rate proceeded simultaneously with dissolution of the conversion coating. It was concluded that the thickness of the coating was not the most important factor to hinder O2 reduction. Corrosion inhibition was attributed to zinc surface passivation byproducts of ionization.

Isothermal oxidation of P/M-TiAl alloys

Abstract: Le comportement en oxydation de trois alliages intermetalliques P/M-TiAl a ete etudie a des temperatures comprises entre 700 et 900°C. Deux alliages contenaient Cr et le troisieme contenait Mn et W. Les composes intermetalliques contenaient des impuretes, en particulier Cl dont la presence doit etre mentionnee en raison de son influence importante sur le comportement en oxydation. Tous ces materiaux ont ete fabriques selon des procedes propres a la metallurgie des poudres. Les differents moyens de production de poudres TiAl comprennent l'alliation par voie mecanique, l'atomisation gazeuse et le melange de poudres pre-alliees suivi de procedes de compactage tels que l'extrusion et la compression isostatique a chaud. La cinetique d'oxydation, la structure des couches oxydees et l'influence des impuretes sont discutees.

Changes in the transport properties of silicon dioxide due to doping by foreign ions

Abstract: Based on the model that in the intrinsic disorder of SiO2 oxygen-ion vacancies dominate at low oxygen pressure and oxygen-ion interstitials at high oxygen pressure, a model is presented which explains the formation of mobile interstitial silicon ions preferentially at high oxygen pressure by doping of SiO2 with Ti3+ and/or Ti2+ ions. Analogously an attempt has been made to understand an increase in the solubility of MoO3 in SiO2 by Al2O3 dissolved in SiO2. Doping and co-dissolution are in particular to be expected in the oxidation of SiO2 forming ceramics because SiO2 has very probable dissolved impurities of the ceramic or/and sinter additives.

Korrosionsschäden am Förderband eines Ofens zur Reinigung von mit organischen Chlorverbindungen kontaminiertem Erdboden

Abstract: Es wird das Korrosionsverhalten von metallischen Werkstoffen beschrieben, die als Komponente des Forderbandes eines Dekontaminationsofens zur Reinigung von mit chlorhaltigen organischen Verbindungen verunreinigtem Erdboden zum Einsatz kamen oder als Testwerkstoffe am Forderband befestigt waren. Insgesamt wurden 1 ferritischer und 8 austenitische Stable, 9 Nickelbasiswerkstoffe, 2 Gusstahlsorten mit 15%, Si bzw. 15%, Si + 5% Cr sowie 3 alitierte Stahle getestet und anschliesend metallographisch und mikroanalytisch untersucht. Die meisten der Werkstoffe versagten durch ausere abtragende und/oder durch innere Korrosion, bevorzugt entlang der Korngrenzen, ausgelost durch aggressive Bodenbestandteile, insbesondere Chlorverbindungen. Als recht bestandig erwiesen sich die beiden legierten Gusstahlsorten. Unerwartet gut hat sich auch der Werkstoff 2.4061 (LC-Ni 99,6) verhalten, obgleich hier noch zu prufen bleibt, ob das Einwandern von C und O an den Korngrenzen nicht eine Versprodung bewirkt. Sehr gute Bestandigkeit zeigten die Werkstoffe 2.4610 und 2.4831. Die Grunde hierfur konnten nicht geklart werden. Ein Grund konnte der relativ hohe Mo-Gehalt von 14-17 bzw. 8,5-9,5 % sein, jedoch wurden andere Werkstoffe mit vergleichbaren Mo-Gehalten wie die Werkstoffe 2.4856 oder 2.4663 stark angegriffen. Ferner bleibt zu klaren, ob die gute Bestandigkeit auch langzeitig erhalten bleibt. Corrosion damage to the conveyor belt of a furnace for the removal of soil from organic chlorine compounds This paper reports the corrosion behaviour of metallic materials which were used in components of a conveyor belt of a decontamination furnace or of which specimens were attached to the belt for testing purposes. The furnace was used for decontaminating soil containing organic chlorine compounds. A total of 1 ferritic and 8 austenitic steels, 9 nickel base alloys, 2 east steels with 15 % Si and 15 % Si + 5 % Cr respectively, and 3 aluminised steels were tested and afterwards investigated by metallographic and micro-analytical methods. Most of the materials failed due to external and/or internal corrosion, preferentially along grain boundaries. The 2 cast steels revealed fairly good corrosion resistance. The material 2.4061 (LC-Ni 99.6) displayed surprisingly good behaviour, however it remains to be seen whether the inward migration of O and C along grain boundaries causes embrittlement. The materials 2.4610 and 2.4831, containing 14-17 % Mo and 8.5-9,5 % Mo respectively, showed good resistance. The reason for this good corrosion resistance could not be clarified. One reason may be the relatively high Mo content but other materials with a comparable Mo content, like materials 2.4856 and 2.4663, were significantly attacked. It appears necessary to check the long-term behaviour of these two materials.

