Oxidation Protection of MgO-C Refractories by Means of Al8B4C7

Article abstract of DOI:10.1111/j.1151-2916.2001.tb00701.x

The effect of Al₈B₄C₇ used as an antioxidant in MgO-C refractories and the behavior of Al₈B₄C₇ in CO gas were investigated in the present study. Al₈B₄C₇ was found to react with CO gas, to form Al₂O₃(s), B₂O₃(l), and C(s), at temperatures > 1100°C. The Al₂O₃ reacts with MgO to form MgAl₂O₄ near the surface of the material. At the same time, B₂O₃(l) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B₂O₃ and MgO. The coexistence of the liquid and MgAl₂O₄ makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C.

Full text of DOI:10.1111/j.1151-2916.2001.tb00701.x

J. Am. Ceram. Soc., 84 [3] 577–82 (2001)
Oxidation Protection of MgO–C Refractories by Means of Al8B4C7
Tianming Wang and Akira Yamaguchi*
journal
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku,
Nagoya, 466-8555, Japan
The effect of Al8B4C7 used as an antioxidant in MgO–C
refractories and the behavior of Al8B4C7 in CO gas were
investigated in the present study. Al8B4C7 was found to react
with CO gas, to form Al2O3(s), B2O3(l), and C(s), at temper-
atures >1100°C. The Al2O3 reacts with MgO to form
MgAl2O4 near the surface of the material. At the same time,
B2O3(l) evaporates and reacts with MgO, to form a liquid
phase, at >1333°C, the eutectic point between 3MgO⅐B2O3 and
MgO. The coexistence of the liquid and MgAl2O4 makes the
protective layer more dense, thus inhibiting oxidation of the
refractory. At >1333°C, the process apparently is controlled
by oxygen diffusion, whereas it is controlled by chemical
reaction when the temperature is <1333°C.
100 h. In addition, Al8B4C7 performed better than aluminum in
preventing carbon oxidation.
The present paper is intended to clarify the chemical process
and, hence, the mechanism by which Al8B4C7 works as an
antioxidant in MgO–C refractories.
II. Experimental Procedure
(1) Raw Materials
The raw materials used in the present study were carbon
(natural graphite, Ͼ99% purity), MgO (fused and sintered, Ͼ99%
purity), and Al8B4C7, prepared by firing a mixture of aluminum
(99.9% purity, Ϲ10 ␮m), B4C (99% purity, Ϲ10 ␮m) and graphite
(99.9% purity, Ϲ5 ␮m).
A mixture with the composition ratio of Al8B4C7 was wet-
mixed with ethanol, dried, and the process repeated three times.
The powders were uniaxial pressed at 73.5 MPa, to form a
compact measuring 20 mm ϫ 20 mm ϫ 8–10 mm, which was then
cold isostatically pressed at 100 MPa. The shaped compact was
dried and heated to 1650°C, at a rate of 10°C/min, for 2 h in
flowing argon, at a flow rate of 0.2 L/min, then crushed, shaped,
and reheated at the same temperature for 6 h. The X-ray diffraction
(XRD) pattern of the reheated sample is shown in Fig. 1. The fired
compact was crushed and ground, using a ball-mill, to a Ͻ50 ␮m
powder and used as an additive to refractory compositions.
I. Introduction
HE role of carbon as an important component of refractories is
undisputed. Carbon has excellent slag resistance, high thermal
T
conductivity, and a low thermal expansion coefficient; therefore, it
contributes to improving many key properties of refractory brick.
However, carbon has one main shortcoming: its susceptibility to
oxidation. To improve the oxidation resistance of carbon-
containing refractories and to maintain the positive effects of the
carbon for as long as possible, antioxidants, such as metals (Mg,
Al, Si), alloys (Mg–Al), and carbides (B4C, SiC), are often added
to the refractories. Aluminum is the most widely used antioxidant,
but it has some drawbacks, especially when in contact with carbon.
The formation of Al4C3 can be a problem, because that phase is
easily hydrated, even at room temperature. The hydration of Al4C3
will cause a large volume expansion and deterioration of the
material during reheating. This concern underlies the necessity for
developing new aluminum-containing antioxidants with enhanced
properties.
