Note: Descriptions are shown in the official language in which they were submitted.
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MULTI-LAYER NON-CARBON METAL-BASED ANODES FOR
ALUMINIUM PRODUCTION CELLS
Field of the Invention
This invention relates to multi-layer non-carbon,
metal-based anodes, for use in cells for the
electrowinning of aluminium by the electrolysis of
alumina dissolved in a molten fluoride-containing
electrolyte, and to methods for their fabrication and
reconditioning, as well as to electrowinning cells
containing such anodes and their use to produce
aluminium.
Background Art
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten
cryolite, at temperatures around 950 C is more than one
hundred years old.
This process, conceived almost simultaneously by
Hall and H6roult, has not evolved as many other
electrochemical processes.
The anodes are still made of carbonaceous
material and must be replaced every few weeks. During
electrolysis the oxygen which should evolve on the anode
surface combines with the carbon to form polluting CO2
and small amounts of CO and fluorine-containing dangerous
gases. The actual consumption of the anode is as much as
450 Kg/Ton of aluminium produced which is more than 1/3
higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning
cells would drastically improve the aluminium process by
reducing pollution and the cost of aluminium production.
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US Patent 4,614,569 (Duruz/Derivaz/Debely/
Adorian) describes anodes for aluminium electrowinning
coated with a protective coating of cerium oxyfluoride,
formed in-situ in the cell or pre-applied, this coating
being maintained by the addition of cerium to the molten
cryolite electrolyte. This made it possible to have a
protection of the surface only from the electrolyte
attack and to a certain extent from the gaseous oxygen
but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/
Doan) describes anodes composed of a chromium, nickel,
cobalt and/or iron based substrate covered with an oxygen
barrier layer and a ceramic coating of nickel, copper
and/or manganese oxide which may be further covered with
an in-situ formed protective cerium oxyfluoride layer.
Likewise, US Patents 5,069,771, 4,960,494 and
4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium
production anodes with an oxidised copper-nickel surface
on an alloy substrate with a protective oxygen barrier
layer. However, full protection of the alloy substrate
was difficult to achieve.
Metal or metal-based anodes are highly desirable
in aluminium electrowinning cells instead of carbon-based
anodes. As mentioned hereabove, many attempts were made
to use metallic anodes for aluminium production, however
they were never adopted by the aluminium industry.
Ob-iects of the Invention
An object of the invention is to provide a multi-
layer functionally graded coating for metal-based anodes
for aluminium electrowinning cells which is substantially
impervious to molecular oxygen and also to monoatomic
oxygen and is electrochemically active for the oxidation
reaction of oxygen ions present at the anode/electrolyte
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interface into monoatomic oxygen, as well as for
subsequent reaction for the formation of biatomic
molecular oxygen evolving as gas.
Another object of the invention is to provide a
coating for metal-based anodes for aluminium
electrowinning cells which has a high electrochemical
activity, a long life and which can easily be applied
onto a metal-based anode substrate.
A further object of the invention is to reduce
substantially the consumption of the active anode surface
of metal-based anodes for aluminium electrowinning cells
which is attacked by the nascent oxygen produced by
enhancing the reaction of nascent oxygen to gaseous
oxygen which is much less active in oxidising metal
anodes of aluminium electrowinning cells.
A major object of the invention is to provide an
anode for aluminium electrowinning cells which has no
carbon so as to eliminate carbon-generated pollution and
eliminate the high carbon anode cost.
Summary of the Invention
The invention relates to a composite, high-
temperature resistant, non-carbon, metal-based, oxygen-
evolving anode of a cell for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a
molten fluoride-containing electrolyte. The anode
comprises a metal-based core structure of low electrical
resistance, for connecting the anode to a positive
current supply, coated with a series of superimposed,
adherent, electrically conductive layers. The conductive
layers consist of:
a) at least one layer on the metal-based core structure
constituting during electrolysis a barrier
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substantially impervious to molecular oxygen and also
monoatomic oxygen;
b) one or more intermediate protective layers on the
oxygen barrier to protect the oxygen barrier against
dissolution, which intermediate layer(s) during
electrolysis remain inactive in the reactions for the
evolution of oxygen gas; and
c) an electrochemically active layer on the outermost
intermediate layer, for the oxidation reaction of
oxygen ions present at the anode/electrolyte
interface into nascent monoatomic oxygen, as well as
for subsequent reaction for the formation of gaseous
biatomic molecular oxygen evolving as gas, the active
layer protecting the intermediate layer(s) against
dissolution.
