Volume 6 Preprint 44
Investigation on Corrosion of Magnesium and its Alloys
Keywords: corrosion, magnesium, electrochemistry
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Volume 6 Paper C104
Investigation on Corrosion of Magnesium and
CRC for Cast Metals Manufacturing (CAST), Division of Materials
Engineering, School of Engineering, The University of
Queensland,Brisbane, QLD 4072, Australia, email@example.com
Magnesium alloys are well known for their high specific strength.
There is growing interest in the alloys as structural materials in the
automotive, aerospace and electronic industries. However, the
corrosion performance of most magnesium alloys is not satisfactory
for some practical applications. The Cooperative Research Centre for
Cast Metals Manufacturing (CAST) has been investigating the corrosion
behaviour of magnesium and its alloys since 1995. This paper
presents some of the major achievements in this area recently made
by CAST. The following issues are addressed: Negative Difference
Effect (NDE), role of alloying elements, effect of microstructure and
galvanic corrosion. It was found that: (a) the NDE is a common
phenomenon for all alloys investigated; (b) alloying with aluminium
and zirconium can stabilise the matrix phase of magnesium alloys in
corrosion; (c) in the aluminium containing alloys the β phase appears
to have a beneficial effect on the corrosion resistance especially where
a fine grain size is produced, but in some cases, its presence in an
alloy accelerates the corrosion of the alloy; (d) galvanic corrosion of
magnesium alloys strongly depends on the cathode material and the
manner by which the galvanic couples are exposed to the
Keywords: corrosion, magnesium, electrochemistry
The density of magnesium is only two thirds that of aluminium.
Because of their high strength-to-weight ratio, magnesium alloys are
being increasingly used, particularly in the automotive industry. Over
the past 10 years, the weight of magnesium applications in North
American automobiles has increased about 4 times .
However, the relatively poor corrosion performance of magnesium
alloys [1, 2, 3, 4] is a major issue which limits their application.
Currently, the use of magnesium alloys in the automotive industry is
mainly limited to components in mild service environments . For
those parts exposed to outside environments, corrosion is still a
serious issue. At a recent international conference “Magnesium Alloys
2003, Japan”, corrosion was listed as one of the areas where technical
progress is necessary for further expansion of magnesium use by
The Cooperative Research Centre for Cast Metals Manufacturing (CAST)
was established as part of the Australian Commonwealth Government's
Cooperative Research Centre's Program and commenced operations in
July 1999. The Centre continues and extends research related
activities on light metals previously carried out by the CRC for Alloy
and Solidification Technology that operated between 1993 and 1999.
The corrosion and prevention of magnesium and its alloys has been
one of the most important areas under the CAST’s research program
since 1995. The research effort in this area is focused on solving
corrosion problems relative to the application of magnesium alloys in
the automotive industries. Nevertheless, encompassed by the
requirements of the applied research, some essential fundamental
studies have also been conducted in CAST. This paper presents a
summary of some of the fundamental research achievements in this
area recently made by CAST.
Negative Difference Effect (NDE)
Magnesium and its alloys have a special electrochemical behaviour, the
Negative Different Effect (NDE). The NDE is usually defined by the
difference ∆ for a galvanostatic applied
∆ = Icorr - IH,m
current density as
where Icorr is the corrosion rate or the spontaneous hydrogen
evolution rate (HER) at the corrosion potential; IH,m is the measured
HER at an applied anodic current density or potential. The polarization
behaviour is called NDE when ∆<0. According to this definition, the
NDE is a phenomenon under an anodic polarisation condition.
For most conventional metals, such as iron, copper, nickel, etc, the
cathodic process normally slows down while the anodic dissolution of
the metal is accelerated by anodic polarisation. However, for
magnesium or its alloys, HER increases as the applied polarisation
potential becomes more positive. Figure 1 displays a typical NDE case
for pure magnesium in a NaCl solution [6, 7]. It can be seen that HER
increases with increase in anodic polarisation current density.