Carbides have been used as antioxidants because they yield
more C than do metals and alloys during oxidation.1 Al8B4C7
should be a successful antioxidant in carbon-containing refracto-
ries, because it has the composition 2Al4C3⅐B4C. Both Al4C3 and
B4C are carbides, and B4C has excellent antioxidation properties.
B4C has been proved able to accelerate the crystallization of
carbon from resin.2 The mechanism by which B4C acts as an
effective antioxidant in carbon-containing refractories has been
studied,3 and a boron-containing liquid phase has been found in the
firing process. This phase forms a dense layer near the surface of
the material and thus protects the carbon against oxygen ingress.
The hydration resistance and the effect of adding Al8B4C7
powder to carbon-containing refractories, compared with the
effects of using aluminum as additive and using no additive in
MgO–C refractories, have been investigated,4 and no hydration
was found with Al8B4C7 addition at 40°C and 90% humidity for
(2) Sample Preparation and Examination
(A) Behavior of Al8B4C7 Heated in CO(g): Approximately
1.0 g of Al8B4C7 powder was heated in an electric furnace, at a
rate of 10°C/min. Argon gas was flowed during heating, and CO
gas was flowed after the set temperature had been reached. The
flow rate of the gases was 0.2 L/min. The powder sample was
heated at various temperatures for 2 h and at 1500°C for 2–8 h.
The heated samples, cooled to room temperature at a rate of
10°C/min, were analyzed by XRD, and their microstructures were
observed by scanning electron microscopy (SEM).
(B) Oxidation Resistance of MgO–C Refractories at Various
Temperatures: MgO–C brick samples with the composition
shown in Table I were prepared using the above-listed raw
materials. Phenol resin (4 mass%) was used as a binder.
Samples measuring 20 mm ϫ 20 mm ϫ 20 mm were cut from
the brick samples and quenched at different temperatures for 3 h.
The average thickness of the decarbonized layers was determined
by measuring the side length of the black core (almost square at the
cross section) at two different positions.
Samples measuring 15 mm ϫ 15 mm ϫ 15 mm were heated in
graphite powder at various temperatures for 3 h, and the open
porosity of the samples was examined by mercury porosimetery.
Samples of the same size also were heated from room temperature
to 1500°C in air for 4 h at a heating rate of 10°C/min, and their
open porosity was examined by the same method.
(C) Stability of the MgO–Al2O3–B2O3 System: Al18B4O33
(30 wt%, obtained by heating Al8B4C7 at 1200°C for 6 h in air,
with XRD result verifying that the remnant was absolutely
Al18B4O33) and MgO (70 wt%, prepared by heating
4MgCO3⅐Mg(OH)2⅐5H2O at 900°C for 4 h) were well mixed and
A. Stett—contributing editor
Manuscript No. 189142. Received August 30, 1999; approved August 28, 2000.
*Member, American Ceramic Society.
577

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Journal of the American Ceramic Society—Wang and Yamaguchi
Vol. 84, No. 3
Al18B4O33 formed instead. At the same time, the intensities of
Al2O3 and carbon increased continuously, reaching a maximum at
1500°C after 4–8 h. Al18B4O33 increased sharply at first, at
1500°C in air, whereas Al2O3 decreased, but then reached a
maximum after 2 h, and Al18B4O33 almost disappeared.
Figure 3 shows the SEM photographs of Al8B4C7 powder (a)
unheated, (b) heated in CO gas at 1500°C for 8 h, (c) further heated
in air at 1500°C for 1 h, and (d) further heated in air at 1500°C for
2 h. Some pillarlike crystals appear after heating and are believed
to be Al18B4O33; the round shapes are Al2O3, and the irregular
shapes are thought to be carbon (see Fig. 3(b)). Both Al18B4O33
and Al2O3 are easily distinguished in Fig. 3(c), and a small amount
of Al18B4O33 appears in Fig. 3(d).
(2) Oxidation Resistance of MgO–C Refractories at Various
Temperatures
Fig. 1. XRD parttern of the Al8B4C7 powder.
The oxidation resistance of the MgO–C samples heated at
various temperatures both alone and with the addition of alumi-
num, Al ϩ B4C, and Al8B4C7 was studied to compare the effect of
the three additives and to understand the behavior of Al8B4C7 in a
MgO–C refractory. The thicknesses of the oxidized layers treated
at various temperatures for 3 h in air are compared in Fig. 4.