The active layer comprises at least one
transition metal and/or an oxide thereof (excluding the
lanthanides and actinides and their oxides alone), for
instance iron, cobalt, nickel, copper, chromium or
titanium as metals and/or oxides.
The active layer may be slowly consumable during
electrolysis.
In this context, metal-based anode means that the
anode contains at least one metal in the anode core
structure and/or in the protective layers as such or as
alloys, intermetallics and/or cermets.
The core structure may comprise at least one
metal selected from nickel, copper, cobalt, chromium,
molybdenum, tantalum, niobium or iron. For instance, the
core structure may be made of an alloy consisting of 10
to 30 weight% of chromium, 55 to 90% of at least one of
nickel, cobalt or iron, and 0 to 15% of aluminium,
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titanium, zirconium, yttrium, hafnium or niobium.
Alternatively, the core may be nickel plated copper.
Possibly, the core structure may comprise an
alloy or intermetallic compound containing at least two
metals selected from nickel, cobalt, iron and aluminium.
Alternatively, the core structure can comprise a
cermet containing copper and/or nickel as a metal, and a
ceramic phase.
Usually, the oxygen barrier layer comprises at
least one oxide selected from chromium, niobium and
nickel oxide. Advantageously, the oxygen barrier layer
may be formed on the core structure by surface oxidation.
However, it is also possible to form an oxygen barrier by
slurry application techniques, arc spraying or plasma
spraying. The oxygen barrier may optionally be formed by
applying a precursor which is then converted into a
functional barrier by heat treatment, such as applying a
layer of chromium, niobium or nickel metal on the core
which can then be oxidised.
One of the intermediate layers normally contains
copper, or copper with at least one of nickel and cobalt,
and/or oxide(s) thereof. An intermediate layer may also
comprise iron cuprate, nickel ferrite and/or cobalt
ferrite.
Typically, one of the intermediate layers
comprises an oxidised alloy containing 20 to 60 weight%
of copper with one or more further metals forming a solid
solution with copper, such metals being generally nickel
and/or cobalt.
Usually, the electrochemically active layer
comprises at least one oxide which may slowly wear away
during electrolysis. Optionally but not necessarily the
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electrochemically active layer comprises (an) oxide(s)
throughout its thickness.
An oxide may be present in the electrochemically
active layer as such, or in a multi-compound mixed oxide
and/or in a solid solution of oxides. The oxide may be in
the form of a simple, double and/or multiple oxide,
and/or in the form of a stoichiometric or non-
stoichiometric oxide.
The electrochemically active layer may for
instance comprise a metal, alloy, intermetallic compound
or cermet which during normal operation in the cell is
slowly consumable by oxidation of its surface and
dissolution into the electrolyte of the formed surface
oxide. In this case the rate of oxidation may be
substantially equal to the rate of dissolution.
Advantageously, the electrochemically active
layer containing metals is pre-oxidised prior to
electrolysis. The metals of the electrochemically active
layer may be iron with at least one metal selected from
nickel, copper, cobalt, aluminium and zinc.
Optionally, the electrochemically active layer
may further comprise at least one additive selected from
beryllium, magnesium, yttrium, titanium, zirconium,
vanadium, niobium, tantalum, chromium, molybdenum,
tungsten, manganese, rhodium, silver, hafnium, lithium,
cerium and other Lanthanides.
Advantageously, the electrochemically active
layer may also comprise at least one electrocatalyst for
the anode reaction selected from iridium, palladium,
platinum, rhodium, ruthenium, silicon, tin, mischmetal
and metals of the Lanthanide series, and mixture, oxides
and compounds thereof, for example as disclosed in
W099/36592 (de Nora).
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The electrochemically active layer may be a
surface oxidised iron-nickel layer, the surface
containing iron oxide, nickel oxide or a mixture thereof.
Alternatively, the electrochemically active layer
comprises spinels and/or perovskites. In particular, the
electrochemically active layer may comprise ferrites,
such as ferrites selected from the group consisting of
cobalt, copper, manganese, magnesium, nickel and zinc
ferrite, and mixtures thereof, in particular nickel
ferrite partially substituted with Fe2+. Additionally,
the ferrite may be doped with at least one oxide selected
from chromium, titanium, tin and zirconium oxide.
