Materials Transactions, Vol. 57, No. 2 (2016) pp. 143 to 147
© 2016 The Japan Institute of Metals and Materials
Solubility Measurements of Fe2O3 and Cr2O3
in Fused Na2B4O7-B2O3 in Air at 1173 K+
Ryutaro Toki1, Takashi Doi1 and Nobuo Otsuka2
1
Advanced Technology Research Laboratories, Technical Research & Development Bureau,
Nippon Steel & Sumitomo Metal Corp., Amagasaki 660-0891, Japan
2
Nippon Steel & Sumikin Technology Co., Ltd., Amagasaki 660-0891, Japan
The solubility of Fe2O3 and Cr2O3 powder in a binary fused Na2B4O7-B2O3 salt system was measured at 1173 K in air. The melt was
continuously monitored via a basicity sensor with an oxide ion probe and a zirconia oxygen probe. The basicity of the fused salt was varied from
10.3 to 14.5 by adding B2O3 powder to Na2B4O7. For the entire range of the measured basicity, solubility of the Fe2O3 was one order of
magnitude higher than that of the Cr2O3. The solubility of both the Fe2O3 and the Cr2O3 decreased with an increase in the melt basicity. From the
solubility dependence on the melt basicity, both Fe2O3 and Cr2O3 are expected to dissolve via a basic dissolution reaction.
[doi:10.2320/matertrans.M2015275]
(Received July 6, 2015; Accepted November 11, 2015; Published January 18, 2016)
Keywords: basicity, Na2B4O7-B2O3, solubility, metal oxides
1.
Introduction
Borate glass compounds are used in a wide range of metal
production processes, as cleaning agents for oxide removal
from metal surfaces,1) and as bath salts for high temperature
boronizing of steel surfaces.2) However, thus far, information
on the dissolution reaction of metal oxides in the binary fused
Na2O-B2O3 salt systems, such as Na2B4O7 (Na2O-2B2O3 has
the melting point of about 1018 K3)), is not sufficient.
Activity of O2¹ (equivalent for the basicity) of the fused
salts is one of the dominant influencing factors in dissolution
reactions of metal oxides.4) In the range of high activity of
O2¹ in fused salts, metal oxides are dissolved via a basic
dissolution reaction, and in the range of low activity of O2¹,
an acidic dissolution reaction dominates in melts.
Dissolution of metal oxides in molten Na2SO4 is well
known as an example of reactions of metal oxides with fused
salts. To clarify the dissolution behavior of protective oxide
scales in molten sulfate film under the corrosion environment
of gas turbine blades and vanes, the dissolution behavior of
metal oxides such as Fe2O3 and Cr2O3 in molten Na2SO4 was
investigated.4,5)
For the sodium borate salt, Itoh et al.6) measured its
basicity by the following cell arrangement using ¢-alumina as
the solid state sodium cation conductor at around 1123 K.
ðÞ Ptj0:05Na2 O-0:95B2 O3 ðReference meltÞj jð1 xÞNa2 O-xB2 O3 ðSample meltÞjPt ð+Þ ðx ¼ 0:667 0:999Þ
"
"
"
¢-alumina
electrode
½A
Na !
Here, the basicity B of the melt is defined by the Na2O
activity (aNa2 O ) of the melt as follows.
B ¼ log aNa2 O
ð1Þ
The Na2O activity corresponds to the O2¹ activity in the
melt because Na2O dissociates into Na+ and O2¹ in the
melt.4) In the case of the binary Na2O-B2O3 melt, dissociation
reactions indicated in eqs. (2) and (3) can be expected, and
changes in the basicity in the melt and the structure of B2O3
molecule as a function of the mole fraction of Na2O were
reported.6­8)
þ
Na2 O ¼ 2Na þ O
B2 O3 þ O
+
electrode
þ
2
¼
2
ð2Þ
2BO
2
ð3Þ
This Paper was Originally Published in Japanese in J. Japan Inst. Met.
Mater. 78 (2014) 395­399. In order to more precisely explain the
background, the experimental procedures, and the results, some parts of
the contents were revised. The Keywords, the sentences of abstract, the
eqs of (15)­(21), and the titles of Figs. 2­5 were slightly modified. The
schematic representation of the ceramic cement was added in Fig. 1.