Mg dissolved (mol/s)
applied current density (mA/cm )
Figure 1. Hydrogen evolution rate and anodic dissolution rate for pure
magnesium in 1 N NaCl of pH11 under different polarisation current
The NDE is a common phenomenon for magnesium alloys [8, 9, 10,
11, 12]. Figure 2 presents the NDE phenomena for some magnesium
applied Current density (mA/cm )
applied Current density (mA/cm )
(c) AZ91D Diecast
hydrogen evolution rate (ml/min)
Applied current density (mA/cm )
applied Current density (mA/cm )
Figure 2. Hydrogen evolution rate and anodic dissolution rate under
different polarisation current densities for (a) AZ21 (Al2Zn1) in 1N
NaCl of pH11, (b) AZ91 (Al9Zn1) in 1N NaCl of pH11 (c) diecast AZ91
(Al9Zn0.8Mn0.3) in 1N NaCl of pH11 and (d) sand cast MEZ (REZn) in
The basic feature of the NDE phenomenon is that HER increases as the
polarisation current density becomes more positive. As shown in
Figure 1 and Figure 2 (b) and (c), this special hydrogen evolution
behaviour actually starts at a cathodic current density. This means
that the NDE can occur under a cathodic polarisation condition, not
only under anodic polarisation as specified in the definition. The NDE
is not always associated with anodic polarisation. This point has not
been clearly illustrated before.
The above figures also show another interesting phenomenon; the HER
first decreases and then increases as the polarisation current changes
from a negative to a positive value. These two different hydrogen
processes are termed as “cathodic hydrogen evolution” and “anodic
hydrogen evolution” respectively [#ref11] in this paper. This is
because the first one occurs when the electrode is under cathodic
polarisation and the second one is closely associated with the anodic
shift of the applied polarisation potential or current density and it
mainly occurs in the anodic polarisation region. The different trends
of the dependence of the HER on polarisation current suggest that
different mechanisms are controlling the “cathodic” and “anodic”
hydrogen evolution processes. It is a normal mechanism by which HER
decreases as polarisation current or potential becomes more positive.
However, the increase in HER with increasing anodic current density or
anodic polarisation potential is an abnormal behaviour from an
electrochemical point of view. It means that this hydrogen evolution
process could follow a different mechanism.
The differences in these two HER behaviours can be further illustrated
by in-situe observation [#ref11] of a polarised MEZ magnesium alloy
(0.18wt%Ce, 0.34wt%Nd, 0.9wt%Pr,0.65wt%La, 0.54wt%Zn) in a NaCl
solution (see Figure 3). When MEZ was cathodically polarised to –
1.8V/SCE, “cathodic hydrogen evolution” was observed from many
sites (e.g. particles) in grains or grain boundaries. When the
polarisation potential became less negative, e.g. at -1.7V/SCE, less
sites produced hydrogen bubbles, and the hydrogen evolution rate
was lower. So far, “anodic hydrogen evolution” was not observed. If
the polarisation potential was increased to a more noble value, e.g.
-1.4V/SCE, then “cathodic hydrogen evolution” became even less and
almost stopped. Meanwhile in another area where corrosion was
occurring, “anodic hydrogen evolution” was observed to be very
intensive. When the specimen was catholically polarised back to a
cathodic potential, “anodic hydrogen evolution” became weaker and
weaker, while the “cathodic hydrogen evolution” became faster again.
At -2V/SCE, “anodic hydrogen evolution” stopped in the corroded area
(corrosion also stopped there at this potential), but “cathodic hydrogen
evolution” was even more intensive in a number of sites in the
These phenomena strongly suggest that “cathodic hydrogen evolution”
is mainly from the uncorroded area of a magnesium alloy, which is
responsible for the cathodic polarisation behaviour as represented by
the cathodic branch of the polarisation curve of the alloy; and the
Figure 3 Hydrogen evolution
and corrosion of polarised MEZ
magnesium alloy: (a) at cathodic
potential –1.8V/SCE in an area;
(b) at cathodic potential –
1.7V/SCE in the same area as (a);
(c) at anodic potential –1.4V/SCE
in the same area as (a); (d) at
anodic potential –1.4/SCE in
another area; (e) at cathodic potential –2V/SCE in the same area as (d)
after being anodically polarised at –1.4V/SCE. The cloudy “trails” of
hydrogen bubbles and the trapped hydrogen bubbles associated with
“cathodic hydrogen evolution” are clearly observed in (a) to (e). The
cloudy zones along the edges of black (corroding) areas associated
with “anodic hydrogen evolution” are observed in (d) and (e).
anodic dissolution of a magnesium alloy is closely associated with
“anodic hydrogen evolution” from corroding areas of the alloy, and the
anodic dissolution and “anodic hydrogen evolution” are responsible for
the anodic polarisation behaviour of the magnesium alloy.
