CORROSION OF POTLINING REFRACTORIES: A COMPARISON
BETWEEN DIFFERENT CELL LINING DESIGNS USING
A UNIFIED APPROACH
R. Pelletier and C. Allaire
CIREP-CRNF
Dept. Of Eng. Physics & Materials Eng.
Ecole Polytechnique (CRIQ campus)
8475 Christophe-Colomb Street
Montreal, Quebec, Canada, H2M 2N9
ABSTRACT
The corrosion chemistry of aluminosilicate potlining refractories has been
studied from post-mortem analysis according to a unified approach that considers
both metallic sodium and molten bath has the corrosive agents. The results show
that the cell lining design has a great influence on the type of corrosion involved.
Cells containing semi- graphitized cathode blocks are more prone, with time, to the
attack of their refractory potlining by the action of the molten bath. Unlike this
case, the use of amorphous blocks in such cells promotes the corrosion of their
potlining refractories mostly by the action of the metallic sodium, irrespectively of
the cell’s age.
INTRODUCTION
In the past, two corrosive agents have been identified as the main responsible
for aluminosilicate corrosion in potlining: liquid sodium fluoride and metallic
sodium. Two distinct theoretical approaches have been developed to interpret
laboratories and post-mortem observations. However, each approach can only
explain a fraction of the wide range of observations made in used potlinings.
These two approaches have recently been unified, resulting in a better
understanding of real life conditions. The theoretical strategy that has permit to
reach such unification has been presented elsewhere [1] and is resumed in the
following.
UNIFIED APPROACH
The following development was inspired from the work of Schøning et al. [2].
In the approach that proposes molten NaF as the corrosive agent, the reaction
pattern is:
NaF + [aluminosilicate refractory]
Na3 AlF 6 + [Al-Si-Na-O] compounds (1)
Basically, NaF attacks alumina to produce cryolite and Na2 O, which combines
with the remaining refractory. In the other approach, the reaction pattern is:
Na(g) + [atm] + [aluminosilicate refractory]
[Al-Si-Na-O] compounds
(2)
The sodium reacts with the atmosphere (gaseous oxygen or carbon monoxide)
and/or with the silica contained in the refractory to produce Na2 O, which reacts
with the remaining refractory. The reaction with the atmosphere should be
favored if the gaseous oxygen renewal in the lining is fast. The availability of
gaseous oxygen depends on the lining permeability and on the gas tightness of the
cell. In the unified approach, it was assumed that all the sodium is oxidized by the
atmosphere. Under this assumption, the reaction pattern becomes:
Na2 O + [aluminosilicate refractory]
[Al-Si-Na-O compounds]
(3)
The reaction patterns presented in equations (1) and (3) are linked by the
following basic reaction:
3Na2 O + 2AlF 3
6NaF + Al2 O3
(4)
Combining reaction patterns (1) and (3) with the help of the reaction (4) results
in the following unified corrosion pattern:
Na2 O + NaF + AlF 3 + [aluminosilicate refractory]
[Na-Al-F compounds] + [Al-Si-Na-O compounds]
(5)
