Available online at www.sciencedirect.com
Journal of Magnesium and Alloys 8 (2020) 219–230
www.elsevier.com/locate/jma
On the dynamically formed oxide films in molten Mg
Mohammad Mahdi Jalilvand, Mehdi Akbarifar, Mehdi Divandari∗, Hassan Saghafian
School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, 16846-13114, Iran
Received 6 November 2019; received in revised form 26 December 2019; accepted 26 December 2019
Available online 22 January 2020
Abstract
The so-called “Oxide/Metal/Oxide sandwich” method is one of the technique used to investigate the dynamic oxidation of metals which
happens during the casting process. In this study, characteristics of the oxide films formed on the molten magnesium in dynamic conditions
have been investigated using the aforementioned method. The air bubbles were released into the cast sample at the pressure of 0.2 atm
through a quartz tube of 1 mm internal diameter. The interface of two adjacent entrapped bubbles is considered as the Oxide/Metal/Oxide
(OMO) sandwich. The sandwiches were characterized by the aid of the optical and scanning electron microscopy and also X-Ray diffraction
analyses. Two different approaches, including measuring the width of the folds formed on the oxide films and the edge of the sandwiches,
were used to estimate the thickness of the films. The thicknesses were estimated to be in the range of 200 to 800 nm. The features such
as fold, wrinkle, and crack were observed on the OMO sandwiches. On the microscopic scale, the oxide films were rough and porous.
This is attributed to the non-protective behavior of oxide films. The XRD patterns indicated that the oxide films formed on the pure molten
magnesium in dynamic conditions are crystallized MgO.
© 2020 Published by Elsevier B.V. on behalf of Chongqing University.
This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/)
Peer review under responsibility of Chongqing University
Keywords: Dynamic oxidation; Bifilm; Magnesium; Oxide films; Casting defects.
1. Introduction
Magnesium and its alloys show high affinity to oxidation,
especially in the molten state. Literature indicates that at temperatures below 250 °C, a protective oxide film forms on the
solid magnesium surface and kinetics of the oxidation follows a parabolic law [1]. At the higher temperatures, the film
becomes porous. As a result of that, the surface can’t be protected against the atmosphere and the oxidation will continue
with a linear dependence with the time [1,2].
In an unprotected atmosphere, magnesium melt tends to
continuously react with oxygen until all the Mg content turns
to oxides [3–5]. Thermodynamics calculations suggest that
at 700 °C, an oxygen partial pressure of 5 × 10−54 atm is
enough for magnesium oxidation [6]. In order to tackle the
problem, taking the benefit of the protective fluxes and gases
∗
Corresponding author.
E-mail address: divandari@iust.ac.ir (M. Divandari).
[7], vacuum melting [8], and designing of the alloys with
higher ignition temperature have been proposed [9–12].
The majority of the researches on Mg oxidation are concerned with the oxidation in the solid state. Due to the high
reactivity of the Mg melt, studying the oxidation of the molten
Mg have been carried out in a protective atmosphere [13–16].
However, during the casting process, oxidation of the molten
metal occurs in two stages including: a) melting, and b) pouring stages. The oxidation of metal during the melting step is
categorized as static oxidation. While, the pouring step oxidation in which the surface turbulence changes the state of
the oxidation, is known as dynamic oxidation. The last case
would lead to formation of the double layer oxides (Bifilms)
in casting parts [17]. The double oxide layers not only deteriorate the mechanical performances of the castings but also
can act as the preferential sites for the nucleation and growth
of gas and shrinkage porosities [18,19].
During the dynamic conditions i.e. melt pouring step of
the casting process, the oxide films are being subjected to
deformation forces which causes the characteristics of the
https://doi.org/10.1016/j.jma.2019.12.003
2213-9567/© 2020 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license.
(http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University
220
M.M. Jalilvand, M. Akbarifar and M. Divandari et al. / Journal of Magnesium and Alloys 8 (2020) 219–230
Fig. 3. Macroscopic picture of pure Mg OMO sandwich with a length and
width of about 8 mm and 6 mm, respectively.
in dynamic conditions [22]. Due to the fact that these oxide
films form in a very short period of time, they are considered
as the short time oxide films [23].
The aim of this work is to investigate the characteristics of
the dynamically formed oxide films in the unprotected pure
magnesium melt using the aforementioned method. The morphology and thickness of these films have been carefully investigated.