The electrochemically active layer can also
comprise ceramic oxides containing combinations of
divalent nickel, cobalt, magnesium, manganese, copper and
zinc with divalent/trivalent nickel, cobalt, manganese
and/or iron. The electrochemically active layer may for
instance have doped, non-stoichiometric and/or partially
substituted spinels, the doped spinels comprising dopants
selected from Ti4+, Zr4+, Sn4+ Fe4+, Hf4+, Mn4+, Fe3+, Ni3+,
C03+, Mn3+, A13+, Cr3+, Fe2+, Ni2+, C02+, lvjg2+. Mn2+, Cu2+,
Zn2+ and Li+.
For instance, the electrochemically active layer
may be made of an oxidised nickel-cobalt alloy. Such an
alloy forms upon oxidation complex oxides, in particular
(NiXCo1_X)O, having semi-conducting properties.
Furthermore, nickel-cobalt oxides provide an
advantage over conventional nickel ferrite. Whereas
trivalent iron ions of nickel ferrite are slowly replaced
by trivalent aluminium ions in the octahedral sites of
the spinel lattice, which leads to a loss of conductivity
and of mechanical stability, nickel-cobalt alloys
oxidised in oxygen at 1000 C lead to a semi-conducting
mixed oxide structure of NiCo2O4 and Co304 spinels which
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is similar to the NaCl lattice. In these spinels, a
replacement of trivalent cobalt ions by trivalent
aluminium ions is unlikely.
The cobalt nickel atomic ratio is preferably
chosen in the range 2 to 2.7.
Further materials which may be used for forming
the electrochemically active layer include high-strength
low-alloy (HSLA) steels.
It has been observed that low-carbon HSLA steels
such as Cor-TenTM, even at high temperature, form under
oxidising conditions an iron oxide-based surface layer
which is dense, electrically conductive,
electrochemically active for oxygen evolution and, as
opposed to oxide layers formed on standard steels or
other iron alloys, is highly adherent and less exposed to
delamination and limits diffusion of ionic, monoatomic
and molecular oxygen.
HSLA steels are known for their strength and
resistance to atmospheric corrosion especially at lower
temperatures (below 0 C) in different areas of technology
such as civil engineering (bridges, dock walls, sea
walls, piping), architecture (buildings, frames) and
mechanical engineering (welded/bolted/riveted structures,
car and railway industry, high pressure vessels).
However, these HSLA steels have never been proposed for
applications at high temperature, especially under
oxidising or corrosive conditions, in particular in cells
for the electrowinning of aluminium.
It has been found that the iron oxide-based
surface layer formed on the surface of a HSLA steel under
oxidising conditions limits also at elevated temperatures
the diffusion of oxygen oxidising the surface of the HSLA
steel. Thus, diffusion of oxygen through the surface
layer decreases with an increasing thickness thereof.
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If the HSLA steel is exposed to an environment
promoting dissolution or delamination of the surface
layer, in particular in an aluminium electrowinning cell,
the rate of formation of the iron oxide-based surface
layer (by oxidation of the surface of the HSLA steel)
reaches the rate of dissolution or delamination of the
surface layer after a transitional period during which
the surface layer grows or decreases to reach an
equilibrium thickness in the specific environment.
High-strength low-alloy (HSLA) steels are a group
of low-carbon steels (typically up to 0.5 weight% carbon
of the total) that contain small amounts of alloying
elements. These steels have better mechanical properties
and sometimes better corrosion resistance than carbon
steels.
The surface of a high-strength low-alloy steel
electrochemically active layer may be oxidised in an
electrolytic cell or in an oxidising atmosphere, in
particular a relatively pure oxygen atmosphere. For
instance the surface of the high-strength low-alloy steel
layer may be oxidised in a first electrolytic cell and
then transferred to an aluminium production cell. In an
electrolytic cell, oxidation would typically last 5 to 15
hours at 800 to 1000 C. Alternatively, the oxidation
treatment may take place in air or in oxygen for 5 to 25
hours at 750 to 1150 C.
In order to prevent thermal shocks causing
mechanical stresses, a high-strength low-alloy steel
layer may be tempered or annealed after pre-oxidation.