A good mean of visualising the predictions resulting from this corrosion pattern
is by drawing what could be called "corrosion maps".
CORROSION MAPS
Corrosion maps are designed to visualize the evolution of the mineral
composition of refractories during the corrosion process. Fundamentally, they are
projected views of coexistence diagrams along certain paths. Figure 1 shows the
trigonal composition prism proposed by Rutlin and Grande [3], which summarizes
the coexistence lines of the system Na6 F6-Si3/2 F6-Al2 F6 -Na6O3-Si3/2 O3-Al2 O3 . In
this diagram, all the aluminosilicate refractories lie on the Si3/2 O3 -Al2 O3 line,
which is the silica-alumina line.
Si 3/2 O 3
Na2 Si2 O 5
Na2 SiO3
NaAlSi 3 O8
NaAlSiO4
Na4 SiO4
Al 6Si 2 O 13
NaAl 11 O17
Al 2O 3
NaAlO2
Na6O3
Si3/2F6
Na2 SiF6
Al 2F6
Na3 AlF6
Na5 Al3F14
Na 6F 6
Figure 1: Coexistence lines in the quaternary reciprocal system Na2 O-SiO 2 -Al2 O3 NaF-SiF 4 -AlF 3 at subsolidus temperature proposed by Rutlin and Grande [3].
During the corrosion process, at least those described by the corrosion pattern
(5), the mineral composition of a refractory shifts from a point on this line (initial
composition) to another located in the Na6 O3-Na6 F6-Al2 F6 plane (ultimate
composition). The exact location of the latter point depends on the proportion of
Na2 O, NaF and AlF 3 in the mixture corroding the refractory. These two points
delimit a line corresponding to a corrosion path. Each path is different because it
passes through a certain number of domains at specific proportions of refractory
and corrosive agents. Note that in coexistence diagrams such as the one shown in
figure 1, the domains are bounded by four triangular planes that are each delimited
by three coexistence lines. Because all the paths that converge to the same
ultimate composition form a plane, they can be represented on a single corrosion
map. Therefore, a map shows the behavior of all the aluminosilicate refractories
in a specific corrosion condition. With only a few maps, it is possible to predict
the corrosion path of any aluminosilicate refractory in all the conditions taken into
account by the corrosion pattern (equation (5)). To draw these maps, all the
coexistence domains have to be expressed as mathematical functions.
Reference [1] presents an example of how each boundary on the corrosion map
is calculated, starting from the following equation as the reactive side of the
corrosion pattern:
Z
Y
Y
X
(100− X)
⋅ Na2 O +
⋅ NaF +
⋅ AlF3 +
⋅ SiO2 +
⋅ Al2 O3 ⇒
MNa2O
MNaF
MAlF3 ⋅ R B
MSiO2
MAl2O3
(6)
In this equation, X, Y and Z are the number of grams of each chemical, the Mi's
are their molecular weights and RB is the NaF/AlF 3 mass ratio of the initial
fluorides melt. This equation remains the same for all the domains. For the
products side, we have to go systematically through all the existing triangular
planes inside the trigonal prism of the coexistence diagram shown in Figure 1.
The three compounds delimiting each plane are used as reaction products to
complete the reaction.
One more equation is needed to solve the system. It is the following:
R Na =
2⋅ Z
M Na 2 O
 2 ⋅Z
Y 

