Volume 20 Preprint 44
Peculiarities of surface interaction of Al+REM alloys with air and water
Akashev L.A., Popov N.A., Shevchenko V.G., Grigorov I.G.
Keywords: Al+REM, thermal oxidation, water
The effect of the phase and chemical composition of aluminium+rare earth metal (1-2.5%R, ~22%R) polycrystalline alloys (Al+REM) on the rate of their surface film growth in air (at temperatures 400, 500, 600Â°C) and in water (~100 Â°C) was studied. It is shown that in the temperature range 500-600Â°C the oxidation of 1-2.5%R alloys in air is enhanced due to the increasing amount of REM oxide phases and crystallization of amorphous Al2O3. Al+1at.%Yb alloy shows the lowest oxidation stability in this temperature range owing to the formation of the greatest amount of REM oxides. Oxidation of Al+REM (~22%R) alloys in air begins at a temperature below 400Â°C. Their oxidation rate depends on the type and amount of dopant metal and the phase composition: the presence of REM-rich intermetallics in the alloy dramatically increases its reactivity. It is established that in the interaction of Al+REM alloys with boiling water, the active reacting phase is aluminum.
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Peculiarities of surface interaction of Al+REM alloys with air and water
Akashev L.A., Popov N.A., Shevchenko V.G., Grigorov I.G.
Institute of Solid State Chemistry, Ural Branch of RAS, 620990 Ekaterinburg, Russia
The effect of the phase and chemical composition of aluminium+rare earth metal
(1-2.5%R, ~22%R) polycrystalline alloys (Al+REM) on the rate of their surface film
growth in air (at temperatures 400, 500, 600°C) and in water (~100 °C) was studied.
It is shown that in the temperature range 500-600°C the oxidation of 1-2.5%R
alloys in air is enhanced due to the increasing amount of REM oxide phases and
crystallization of amorphous Al2O3. Al+1at.%Yb alloy shows the lowest oxidation
stability in this temperature range owing to the formation of the greatest amount of
REM oxides. Oxidation of Al+REM (~22%R) alloys in air begins at a temperature
below 400°C. Their oxidation rate depends on the type and amount of dopant metal
and the phase composition: the presence of REM-rich intermetallics in the alloy
dramatically increases its reactivity. It is established that in the interaction of
Al+REM alloys with boiling water, the active reacting phase is aluminum.
Aluminum alloys with rare earth metals (REM) find wide application in electrical
engineering. Small amounts (up to at.5%) of transition (Zr) or rare-earth metal (Sc, Y
and lanthanides) dopants increase the strength of aluminum applied in highstrength power lines. Due to low solubility of these metals in aluminum, its
electrical conductivity is preserved. The finely dispersed phase of intermetallic
compounds Al11R3 (Al3R) formed in the aluminum alloy increases its strength almost
twice . Additives reinforcing aluminum (for example, copper in duralumin) often
impair its resistance to corrosion. One of the reasons of this is a variation in the
chemical composition and structure of the passivating oxide films. Therefore, the
study of the protective properties of oxide films of aluminum alloys is not less
important than the study of their strength and electrical properties. The information
about the properties of protective passivating oxide films at elevated temperatures
can be used for the development of new high-temperature (up to 300°C)
uninsulated wires and wires operating at elevated humidity.
Besides, the prospects of using rare-earth metals as alloying additions increasing
the oxidation rate of aluminum powder in rocket propellants were shown by authors
of works [2,3]. Small additions of rare earth metals improve the burning rate and
other impulse characteristics of disperse aluminum by reducing the protective
properties of its oxide film. Because of this, it is important to study the effect of
rare earth additives on the thermokinetic characteristics of aluminum, including
cracking of the barrier layer of oxidation products on the metal surface.
