Volume 4 Preprint 4
Corrosion Behaviour Of Magnesium Sacrificial Anodes In Tap Water
F. Di Gabriele and J. D. Scantlebury
Keywords: Sacrificial anodes, AZ63 magnesium alloy, cathodic protection, anode efficiency, current capacity, hydrogen evolution, negative difference effect
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CORROSION BEHAVIOUR OF MAGNESIUM SACRIFICIAL
ANODES IN TAP WATER
F. Di Gabriele and J. D. Scantlebury
Corrosion and Protection Centre, UMIST, PO Box 88, Manchester, M60 1QD, UK
Cathodic protection of potable water storage containers, with magnesium sacrificial
anodes, is commonly used. Magnesium alloys are usually preferred to aluminium- and zinc-
based alloys in high-resistivity environments, because they have a lower operating potential.
To evaluate the corrosion properties of the anodes, galvanostatic and potentiodynamic
polarisation tests were carried out to determine anode efficiency and corrosion rate. The
effects of applied current, testing time and microstructure on the electrochemical properties
of sacrificial anodes of alloy AZ63, in potable water, were evaluated. Optical and electron
microscopy (SEM) techniques were used to examine and analyse the microstructure of the
specimens and, correlate it with their corrosion behaviour. The contribution of phenomena
such as mechanical material loss and hydrogen evolution on the current wastage was
investigated. The effect of a different microstructure, after heat treatment, to reduce the
intermetallic phase, was also tested and the main differences highlighted.
Sacrificial anodes, AZ63 magnesium alloy, cathodic protection, anode efficiency,
current capacity, hydrogen evolution, negative difference effect.
Alloys of aluminium, magnesium and zinc are used as sacrificial anodes for metal
protection in aqueous and soil environments. Aluminium and zinc-based alloys are widely
used for CP of steel in marine environments, while magnesium-based alloys are more
appropriate in high resistivity environments, because of their theoretical low half-cell
potential, as well as the possibility of high current capacity.
The present study has focused on the use of magnesium alloys for the protection of
tap water storage systems. One of the most used magnesium alloy is the cast AZ63, with a
sufficient amount of aluminium to give rise to the secondary β phase, Mg
anodes were tested with galvanostatic and potentiodynamic methods, in order to establish
their efficiency and their corrosion behaviour in the conditions imposed.
The actual limit in the use of magnesium-based sacrificial anodes is their relatively
low efficiency, giving rise to the loss of a substantial part of the required current capacity.
Phenomena affecting the magnesium anodes efficiency, such as local cell action (LCA) ,
chunck effect (CE) , film breakdown  and uncommon ion valence , were considered.
Another characteristic of magnesium alloys, the Negative Difference Effect , was verified
during the experiments carried out. Moreover, a simple relationship was established
attempting to quantify the amount of charge lost on non-electrochemical phenomena like
LCA and CE.
Correlation between alloys' microstructure and corrosion behaviour of anodes was
characterised. The presence of aluminium, in the magnesium alloys, leads to precipitation of
an intermetallic phase, Mg
. This secondary phase has an important role in galvanic
corrosion of magnesium alloys, since the β-phase seems to be more resistant than the
surrounding matrix alloy . A heat treatment was employed to modify the microstructure of
the alloy. The β-phase was partially dissolved; in consequence, the corrosion behaviour of
the anodes was modified. Anode efficiency, current capacity and corrosion parameters were
also calculated for the heat-treated alloy.
AZ63 ingots were melted in a steel crucible at a temperature of 690°±10°C. The
chemical composition of the alloy, as provided by the manufacturer, is listed in Table 1.
3The anodes were provided in form of bars, diameter 22 mm, length 220 mm. They
were cut in cylinders, 70 mm long. The cross section of each specimen was covered with
beeswax, in order to expose to the electrolyte only the lateral surface of the anodes.
In the galvanostatic tests, the counter electrode was a mesh of galvanised steel. The
mesh was adjusted around the anode to keep the active metal in the centre of the cell.
Attention was paid on maintaining the same distance from each point between anode and
electrode, in order to avoid non-uniform current distribution. The reference electrode used for
the galvanostatic tests was a SCE, with a Luggin capillary to minimise the iR drop.