+
 M Na O M NaF 

2

(7)
The parameter RNa is defined as the molar fraction of the sodium introduced as
oxide, with respect to all sources of sodium (Na2 O and NaF).
Many combinations of parameters can be used as axes when drawing corrosion
maps. A good choice is the weight percent of corrosive agents, i.e. Na3 AlF 6 + NaF
+ Na2 O, introduced (Y-axis) as a function of the silica content of the refractory (X-
axis). This choice for the Y-axis is particularly useful in an engineering point of
view. Since the quantity of corrosive agents under carbon blocks increases as the
potlining ages, the Y-axis can be seen as time. For a similar reason, it can also be
seen as the vertical position into the refractory lining.
Depending on the corrosion conditions involved, the number of domains visible
and their shape vary significantly. Two important parameters control the number
and the shape of domains: RNa and the bath ratio, RB. The parameter RNa has a
much more dramatic effect on the corrosion maps. Figure 2 shows the corrosion
maps obtained with RNa = 0 or 1, at RB = 3.2 (in wt.). On such maps, the thin lines
represent equilibria involving only cryolite as fluoride, while the thick lines
correspond to equilibria involving only villiaumite (NaF) as fluoride. Therefore,
when a domain is surrounded by only one type of line, the assemblage contains
only one fluoride, either cryolite or villiaumite.
Consequently, by varying RNa from 0 to 1, a very wide range of conditions can
be covered with the proposed unified corrosion pattern. This shows that the two
previously antagonist approaches constitute in fact the limiting cases of a more
general corrosion pattern.
DRY CELLS VERSUS WET CELLS
It was previously shown [1] that the parameter RNa is useful to differentiate
between two major types of cells, i.e., wet cells or dry cells, with respect to the
unified approach predictions. Wet cells are those in which the potlining
refractories are ultimately corroded mostly by the action of molten bath. In such a
case, the presence of both cryolite (Na3 AlF 6 ) and villiaumite (NaF) in the
corrosion products is predicted. Unlike this case, the potlining refractories in dry
cells are mostly corroded by the action of metallic sodium, such that ultimately
cryolite is not predicted to coexist inside the corrosion products. The maximum
value of RNa leading to the presence of coexisting cryolite in the corrosion
products, irrespectively of the corrosive agents content is:
RNa** = 3/(2RB + 3)
(8)
Thus, cells having their RNa value higher or lower than RNa** can be defined as dry
or wet cells, respectively.
POST-MORTEM ANALYSIS
Based on the above-mentioned unified approach, corrosion maps were obtained
corrosion agents [wt%]
100
R Na = 0.0
R B = 3.2
80
NAS2
60
NAS 6
40
β-A
20
A
A3S 2
S
0
0
20
40
60
80
SiO2 in the refractory [wt%]
100
corrosion agents [wt%]
(a)
100
RN a = 1.0
RB = 3.2
80
N 2S
N3 S2
60
NS
NA
40
NS2
20
β -A
A
0
0
NAS2
A3 S2
NAS6
20
40
60
80
SiO2 in the refractory [wt%]
S
100
(b)
Figure 2: Corrosion maps obtained for (a) RNa = 0 and (b) RNa = 1, at RB = 3.2 (in
wt.).
from corroded refractories taken from the potlining of two AP-30 cells. Both cells
were using the same 130 mm thick insulating refractory material underneath a 130
mm thick protective refractory and a bedding mix. The original characteristics of
these materials are given in Table I. Although the protective refractories were
different in both cells, they were involving similar compositions, as shown in
Table I. The major difference between the two cells was the type of cathode blocks
they were using. Cell “A” was lined with amorphous cathode blocks while cell
“B” had semi- graphitized cathode blocks. The properties of these blocks are given
in table I.
Table I: Characteristics of the cells potlining materials (from the manufacturers)
Materials
Cell
Chemical
composition
[wt. %]
SiO2
Al2 O3
Fe2 O3
MgO
CaO
Na2 O
K2 O
TiO2
Al2 O3 +TiO2
Bulk density
[g/cm3 ]
Porosity [%]
Amorphous
SemiBedding
blocks
graphitized
mix
blocks
A
B
A and B
Protective refractory
Insulation
A
B
A and B
60 - 64
---------------------29 - 33
1.97 – 2.08
74
8.2
6.5
1.2
5.0
0.4
1.5
0.7
---0.57
20 - 25 b
76 b
---------------------------1.54
---------------------------1.63
0.01
99.5
0.01
0.01
0.04
0.39
------1.15
60 - 65
26 - 32
5-7
0.5 – 1.0
0.5 – 1.0
------1-2
27 - 34
1.84 – 1.92
16 - 18 a
26 a
----
----
a : Total porosity
b: Apparent porosity
The refractory core samples were taken in both cells on the central longitudinal
axis. They were then submitted to semi-quantitative XRD and XRF analyses. Cells
“A” and “B” were autopsied after 1361 and 1710 days, respectively. Both cells
were operating with a bath ratio (RB) of about 1.1 (in wt.).
To obtain the corrosion map associated to an analyzed sample, its RNa value has
to be determined. To achieve this, the mixture of oxides in the corrosion products
has to be considered as a single non- stoechiometric compound. In such a case, the
products side of reaction (5) becomes:
⇒
A
A
⋅ NaF +
⋅ AlF 3 + B ⋅ NaAl a Si b O (3 a + 4 b +1 ) 2
M NaF
M AlF3 ⋅ R F
(9)
In equation (9), a and b are respectively the Al/Na and Si/Na molar ratios of the
oxides found in the analyzed sample. The parameter RF is the NaF/AlF 3 mass
ratio of the fluorides also found in the corroded sample. Using the same technique
than the one used to build the corrosion maps, it is possible to obtain a
mathematical function relating the chemical composition of the corroded sample
(X*) with the corrosion conditions. This function is the following:
X* =
where:
100 ⋅ (2 b ⋅ H)
(10)
 M Al O 