Figure 5 shows the open porosity of the MgO–C samples with
different additives after heating at various temperatures for 3 h in
carbon powder. For comparison, the open porosity of the same
samples after heating at 1500°C in air also was measured; those
results are given in Fig. 6. All of the samples were thoroughly
oxidized after 4 h.
Table I. Composition of the MgO–C Brick Samples
Composition (mass%)
1
2
3
4
MgO
C
Fused MgO
1–2 mm
60 60 60 60
20 20 20 20
Sintered MgO Ͻ65 ␮m
Graphite
180–300 ␮m
Ͻ180 ␮m
10 10 10 10
10 10 10 10
Al8B4C7
Al
Ͻ50 ␮m
Ͻ40 ␮m
Ͻ10 ␮m
4
4
2.52
0.64
B4C
(3) Stability of the MgO–Al2O3–B2O3 System
Figures 7 and 8 show the XRD results for the 70MgO ϩ
30Al18B4O33 (wt%) mixture heated at various temperatures for 2 h
and SEM photographs of the cross sections heated at various
temperatures for 2 h, respectively.
pressed under 73.5 MPa pressure. The samples were heated at
various temperatures for 2 h in air. The cross sections of the
samples were examined by SEM, and the samples then were
crushed and subjected to XRD.
IV. Discussion
III. Results
(1) Behavior of Al8B4C7 in CO Gas
The reaction between Al8B4C7 and CO can be described as
follows:
(1) Behavior of Al8B4C7 in CO(g)
Al8B4C7 powder was heated in CO gas at various temperatures
for 2 h and at 1500°C for 2–8 h. The product then was further
heated at 1500°C, for up to 2 h, in air. Phase changes detected in
the samples by XRD are shown in Fig. 2. Al2O3, Al18B4O33, and
C were first identified at Ͼ1100°C. The intensity of the Al2O3 and
carbon increased rapidly from 1100° to 1500°C, whereas Al8B4C7
decreased sharply. Al8B4C7 disappeared after 2 h at 1400°C, and
Al8B4C7͑s͒ ϩ 18CO͑g͒ ϭ 4Al2O3͑s͒ ϩ 2B2O3͑l͒ ϩ 25C͑s͒
(1)
Figure 9 shows the stability domains of the condensed phases in
the Al–B–C–O system. Because the thermodynamic data for
Al8B4C7 are not established, these data have been substituted by
data for Al4C3 and B4C. The partial pressure of the CO(g) inside
the carbon-containing refractories is taken as ϳ0.1 MPa under
oxidizing conditions.5 Figure 9 shows that Al2O3, B2O3, and
carbon are stable when PCO Ͼ 0.03 MPa at a temperature as high
as 1500°C and that Al2O3 reacts with B2O3 to form Al18B4O33.
As shown in Fig. 2, the obvious reaction occurs when the
temperature is Ͼ1200°C. Figure 9 also reveals that Al8B4C7 reacts
with CO to form Al18B4O33 in the following sequence:
Al8B4C7͑s͒ ϩ 12CO͑g͒ ϭ 4Al2O3͑s͒ ϩ B4C͑s͒ ϩ 18C͑s͒
(2)
B4C͑s͒ ϩ 6CO͑g͒ ϭ 2B2O3͑l͒ ϩ 7C͑s͒
9Al2O3͑s͒ ϩ 2B2O3͑l͒ ϭ Al18B4O33͑s͒
(3)
(4)
Although B2O3 exhibits a higher partial pressure at higher tem-
peratures, as shown in Fig. 10, and was not detected by XRD, as
illustrated in Fig. 2, either because it did not crystallize or because
too little was present, the present result proves that B2O3 does exist
in the reaction product of Al8B4C7 and CO(g). Little Al18B4O33
formed in the remnant. This result can be explained by the
formation of a large amount of carbon (35.4 mass%, in theory),
Fig. 2. Phase changes of Al8B4C7 powder heated at various temperatures
for 2 h and at 1500°C for 2 to 8 h in CO gas, and further heated at 1500°C
up to 2 h in air.

