Fig. 1 Examples of the tested tubes

Tubes Made from Al 2 O 3 Ceramic for High-Temperature Technology an Orientational Comparison Abstract Tubes made from high-quality, dense-sintered Al 2 O 3 ceramic are reliable components in the plants of the glass and metal producing and processing industries, industrial kiln engineering and the basic chemical industry. In such plants, the service conditions often require an extremely high strength of the ceramics shape in a corrosive environment and a high thermal shock resistance. Commercially available tubes from six manufacturers were tested in an orientational study, with regard to these properties. The results of the tests revealed some drastic differences between the various types. Among the dense-sintered ceramics, the materials and products behaviour of Degussit demonstrated the optimum combination of these three properties. The Degussit strength of shape is only surpassed by Degussit, a material especially developed for the high-temperature technology. Introduction The reliable function of tubes made from dense-sintered Al 2 O 3 ceramic, as used, for instance, for thermocouples in high-temperature technology, necessitates a high quality of materials and products, especially at temperature levels above 15 C. Only in that case acceptable service lifetimes can be expected under the conditions of application. In order to describe the technical performance of such tubes today, Degussit and products from three other German manufacturers (D1, D2, D3), one US American manufacturer (U) and one Chinese (C) manufacturer were tested. In addition, Degussit, a material especially developed for the hightemperature technology and proven over decades, was included in the comparison. differs from the other materials types with its up to 5 vol.-% open porosity. As test specimens, tubes closed at one end with Ø ou Ø i = 19 mm 14 mm; 15 mm 1 mm; D2 U D3 D1 Fig. 1 Examples of the tested tubes 8 mm 5 mm; 7 mm 5 mm and a length of 1 mm were used. The type C specimen was a 3 mm long tube open at both ends with Ø ou Ø i = 19 mm 13,5 mm. Fig. 1 shows a selection of the tubes. The results described in the following are predominantly based on the investigation of one specimen from each material. For that reason, the results tend to be more of a guide. However, they can provide assistance in the selection of suitable materials for sophisticated applications in the high-temperature range. Test Methods Their use at high temperatures demands from alumina tubes a high strength of shape and corrosion resistance and, for installation in up and running plants, also acceptable thermal shock resistance. The tests were therefore conducted with a focus on these properties. The characterization of the materials and products has been done by the following methods: Microstructure: incident-light microscopy on thermally etched polished sections Density: Archimedes buoyancy method (demineralized water, 2 C) Chemical composition: ICP-OES analysis Behaviour on corrosive load: boiling 1,8 n sulphuric acid, 72 h Thermal shock resistance: thermal shock in air Strength of shape: sagging of rods at 17 C in air. Results Microstructure The microstructure of the ceramics has a core function with regard to the tubes resistance to deformation and corrosion at high temperature as well as to thermal shock. Both the size distribution and the morphology of the crystallites and pores as well as the degree of inter- and intracrystalline porosity are crucial factors for the stability of the materials and the service lifetime of the tubes [1]. Figs. 2 8 show clear differences between the microstructures depending on the manufacturer. Both mono- and bimodal types of microstructure with globular and sometimes longitudinal grain morphology can be identified. The maximum grain size of the materials most amounts to 5 µm, however 1 µm has been found in D2. In D3, a pronounced difference of the microstructure exists between tube (T) and bottom (B), which was not observed in the other materials. C is a material with a marked globular morphology of the crystallites and comparatively wide grain bound- Helmut Mayer FRIATEC AG 68229 Mannheim, Germany E-mail: helmut.mayer@friatec.de www.friatec.de cfi/ber. DKG 89 (212) No. 4 E 23