Fig. 1. A view of the designed model for the casting practice.
Fig. 2. Schematic of bubbles entrapment and formation of the OMO sandwiches.
dynamically formed oxides to be different from the stagnant
oxides [20]. To simulate and provide the dynamic conditions
during pouring step, Divandari and Campbell [21] have
devised a technique in which artificial bubbles are introduced
to the melt. In this method, releasing the bubbles into the
melt leads to the formation of an oxide/metal/oxide sample.
The mutual layer between two adjacent bubbles hands out
unique information about the oxide films formed on the melt
2. Experimental method
To generate artificial bubbles in a certain shape and size
and blowing them at the predefined time intervals of 2 bubbles
per second, a system was designed based on pervious works
[21–26] with minor changes described here. In this setup,
the compressor produces the compressed air and its pressure
reaches the specific value passing through a few instruments
including solenoid and control valves. Eventually, the air is
introduced to the fluid through a quartz tube with a pressure
of 0.2 atm. The internal and external diameter of this tube is
1 mm and 3 mm, respectively.
As shown in Fig. 1, the casting model is a thin simple
plate. The top feeder is designed to postpone the solidification
process in order to increase the chance of bubbles entrapment.
Apart from that, to prevent solidification of the melt surrounding the quartz tube, a trapezium-shape feeder was designed
at the bottom of the plate. A bottom pour gating system was
designed and performed to minimize the melt surface turbulence during the filling stage. Reducing the entrance velocity
of the melt into the mold was the main concern. The mold
material consisted of silica sand, 4%wt. sodium silicate as the
binder and 1% dry sulfur powder.
The commercially pure magnesium bars were melted in
an electrical resistance furnace. To protect the melt against
R
ignition, MagRex
coverall flux with a grain size of 60 was
used. All the tool e.g. crucible, mold, and etc. used in the
casting process were preheated up to 100 °C. The pouring
temperature was 50 °C higher than the melting point of the
pure Mg.
M.M. Jalilvand, M. Akbarifar and M. Divandari et al. / Journal of Magnesium and Alloys 8 (2020) 219–230
221
Fig. 4. Backscattered SEM images of OMO sandwich of pure Mg.
Table 1
Chemical composition of the commercially pure Mg.
Element
Mg
Al
Cu
Zn
Fe
Wt.%
99.86
0.056
0.068
0.013
0.00
Considering Fig. 2, the process of formation of the oxide/metal/oxide samples was done. During melt pouring, the
artificial bubbles were released to the mold with specified
time intervals. After successfully performing the procedure,
the final cast was cooled in the air and cut in pieces. The
chemical composition of the sample after the casting process
is presented in Table 1.
Eventually, the interfaces of the two entrapped bubbles
were carefully taken for the examinations. These samples are
called oxide/metal/oxide sandwiches and in the following text,
it will be referred as OMO sandwiches. The samples were investigated using X-Ray diffraction (XRD), scanning electron
microscopy (SEM) and energy dispersive spectroscopy (EDS)
analyses.
3. Results and discussion
3.1. Macroscopic investigations
Fig. 3 shows a macroscopic image of the shiny surface of
OMO sandwich. The average surface size of produced OMO
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Fig. 5. SEM image of the pure Mg OMO sandwich showing some details of the microstructure. The bright areas in image (b) consists of oxide films and the
dark areas are formed as a result of metal entrapment between oxide layers.
Fig. 6. (a) and (b) SEM pictures of pure Mg OMO sandwich. (1) And (2) are EDS results regarding to the marked points in image (b).
M.M. Jalilvand, M. Akbarifar and M. Divandari et al. / Journal of Magnesium and Alloys 8 (2020) 219–230
223
Fig. 7. Schematic views of pure Mg OMO sandwich (a). Top view and (b).
Cross-section view of the dashed line in (a).
Fig. 8. XRD pattern of pure Mg OMO sandwich.
sandwiches was less than 50 mm2 . Producing larger OMO
sandwiches was not practical since it requires higher blowing
pressure. Higher pressure increases the bubble rising velocity,
which lowers the chance of its entrapment in the casting.
3.2. Microscopic investigations
Fig. 4 shows backscattered SEM images of a pure Mg
OMO sandwich. It seems a thick oxide film covers the
Fig. 9. SEM image of the cracks formed on the pure Mg OMO sandwich.
Arrows have marked the cracks and teared areas of the sandwich.