The “cathodic hydrogen evolution” can be easily understood as a
normal cathodic electrochemical process.
However, the “anodic
hydrogen evolution” appears to be a cathodic reaction that is
“abnormally” accelerated as polarisation potential becomes more
positive. To explain such an electrochemical behaviour, the following
model is proposed [#ref4, #ref7] (see Figure 4):
Mg2+ + H2
Mg 2+ + H 2
Figure 4. Electrochemical corrosion and Negative Difference Effect on
the magnesium surface: (a) at a very negative cathodic potential; (b) at
the “pitting potential”; (c ) at an potential more positive than the
At a very negative potential or current density, the surface film on
magnesium is intact. There is no film-free area, so the anodic
dissolution of magnesium is very low, almost zero. “Cathodic
hydrogen evolution” can still proceed on the surface film at such a
negative potential. The hydrogen evolution rate decreases as the
potential becomes more positive until a “pitting potential” is reached.
At the “pitting potential”, the surface film starts to breakdown, and
both hydrogen evolution and magnesium dissolution become much
easier in the defects or film-free areas. In these areas, magnesium
corrosion occurs, which produces univalent magnesium ions and
subsequently leads to generation of hydrogen:
Mg = Mg+ + eMg+ + H2O = Mg2+ + OH- + ½ H2
The anodic dissolution rates of reactions (2) increase as polarisation
potential or current density becomes more positive. Therefore, more
hydrogen is produced at a higher potential by reaction (3). This
explains the “anodic hydrogen evolution” phenomenon. The “pitting
potential” is not necessarily the same as the corrosion potential and
could be more negative than the corrosion potential in most cases. So
NDE can occur even at a cathodic potential. When the electrode is
polarised back to a very negative potential, reaction (2) will stop. As a
result, reaction (3) slows down, and the “anodic hydrogen evolution”
ceases too. Certainly, in the later stage after the surface has been
severely corroded, undermining of magnesium particles would occur.
It is quite clear that the “special” electrochemical behaviour is caused
by complicated reactions. At the corrosion potential, there are at least
three reactions occurring on the surface: anodic dissolution of
magnesium, “anodic hydrogen evolution” and “cathodic hydrogen
evolution”. The “cathodic hydrogen evolution” from uncorroded areas
should be different from that from corroding areas. Therefore, the
polarisation curve may not necessarily follow a simple Tafel equation
and the corrosion rate at the corrosion potential cannot be estimated
through Tafel extrapolation.
Figure 5. Polarisation curves for sand cast MEZ and diecast AZ91D in
5% NaCl (pH11)
For example, the polarisation curves of sand cast MEZ and diecast
AZ91D (Ai9Zn0.8Mn0.3) shown in Figure 5 indicate that the corrosion
rates of these two alloys are almost the same if estimated through
“Tafel” extrapolation. However, the weight-loss measurements
showed a very big difference between their corrosion rates. The
weight-loss rate of MEZ is about 25 times higher than that of AZ91D
. The estimation based on the polarisation curves alone is
Effect of alloying elements
Magnesium alloys in general can be divided into two groups: 1) those
containing aluminium as the primary alloying element; and 2) those
free of aluminium and containing a small amount of zirconium for the
purpose of grain-refinement.
Weight loss rate (mg/cm /day)
Aluminium concentration of alloy (wt%)
Figure 6. Average weight loss rates of Mg-Al single phase alloys with
various aluminium contents after immersion in 5wt.% NaCl solution for
Aluminium is the most important alloying element in the first group of
magnesium alloys. The addition of a certain amount of aluminium can
introduce a secondary phase (β-Mg17Al12 intermetallic) into the
magnesium alloys. In most cases, the improved corrosion resistance is
principally attributed to the secondary phase. This will be addressed
later in this paper.