Process Engineering Fig. 2 Microstructure Fig. 3 Microstructure D1 Fig. 4 Microstructure D2 Fig. 5 Microstructure D3, tube Fig. 6 Microstructure D3, bottom Fig. 7 Microstructure U Fig. 8 Microstructure C Fig. 9 Microstructure Tab. 1 Material and test specimen data industrial plants. For the materials, high purity is therefore essential, as the composition of the grain boundary phase of the microstructure often has a crucial influence on the degree of corrosion resistance and therefore on the lifetime of the materials [2]. In this connection, the Si content of the materials is a key parameter. Especially with the presence of alkali and alkaline earth elements, amorphous silicate containing phases can exist at the grain boundaries, which are detrimental to corrosion resistance [3]. With tube segments of the materials, a simplified qualitative test was conducted to detect the corrodible grain boundary substance. After documentation of the starting conditions by use of Ardrox 9VF2, a dye penetrant [4], the specimens were first exposed to boiling 1,8 n H2SO4 for 72 h. They were then rinsed with demineralized water, dried and, in the next step, soaked for 16 h in an alcoholic methylene blue solution. Finally, they were cross-sectioned to make the infiltration of the leached grain boundaries visible. Type Purity Density [mass-%] [g/cm3] Penetration Depth [µm] C 98,43 3,75 36 99,7 3,81 D1 99,7 3,84 18 D2 99,73 3,91 tube base 24 D3 99,74 3,9 U 99,74 3,93 18 99,77 3,59 aries. exhibits a very different microstructure with grain and pore sizes up to 1 µm and partly jagged grain surfaces (Fig. 9). The different microstructures are the reason for the measured differences of the materials density, despite their comparable purity. Tab. 1 summarizes the data. Resistance to Corrosion High stability in a corrosive environment is often a major requirement for oxide ceramic tubes used in E 24 The result is shown in Fig. 1. This shows the state of the segments before and after the test for each material. Degussit and D2 proved corrosion resistance under the test conditions. This also applies to Degussit, which is coloured only in the open pores. In D1 and U an attack has taken place, while corrosion is very pronounced in C. Tab. 1 lists the rough penetration depths of the methylene blue solution in the leached grain boundaries. One special case is type D3, which is resistant in T, but not in B. The microstructure of the two areas shows distinct differences (Figs. 5 and 6). Obviously in B, the much coarser crystalline microstructure there and the associated higher concentration of corrodible substance at the grain boundaries enable their leaching. Presumably, this process would not have taken place in a microstructure corresponding to T. The materials were analysed with ICP-OES [5], as their chemical composition was expected to be a substantial reason for their different behaviour in the corrosion test. From the results (Fig. 11), assuming a representative sample state, the purities shown in Tab. 1 can be derived. In Fig. 11, only those substances are included, which are represented in the material with at least 1 ppm. Degussit, D1, D2, D3 and U lie close together in terms of their purity, differ, however, sometimes considerably with regard to the accessory substances. One particularity, which is not shown on Fig. 11, is that Type C contains a significant cfi/ber. DKG 89 (212) No. 4

Process Engineering amount of Zr, Hf and Y, maybe as a result of the raw materials milling by use of ZrO2 grinding media. The highest purity was found in this analysis for Degussit. A direct correlation with the behaviour of the materials in the corrosion test can hardly be derived from the analyses, as not only the amounts, but also the percentages of the accessory substances and the microstructure condition are determining factors. The corrosion resistance of Al2O3 ceramic materials from different manufacturers is not necessarily on a comparable level, even though they have identical purity. The results of the corrosion test can be evaluated as a general indication of the corrosion resistance of the materials in high-temperature applications, as corrosion processes under the prevailing conditions, as experience shows, progress faster via the grain boundaries than as a result of a possible attack on the base crystal. before after D1 before D1 after D2 before D2 after D3 before D3 after U before U after C before C after before after Resistance to Thermal Shock Resistance to thermal shock was investigated in the following test with Degussit, D1, D2, D3, U and in different dimensions. For this purpose, the tube specimen was inserted in around 1 s through an aperture into the hot zone of an electric laboratory furnace at set temperature. The open end of the tube remained outside the furnace at room temperature. A hold-up time of 3 min was followed by natural cooling in this set-up down to room temperature. Starting at 12 C and with an increase of 1 K/cycle, this procedure was repeated until the first cracks became visible in the ceramic. The results are shown in Fig. 12. In each case the temperatures are specified where the first damage of the components could be macroscopically identified after dyeing with Ardrox 9VF2. Degussit, D1, D2 and D3, each with 1,5 mm wall thickness, did not reach the level at which the first cracks could be identified even with thermal shock cycles from room temperature up to 17 C. Higher temperature differences could not be realized with that furnace. The drastic differences of the microstructures between T and B in D3 did not create any damage of the tube in this test. In Degussit, after thermal shocks to 14 C, the first defects were observed. They can be attributed primarily to the higher cfi/ber. DKG 89 (212) No. 4 Fig. 1 Specimens before and after the corrosion test:, D1, D2, D3, U, C, and E 25