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Fig. 10. Schematic for the growth of the cracks in OMO sandwiches.
Fig. 11. Appearance of folds on the surface of pure Mg OMO sandwich.
The larger folds have a width range of about 3 to 6 μm and the smaller ones
are below 1 μm in width.
magnesium dendrite’s tip. Several coarse folds and wrinkles
can be seen in Fig. 4. The main difference between the
morphology of the oxide films formed in dynamic conditions
and those formed in a static situation is the presence of such
folds and wrinkles [20–24]. Moreover, regarding Fig. 4, in
some places on the sandwich’s surface, several discontinuities
and micro-cracks are visible. Apparently, OMO sandwich has
torn in these locations due to the presence of the mechanical
stresses caused by the dynamic motion of the bubbles in the
melt. Moreover, shrinkage stresses caused by the difference
between the thermal expansion coefficient of magnesium and
its covering oxide can be another source.
Fig. 12. Schematics for the re-oxidation of surface in an OMO sandwich.
(a). A crack appears on the oxide surface due to the mechanical stress. (b)
Opening of the crack, which leads the melt to be exposed to atmosphere. (c)
Re-oxidation of the exposed surface and elimination of the crack.
The close view of SEM images of the interface between
two entrapped bubbles are presented in Fig. 5. One can
see that there is some amount of solidified metal entrapped
between two oxide layers. These areas can be seen as the
darker and thicker areas in the Fig. 5(b). The adjacent brighter
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Fig. 13. Formation mechanism of shrinkage folds and wrinkles. The difference in volume reduction of the metal and oxide would be compensated by
the formation of folds and wrinkles.
225
regions on the sandwich are the areas in which two oxide
layers have touched each other on a dry surface [21,24].
Fig. 6 presents the EDS results obtained from two points
marked in Fig. 6(b). Clearly, the intensity of oxygen picks
in EDS result of point 2 is relatively high. Besides, presence of the cracks in the bright areas (point 2), indicates that
these areas consist of oxide layers, which are overlapped on
each other. Thus, it can be confirmed that the dark areas are
formed as a consequence of metal entrapment between oxide
layers during solidification of the molten metal. As the solidification proceeds, the entrapped molten metal between two
oxide layers undergoes a volume shrinkage. Consequently,
the solidified metal would be encapsulated at different local
points. This creates such a contrast in SEM images. Besides,
many folds are visible in SEM micrographs showed in Fig. 5,
which are probably formed due to the presence of mechanical
stresses [20,24].
Schematic views, top and cross-section, of the pure Mg
OMO sandwich is depicted in Fig. 7. The molten metal was
suctioned in because of the solidification shrinkage and then
Fig. 14. SEM images showing the blisters on the OMO sandwich surface. Fig. (d) is a magnified picture of the rectangular area marked in (c).
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Table 2
Linear expansion coefficients for Mg and MgO at room temperature [27].
Fig. 15. Schematics of folds on OMO Sandwiches. (a) A fold with two oxide
surface touching each other (b). A fold with metal entrapment between two
layers of oxides, which causes error in the thickness estimation.
entrapped between two layers of oxide. This dual-phase microstructure has also been observed in aluminum and zincbased alloys [20,23–25]. In the case of Al alloys, the contrast
of these two phases is more noticeable in SEM images [24].
The lower thickness of the amorphous oxide films formed on
molten Al alloys can be the reason behind. Moreover, in comparison to the dynamically formed oxide films of Al alloys,
Mg-based alloys form a less wrinkled oxide film [23,25].
The XRD pattern of the sandwich is presented in Fig. 8.
The majority of the peaks are related to pure Mg. However,
few peaks, which are attributed to the MgO, are also observed
in the XRD pattern. The low amount and thickness of the
oxide formed on the sandwich are the reasons behind the low
intensity of the MgO oxides peaks. Regarding the XRD result,
it can be concluded that short time oxidation of the molten
pure Mg led to the formation of the crystalline MgO.
As illustrated in Fig. 9, a closer look at the SEM images
taken from the sandwiches, indicates the higher probability of
crack formation in places where two oxide layers are in touch.
The oxide films are brittle in nature and cannot be as flexible
as metals facing mechanical stresses. Areas where two oxide
layers are directly in contact and those with the minimum
entrapped metal are more inclined to host the crack. This
Material
Linear expansion coefficients × 10−6 (1/k)
Mg
MgO
25–26.9
13.5
concept is schematically depicted in Fig. 10. If the oxide film
is thin, about a few nanometers, the forces and stresses will
be exerted on the film in only two directions.