To determine the quantitative effect of the chemical and phase composition of
alloys on the intensity of their interaction with reactive media (air, water), two
groups of alloys were synthesized: alloys close to eutectics (Al+1-2,5%R) and alloys
with chemical composition close to aluminum in intermetallides Al11R3 (~22%R). All
samples were synthesized by melting in a vacuum furnace in helium environment
(high purity grade) at 1500°C followed by cooling in alundum crucibles. The
composition of the alloys was refined by mass spectrometry (Spectromass 2000)
and X-ray fluorescence spectroscopy (XRF-1800). The investigated sample surface
was the top plane polished mechanically. The alloys were polished by diamond
pastes applied on soft fabrics until a specular reflection was obtained (surface
roughness was Ra = 0.047-0.051 mcm). Then the samples were washed thoroughly
with alcohol and acetone with additional annealing in vacuum (~0,01 Pa) at 300°C
for 1 h. The alloys were oxidized in air in a muffle furnace at temperatures of 400,
500 and 600°C. In all the experiments, the oxidation of alloys took place stepwise at
the same temperature at 5, 10, 15 etc. min. intervals with windows for
measurements. The interaction of water with the surface of Al+REM alloys was
realized by immersing the samples in boiling distilled water at 100°C with the same
intervals for ellipsometric measurements in air.
The ellipsometric parameters Δ and ѱ were measured by the compensating (zero)
method with a one-wave ellipsometer LEF-3M (λ = 0.6328 mcm) after the samples
were cooled in air to room temperature. Thereby the time dependence of the
ellipsometric parameters of the examined surface in quasi-isothermal conditions
To determine the kinetics of surface film growth (thickness against time), at first the
optical constants (n, k) of the alloys and their natural passivation films were found.
For this purpose, we used the ellipsometric immersion method [4,5]. The immersion
liquids were anise oil (n = 1.5) and alcohol (n = 1.364).
RESULTS AND DISCUSSION
Thermal oxidation of the alloys in air
Table 1 lists the refractive indices (n2) and extinction coefficients (k2) of pure
aluminum, alloys of intermetallic and eutectic compositions, as well as of their
natural oxide films (n1, k1) with the initial thickness d. It is seen that the thickness
of the oxide layer on the alloys is always higher than on pure aluminum. Besides,
the optical constants of substrates for the two groups of alloys differ considerably
from each other due to different phase composition.
Chemical composition and optical constants of Al+REM alloys
Natural passivating film
Figure 1(a,b) presents the temperature and time dependences of the thickness
growth of oxide films as oxidation products. The thickness of the oxide film on the
Al+1-2.5%R alloys is higher than on pure Al and increases with temperature. There
are several stages at each temperature. The oxidation of the Al+1-2.5%R alloys at
400°C (Fig.1a) for 15 min is characterized by thermal desorption of water molecules
and carbonates adsorbed from air. During long-term annealing, the increase in the
oxide thickness is less than 4 nm per hour. When the temperature is raised to
500°C, a more active growth of the oxide film is observed. Thermal desorption takes
less than 5 min, therefore it is not marked on the curves. After 1 h exposure at
500°C and 600°C (fig.1b), the greatest increase in the film thickness was observed
for the alloy with ytterbium (1%Yb). At 600°C, during the first 25 minutes of
exposure, a sharp increase in the oxide film thickness takes place for the majority
of Al + 1-2.5at.% R alloys (Fig. 1b).
Figure 1. Temperature and time dependences of the oxide film thickness growth on
Al+1-2.5%R alloys at 400, 500 (a), and 600°С (b) in air.
Detailed analysis of the curves shows that the sharp thickness growth is due
to crystallization of the natural (amorphous) aluminum oxide film in γ-Al2O3.
Aluminum oxide is a predominant component of the examined oxide films; it
crystallizes forming grain boundaries that give way to oxygen particles to the alloy
surface. According to different literature data [6-12], the temperature of the phase
transformation of aluminum oxide falls within the range of 475-550 °C.
Figures 2, 3 show the literature data proving that in the temperature interval
475-550°C, on the surface of bulk  and finely-dispersed aluminum , the
amorphous surface oxide film transforms into a crystalline gamma modification.
Figure 2. The growth of oxide film on bulk Al as a function of temperature .
Figure 3. XRD diagram of aluminum
nanopowder samples annealed in air for 1.5
h at 350°C and 550°C .