Solid solution aluminium in the matrix phase plays an important role
in the corrosion of this group of magnesium alloys. The beneficial
effect of aluminium in the α phase can be elucidated by the corrosion
rates of several single α-phase alloys with various aluminium contents
(see Figure 6) . The corrosion resistance of the α phase matrix
increases with increasing solid solution aluminium level. This is
contrary to the explanation by Lunder et al  that the matrix phase
becomes more anodically active as the aluminium content increases up
For an AZ (Al-Zn) alloy, the solid solution aluminium content can vary
from 1.5wt % in the grain centre to about 12wt % in the vicinity of the β
phase . Therefore, the corrosion behaviour could be very different
in a grain of the α matrix phase. The evidence for this difference is
the corrosion morphologies of AZ91E as shown in Figure 7 .
Corrosion mainly occurred in the interior of the α grain, and developed
from grain to grain. In many cases, corrosion stopped at the grain
boundary before it reached the β phase, where the aluminium content
is much higher than the grain centre.
Figure 7. Corrosion morphologies after 4 hour immersion in 5% NaCl.
(a) sand cast AZ91E and (b) sand cast AZ91E (Ca)
In the second group of alloys, the role of zirconium is, to some extent,
as important as aluminium in the first group of alloys. The beneficial
effect of zirconium is remarkable.
Zirconium is not only a powerful grain refiner, but also a very effective
purifier for magnesium. Iron is an unavoidable impurity that can be
easily introduced into most magnesium alloys during melting and
casting processes. A trace level of iron impurity can significantly
deteriorate the corrosion performance of a magnesium alloy.
Zirconium can react with iron in molten magnesium to form ironzirconium intermetalics which quickly settle out due to their high
density. Therefore, the addition of zirconium in the second group of
magnesium alloys can lead to higher purity and hence a more
corrosion resistant magnesium alloy.
An example for the relationship between the zirconium addition, purity
and the corrosion resistance of a magnesium alloy is presented in
Table 1[#ref12]. A higher concentration of zirconium results in a
lower iron impurity and slower corrosion of the magnesium alloy.
Table 1. Zirconium additions, iron impurity levels and corrosion rates
of MEZ and grain refined MEZ magnesium alloys
5 day B117 salt spray corrosion rate
Grain refined MEZ
Al (w t %)
Ce (w t %)
La (w t %)
Nd (w t %)
Fe (w t %)
Mn (w t %)
Zr (w t %)
Figure 8 Elemental distribution across a grain boundary in a grain
refined MEZ magnesium alloy
Similar to aluminium in the first group of alloys, the distribution of
zirconium in the grain of the second group of alloys is not uniform.
The grain centre is rich in zirconium (see Figure 8) [#ref12].
The enrichment of zirconium in the grain centre leads to higher
corrosion resistance of this zone. A simple optical observation of the
corrosion morphology (see Figure 9) of the grain refined MEZ
magnesium alloy has verified the relationship between the zirconium
enrichment and the corrosion resistance of the grain interior[#ref12].
From this photo, it can be clearly seen that many grain central areas
remain uncorroded while the grain boundaries have been severely
corroded. This indicates that zirconium can stabilise the magnesium
matrix phase and reduce its corrosion rate.
Figure 9 Optical micrograph of the surface of grain refined MEZ after
immersion in 5% NaCl for 3 hours
The Effect of microstructure
The microstructure of an alloy usually refers to the phase constituents
and their distribution as well as the grain size. All these have a
significant influence on the corrosion performance of a magnesium
The secondary phase in a magnesium alloy is normally inert and stable
in corrosion. For example, in AZ alloys, β-phase is very corrosion
resistant and normally not corroded if exposed to a sodium chloride
solution [#ref9]. This can be clearly illustrated by the corrosion
morphologies of AZ91E as shown in Figure 7. No corroded β-phase
can be observed in the alloy. In most cases, even in the corroded
areas where the matrix phase has been severely removed, the β-phase
is still intact. Similarly, the secondary phase in the MEZ alloy can not
be corroded [#ref11, #ref12]. In Figure 9, the secondary phase is also
intact in the corroded areas.
log|I| (mA/cm )
Figure 10. Polarisation curves for β phase (AZ501), α phase (AZ21) and
α-β binary phase (AZ91) alloys in 1N NaCl (pH11)
The β-phase has an electrochemical polarisation behaviour different
from the α matrix phase or the α-β binary phase alloys [#ref9]. Their
differences are compared in Figure 10. AZ501 is a β-phase alloy,
AZ21 is an α-phase alloy and AZ91 is an α-β binary phase alloy.