The presence of the folds in different sizes is the most
significant feature of the dynamically formed oxide films in
the case of pure Mg. Folding of the oxide films may occur
either above or below the melting point of the metal. Thus,
the folds can fall into the two categories, (a) those that form
above the melting point including folds and wrinkles and (b)
those that form below the point which are only wrinkles. The
width of the folds is greater than the wrinkles [25].
As shown in Fig. 11, smaller folds have formed within the
areas between the bigger folds. Re-oxidation of the surface
oxide film is the reason behind such a morphology. Considering Fig. 12, tension stresses exerted on the oxide film
leads it to be torn. By increasing the size of the crack, the
molten metal, which has been entrapped in the OMO sandwich, would be exposed to the atmosphere of the bubbles. The
fresh and uncovered melt would be immediately re-oxidized
and the crack will be sewn. These freshly formed films are
very thin comparing to the old oxide layers. In addition, one
can see that folds and wrinkles on the oxide layer are formed
in the same direction. Since the direction of the stress which
has caused the oxide films to be cracked, is still the same,
the new oxide film experiences it with the same axis. Thus,
the arrangement of the folds and wrinkles formed on the new
oxide layers remains constant. Moreover, due to the lower
thickness of the new oxide film, the wrinkles are also smaller
than those on the older film are.
Mechanical stresses during formation of the OMO sandwich are one of the main sources for the generation of
folds [20]. The high-velocity upward motion of the bubbles,
pressure difference between them and turbulence of the
melt surrounding the bubbles can cause mechanical stresses.
Formation of the large folds and wrinkles is to be expected
due to such stresses [20]. The magnitude of this stress is high
enough to deform the entrapped metal between the oxide
layers and fold it. Moreover, the mechanical stresses lead
to the formation of large cracks in some areas of the OMO
sandwich. Table 2 presents the values of the linear expansion
coefficient for the Mg and MgO at room temperature. The
difference between the volumetric thermal expansion of the
MgO film and Mg metal is another source of stress.
The formation mechanism of the shrinkage folds and wrinkles is schematically shown in Fig. 13. Decreasing temperature causes both the entrapped metal and oxide film to shrink.
As illustrated in Fig. 13(b), the substrate metal experiences
a greater reduction in volume compared to the shrinkage of
oxide film. Consequently, the folds and wrinkles would be appeared on oxide film to compensate the difference between the
M.M. Jalilvand, M. Akbarifar and M. Divandari et al. / Journal of Magnesium and Alloys 8 (2020) 219–230
227
Fig. 16. Estimation of the oxide thickness by measuring the width of the folds in SEM images from different areas of OMO sandwich. The images (b) and
(d) are the magnified images of the marked area in images (a) and (c), respectively.
surface area of the film and substrate. The Shrinkage forces
act on the OMO sandwich in all directions. Therefore, wrinkles can form in different directions on the OMO sandwich.
The swells or blisters found on the oxide layers are
other notable features of the pure Mg OMO sandwich.
Fig. 14 shows some of these swells from different views.
Three possible mechanisms can cause the surface swelling or
blisters on the oxide films. The first mechanism can be based
on the entrapment of the gaseous hydrogen molecules under
the oxide skin. Hydrogen is the only gas, which can be dissolved in Mg melt. The maximum solubility of hydrogen in
liquid and solid states of magnesium is 27 and 19 cm3 per
100 g, respectively [28]. Molten Mg rejects only 32% of its
hydrogen content while in the case of Al this value is about
95% [29]. This implies the higher rejection of hydrogen in
Al melt than Mg [29,30]. Anyway, the extra amounts of the
dissolved hydrogen atoms in the melt would be rejected during the solidification process. These rejected atoms join each
other and form the molecular hydrogen. The molecular hydrogen can be entrapped under the oxide skin and create blisters
on the OMO sandwich. The second explanation for the formation of the blisters can be attributed to Mg vapor. The vapor
pressure of the molten magnesium in the atmosphere is equal
to 1 atm [31]. Therefore, there is a chance for the presence
and entrapment of the Mg vapors under the oxide layers. In
other words, the vaporization of the entrapped molten Mg
can be one of the sources for the formation of blisters. The
third mechanism which sounds more conservative and acceptable among other mechanisms is the oxide film defect. The
first and second mechanisms suggest creating a hollow swell.