Alternatively, the high-strength low-alloy steel layer
may be maintained at elevated temperature after pre-
oxidation until immersion into the molten electrolyte of
an aluminium production cell.
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The high-strength low-alloy steel layer may
comprise 94 to 98 weight% iron and carbon, the remaining
constituents being one or more further metals selected
from chromium, copper, nickel, silicon, titanium,
tantalum, tungsten, vanadium, zirconium, aluminium,
molybdenum, manganese and niobium, and optionally a small
amount of at least one additive selected from boron,
sulfur, phosphorus and nitrogen.
Advantageously, the electrochemically active
layer is initially sufficiently thick to constitute an
impermeable barrier to gaseous oxygen penetration, and
even to nascent, mono-atomic oxygen.
Any of these layers may be slurry applied, for
instance by applying a precursor slurry. The layers may
also be applied in the form a precursor powder followed
by heat-treating.
Several techniques may be used to apply the
layers, such as dipping, spraying, painting, brushing,
arc spraying, plasma spraying, electro-chemical
deposition, physical vapour deposition, chemical vapour
deposition or calendar rolling.
The invention also relates to a method of
manufacturing an anode as described above. The method
comprises the steps of formation of the oxygen barrier
layer(s), of the intermediate layer(s) and of the
electrochemically active layer. It is possible to form
the oxygen barrier by substrate oxidation after the
intermediate barrier has been applied onto the substrate.
The method for manufacturing such an anode may
also be used for reconditioning an anode whose
electrochemically active layer is worn or damaged. The
method comprises clearing at least worn and/or damaged
parts of the active surface from the core structure or
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from the outermost intermediate layer to which it adheres
and then reconstituting at least the electrochemically
active layer.
Another aspect of the invention is a cell for the
production of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte
comprising at least one composite anode as described
above.
Advantageously, the cell may comprise at least
one aluminium-wettable cathode which can be a drained
cathode Qn which aluminium is produced and from which it
continuously drains.
Usually, the cell is in a monopolar, multi-
monopolar or in a bipolar configuration. Bipolar cells
may comprise the anodes as described above as the anodic
side of at least one bipolar electrode and/or as a
terminal anode.
In such a bipolar cell an electric current is
passed from the surface of the terminal cathode to the
surface of the terminal anode as ionic current in the
electrolyte and as electronic current through the bipolar
electrodes, thereby electrolysing the alumina dissolved
in the electrolyte to produce aluminium on each cathode
surface and oxygen on each anode surface.
Preferably, the cell comprises means to improve
the circulation of the electrolyte between the anodes and
facing cathodes and/or means to facilitate dissolution of
alumina in the electrolyte. Such means can -for instance
be provided by the geometry.ojE the_cell as described in
co-pending application W099/41429 (de Nora/Duruz) or
by periodically moving t_he anodes as described in co-
pending application W099/41430 (Duruz/Bello).
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The cell may be operated with the electrolyte at
conventional temperatures, such as 950 to 970 C, or at
reduced temperatures as low as 700 C.
Yet another aspect of the invention is a method
of producing aluminium in such an aluminium
electrowinning cell, wherein alumina is dissolved in the
molten fluoride-containing electrolyte and then
electrolysed to produce aluminium.
Advantageously, during electrolysis the active
layer of the anode may be protected by an electrolyte-
generated oxyfluoride-containing layer, such as cerium
oxyfluoride self-formed on the electrochemically active
layer as described in US Patent 4,614,569 (Duruz/Derivaz/
Debely/Adorian).
Detailed Description
The invention will be further described in the
following Examples:
Example 1
A test anode was made by coating by electro-
deposition a core structure in the shape of a rod having
a diameter of 12 mm consisting of 74 weight% nickel, 17
weight% chromium and 9 weight% iron, such as Inconel ,
first with a nickel layer about 200 micron thick and then
a copper layer about 100 micron thick.
The coated structure was heat treated at 1000 C
in argon for 5 hours. This heat treatment provides for
the interdiffusion of nickel and copper to form an
intermediate layer. The structure was then heat treated
for 24 hours at 1000 C in air to form a chromium oxide
(Cr203) barrier layer on the core structure and oxidising
at least partly the interdiffused nickel-copper layer
thereby completing formation of the intermediate layer.