The galvanostatic tests and the weight loss measurements were performed according
to the ASTM G97 standard . The standard suggests a current of 0.39 A/m
; as well two
other impressed current densities (0.05 and 1 A/m
) were chosen. The samples were exposed
to the galvanostatic test for three different times: 1, 2 and 4 weeks. The experimental
equipment consisted of a galvanostat, a voltmeter and a coulometer. The coulometer was
built according to ASTM G97 .
For the weight loss tests, the samples were connected in series as shown in Fig. 1. The
first anode of the sequence was connected to the plating copper wire in the coulometer, while
the last steel mesh was linked to the galvanostat. The copper sheets in the coulometer were
finally connected to the anode pole of the current supply.
The electrochemical potential of the anodes was measured during the testing time.
The potential was measured at the moment the system was switched on and also during the
first hours of polarisation. After that, there was a reading every 24 hours. For the long term
tests the potential was read every week. At the end of the galvanostatic tests a potential
measurement was taken before and one hour after the system was switched off (potential on
open circuit, OC).
At the end of the weight loss tests the anodes samples were cleaned of the corrosion
products in their surface, by immersing them in a 250g/l CrO
post-cleaning bath solution, for
30 minutes. After that, the magnesium anodes were rinsed, dried and weighed. At the same
time the copper wires of the coulometer, weighed before the test, were extracted, rinsed,
dried and weighed. Evaluation of the mass loss for the anodes and weight gain for the copper
wires allowed determining the efficiency of the anodes and their current capacity.
The NACE TM0190-90  standard was adopted to conduct the hydrogen evolution
tests. The volume of evolved hydrogen collected from the surface of the anodes during these
tests allowed the calculation of charge lost because of hydrogen reduction reaction.
4The potentiodynamic tests were carried out to evaluate the corrosion behaviour of
magnesium anodes. The working electrode was constructed by mounting a small specimen
(10x10x8mm) in cold-setting resin. The metal surface was polished till 1200 paper grid. A Pt
electrode was used as a counter electrode and a calomel electrode, as reference electrode. The
potentiodynamic curves were obtained using a PAR 273 potentiostat, with a voltage scan rate
of 0.2 mV/s. All the electrochemical tests were performed in artificial tap water prepared in
order to simulate the actual tap water, with medium value of hardness, neutral pH, and
relatively high resistivity (low conductivity). The chemical composition and the
electrochemical properties of the artificial tap water are given in Table 2.
A heat treatment was imposed in order to change the microstructure of the alloy. The
samples were heated in a furnace to 385°C, maintained at temperature for 10 hours, then
quenched in water at 20°C. Temper designations were in accordance with ASTM B 296 .
Samples examined in the optical and electron microscopes were metallographically
finished. The etching solution used to highlight the general microstructure was a nital 3%
RESULTS AND DISCUSSION
The galvanostatic tests are designed to simulate the service operating conditions of
sacrificial magnesium anodes. The fundamental properties obtained by these tests are anode
potential, current capacity and anode efficiency.
The values of the potential, monitored during the galvanostatic test, showed that at the
very beginning of the experiment, the potential of the anodes was approximately -1.50 ± 0.05
V vs. SCE. The value increased in the first few hours until it reached a relatively stable level
of circa -1.33 V for all the remaining exposure time, for all the specimens.
The tests were performed at several exposure times (7, 14 and 28 days respectively).
The potential values differed slightly, but only at the beginning of the tests. Moreover, the
anodes working at low current density reached a stable level quite soon, while the others
needed a longer time to achieve a steady state. At the end of each experiment, the potential at
open circuit (one hour after switching off the system) was V
= -1.35 ± 0.03V vs. SCE. This
value, nobler than the ones measured at the beginning, is due to the white layer of the
corrosion product built-up on the anode surface. However, the differences on potential
values, at different impressed anodic currents and different testing time, were quite small.
5Then, it could be assumed that the magnitude of the working current and the length of the
tests (4 weeks the longest exposure) did not affect the anode potential.
The current capacity assessed by weight loss during the galvanostatic test
was evaluated as established by the ASTM G97 standard . Thus, the actual current
capacity of the anodes, CC, is obtained according to the following formula:
Mis the initial mass of Magnesium anode and
is the final mass (mass in
grams). The theoretical current capacity, evaluated for this alloy, is
The anode efficiency is defined as the ratio between actual and theoretical values of
current capacity, and it is evaluated by the following expression:
where 2204 [Ah/kg] is the theoretical equivalent weight for pure Magnesium.