March 2001
Oxidation Protection of MgO–C Refractories by Means of Al8B4C7
579
Fig. 3. SEM photographs of Al8B4C7: (a) as-received, (b) heated at 1500°C for 8 h in CO gas, (c) further heated (b) at 1500°C for 1 h in air, and (d) further
heated (b) at 1500°C for 2 h in air.
Fig. 4. Thickness of decarbonized layers of the MgO–C samples heated
at different temperatures for 3 h in air.
Fig. 5. Open porosity of the MgO–C samples heated at different
temperatures, embedded in carbon powder.
which greatly decreased the chance contact between Al2O3 and
B2O3, therefore retarding the reaction between those materials, and
also the vaporization of B2O3. The vaporization of B2O3 became
rapid when carbon disappeared, which accelerated the decompo-
sition of Al18B4O33. Thus, little Al18B4O33 remained after 2 h at
1500°C.
Reaction (1) is achieved via the gases that formed in the system.
For example, Fig. 11 shows condensed stable phases and equilib-
rium partial pressures of the gases in the Al–B–C–O system at
1700 K. Many types of gases exist, but Al(g), B2O2(g), and
B2O3(g) have relatively higher pressures at the partial pressures of
CO at which Al8B4C7, Al2O3 ϩ B4C, and Al2O3 ϩ B2O3 are
stable, respectively, and therefore are considered as the main
gases. Thus, the whole process apparently proceeds in the follow-
ing manner. Al(g) evaporates at first:
Al8B4C7͑s͒ ϭ 8Al͑g͒ ϩ B4C͑s͒ ϩ 6C͑s͒
(5)
As Al(g) diffuses to the surrounding area, where the partial
pressure of CO(g) is high enough, it reacts with CO and condenses:
2Al͑g͒ ϩ 3CO͑g͒ ϭ Al2O3͑s͒ ϩ 3C͑s͒
(6)
When Al8B4C7 disappears, B4C(s) reacts with CO(g), to form
B2O2(g) and C(s):
B4C͑s͒ ϩ 4CO͑g͒ ϭ 2B2O2͑g͒ ϩ 5C͑s͒
(7)

580
Journal of the American Ceramic Society—Wang and Yamaguchi
Vol. 84, No. 3
Fig. 6. Open porosity of the MgO–C samples heated at 1500°C for 4 h in
air.
Fig. 7. XRD results of Al18B4O33 ϩ MgO mixture heated at different
temperatures for 2 h in air.
Fig. 8. SEM photographs of 30% Al18B4O33 ϩ 70% MgO mixture
heated at (a) 1000°, (b) 1200°, and (c) 1400°C for 2 h in air.
The formed B2O2(g) further reacts with CO(g), to form B2O3(l)
and condenses:
B2O2͑g͒ ϩ CO͑g͒ ϭ B2O3͑l͒ ϩ C͑s͒
(8)
Investigation of the use of B4C as an antioxidant in MgO–C
refractories indicates that the protective mechanism involves the
formation of a dense borate layer. Correspondingly, the mecha-
nism by which carbon oxidation is suppressed in MgO–C refrac-
tories by additives involves the formation of a dense layer near the
surface of the material. Therefore, the oxidation should be closely
related to the open porosity of the material and also the structure
of the pores. The comparison in Figs. 5 and 6 indicates that the
open porosity is far higher in the oxidized layer than in the black
core, which means that oxygen can pass through the oxidized layer
more easily. The controlling step in carbon oxidation is the
diffusion of oxygen through the surface of the black core.
Therefore, the open porosity of the samples embedded in carbon
powder is used to explain the effect of additives.
(2) Mechanism for the Suppression of Carbon Oxidation by
Al8B4C7
Thermodynamically and kinetically it has been proved that the
carbon oxidizes more rapidly at higher temperature. That the
oxidized layer was thicker at 1300°C than at 1400°C in the present
study indicates a different mechanism. Normally, aluminum is
used as the additive to suppress carbon oxidation in MgO–C
refractories. The mechanism by which aluminum acts involves
reducing CO(g) to C(s) and, therefore, controlling the decreasing
of carbon. The formed Al2O3(s) further reacts with MgO to yield
MgAl2O4 through gas phases, and the volume expansion that
accompanies this process, theoretically 240%, promotes the for-
mation of a dense protective layer near the surface of the
refractory.6
As mentioned, B2O3(l) forms during the oxidation of Al8B4C7.