Preparation of silicon-rich surfaces on alloys

Abstract: Preparation of silicon-rich surface zones on alpha2-Ti3Al and nu-TiAl alloys, comprises calcining the alloys in a vacuum with titanium chips and parts of silicon dioxide at 900-1150 deg C.

Impedanzspektrometrische Untersuchungen zur Wirkungsweise von Inhibitoren und Tensiden bei der CO2-Korrosion von Stahl unter Erdgasförderbedingungen

Abstract: Inhibitionsmechanismen der CO2 Korrosion von niedriglegiertem C-Stahl (38 Mn 6) warden unter den Bedingungen der Forderung suser Erdgase (1 M NaCl + 0,1 M CaCl2 I und 5 bar CO2 80°C) in Gegenwart von grenzflachenaktiven Stoffen (Inhibitoren, Tenside) mit Hilfe der elektrochemischen Impedanzspektrometrie studiert. Auf Basis eines Porenmodells wurde ein Ersatzschaltbild entwickelt, mit welchem die Mechanismen der untersuchten Werkstoffe beschrieben werden konnen. Fettamine und Imidazoline hemmen die Phasengrenzreaktion am Grunde von Deckschichtporen durch Adsorption. Nichtionische Tenside konnen die Wirksamkeit der N-haltigen Inhibitoren beeintrachtigen. 1-Octin-3-ol wirkt durch Polymerisatfilmbildung und Reduzierung der Deckschichtporositat. Alle Wirkstoffe beeinflussen die Morphologie der Deckschicht und ihre Bildungskinetik. AC impedance investigations on the performance of inhibitors and surfactants in CO2 corrosion of steel under natural gas production conditions Inhibition mechanisms of CO2, corrosion of low alloy carbon steel (38 Mn 6) were studied under production conditions of sweet natural gas (1 M NaCl + 0.1 M CaCl2; 1 and 5 bar CO2; 80°C) in the presence of surface active compounds (inhibitors, surfactants) with AC impedance spectrometry. Based on a pore model an equivalent circuit was developed which explains the action mechanisms of the substances tested. Fatty amines and imidazolines inhibit the interface reaction at the bottom of pores in the corrosion product scale due to adsorption. Nonionic surfactants can impair the effectivity of N-containing inhibitors. 1-Octyne-3-ol acts through formation of polymer films and reduces the porosity of the scale. All substances under investigation influence the morphology of the scale and the kinetic of its formation.

Textural, Structural, and Acid Properties of a Catalytically Active Mesoporous Aluminosilicate MCM‐41.

Abstract: As-prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterised by N2 adsorption/desorption at 77 K, 1H, 27Al and 29Si MAS NMR as well as by temperature-programmed ammonia desorption (TPAD) with in situ FTIR. 29Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting a broad range of Si—O—Si bond angles in this material. The obtained results from 1H MAS NMR and a combination of FTIR and TPAD investigations indicate that the washed, template-free MCM-41 and its protonic form behave like a solid-state acid with a broad acid strength distribution. With regard to the presence of bridging hydroxy groups in the investigated MCM-41 materials, the findings of these two methods are not consistent. Therefore, it is assumed that the broad absorption between 3650 and 3400 cm–1 in the FTIR spectra is connected with hydrogen-bonded vicinal silanol pairs. A signal in the 27Al MAS NMR spectra at 40 ppm is identified as a separate resonance, assigned preferentially to an isolated extranetwork tetrahedral aluminium species, whereby the assignment to a pentahedrally coordinated network aluminium species is also considered. When calcined, these species are reinserted into network positions. Subsequent catalytic testing leads to partial recovery of the former resonance. The results of the catalytic testing imply that the weak Bronsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites form.