2 3 

⋅
{
a
⋅
H
+
(
R
−
R
)
}
+
2
b
⋅
H


B
F
 M SiO2 

R R

H =  B Na ⋅ (2 R F + 3) + 3 ⋅ (R B − R F )
 (1 − R Na )

(11)
Reorganizing equation (10) allows to obtain the sodium ratio, R* Na, necessary so
that X* = X°, where X° is the uncorroded refractory composition. The expression
for R* Na is the following:
(12)
 *  M Al2 O3 

*
 ⋅ (3a + 1) − 100 − X ⋅ 6b
(R B − R F ) ⋅ X ⋅ 

M


 SiO2 
*

R Na =
 *  M Al2O3 

*


X
⋅
⋅
[
(
R
−
R
)
−
a
⋅
R
⋅
(
2
R
+
3
)
]
+
100
−
X
⋅
2
b
⋅
R
⋅
(
2
R
+
3
)


B
F
F
B
F
B
 M SiO 



2 
(
(
)
)
Equation (12) was used to calculate the RNa* value of each corroded sample.
This allowed to build a corrosion map specific to each sample. Then, each sample
was located on its corresponding map. The initial (uncorroded) composition of a
sample (Xo ) was taken as its X-coordinate, while its Y-coordinate, identified as
Y*, was calculated using the following equation:
Y* =
where:
Z=
[Z + Y ⋅ (1 +1 RB )]
[Z + Y ⋅ (1 +1 RB ) +100]
M Na 2 O
2 ⋅ M NaF
⋅
R Na
⋅Y
(1 − R Na )
(13)
(14)
and:
 MAlF3 ⋅ R B ⋅ X* 

 ⋅ (2 ⋅ R F + 3)
 MSiO ⋅ b 

2

Y=
*
 2 ⋅ R B ⋅ R Na

⋅ (2 ⋅ R F + 3) + 6 ⋅ (R B − R F )

*
 (1− R Na )