The active thickness growth of the
oxide film on Al+1%Yb alloy (Figure 1a,b)
can be explained as follows: in addition to
individual oxide phases RxOy are formed in the films at these temperatures. The
formation of RxOy phases affects the continuity and overall thickness of the oxide
film in the same way. This is particularly evident in the case of Yb oxides during
oxidation of Al+1%Yb alloy. In ref 10, the study of ytterbium powder oxidation in air
revealed that three oxide phases are present at once in the powder in a wide
temperature range (from 250 to 650°C and higher): lower oxide YbO crystallizing in
fcc structure (unlike lower oxides of other rare-earth metals formed in the bcc
lattice), orthorhombic Yb3O4 found in the interval 300-650°C, and sesquioxide bcc
oxide Yb2O3 (>400°C).
Figure 4. XRD diagrams of
ytterbium powder at different
Thus, in the temperatures interval 550-600°C the multiphase oxide film may
contain at once three stoichiometric oxide phases (Figure 4), which was also
confirmed by the maxima on the thermogravimetric curves .
Analysis of the available literature data [13-15] and our experimental findings
shows that in the investigated temperature range, the oxidation of other rare earth
stoichiometric oxide phases (Figure 4). So, oxides of elements from Nd to Lu are
usually formed as higher R2O3 oxide (typical valence of REM is 3+) having a bcc
lattice. In the case of La, Ce, Pr, the following structural phase transitions are
possible: La (bcc La2O3
hcp La2O3), Ce (bcc Ce2O3
fcc CeO2), Pr (bcc
hcp Pr2O3). Thus, it is shown that the main reason of increasing the
oхidation rate of Al+1-2.5%R alloys at t> 500°C is the growth of interfaces of
Oxidation of the alloys with a high content of REM (~22 at.%R) begins at lower
temperatures (400°C) (Fig. 5). The time dependences of the films thicknesses
contain the curves for the alloys with large and small additions of REM at 400°C and
500°C. It is seen that the increase in temperature by 100 degrees leads to 2-3 times
enhancement of the oxide film thickness. At 600°C, after 5 min exposure in the
furnace, a strong interference of the reflected beam is observed for the surface of
intermetallic compounds, which is associated with a dramatic thickening of the
Figure 5. Time dependence of the thickness of oxide films on aluminum alloys Al +
~ 22at.%R at 400 (a) and 500°C (b) in air.
Analysis of the equilibrium diagrams  and micrographs of the alloy
surface showed that the film growth rate depends on the phase composition of the
alloy. The main phase in the structure of Al+1-2.5%R alloys (Figure 6) is aluminum
(rare earth metals have a very low solubility in aluminum). The rare earth metal is
present in the structure of these alloys in the form of intermetallic inclusions.
Al+2.57at.%La; b) Al+2.57at.%Ce; c) Al+2.54at.%Pr; d) Al+1.5at.%Sm
The structures of the alloys with a high content of REM differ from each other.
So, in the structure of Al+21.5%La and Al+22.4%Sm alloys (Figure 7 a, d), grains of
intermetallic compounds dominate (hypereutectic region of the diagram) – these
phases have the greatest affinity for atmospheric oxygen. Therefore, the film
thickness on their surface is considerably higher (Figure 5).
On the phase diagram, the alloys Al+22.8%Pr (Figure 7b) and Al+22.5%Nd are
exclusively in the region of intermetallic compounds, as seen from their structure.
The thickness of their oxide films will be slightly higher than in the hypereutectic
region. When the rare earth metal content increases further as in the Al+28.9%Ce
alloy, the alloy enters into the two-phase region Al3R - Al2R. Due to the presence of
Al2Ce phase, the Al+28.9%Ce alloy exhibits a high activity during thermal oxidation
in air (Figure 5). Thus, it is seen that the presence of REM-rich phases dramatically
increases the oxidation rate of Al+R alloys.