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A nickel-ferrite powder was made by drying and
calcining at 900 C the gel product obtained from an
inorganic polymer precursor solution consisting of a
mixture of molten Fe(N03)3.9 H20 with a stoichiometric
amount of Ni(C03)2.6 H20. A thick paste was made by mixing
1 g of this nickel-ferrite powder with 0.85 g of a nickel
aluminate polymer solution containing the equivalent of
0.15 g of nickel oxide. This thick paste was then diluted
with 1 ml of water and ground in a pestle and mortar to
obtain a suitable viscosity to form a nickel-based paint.
An electrochemically active oxide layer was
obtained on the core structure by applying the nickel-
based paint onto the core structure with a brush. The
painted structure was allowed to dry for 30 minutes
before heat treating it at 500 C for 1 hour to decompose
volatile components and to consolidate the oxide coating.
The heat treated coating layer was about 15
micron thick. Further coating layers were applied
following the same procedure in order to obtain a 200
micron thick electrochemically active coating covering
the intermediate layer and barrier layer on the core
structure.
The anode was then tested in a cryolite melt
containing approximately 6 weight% alumina at 970 C by
passing a current at a current density of about 0.8
A/cm2. After 100 hours the anode was extracted from the
cryolite and showed no significant internal corrosion
after microscopic examination of a cross-section of the
anode sample.
The Example can be repeated with an
electrochemically active layer obtained from a feed
prepared by slurrying nickel ferrite powder in an
inorganic polymer solution having the required
composition for the formation of NiFe2O4. The powder to
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polymer ratio was 1 to 0.25. Several layers of the
coating feed can be brushed onto the nickel-copper layer
and heat treated to form the electrochemically active
layer on the intermediate layer.
Alternatively, the Example can be repeated with
an electrochemically active layer obtained from an amount
of 1 g of commercially available nickel ferrite powder
slurried with 1 g of an inorganic polymer consisting of a
precursor of 0.25 g equivalent nickel-ferrite per 1 ml.
An amount corresponding to 5 weight% of IrO2 acting as an
electrocatalyst for the rapid conversion of oxygen ions
into monoatomic oxygen and subsequently gaseous oxygen
can be added to the slurry as IrCl4, as described in
W099/36592 (de Nora). The slurry can be brush-coated onto
the interdiffused and at least partly oxidised nickel
copper alloy layer by applying 3 successive 50 micron
thick layers of the slurry, each slurry-applied layer
should be allowed to dry by heat-treating the anode at
500 C for 15 minutes between each layer application.
Examole 2
A nickel metal core structure was heated in air
at 1100 C for 16 hours to form an oxidised surface layer
having a thickness of about 35 micron. The surface layer
was black showing the presence of nickel oxide (NiOl+X)
which is known to act as an oxygen barrier layer and to
be electrically conductive.
An interdiffused nickel-copper layer was then
applied onto the oxygen barrier and oxidised as described
in Example 1.
A mixture of nickel-ferrite and copper-ferrite
powder was slurried in an inorganic polymer solution
having the required composition for the formation of
CuFe2O4 and NiFe2O4. The polymer solution had a
concentration of 350g/l oxide equivalent and the powder
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to polymer ratio was 1 to 0.25. The slurry was used as a
coating feed and brushed onto the nickel oxide surface
layer of the core structure to form a ferrite-based
electrochemically active layer on the nickel oxide layer.
After drying the ferrite-based layer at 105 C, the core
structure was submitted to a heat treatment at 500 C in
air to consolidate the coating.
Several ferrite-based layers were applied, with
each applied layer being heat treated before applying a
subsequent layer, to form a consolidated coating of more
than 100 micron thick.
Examnle 3
A steel core structure was coated with a slurry
prepared by suspending chromium oxide (Cr203) in an
inorganic Cr3+ polymer solution. The feed concentration
was greater than 500 g/l of Cr203.
After heat-treating to consolidate the chromium
oxide (Cr203) applied layer, thereby forming a barrier
layer on the steel structure, a second intermediate layer
of interdiffused nickel-copper was applied as described
in Example 1 on the barrier layer. Finally the
intermediate layer was coated with several
electrochemically active layers of CuFe2O4 and NiFe204 as
described in Example 2.
Example 4
A test anode was obtained by coating an Inconel
metal core structure with a nickel copper alloy layer and
heat-treating it as described in Example 1 to form a
barrier layer and an intermediate layer on the metal core
structure.