(15)
RESULTS AND DISCUSSION
The designation and location of the analyzed samples taken from the two
autopsied cells are given in Table II. Their XRD and XRF analyses are presented
in Tables II and III, respectively. The corrosion maps of the samples taken from
cells "A" and "B" are shown on Figure 3 and 4, respectively. As shown in Table II,
there is a good correlation between the predicted minerals in the corroded samples
and those identified by XRD. However, in some samples, the presence of
compounds such as Na-Al-Si-O, Na-Ca-Al-Si-O and Na-K-Al-Si-O were detected
by XRD instead of pure Nepheline (NaAlSiO 4 ). As expected, nor Albite
(NaAlSi3 O8 ) or sodium disilicate (NaSi2 O5 ) were detected by XRD since these
minerals are most often present as amorphous phases in corroded refractories.
An important result concerns the values of RNa and RF obtained from the
corroded samples of both cells. As shown in Table II, the RNa value for cell “A”
lies between 0.85 and 0.90, while the RF value is infinite in all cases. This suggests
that cell "A" can be classified as a dry cell. Since RB for both cells is 1.1 (in wt.),
their corresponding RNa** value is 0.58 (see equation (8)). The latter is in fact
much lower than the three above RNa values. Unlike in cell “A”, the RNa values
obtained for samples B-1 and B-2 (0.25 and 0.30, respectively) in cell B are much
lower than RNa** . The RF values for these two samples are also much lower than in
cell “A”, i.e., 1.9 and 1.5 (in wt.), respectively. These results are typical of a wet
cell which alloys more melt percolation through the cathode blocks during
operation. As shown in Table I, the porosity of the semi- graphitized blocks used in
cell “B” is about 1.5 times higher than that of the amorphous blocks used in cell
“A”. An interesting point is that sample B-3 (cell “B”) shows RNa and RF values
more typical of a dry cell, i.e., 0.89 and 213, respectively. Since the sample B-3
was taken closer to the cathode blocks than samples B-1 and B-2, this result
suggests that even a wet cell first behave like a dry cell during early operation.
This would be consistent with the fact that after a cell start-up, the molten bath can
percolate through the cathode blocks only after the latter have been infiltrated by
sodium [4]. Thus the refractories underneath these blocks should theoretically first
be corroded by the action of Na2 O before the percolating melt reaches them.
Table II: Comparison between the unified approach and the observations.
Cell
Sample
A
A-1
A
A
B
B
A-2
A-3
B-1
B-2
Position a
[mm]
114
244
293
114
141
RF
RNa*
Prediction
Analysis, XRD b
∞
0.90
Na2 Si2 O5
+ Albite
Nepheline
Glass
∞
∞
1.9
1.5
0.88
0.85
0.25
0.30
NaF
Na-Al-Si-O (56)
+ Nepheline (25)
Others
Glass
Nepheline (42)
+ Na-Al-Si-O (2)
NaF (39)
Na2 SiO3
----
Na2 SiO3 (13)
Others
Na2 Si2 O5
Nepheline
Glass
Nepheline (49)
+ Na-Al-Si-O (4)
Na2 SiO3
Na2 SiO3 (14)
NaF
Albite
Nepheline
NaF (34)
Glass
Na-Ca-Al-Si-O (13)
Cryolite
Cryolite (15)
NaF
------------Albite
Nepheline
NaF (14)
Chiolite (27)
Fluorite (12)
Cristobalite (11)
Others
Glass
Na-Ca-Al-Si-O (6)
+ Na-K-Al-Si-O (70)
Cryolite (6)
NaF
Na2 Si2 O5
Nepheline
Cryolite
B
B-3
228
213
0.89
Corundum
------Albite
+ Na2 Si2 O5
Nepheline
NaF
----
Corundum (4)
Cristobalite (5)
Others
Glass
Na-K-Al-Si-O (41)
NaF (35)
Others
a: Position of the sample. The distances are given with respect to the bottom of
the pot shell.
b: Values between parentheses are weight percentages of the crystalline phases
estimated using a semi-quantitative method.
Table III: Chemical composition of the corroded samples (XRF).
Chemical
composition
[wt. %]
SiO2
Al2 O3
Na2 O
TiO2
CaO
Fe2 O3
F
A-1
A-2
A-3
B-1
B-2
B-3
36.6
5.9
19.5
0.4
3.1
33
1.0
43.9
18.4
30.9
1.0
1.4
2.7
6.6
41.1
22.8
29.5
0.6
1.6
2.3
4.3
16.5
24.7
27.3
0.3
4.7
1.1
31.4
35.6
27.1
19.9
0.5
2.6
3.1
12.2
43.8
24.0
26.3
1.1
1.8
1.2
4.2
Based on the above overall results, potlining refractories in a dry cell tend to be