According to this figure, the corrosion potential of the β-phase is
much more positive than the α-phase. This means that indicates that
the β-phase is cathodic to the α phase and can accelerate the anodic
dissolution of the α-phase through the galvanic effect in a binary
Even worse, The β-phase is a very effective cathode if it is coupled
with the α-phase as its cathodic polarisation curve of the β-phase is
much higher than the α-phase. Figure 11 shows that the corrosion
rate increases after the introduction of β-phase into a Mg-Al matrix
phase. The alloys presented in the figure were permanent mould cast.
The microstructure of the Mg-1%Al alloy is a single α-phase matrix;
the Mg-5%Al alloy has some β precipitates along the grain boundary;
and more β precipitates are present along the grain boundary in the
Average corrosion rate (mg/cm2/day)
Figure 11. Influence on corrosion rate of the introduction of β-phase
into the permanent mould cast matrix phase
This is contradictory to some other experimental results [#ref10,
#ref17, 18]. For example, diecast AZ91D specimen has different
microstructures in its interior and skin (see Figure 12). The skin has a
greater amount of the β-phase than the interior, and the corrosion
rate of the skin is about 10 times lower than the interior under the
same salt solution (see Table 2).
Figure 12. Back scattered SEM images of die cast AZ91D. (A) skin, (B)
Table 2. Dissolution rates under open circuit conditions.
penetration rate in 1N NaCl (pH11) (mm/y)
Interior of diecast AZ91D
Skin of diecast AZ91D
A proposed explanation for these contradictory results is that the βphase has a dual role in corrosion. The β-phase in an AZ alloy can act
as either a corrosion barrier or a galvanic cathode accelerating
corrosion, depending on the amount and distribution of the β-phase.
Finely and continuously distributed β-phase can more effectively stop
the development of corrosion in an AZ alloy. Otherwise, the presence
of β-phase will accelerate corrosion.
It should be stressed that the distribution of a secondary phase is
closely associated with the grain size. Normally, a grain-refined
microstructure has a more uniform and continuous secondary phase
along the grain boundary. This is clearly demonstrated by the
microstructures of diecast AZ91D and MEZ alloys (Figure 9 and Figure
12). Particularly, Figure 12 clearly shows that the continuously
distributed secondary phase effectively confines corrosion in the
grains. This signifies that grain refinement is an effective approach of
improving corrosion resistance of a magnesium alloy.
Galvanic corrosion is one of the major obstacles to the use of
magnesium parts in the automobile industry . This is because
magnesium is the most active metal in the galvanic series , and a
magnesium alloy component is always an active anode if it is in
contact with other metals. Theoretically, galvanic corrosion can be
eliminated by insulating or blocking the direct electrical contact
between magnesium alloys and other metals. Unfortunately, direct
electrical contacts are sometimes required or unavoidable in the
practical designs because of mechanical and electrical demands.
In practice, aluminium, steel and galvanised steel are popular
materials, and magnesium alloys will unavoidably be in contact with
them. In this case, the metal that forms a galvanic couple with a
magnesium alloy has a significant influence on galvanic corrosion.
Distance from the cathode|anode junction (cm)
Figure 13. The distribution of galvanic current density on “Al|Mg” ,
“Steel|Mg” and “Zn|Mg” galvanic couples.
Figure 13  shows typical distributions of the galvanic current
density on “Al|Mg”, “Zn|Mg” and “Steel|Mg” galvanic couples. The
galvanic current density is the highest when AZ91D, which is the most
widely used magnesium alloy, is used in practice, is in contact with
steel. The least severe is the galvanic corrosion of AZ91D in contact
with aluminium. The galvanic current densities on the magnesium
side and the coupling cathode metal side both decrease with
increasing distance from the “anode|cathode” junction. This indicates
that galvanic corrosion is more severe on magnesium in the area
adjacent to the cathode and the cathode metal is better protected from
corrosion attack in the area adjacent to the magnesium anode.