The current capacities (CC) measured for the anodes allowed to gain the values of
efficiency listed in Table 3. The values of CC were found to be very low, but they increased
with current density and time. The values of anode efficiencies indicated that the anodes are
able to supply only a partial amount of current for protection. The missing percentage
represents the amount of charge lost, which is no longer useful for CP.
The current delivered for CP is very low (the anode efficiencies being very small) and
acceptable values are obtained only at high impressed current densities. This condition does
not imply a better performance for anodes working at elevated current densities, since they
dissolve much faster than the ones operating at low current densities.
The principal factors affecting anode wastage are the hydrogen evolution, mainly due to
local cell action (LCA), and the mechanical loss of metal pieces from the anode surface. The
latter phenomenon is known as
chunk effect (CE). The presence of impurities in the alloy,
usually heavy metals particles (Fe, Ni, Cu) and second phases, leads to the formation of
micro-cells on the anode surface [10, 6]. Fagbagyi  asserted that at increasing anodic
polarisation of metals, the effect of cathodic current due to micro-cells should be reduced.
Instead of that, with Mg anodes, at increasing impressed current, the hydrogen evolution
increased, as it will be shown later.
6Chunk effect (CE) can be defined as the complete physical detachment of metal
portions from the bulk alloy, during the period of anodic polarisation . CE is a damaging
mechanical process giving rise to a mass loss, then, the missing metal is no longer useful for
CP  and the efficiency is lowered. In case of pure magnesium, Hoey et al  measured
the particles of metal detached from the bulk and found that they are particles of
mm. They are held together by Mg(OH)
. In the case of magnesium
alloys, the particles falling down from anodes are mainly magnesium, together with the
impurities (e. g. iron) on the anode surface and, in time, the internal particles of the secondary
phase. During anodic polarisation, the metal reacts preferentially along grain boundaries, slip
planes, dislocation lines and/or along concentrations of vacancies and impurities. In this way,
when the anodic current is concentrated along the grain boundaries, the metal particles are
detached because of partial undermining.
The microstructure of AZ63 alloy (fig. 2a) shows an equiaxed arrangement, with
eutectic precipitation of β-phase (fig.2b) located homogeneously along the grain boundary as
well as inside the grains. SEM examination on β-phase particles indicates the presence of
nuclei of Mg
surrounded by the eutectic lamellae of the same hard phase. The average
size of β particles was around 15µm. Dimensions of grains were very large and not uniform,
in a range of values between 100 and 350 µm. The large grains were principally located in
the core of the anode bar, while in the external surface there were the small ones. This
distribution was mainly due to the process of production of the anodes.
Figure 3 represents cross sections of the anodes, before (fig.3A) and after (fig.3B)
exposure to the galvanostatic test. Optical examination, after each test, showed that they had
common features independently from impressed current and exposure time. The corrosion
process seemed to occur, preferentially, in the magnesium-base matrix, while the β-phase
was standing, unreacted. Figure 3B shows that the metal surrounding the second phase
particles was dissolved in the galvanic test. Eventually, the β-phase particles fell off when all
the adjacent metal was corroded. The most probable explanation to this phenomenon is in the
fact that the β-phase played the role of cathodic site, while the α-phase (metal matrix) is
anodic to the former one and consequently corroded. The corrosive attack acts preferentially
on the matrix, where metallic magnesium drops in the form of chunks, detached from the
matrix in those points where the metallic bonding is weakened (defects, grain boundaries,...).
The second phase in the magnesium cast alloys is generally more noble than the
surrounding matrix alloy . Then, the eutectic Mg
particles represented the cathodic
7sites for the micro-cells formed on the anode surface (LCA). Hydrogen reduction took place
on the cathodic sites and it is believed that the gas evolved was probably able to impress a
mechanical stress to undermine pieces of metal from the bulk. At the time the chunks left the
bulk, a more extended area is exposed to the electrolytic solution which was then able to give
rise to new cathodic reactions.
Anode efficiency could be evaluated measuring the hydrogen volume evolved during
the galvanostatic tests. In accordance to the NACE TM0190 standard , the following
expression was used to evaluate the efficiency, E%, of the anode:
I is the impressed current, V is the volume of hydrogen and t is the elapsed time.