½B
In this study, the electrochemical electro motive force
(emf ) measuring cell used to measure the basicity of sodium
sulfate melt5) was tested whether the basicity of the Na2B4O7B2O3 melt can be measured at 1173 K. And the results were
compared with the previous study.6) Then, correlation
between the basicity and the solubility of the relevant oxides
in the Na2B4O7-B2O3 melt was investigated.
2.
Experimental
2.1 Basicity sensor
The schematic representation of the basicity sensor is
shown in Fig. 1. It comprised two types of sensors which
were taken from the one used successfully for the Na2SO4
melt.4,5,9) Details of these sensors are explained as follows.
Mullite,4,5,9) ¢-alumina,6) and quartz10,11) are well known as
the sodium ion electrolyte, and they were used to measure
the sodium potentials of the melts at high temperatures.
However, it is difficult to use ¢-alumina in the Na2O-B2O3
melts because it significantly dissolved into the Na2O-B2O3
144
R. Toki, T. Doi and N. Otsuka
melts.7) Similarly, we found that mullite (Nikkato, NC Tammann tube) dissolved as well to the Na2B4O7 melt at 1323 K. For
these reasons, a quartz glass was tested in this melt.
The arrangement of the cell I for the oxide ion sensor
ðÞ AgjNa2 SO4 -10 mol%Ag2 SO4 j jNa2 B4 O7 -B2 O3 jPt ðþÞ, Air
"
"
"
electrode
½A
quartz
Naþ !
It comprised one-end closed quartz glass tube (Eiko, OVISIL,
outside diameter: 10 mm, inside diameter: 7 mm, length:
200 mm). As shown in Fig. 1, 0.4 g mixture of Na2SO4
(Wako Pure Chemical Industries, Wako 1st Grade) ¹10
mol%Ag2SO4 (Merck, 99.5 mass% pure) was placed at the
bottom of the tube which contacted a pure Ag wire (Nilaco,
99.99 mass% pure, diameter of 2 mm) welded with a
platinum lead (Nilaco, 99.98 mass% pure, diameter of 0.3
mm). The open side of the quartz tube was sealed with the
ceramic cement (Asahi Chemical Industry, S-10A).
The electrochemical reactions of the electrodes [A] and [B]
are given as follows.
Electrode ½A Agþ þ e ¼ Ag
ð4Þ
1
Electrode ½B O2 þ 2e ¼ O2
ð5Þ
2
The electrochemical reactions of both electrodes can be
represented as follows.
Electrode ½A Ag2 SO4 þ 2e ¼ 2Ag þ SO2
ð6Þ
4
1
Electrode ½B 2Naþ þ O2 þ 2e ¼ Na2 O
ð7Þ
2
From eqs. (6) and (7), the overall cell reaction can be
described, shown in eq. (8).
Na2 SO4 þ 2Ag þ
1
O2 ¼ Na2 O þ Ag2 SO4
2
ð8Þ
Designating Na2SO4 as component 1, Ag2SO4 as component
2, and Na2O as component 3 (1 : Na2SO4, 2 : Ag2SO4,
V1
V2
Pt wire
Ceramic cement
ZrO2 tube
Quartz tube
Oxide ion sensor
(Cell 䠥)
Fig. 1
3 : Na2O), the Gibbs free energy change of eq. (8), ¦G, is
written as
G ¼ ½Go3 þ Go2 Go1 RT lnða1 =a2 Þ
þ RT lnða3 =PO2 ½B1=2 Þ
Pt paste
Oxygen sensor
(Cell 䠥䠥)
Schematic representation of the basicity sensor.