Figure 7. The micrographs of the surfaces of Al+~22at.%R alloys: а) Al+21.5 at.%La,
b) Al+28.9 at.%Ce, c) Al+22.8 at.%Pr, d)Al+22.4 at.%Sm
Interaction with boiling water
The opposite character of interaction was observed in the study of the interaction of
Al+REM surface with boiling water. According to the IR spectroscopy and X-ray
analysis results, the main product of the reaction on the Al and Al+REM alloys
surface in these conditions is the AlOOH hydroxide film. The thicknesses were
calculated from the ellipsometric parameters Δ and ѱ on the basis of the two-layer
model: substrate - oxide film - hydroxide film (n=1.56). Figure 8 shows the time
dependence of the surface film thickness growth. It is seen that the rate of the
formation of reaction products on the surface of pure aluminum and Al+1-2.5%R
alloys is higher than on the surface of intermetallic compounds. Small additions of
REM in aluminum affect little the thickness of films produced in these conditions on
its surface. The ellipsometric parameters at the initial stages of interaction (1-3
min) are described well by a two-layer model with an inhomogeneous (porous) film
of the upper layer of aluminum hydroxide. At further boiling, the layer is
compacted, as a result of which it grows already at a slower rate. After 65 min
interaction with distilled water, the film thickness on aluminum reaches 270 nm,
while on the Al+1-2.5%R alloys it is slightly smaller. As can be seen, after 65 min
boiling the formation of films on the alloys is not completed.
Figure. 8. The time dependence of thicknesses of oxide-hydroxide films on Al and
Al+REM alloys during interaction with distilled water at 100°C.
For Al+~22%R alloys, the interaction takes place at a much lower rate due to a
significantly smaller content of aluminum metal (solid solution) in the bulk and on
the surface of these alloys. The films on their surface turned out to be 2.5 times
thinner during this period of time, thus there was no noticeable film growth (Figure
The presence of a porous film formed on the surface of eutectic alloys and pure
aluminum after 1-3 min boiling in distilled water was confirmed by atomic force
microscopy. As seen in Figure 9, a raised roughness is detected on the aluminum
surface after 1.5 min interaction (average roughness Ra = ~ 12.7 nm), which may be
indicative of a porous structure of aluminum hydroxide on the surface at the initial
stage of interaction. The stripes are the traces of pre-mechanical polishing of the
surface. The sample of aluminum intermetallic with praseodymium (Fig. 9b) has a
much smaller average roughness Ra = ~6.3nm under these conditions.
Figure. 9. a) AFM picture of aluminum surface after 1.5 min interaction with
water. b) AFM photograph of the surface of the intermetallic compound Al +
22.8at.% Pr after 1.5 min interaction with water (the stripes are polishing defects).
Note that before and after 1.5 min interaction the Al+~ 22at.%R alloys visually
retain their mirror surface. The surface of aluminum and Al+1-2.5%R quickly gets
The alloy powders prepared by mechanical grinding of monolithic ingots in a
mortar and by screening on laboratory sieves (diameter of grains <63mcm) were
boiled for 65 min under the same conditions. Figure 10 shows the IR spectra of the
powders obtained for all the investigated alloys. The intensity of the absorption
bands is weak because of low dispersion. However, on the spectra of Al+1-2.5at%R
powders one can identify the absorption bands corresponding to stretching,
deformation and libration vibrations of OH-bonds in aluminum hydroxide (3433
cm-1, 1631 cm-1, 1078 cm-1, respectively). In all the IR spectra of Al+~22%R alloys,
the absorbtion band at 1078cm-1 is faint or absent, which is indicative of a small
amount of hydroxide forming the surface film. Proceeding from the surface
morphology of the samples, it can be supposed that metallic aluminum in these
alloys in water at 100°C is the main reactive phase, and when its content in the bulk
and on the alloy surface decreases, the amount of hydroxide forming the surface
Figure 10. The infrared transmission spectra of powders of Al+REM alloys after
65 min contact with boiling water.
The peculiarities of the surface interaction of Al+REM alloys with oxygen at 400,
500, and 600°C and with boiling water were studied. For Al+1-2.5%R alloys, in the
temperature range from 500 to 600°C oxidation in air is enhanced due to the
increasing amount of RxOy oxide phases and crystallization of amorphous Al2O3
inside the surface film. The Al+1%Yb alloy has the lowest oxidation stability in this
temperature range owing to the formation of the greatest amount of REM oxides.
The oxidation of Al+~22%R alloy in air begins at a temperature below 400°C.
The oxidation rate depends on the type and amount of the alloying metal and the
phase composition: the presence of REM-rich phase dramatically increases the
oxidation rate of Al+R alloys.
It is found that in the interaction of Al+REM alloys with boiling water, the active
reacting phase is pure aluminum.
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