A further layer of a nickel-iron based alloy
consisting of 79 weight% nickel, 10 weight% iron and 11
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weight% copper of a thickness of approximately 1 mm was
then applied on the interdiffused and at least partly
oxidised nickel copper layer by plasma spraying.
This alloy layer was then pre-oxidised at 1100 C
for 5 hours for the formation of an electrochemically
active oxide layer on the alloy layer. Although pre-
oxidation of the alloy layer is preferred, the treatment
is not necessary before using the anode in the cell to
produce aluminium.
The test anode was then tested in a cell as
described in Example 1. During electrolysis the alloy
layer was further oxidised at the alloy layer/active
layer interface, self-forming the electrochemically
active layer. Simultaneously, the active layer was slowly
dissolved into the electrolyte at the active
layer/electrolyte interface at substantially the same
rate as its rate of formation at the alloy layer/active
layer interface, thereby maintaining the thickness of the
oxide layer substantially constant, as the alloy layer
wears away.
When the alloy layer is worn or damaged, the
anode can be reconditioned by clearing at least the worn
or damaged parts and reconstituting at least the alloy
layer.
The Example can be repeated by applying on the
interdiffused nickel copper intermediate layer a nickel-
iron alloy consisting of 30 weight% nickel and 70 weight%
iron having a thickness of about 0.5 mm by arc spraying
or plasma spraying. The nickel-iron alloy layer can be
pre-oxidised in air at 1100 C for 6 hours to form a dense
iron oxide-based electrochemically active outer surface
layer on the intermediate layer.
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Example 5
A test anode was obtained by electrodepositing
onto a copper metal core structure a series of successive
metallic layers consisting of a nickel layer (10 micron
thick) which is known to be well adherent to copper and
chromium, a chromium layer (25 micron thick), a nickel
layer (50 micron thick) and a copper layer (50 micron
thick) and heat treating first in argon and then in
oxygen as described in Example 1 to interdiffuse and
oxidise the nickel and the copper layers to form an
intermediate layer, and oxidise the chromium layer to
form an oxygen barrier layer.
An iron layer (200 micron thick) was then
electrodeposited onto the interdiffused nickel-copper
layer and pre-oxidised at 1100 C in air for 6 hours to
form a dense iron oxide-based electrochemically active
outer surface layer on the intermediate layer.
The anode was then tested in molten electrolyte
containing approximately 6 weight% alumina at 850 C at a
current density of about 0.8 A/cm2. The anode was
extracted from the cryolite after 100 hours and showed no
sign of significant internal or external corrosion after
microscopic examination of a cross-section of the anode
sample.
Example 6
Examples 1 to 5 have been repeated by replacing
the electrochemically active layer by a Cor-TenTM type
low-carbon high-strength (HSLA) steel layer doped with
niobium, titanium, chromium and copper in a total amount
of less than 4 weight% which is also electrochemically
active upon oxidation. The anodes were pre-oxidised in
air at about 1050 C for 15 hours for the formation of a
dense hematite-based outer layer constituting an oxide-
based surface layer on an un-oxidised anode body.
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The anodes were then tested in a fluoride-
containing molten electrolyte at 850 C containing
cryolite and 25 weight% excess of A1F3 and approximately
3 weight% alumina at a current density of about 0.7
A/cm2.
To maintain the concentration of dissolved
alumina in the electrolyte, fresh alumina was
periodically fed into the cell. The alumina feed
contained sufficient iron oxide to slow down the
dissolution of the hematite-based electrochemically
active anode layer.
After 140 hours electrolysis was interrupted and
the anode extracted. Upon cooling the anode was examined
externally and in cross-section. No corrosion was
observed at or near the surface of the anode.
The produced aluminium was also analysed and
showed an iron contamination of about 700 ppm which is
below the tolerated iron contamination in commercial
aluminium production.
The Example can be repeated with different HSLA
steel layers such as an HSLA steel layer doped with
manganese 0.4 weight%, niobium 0.02 weight%, molybdenum
0.02 weight%, copper 0.3 weight%, nickel 0.45 weight% and
chromium 0.8 weight%, or an HSLA steel layer doped with
nickel, copper and silicon in a total amount of less than
1.5 weight%.