ð9Þ
Where R is the gas constant, T is the absolute temperature, ai
is the activity of component i, Goi is their standard free
energy of formation, and PO2 ½B is the oxygen partial
pressure at electrode [B]. Assuming regular solution behavior
for the binary Na2SO4-Ag2SO4 melt, a1 =a2 can be calculated
using the interaction coefficient ¡ of the Na2SO4-Ag2SO4
melt at 1173 K.10) Here, £ i is the activity coefficient of
component i, and Ni is its mole fraction.
a1 =a2 ¼ £ 1 N1 =£ 2 N2
¼ ðN1 =N2 Þð£ 1 =£ 2 Þ
¼ ðN1 =N2 Þ exp½¡ðN22 N12 Þ=ðRT Þ
* £ 1 =£ 2 ¼
expb¡ðN22
N12 Þ=ðRT Þc
ð10Þ
ð11Þ
By substituting eq. (10) into eq. (9) and using values of the
standard free energy of formation of Na2SO4, Ag2SO4, and
Na2O at 1173 K,12) emf V1 of the cell I can be obtained
according to eq. (12).
V1 ¼ G=ð2FÞ
aNa2 O
¼ 1:448 0:116 log
ðPO2 ½BÞ1=2
ðat 1173 K; V1 in VÞ
ð12Þ
Here, F is the Faraday constant.
Stabilized zirconia, such as Y2O3-ZrO2, MgO-ZrO2, and
CaO-ZrO2 have often been employed for an oxygen sensor.
The cell II arrangement for measuring the oxygen partial
pressure was
ðÞ Ptjair ðPO2 ¼ 0:21 atmÞj jNa2 B4 O7 -B2 O3 jPt ð+Þ
"
"
"
electrode
ZrO2
electrode
½A
O !
½B
As shown in Fig. 1, a platinum wire (diameter: 0.3 mm) was
attached to the inside surface of the bottom of the stabilized
zirconia tube (Nikkato, ZR-9M, The outside diameter of
10 mm and the inside diameter of 7 mm. The length was
200 mm) with platinum paste (Tanaka Kikinzoku Kogyo, TR7907). The tube was heated at 1173 K for 3.6 ks before the
measurements to stabilize the paste.
From the Nernst equation, emf V2 of the cell II can be
expressed by the following eq. (13).
RT
0:21
ln
V2 ¼
ð13Þ
4F
PO2 ½B
2
Ag
Na2SO4
-10mol%Ag2SO4
electrode
½B
Solubility Measurements of Fe2O3 and Cr2O3 in Fused Na2B4O7-B2O3 in Air at 1173 K
Oxide ion sensor
Pt wire
Table 2 Salt compositions used for oxide solubility measurement at 1173 K
in air.
Oxygen sensor
Thermocouple
Insula ng tube
The amount of B2O3
added to the melt.
Oxide
Molten
powder Na2B4O7-B2O3
Fig. 2 Experimental setup of melt basicity and oxide solubility measurements in Na2B4O7-B2O3 melt.
Salt compositions used for the basicity measurement at 1173 K in
Salt No.
Na2B4O7
1
100
0
2
80
20
3
50
50
4
30
70
B2O3
(in mass%)
Therefore, the basicity of the melt can be measured from the
values of V1 and V2.
2.2 Experimental setup
The experimental setup used in this study is shown in
Fig. 2. Table 1 shows salt compositions used for the basicity
measurement. For each measurement, 15 g of the mixed salt
was placed in a platinum crucible and was heated at 1173 K
in air. From the Na2O-B2O3 phase stability diagram,3) the
tested salts are considered to have been completely melted at
1173 K in the range of the salt composition employed in this
study. Both V1 and V2 were measured by the potentiostat
(Hokuto Denko, HA-151) and were recorded by the data
logger (KEYENCE, NR-500, NR-HA08). Temperature of the
platinum crucible was measured by R-type thermocouple
with a wire diameter of 0.2 mm. To secure the chemical
equilibrium of the salt mixture, measurements were done
after heating it at 1173 K in air for more than 25.2 ks. The emf
measurement was then performed.
In order to measure the solubility of Fe2O3 and Cr2O3 in
the Na2B4O7-B2O3 melts, salts and oxide powders were
placed in a platinum crucible. 6 g of Fe2O3 powder (Strem
Chemicals, 99.8 mass% pure) or 5 g of Cr2O3 powder (Wako
Pure Chemical Industries, Wako 1st Grade) were placed at
the bottom of the crucible filled with 20 g of Na2B4O7. The
melts were heated at 1173 K in air for more than 86.4 ks to
obtain thermal equilibrium. Basicity of the melts was
measured by the basicity sensor, and a coil of platinum wire
Total amount
of B2O3
Salt no.