When B2O3(g) comes in contact with MgO, it reacts with the MgO

March 2001
Oxidation Protection of MgO–C Refractories by Means of Al8B4C7
581
Fig. 9. Stability domains of the condensed phases in Al–B–C–O system.
Fig. 11. Change of condensed phases and equilibrium partial pressures of
the gases in Al–B–C–O system with CO(g) at 1700 K.
(3) Stability of the MgO–Al2O3–B2O3 System
The oxidation of Al8B4C7 results in Al2O3 and B2O3; thus, the
stability of the MgO–Al2O3–B2O3 system should be considered in
the oxidation of Al8B4C7-added MgO–C refractories. Because no
data are available for 3MgO⅐B2O3, the stability is impossible to
predict based on thermodynamic calculations. Actually, MgAl2O4
is the most stable phase in this system. The Gibbs free energy of
the following reaction is calculated as illustrated in Fig. 12:
Fig. 10. Vapor pressure of B2O3(g) over B2O3(l) at various temperatures.
9MgO(s) ϩ Al18B4O33(s) ϭ 9MgAl2O4͑s͒ ϩ 2B2O3(s)
(9)
The Gibbs free energy is Ͻ0, indicating that the reaction moves to
the right in the calculated temperature range. 3MgO⅐B2O3 forms
after the appearance of MgAl2O4. This result is also proved by
powder diffraction experiments. MgAl2O4 is first identified at
ϳ1000°C, but 3MgO⅐B2O3 is not seen until 1200°C.
to form 3MgO⅐B2O3, engendering a liquid phase when it coexists
with MgO at Ͼ1333°C7 (the eutectic point). The protective layer
becomes more dense than MgAl2O4 alone when the temperature
exceeds 1333°C because of the existence of the liquid, as demon-
strated in Fig. 5. The open porosity achieved with the addition of
Al8B4C7 is lower than that created with the addition of aluminum
and Al ϩ B4C at different temperatures. The gentle increase in the
porosity of the Al8B4C7-added sample from 1300° to 1400°C may
be attributable to the formation of a liquid phase. However, the
reaction between MgO and carbon becomes noticeable after
1500°C,8 and the formed 3MgO⅐B2O3 is unstable and reacts
rapidly with carbon at the same temperature.9 These reactions
make the open porosity of the sample higher at 1500°C.
The formed 3MgO⅐B2O3 neither decomposes nor evaporates at
Ͻ1400°C3 and thus greatly obstructs oxygen penetration. The
oxidized layer at 1300°C is slightly thicker than at 1400°C in the
present study, indicating that the controlling step of the process at
higher temperature is oxygen diffusion. Although the porosity is
higher at 1400°C than at 1300°C, the liquid forms a continuous
layer near the surface of the material and prevents the ingress of
oxygen. No liquid phase forms at temperatures Ͻ1333°C, the
composite is porous, and, therefore, the MgO–C sample is more
susceptible to oxidation at 1300°C. The decarbonized layer is
thinner at 1200°C than at 1300°C, proving that oxidation is
controlled by chemical reaction when temperature is below the
eutectic point of 3MgO⅐B2O3 and MgO.
Fig. 12. Standard Gibbs free energy versus temperature for the reaction
between MgO and Al18B4O33
.

582
Journal of the American Ceramic Society—Wang and Yamaguchi
Vol. 84, No. 3
Figures 7 and 8 show that the reaction between MgO and
(1) Al8B4C7 begins to react with CO gas at ϳ1100°C,
forming Al2O3(s), B2O3(l), and C(s). The Al2O3 reacts with MgO,
through gas phases, to form MgAl2O4 near the surface of the
material. The formed B2O3(l) evaporates and also reacts with
MgO, resulting in 3MgO⅐B2O3. When the temperature is
Ͼ1333°C, the liquid dispersed in MgAl2O4 makes the protective
layer more dense and, thus, inhibits oxidation.
Al18B4O33 starts at 1000°C, Al18B4O33 disappears at 1200°C, and
the formed MgAl2O4 and 3MgO⅐B2O3(l) appear at 1400°C.
