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RESEARCH
4 Al (l) + 3 C (s) → Al 4 C 3(s) (1)
3 C (cathode) + 4 AlF 3 (diss) + 12 e - →
Al 4 C 3 (s) + 12 F - (diss) (2)
To improve the physical understanding and to
build competence concerning this branch of
materials performance, we need to combine
data from chemical and electrochemical experiments
with fundamental studies on diffusion
and thermodynamics, and with computer
modelling of transport processes. This paper
aims to give an insight to the wear mechanisms
of the carbon cathode by summarising the
procedures and some of the main results from
our experiments. A more detailed description
can be found in a series of publications in the
corresponding literature [11-18].
Fundamental studies
To understand the formation mechanism(s) of
aluminium carbide it is necessary to examine
a) b)
c) d)
possible influencing factors. As stated above,
the formation of aluminium carbide could be
of either chemical or electrochemical nature.
The simplest system to start with is molten aluminium
and carbon in direct contact (case 1)
using the so called Al-C diffusion couple test.
In the further course of the work, we intend
to include stepwise other parameters such as
presence of cryolite (case 2) and polarisation
(case 3), so as to build up an experiment that
mimics real potline conditions. The test setup
for case 1 and 2 experiments is shown in
Fig. 2. More detailed descriptions can be found
elsewhere [11, 12].
Case 1 experiments revealed that aluminium
carbide indeed forms by a purely chemical
reaction at the Al-C interface. Temperatures
above 1 100 °C were needed for carbide formation,
probably because the reaction was
impeded by a protective Al 2 O 3 layer initially
present at the aluminium surface due to exposure
to air. This oxide layer had to evaporate
and/or disintegrate mechanically by thermal
expansion to ensure appropriate contact between
aluminium and carbon, leading to the
formation of a dense layer of small Al 4 C 3 crystallites.
A possible reaction mechanism for the
first stage of carbide formation was discussed
[11]. Introduction of synthetic cryolite at the
Al-C interface changed the morphology of
the aluminium carbide layer to a more needle-like
structure, and reduced the reaction
temperature to 1 030 °C [12]. This confirms
that cryolite acts as a wetting agent by dissolving
the oxide layer [5, 7]. This is ongoing work,
and studies currently focus mainly on cases 2
(cryolite) and 3 (polarisation).
The authors used the case 1 set-up in a side
study to compare different types of carbon
materials and their influence on aluminium
carbide formation. Sample treatment, temperature,
duration, and argon pressure in the
glass tube were kept constant throughout
the experiments. Afterwards, the quartz tube
shown in Fig. 2a) was quenched in water and
the sample was removed. The spent samples
were embedded in epoxy and wet cut with
100 % ethanol in a precision diamond saw.
Afterwards, the samples were wet-ground and
polished using 100% ethanol as lubricant to
avoid reactions of the aluminium carbide in
the sample with the moisture in air.
Optical microscopy using a polarising filter
revealed the Al-C interface and aluminium
carbide formation. Some of the initial results
are presented in Fig. 3. As can be observed,
all types of carbon produced aluminium carbide
layers with similar appearance, which
leads to the preliminary conclusion that the
carbide formation is independent of the type
of carbon material. Even though the Al-C diffusion
tests are long-term tests and involved
only small amounts of carbide formation, the
authors would like to point out that this result
is in accordance with observations made during
the wear test studies, which will be described
in the following.
Experimental cathode
wear investigations
Fig. 3: Comparison of polished cross sections of different carbon materials: electrode graphite (a), graphitised
carbon of two different types (b, c) and vitreous carbon (d) after the experiments performed at
1 200 °C, 0.8 bar argon atmosphere and 10 days duration. The Al 4 C 3 layer is clearly visible at the aluminium
carbon interfaces as indicated in Fig. 2.
Several authors have described laboratory test
methods for studying the wear mechanism(s)
and to reveal the influence of different experimental
conditions on the wear rate [5,
13-16, 19-26]. Recent attempts have focused
on predicting the behaviour and performance
of commercial cathode materials in industrial
cells. Ranking or comparing cathode materials
requires defining a standardised test with
consistent test parameters. Several types of
laboratory set-ups to ‘accelerate’ the wear
have been tested. The most promising one is
96 ALUMINIUM · 1-2/2013