Na2B4O7
1
20
0
0
2
20
2.2
2.2
3
20
2.8
5.0
4
20
3.6
8.6
5
6
20
20
4.7
6.7
13.3
20.0
Pt crucible
Table 1
air.
145
(in gram)
(wire diameter: 0.3 mm, coil diameter: 3 mm) was immersed
and subsequently pulled out of the melt to quench the salt
sample.
To change the basicity of the melt, a small amount of B2O3
powder (Alfa Aesar, 99 mass% pure) was added to the
Na2B4O7 (Wako Pure Chemical Industries, anhydrous) melt
several times. This is shown in Table 2. First, the basicity of
20 g of Na2B4O7 melt was measured and its quenched sample
was obtained. Then, 2.2 g of B2O3 was added to the melt, and
the melt was heated at 1173 K for more than 86.4 ks.
Quenching a small amount of the salt sample followed
thereafter. This procedure was repeated five times.
To determine the quantity of soluble Fe or Cr in the melt,
the quenched samples were analyzed by ICP-AES (Thermo
Scientific, iCAP6300). The mass of the soluble Fe2O3 and
Cr2O3 in the solvent melts, wFe2 O3 or wCr2 O3 was obtained.
Metal cations were assumed to have been homogeneously
distributed in the entire melt. The solubility of the metal
oxides in the melts, W was determined by the eq. (14).
WðppmÞ ¼
wi
106 ði : Fe2 O3 or Cr2 O3 Þ
ðwmelt wi Þ
ð14Þ
Here, wmelt is the mass of the quenched samples of the melts
in which Fe2O3 or Cr2O3 is dissolved.
3.
Results and Discussions
The basicity changes of the Na2B4O7-B2O3 melts at 1173 K
in air is shown in Fig. 3 as a function of the B2O3 ratio of the
melt. Test results reported by Itoh et al.6) were superimposed.
The abscissa represents the mass fraction of B2O3 of the melt,
XB2 O3 , and the ordinate is the measured basicity of the melt.
Clearly, the basicity of the melt increased with increasing
XB2 O3 . Decrease in the O2¹ activity upon addition of B2O3 to
the melt is considered to have been taken place according to
eq. (3). The measured basicity was in good agreement with
the literature,6) which was derived by a different experimental
setup from this study. The present sensor can be employed to
measure the basicity of the Na2B4O7-B2O3 melt at 1173 K in
air.
The solubility of Fe2O3 and Cr2O3 in the Na2B4O7-B2O3
melt in air at 1173 K is shown in Fig. 4 as a function of the
melt basicity. For the entire range of the tested basicity, the
solubility of Fe2O3 was one order of magnitude higher than
that of Cr2O3. The solubility of both oxides decreased with
increasing the basicity of the melt.
146
R. Toki, T. Doi and N. Otsuka
16
Immersion Time Immersion Time Immersion Time
0.6 ks
0.6 ks
0.3 ks
(a)
(b)
(c)
This work
Itoh et.al ⁶⁾
15
Basicity, B
14
13
12
10 mm
Fe
11
Fe
Cr
Fig. 5 Appearance of specimens immersed in pure Na2B4O7 melt at
1173 K in air for 0.3 ks and 0.6 ks.
10
9
0
Na2B4O7
20
40
60
80
1
log aNaFeO2 ¼ ð log aNa2 O þ C1 Þ;
2
C1 ¼ log K1 log aFe2 O3
1
log aNaCrO2 ¼ ð log aNa2 O þ C2 Þ;
2
C2 ¼ log K2 log aCr2 O3
B2O3 ra o, XB2O3 (mass%)
Fig. 3 The measured basicity of the Na2B4O7-B2O3 melt as a function
of B2O3 ratio of the melt at 1173 K in air. Literature6) data were
superimposed.