MgAl2O4 disperses among the MgO grains, and 3MgO⅐B2O3 acts
as a bridge that connects the MgO grains. The combination of
MgAl2O4 and 3MgO⅐B2O3 is believed to make the composite
more dense.
(2) The formed liquid phase plays an important role in
antioxidation. When the temperature is Ͼ1333°C, the process
seems be controlled by the diffusion of oxygen; the process is
controlled by chemical reaction when the temperature is Ͻ1333°C.
(3) MgAl2O4 is the most stable phase in the MgO–Al2O3–
B2O3 system. 3MgO⅐B2O3 first forms at ϳ1200°C, after the
formation of MgAl2O4.
(4) Advantages of Al8B4C7 as an Antioxidant for MgO–C
Refractories
Besides its value for inhibiting the oxidation of MgO–C
refractories, as mentioned above, Al8B4C7 has the following
advantages:
The addition of aluminum, as mentioned already, may cause
hydration and deterioration of a material. Al8B4C7 has been
reported to have excellent hydration resistance, and its addition can
also effectively prevent a material from hydration. Al8B4C7 is
better in this respect than aluminum or aluminum-containing
additives used in MgO–C refractories.
Investigation of the use of B4C as an antioxidant in MgO–C
refractories revealed that the protective mechanism involves the
formation of a dense borate layer. Further study proved that this
process played an important role. However, the results also
showed that the 3MgO⅐B2O3 layer was not entirely impermeable.9
A more effective way was provided by adding Al ϩ B4C.10
As already mentioned, the presence of aluminum may result in
the formation of MgAl2O4. The formed 3MgO⅐B2O3(l) is dis-
persed in the spinel making the layer more dense. Al8B4C7 proves
its superiority over B4C in this regard. Al8B4C7 itself is a type of
carbide and the carbon remaining after a series of reactions can
effectively compensate for carbon loss in the material.
References
1A. Yamaguchi, “Progress of the Carbon Containing Refractories in Recent Years”
(in Jpn.), Inorg. Mater., 4 [3] 85–91 (1997).
2S. Zhang and A. Yamaguchi, “Effect of B4C on the Crystallization and Oxidation
Resistance of Carbon from Resin,” J. Ceram. Soc. Jpn., 102 [9] 830–34 (1994).
3T. Rymon-Lipinski, R. Fichtner, and T. Benecke, “Study of the Oxidation
Protection of MgO–C Refractories by Means of Boron Carbide,” Steel Res., 63 [11]
493–95 (1992).
4A. Yamaguchi, S. Zhang, J. Yu, and S. Hashimoto, “Behavior of Antioxidants
Added to Carbon-Containing Refractories,” UNITECR’95 Proc., 2, 341–48 (1995).
5A. Yamaguchi, “Control of Oxidation–Reduction Reactions in MgO–C Refracto-
ries” (in Jpn.), Taikabutsu, 35, 365–70 (1983).
6A. Yamaguchi, “Behaviors of SiC and Al Added to Carbon-Containing Refrac-
tories,” Taikabutsu Overseas, 4 [3] 14–18 (1984)
7R. S. Roth, T. Negas, and L. P. Cook; p. 100 in Phase Diagrams for Ceramists.
Edited by G. Smith. American Ceramic Society, Westerville, OH, 1981.
8S. Zhang and A. Yamaguchi, “Effect of CaO and Al2O3 in MgO Clinkers on
MgO–C Reactions,” J. Ceram. Soc. Jpn., 104 [2] 84–88 (1996).
V. Conclusions
9T. Rymon-Lipinski and P. Wolf, “Reaction Process in the Interior of an
MgO–Carbon Brick with Boron Carbide Addition,” Steel Res., 64 [2] 123–27 (1993).
10S. Palco, X. Li, and M. Rigaud, “Synergetic Effects of Boron-Based Antioxidant
Combined with Aluminum Additive in Magnesia–Carbon Refractories,” J. Can.
The behavior and mechanism of Al8B4C7 as an antioxidant in
MgO–C refractories were investigated. The corresponding conclu-
sions are as follows:
Ceram. Soc, 63 [4] 261–65 (1994).
Ⅺ