Distance from the Steel|Mg junction (cm)
Figure 14. The distributions of galvanic current density of “Mg|Steel”
galvanic couple in different exposure configurations: “Steel|Mg” -Steel
and Mg are beside each other; “Steel/Mg” -Steel is above the Mg;
“Mg/Steel” -Mg is above the Steel.
The relative positions of the anode and cathode of a galvanic couple
can also affect galvanic corrosion [#ref21]. Figure 14 shows the
distributions of the galvanic current density of a “Steel|Mg” couple in
different exposure configurations. The galvanic current densities of
this couple in the “Mg/Steel” exposure configuration (Mg above the
Steel) are all relatively lower than in the “Steel|Mg” exposure
configuration (Steel and Mg beside each other), particularly in the
regions adjacent to the Steel-Mg junction. The differences in
corrosion caused by the different exposure configurations could be
ascribed to the fact that the alkalised solution produced on the
magnesium anode surface can easily flow down to the cathode surface
in this exposure configuration. Therefore, the smaller galvanic current
densities in this exposure configuration can be attributed to the
alkalised solution on the cathode surface caused by the corrosion
products coming from the magnesium surface.
In contrast, galvanic current density of the couple in the “Steel/Mg”
(Steel is above the Mg) exposure configuration is higher than in the
“Steel|Mg” exposure configuration. This is because the corrosion
products from the steel cathode are flushed down to the anode
surface, affecting the electrochemical behaviour of the magnesium
anode. A possible mechanism involved in this process is that the iron
ions from the steel surface deposit on to the magnesium surface,
which dramatically increases the impurity level of the magnesium.
Hence, the galvanic corrosion rate increases.
Although corrosion performance is currently a problem for magnesium
alloys, the prospect for magnesium alloys is promising because of
their attractive advantages and potential applications. Particularly,
with the price of magnesium dropping dramatically and approaching
that of aluminium recently, magnesium alloys are becoming a
commercially attractive light material in many industries. How to solve
the corrosion problem has now become an important issue in the
application of magnesium alloys.
The investigation by CAST on the corrosion behaviour of magnesium
and its alloys has clearly elucidated the following points:
1. Hydrogen evolution always accompanies the anodic dissolution of
magnesium alloys and the Negative Difference Effect (NDE) is a
common phenomenon for all investigated alloys.
2. In the aluminium containing alloys the β phase appears to have a
beneficial effect on the corrosion resistance but it can also
accelerate corrosion in some cases. The role of the β-phase
depends on its amount and distribution. The beneficial effect of
the β-phase on corrosion performance can be enhanced by grain
3. Alloying aluminium and zirconium can stabilise the matrix phase of
a magnesium alloy against corrosion. In addition, zirconium is also
an effective purifier for magnesium, and the purification can lead to
significantly improved corrosion resistance of magnesium alloys.
4. The galvanic corrosion of a magnesium alloy depends on the
cathode material. Steel is the worst coupling metal for magnesium
alloys and aluminium is relatively less detrimental. The manner by
which the galvanic couples are exposed is also an important factor
influencing the severity of galvanic corrosion.
More importantly, these fundamental findings by CAST signify that:
1. For magnesium and its alloys, the corrosion rate of can not be
estimated from polarisation curve alone. Electrochemical
measurements should be used with caution in study of the
corrosion of magnesium and its alloys.
2. Since zirconium can purify a magnesium alloy and the corrosion
resistance of the magnesium alloy can be further improved through
grain refinement, an approach for improving corrosion resistance
could be to first purify the raw magnesium ingots with zirconium
and then refine the grain size of the alloy through high pressure die
casting, which would produce a skin with fine grains and a large
amount of continuously distributed secondary phase on the surface
of the casting.
3. In practice, if the electrical contact between a magnesium
component and steel is unavoidable in design, then try to avoid the
rust from the steel depositing onto the magnesium component.
The study was supported by the CRC for Cast Metals Manufacturing
(CAST). CAST was established under and is supported by the
Australian Government’s Cooperative Research Centres Program (CRC).
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