Solubility, log{W (ppm)}
Fe2O3
Cr2O3
-1/2
4
-1/2
3
T=1173 K, PO2 =0.21 atm
2
10
11
12
13
14
15
Basicity, B
Fig. 4 Solubility of Fe2O3 and Cr2O3 in mass ppm in the Na2B4O7-B2O3
melt at 1173 K in air.
From the gradient of the changes in the solubility of Fe2O3
and Cr2O3 with respect to the melt basicity in Fig. 4, Fe2O3
and Cr2O3 are expected to dissolve via a basic dissolution
reaction in the basicity range of 12­14.5 for Fe2O3 and 11.5­
13 for Cr2O3. Basic dissolution reactions of both oxides are
considered as follows5).
Fe2 O3 þ Na2 O ! 2NaFeO2
ð15Þ
Cr2 O3 þ Na2 O ! 2NaCrO2
ð16Þ
Here, the equilibrium constants of eqs. (15) and (16), i.e., K1
and K2, can be written as eqs. (17) and (18).
ðaNaFeO2 Þ2
K1 ¼
ð17Þ
aFe2 O3 aNa2 O
ðaNaCrO2 Þ2
ð18Þ
aCr2 O3 aNa2 O
Taking the common logarithm of both equations, eqs. (17)
and (18) can be expressed as follows.
K2 ¼
ð20Þ
Partial differentiation of eqs. (19) and (20) with respect to
log aNa2 O , yields the following equation.
@ log aNaFeO2
@ log aNaCrO2
1
ð21Þ
¼
¼
2
@ð log aNa2 O Þ
@ð log aNa2 O Þ
6
5
ð19Þ
Experimentally, the slopes of log W vs. basicity lines for the
solutes of NaFeO2 and NaCrO2 in Fig. 4 were 1/2, which
agrees well with eq. (21). This indicates that both Fe2O3 and
Cr2O3 are expected to dissolve via the basic dissolution
reaction represented in eqs. (15) and (16).
As presented above, the solubility of Fe2O3 was one order
of magnitude higher than that of Cr2O3 in the binary fused
Na2B4O7-B2O3 salt system. This suggests that metal Cr can
have better resistance to hot corrosion in binary fused
Na2B4O7-B2O3 salts than metal Fe in air. To clarify this point,
the immersion test was conducted using sheet specimens of
pure Fe and pure Cr. Specimens were immersed in the pure
Na2B4O7 melt at a depth of 5 mm at 1173 K in air for 0.6 ks.
Specimen appearances after the test are shown in Fig. 5.
The Fe specimen dissolved significantly at the gas-liquid
interface of the melt for the 0.3 ks and disappeared upon
reaction for 0.6 ks. In contrast, the Cr specimen showed much
better corrosion resistance than the Fe specimen. Iron oxide
scale seems to have failed to function as a protective layer in
the Na2B4O7 melt because the solubility of Fe2O3 in the melt
is high. In case of Cr, oxide scale seems to have imparted the
total protection to separate metal surface from the melt, since
the solubility of Cr2O3 was one order of magnitude less than
Fe2O3. Thus, it can be accepted that pure Cr specimen
resisted much greater than pure Fe specimen.
4.
Conclusion
The basicity of the Na2B4O7-B2O3 melt was measured by
using two sets of solid state cation conductor in air at 1173 K.
Solubility of Fe2O3 and Cr2O3 powder in the binary fused
Na2B4O7-B2O3 salt was measured at 1173 K in air as a
function of the basicity of melts.
Solubility Measurements of Fe2O3 and Cr2O3 in Fused Na2B4O7-B2O3 in Air at 1173 K
(1) The basicity sensor of quartz oxide ion sensor and
zirconia oxygen sensor can be employed for measuring
the basicity of the binary fused Na2B4O7-B2O3 salts at
1173 K in air.
(2) In the binary fused Na2B4O7-B2O3 salts, the solubility
of Fe2O3 was one order of magnitude higher than that of
Cr2O3 for the basicity range between 10 and 13. The
solubility of both Fe2O3 and Cr2O3 decreased with
increasing the basicity of the melt of 12­14.5. From the
oxide solubility dependence on the melt basicity, both
Fe2O3 and Cr2O3 are expected to dissolve via a basic
dissolution reaction.
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