converted into a mixture of Nepheline and sodium silicate compounds (Na2 SiO 3
and Na2 Si2 O5 ), which coexist with sodium fluoride (see Table II and Figure 3,
samples A-1 to A-3). In a wet cell, the corrosion of the refractories tends to lead
principally to the formation of Nepheline and Albite, coexisting with cryolite in
presence or not of sodium fluoride (see Table II and Figure 4, samples B-1 and B2). For a wet cell, the possible transition from an initial dry to a final wet operating
conditions, on service, may lead to a mixture of the two above phase systems, as
shown in Table II and Figure 4 for sample B-3 (presence of Nepheline, Albite,
Na2 Si2 O5 and NaF).
Based on the ternary Na2 O-Al2 O3-SiO 2 phase diagram, the above phase system
comprising Nepheline, Na2 SiO 3 and Na2 Si2 O5 , which has been associated to the
corroded refractories from cell “A” (dry cell), should involve low melting point
eutectic compounds. In such condition, refractory bricks with high alumina:silica
ratio are required [5]. The purpose of such bricks is to prevent the formation of an
oxide melt during the operation of the cell. However, in wet cells (such as in cell
B), the presence of a melt in the potlining refractories cannot be prevented since an
important amount of molten bath percolates through the cathode blocks during
operation. In such a case, decreasing the refractories alumina:silica ratio should
prevent the penetration of the fluoride melt since it favors the Albite formation
which increases the melt viscosity [2,6]. In intermediate cell operating conditions
where RNa is closed to RNa** , the used of successive protective layers of low and
high alumina refractories maybe required to prevent both fluoride melt penetration
and oxide melt formation [7].
corrosion agents [wt%]
100
RN a = 0.90
RB = 1.1
RF= inf.
80
corrosion agents [wt%]
N2 S
N3 S2
60
NS
NA
NS2
40
20
β-A
NAS2
A3S2
A
NAS6
0
0
100
20
40
R Na = 0.88
R B = 1.1
R F= inf.
80
60
80
S
100
A-2
N2 S
N3 S2
60
NS
NA
NS 2
40
β-A
20
NAS2
A3S2
A
NAS6
0
0
corrosion agents [wt%]
A-1
100
20
40
RN a = 0.85
RB = 1.1
RF = inf.
80
60
80
100
A-3
N2 S
N 3S2
NS
NA
60
S
NS2
40
β-A
20
A
0
0
NAS2
A3S2
NAS6
20
40
60
80
SiO2 in the refractory [wt%]
S
100
Figure 3: Computed corrosion maps of samples A-1, A-2 and A-3 from cell “A”.
corrosion agents [wt%]
100
RNa = 0.25
RB = 1.1
RF = 1.9
80
B-1
NAS2
NAS6
60
β-A
40
20
A3S2
A
S
0
corrosion agents [wt%]
0
20
100
R Na = 0.30
R B = 1.1
R F= 1.5
80
60
80
100
B-2
NAS 2
60
NAS6
β-A
40
20
A3S 2
A
S
0
0
corrosion agents [wt%]
40
100
20
40
RNa = 0.89
RB = 1.1
RF= 213
80
60
80
100
B-3
N 2S
N3 S2
60
NS
NA
NS 2
40
β-A
20
A
0
0
NAS2
A3S2
NAS6
20
40
60
80
SiO 2 in the refractory [wt%]
S
100
Figure 4: Computed corrosion maps of samples B-1, B-2 and B-3 from cell “B”.
CONCLUSIONS
This work shows that the aluminum reduction cell lining design has a great
influence on the type of corrosion imposed to the refractories during service. Cells
using semi- graphitized cathode blocks were shown to be more prone to the attack
of their refractory lining by the action of the molten bath. This is the case of wet
cells, which should benefit of the use of low alumina refractories (A/S ≅ 0.33) to
prevent the penetration of such fluoride melt during operation. Unlike this case,
the use of amorphous blocks in such cells was shown to promote the corrosion of
their potlining refractories mostly by the action of the metallic sodium. This is the
case of dry cell, which should benefit of the use of high alumina refractories (A/S
≅ 0.85) to prevent the formation of an oxide melt on service.
ACKNOWLEDGEMENTS
The authors are very grateful to Norsk-Hydro and Alcoa for their financial
support during the realization of this work. Special acknowledgements are
addressed by the authors to Dr. O.-J. Siljan from Norsk-Hydro, and to Dr. A.
Tabereaux and Mr. P. Clery from Alcoa, for their technical support.
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