Content [ppm] Fig. 11 ICP-OES analysis T [K] Fig. 12 Shock in air Fig. 13 Type U with defects wall thickness (2 mm) compared with the dense-sintered materials (1,5 mm). In practical use, such damage can be prevented with a slower assembling procedure. In U, after the test, systematically arranged transverse cracks could be detected (Fig. 13). They are obviously the result of weakening the material, which pre- Diameter [mm] sumably took place prior to sintering, resulting in an unexpectedly low thermal shock resistance. Repeated shocks up to the occurrence of initial damage were not tested. In practice such a tube is generally only used once over several months and sometimes over years. When thermocouples are changed, a new tube is regularly fitted. This procedure is usually not avoidable especially in the case of horizontal inserts on account of the deformation of the tubes over long periods at high operating temperatures. Strength of Shape at High Temperature In order to ascertain the deformation of the ceramics with the same geometric conditions, from tubes with sufficiently high wall thickness, rods measuring 2 mm 2 mm 15 mm were prepared by means of identical machining. Afterwards they were inserted horizontally in a support made from Al 2 O 3 ceramic with a span of 1 mm and a maximum possible deformation of 12 mm. The entire setup was placed in an electric laboratory furnace, heated with 2 K/h up to 17 C and held there for 3 h. After natural cooling down to room temperature, the deformation of the rods was determined geometrically. The thermal treatment was repeated with another 3 h at 17 C to obtain a higher differentiation of the deformation. In a second furnace campaign type C was examined in comparison with Degussit and Degussit. C exhibited the highest in that test attainable deformation already after 3 h at 17 C. Fig. 15 clearly shows the pronounced deformation of some rods. The evaluation of these results combined with the ICP-OES analyses and the microstructure of the materials shows that by a low content of Si and Ca and a high one of Mg as well as a microstructure that avoids both globular grain morphology and a high fine-crystalline content, the highest strength of shape can be expected [6, 7]. Among the densesintered materials, Degussit obviously comes closest with an optimum combination of these parameters. The density of the materials, on the other hand, tends to have a minor influence on their deformability at high temperature. With Degussit, a material is available, which on account of its high purity and specific microstructure achieves the highest shape stability of the tested materials. Summary In a comparative study of an orientational nature, six types of tubes made from densely sintered Al 2 O 3 ceramic and one type of tube made from an open porous Al 2 O 3 ceramic supplied by different manufacturers were tested with regard to the properties relevant to high-temperature applications. The results are summarized in the following. Resistance to Corrosion Under the conditions of this study, Degussit, D2 and Degussit proved resistant to corrosion E 26 cfi/ber. DKG 89 (212) No. 4

on a comparably high level. D3 achieves this level in certain areas. U, C and the bottom of D3 exhibits the typical characteristics of grain boundary corrosion and accordingly a dependence of the corrosion resistance on the chemical composition of the grain boundary phase and, as clearly identifiable in D3, on the microstructure of the material. As a result of its low purity, C achieves the lowest general resistance. D1 U D3 D2 Resistance to Thermal Shock a) The thermal shock resistance of the tested tubes is generally high and, according to the results of the shock test, generally acceptable for industrial applications. The significant difference between the microstructure of tube and bottom in D3 did not have any detrimental effect on the thermal shock resistance. Presumably as a result of its around 3 % higher wall thickness, Degussit exhibits a lower thermal shock resistance. U makes clear that even apparently minor weakening of the ceramic can lead to premature failure in operation. C was not tested for geometry-related reasons. Test 1: 3 + 3 h/17 C C Strength of Shape at High Temperature The deformation of rods of the same size exhibits substantial differences after 3 and 6 h hold-up time at 17 C. The cause can be derived from the different influences of chemical composition and microstructure. Among the dense-sintered materials, verifies the highest strength of shape, followed by D1, D3 and D2 with a moderately increasing deformation. U and especially C exhibit an extremely low resistance to deformation. In this test Degussit turned out to be the most stable material. Test 2: 3 h / 17 C Fig. 14 a b Sagging Deformation [mm] Type of tube b) References [1] Salmang, H.; Scholze, H.; Telle, R.: Keramik. 7 th ed., Heidelberg 27 [2] Genthe, W.; Hausner, H.: Korrosionsverhalten von Aluminiumoxid in Säuren und Laugen. cfi/ber. DKG 67 (199) 6 1 [3] Mayer, H.: Beständigkeit oxidkeramischer Produkte in korrosiven Flüssigkeiten. In: Kriegesmann, J.: Technische Keramische Werkstoffe, Kapitel 5.4.1.2.1 (1999) [4] Chemetall GmbH Fig. 15 Degree of deformation [5] DIN EN ISO 11885-E22:29-9. Wasserbeschaffenheit Bestimmung von ausgewählten Elementen durch induktiv gekoppelte Plasma Atom Emissionsspektroskopie [6] Mayer, H.: Oxidkeramische Produkte für die Hochtemperaturtechnik. Glasingenieur 15 (25) [7] Petzold, A.; Ulbricht, J.: Aluminiumoxid. Leipzig 1991 cfi/ber. DKG 89 (212) No. 4 E 27