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Fig. 17. Thickness estimation of the oxide layer by measuring the width of the edge layer in SEM images of pure Mg OMO sandwich.
However, regarding Fig. 14(d), there is no evidence to confirm
the hollowness of swells.
3.3. Thickness estimations
A simple method to estimate the thickness of the dynamically formed oxides is to measure the width of the
folds and wrinkles formed on the OMO sandwiches. By
halving the width of these folds, an approximate thickness
for the oxide film would be achieved. However, considering
Fig. 15 entrapment of the melt reduces the accuracy of the
estimation. Accordingly, not every fold would be suitable for
this measurement.
The thickness of the oxide film which measured using folds
shown in Fig. 16(a,b), is determined to be about 700 nm. Researchers have reported values about 2.5 μm for the dynamic
oxide film formed on the AZ91 Mg alloy melt using the same
method [20]. Fig. 16(c,d) shows thickness estimation of the
oxide films which formed because of the re-oxidation or selfsewing phenomena. This type of oxide film formed on the
pure Mg, as discussed before, has a very limited time to grow
and evolve. The re-oxidation phenomenon causes formation of
M.M. Jalilvand, M. Akbarifar and M. Divandari et al. / Journal of Magnesium and Alloys 8 (2020) 219–230
newer and thinner oxide films so that the width of the folds
formed on these new and thin oxide films is lower than that
of older oxide films. The estimated thickness of this film is
in the range of 400 nm to 450 nm, which is about half the
thickness of the old films. The oxide film formed on the Mg
alloys melt is relatively thick and rough. The presence of the
macroscopic and microscopic folds provide the possibility of
air entrapment in the bifilms.
In comparison to the compact and protective oxide layer
formed on the Al alloys melt, the thicker and rougher oxide
films on the Mg alloys require more attention. The Weibull
modulus of the Mg alloys, at the same conditions, is lesser
than the Al alloys [32]. The reduction in mechanical properties of the Mg cast parts due to the presence of the oxide films
depends on the morphology, thickness and probably distribution of these films in the melt and final casting [19,20,33].
Measuring the thickness of the OMO sandwich at the
edges is another way to estimate the oxide film thickness,
eliminating the error caused by metal entrapment in the
folds. Fig. 17 shows the SEM images taken from the edge
of the pure Mg OMO sandwiches. By halving the acquired
values, the film thickness is estimated from 200 to 800 nm.
The scattered values for the oxide film thickness indicate that
the complex conditions for the oxide formation have caused
the thickness of the film to be varied in different areas of
the films. Considering the type of the oxidation process, i.e.
dynamic or static, and the method used for estimating the
thickness of the film, different values have been reported in
the literature. Entrapment of the oxides in the melt due to the
surface turbulence happens very fast so that theses entrapped
oxides have very short times to thicken. For this reason, the
thickness of these films is in the range of a few hundreds of
nanometer [34].
4. Conclusion
In the present study, the oxide films formed on the pure
Mg melt in dynamic conditions were investigated using
Oxide/Metal/Oxide (OMO) method. Following results were
concluded:
(1) The pure Mg OMO sandwiches were shiny in appearance and their average size was less than 50 mm2 . The
SEM images and EDS analyze showed that the pure Mg
OMO sandwiches consist of two different areas, which
were distinguished by the image contrast, overlapped
oxide layers and metal entrapment regions.
(2) A lot of folds, wrinkles, and cracks found on the surface
of OMO sandwiches. Many factors such as mechanical
stresses and difference in thermal expansion coefficients
between the metal and oxide are the source for the formation of such a microstructure.
(3) Based on the SEM images and XRD patterns, the oxide
films formed in these conditions were rough, porous and
crystalline in the microscopic level which implies the
non-protective behavior of the oxide film.
229
(4) The thickness of the oxide film was estimated using two
methods. In the first method, the half width of the folds
formed on OMO sandwich is reported as the film thickness. Using this method the thickness of the old oxide
was estimated to be about 700 nm. Also, the thickness of
the new oxide which is formed due to the re-oxidation
phenomena was determined in the range of 400 nm to
450 nm. In the second method in which the thickness of
the OMO sandwich edges is measured, the range of oxide film thickness was estimated to be between 200 nm
and 800 nm.
Declaration of Competing Interest
None.
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