Tubes made of DEGUSSIT ceramics for High-Temperature

Process Engineering
Tubes Made from Al2O3 Ceramic for
High-Temperature Technology
– an Orientational Comparison
Abstract
Tubes made from high-quality,
dense-sintered Al2O3 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 AL23 demonstrated
the optimum combination of these
three properties. The Degussit AL23
strength of shape is only surpassed
by Degussit AL24, a material especially developed for the high-temperature technology.
Introduction
The reliable function of tubes made
from dense-sintered Al2O3 ceramic,
as used, for instance, for thermocouples in high-temperature technology, necessitates a high quality of
materials and products, especially at
temperature levels above 1500 °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 AL23 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 AL24, a material
especially developed for the hightemperature technology and proven
over decades, was included in the
comparison. AL24 differs from the
other materials types with its up to
5 vol.-% open porosity.
As test specimens, tubes closed
at one end with Øou × Øi =
19 mm × 14 mm; 15 mm × 10 mm;
cfi/Ber. DKG 89 (2012) No. 4
D2
U
D3
D1
AL23
Fig. 1 Examples of the tested tubes
8 mm × 5 mm; 7 mm × 5 mm and a
length of 1000 mm were used. The
type C specimen was a 300 mm
long tube open at both ends
with Øou × Øi = 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,
20 °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 1700 °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 50 µm, however
100 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 boundHelmut Mayer
FRIATEC AG
68229 Mannheim, Germany
E-mail: [email protected]
www.friatec.de
E 23
Process Engineering
Fig. 2 Microstructure AL23
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 AL24
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
360
AL23
99,70
3,81
0
D1
99,70
3,84
180
D2
99,73
3,91
tube 0
base 2400
D3
99,74
3,90
0
U
99,74
3,93
180
AL24
99,77
3,59
0
aries. AL24 exhibits a very different
microstructure with grain and pore
sizes up to 100 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. 10. This
shows the state of the segments
before and after the test for each
material.
Degussit AL23 and D2 proved corrosion resistance under the test conditions. This also applies to Degussit
AL24, 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 10 ppm.
Degussit AL23, 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 (2012) 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 AL24. 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.
AL23 before
AL23 after
D1 before
D1 after
D2 before
D2 after
D3 before
D3 after
U before
U after
C before
C after
AL24 before
AL24 after
Resistance to Thermal Shock
Resistance to thermal shock was
investigated in the following test
with Degussit AL23, D1, D2, D3, U
and AL24 in different dimensions.
For this purpose, the tube specimen
was inserted in around 10 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 30 min was followed by natural cooling in this set-up down to
room temperature. Starting at
1200 °C and with an increase of
100 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 AL23, 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 1700 °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 AL24,
after thermal shocks to 1400 C, the
first defects were observed. They can
be attributed primarily to the higher
cfi/Ber. DKG 89 (2012) No. 4
Fig. 10 Specimens before and after the corrosion test: AL23, D1, D2, D3, U, C,
and AL24
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Content [ppm]
Process Engineering
T [K]
Fig. 11 ICP-OES analysis
Diameter [mm]
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 preE 26
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 geo-
metric conditions, from tubes with
sufficiently high wall thickness, rods
measuring 2 mm × 2 mm × 105 mm
were prepared by means of identical
machining. Afterwards they were
inserted horizontally in a support
made from Al2O3 ceramic with a
span of 100 mm and a maximum
possible deformation of 12 mm. The
entire setup was placed in an electric
laboratory furnace, heated with
200 K/h up to 1700 °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 1700 °C to obtain a
higher differentiation of the deformation. In a second furnace campaign
type C was examined in comparison
with Degussit AL23 and Degussit
AL24. C exhibited the highest in that
test attainable deformation already
after 3 h at 1700 °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 AL23
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 AL24, 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 Al2O3
ceramic and one type of tube made
from an open porous Al2O3 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 AL23, D2 and Degussit
AL24 proved resistant to corrosion
cfi/Ber. DKG 89 (2012) No. 4
Process Engineering
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
AL24
U
D3
D2
AL23
a)
Resistance to Thermal Shock
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
1700 °C. The cause can be derived
from the different influences of
chemical composition and microstructure. Among the dense-sintered
materials, AL23 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 AL24 turned out to be the
most stable material.
Test 1: 3 + 3 h/1700 °C
AL24
AL23
C
b)
Test 2: 3 h / 1700 °C
Fig. 14 a–b Sagging
Type of tube
Deformation [mm]
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 30 %
higher wall thickness, Degussit AL24
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.
References
[1] Salmang, H.; Scholze, H.; Telle, R.:
Keramik. 7th ed., Heidelberg 2007
[2] Genthe, W.; Hausner, H.: Korrosionsverhalten von Aluminiumoxid in Säuren und Laugen. cfi/Ber. DKG 67
(1990) 6–10
[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
cfi/Ber. DKG 89 (2012) No. 4
Fig. 15 Degree of deformation
[5] DIN EN ISO 11885-E22:2009-09.
Wasserbeschaffenheit – Bestimmung
von ausgewählten Elementen durch
induktiv gekoppelte Plasma – Atom –
Emissionsspektroskopie
[6] Mayer, H.: Oxidkeramische Produkte
für die Hochtemperaturtechnik. Glasingenieur 15 (2005)
[7] Petzold, A.; Ulbricht, J.: Aluminiumoxid. Leipzig 1991
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