The values of anode efficiency obtained by hydrogen evolution tests are listed in
The efficiency of the anodes is higher when evaluated with hydrogen evolution tests,
compared to the weight loss ones. The main reason for those results is in the fact that
take into account the total loss of material (there are no distinctions between
electrochemical and mechanical effects); while,
Hydrogen evolution tests evaluate the anode
efficiencies in function only of the electrochemical behaviour of the anode.
The difference between the values obtained with the two measurements decreased at
higher current densities. This may suggest that at high current density the mechanical loss is
less important and/or the electrochemical effect, giving rise to hydrogen evolution, is
In accordance to equation (1), the anode efficiency should decrease at increasing values
of volumes of hydrogen. However, the efficiency increased, suggesting that the LCA effect
was stronger at the beginning of the tests and diminished with time.
The rate of hydrogen evolution,
I, is defined as the charge elapsed due to hydrogen
formation (and then useless for CP).It can be determined by Faraday's law,
mRTPV=, obtaining the following expression:
I, hydrogen evolution rate, and if Y=mX+c , the efficiency can be defined by the
By plotting the Y value versus the impressed current, I, the rate of hydrogen evolution can be
(current density being actually used,
i = I/S).
In the range of analyses carried out, the linear relationship established by Faraday's
law was found to be consistent with the experimental data (Fig.4). Theoretically, the law
establishes that at zero current impressed, there is no hydrogen evolved. The experimental
data, actually, showed that the straight lines, if ideally prolonged to the y axes, would
intercept the axes at positive values. This result may suggest that the anodes would react
spontaneously in the electrolyte without any current applied. This phenomenon is due to the
presence of the micro-galvanic couples on the surface of the alloy (LCA).
It was noticed that, at increasing current density the rate of evolved hydrogen
increased too. The phenomenon is defined as NDE (
Negative Difference Effect). The Positive
is the most common/opposite observable event for most metals. Hoey et al.
 explained this anomalous behaviour as the result of film surface disruption. Where, by
film is defined the corrosion products layer built up on the anode reacting surface, in this case
a layer of magnesium hydroxide. The theory proposed asserts that pieces of corrosion
products break away, due to mechanical effects. The underlying bare metal is, then, directly
exposed to the electrolyte. This implies that a larger area for reduction reactions is available.
Heat treatment was carried out on a second set of specimens. The anodes were
exposed at 385°C for 10h and then quenched in water. The
β-phase precipitate, after
9treatment, to particles of circa 3
µm (fig.5). surface of the specimens was expected to change
the electrochemical properties, due to the change in the ratio between anodic and cathodic
Galvanostatic and hydrogen evolution tests were repeated with the heat-treated
anodes. Representative results for 2 week tests at 0.39 A/cm
are here discussed. Weight loss
tests showed a reduction of the anode efficiency compared to the untreated anodes (table 5).
The difference in the values before and after the treatment is lower at increasing current
applied; the difference between the values varies between 28 and 7%. The decrease was
higher in case of efficiency calculated by hydrogen evolution (table 6), since the amount of
gas evolved was higher than in the untreated specimens. The difference between the values
varies between 30 and 11%.
However, changing the alloy microstructure by dispersing the eutectic particles with a
heat treatment did not improve the anode performances. The results indicate that the
intermetallic phase, Mg
plays an important role on the corrosion behaviour of the
AZ63 alloy. The function of
β-phase has already been reported for other Mg-Al-Zn alloys
An AZ63 alloy was used for the magnesium sacrificial anodes tested in potable water.
The anodes were tested in order to evaluate main properties such as efficiency, current
capacity and charge loss by hydrogen evolution.
The results showed relatively low efficiency and current capacity, especially at low
current densities and short testing time. The processes that contribute to the reduction of the
anode efficiency are believed to be LCA and CE.
Linearity of Faraday's law was verified in each case, but changes in slopes and zero-
current output were modified because of the phenomenon mentioned above.
A heat treatment was carried out were the size of the secondary phase particles was
reduced. Results showed that the performances of these anodes were lower compared to the
original cast alloy, indicating that the
β-phase, even if cathodic to the base alloy, played an
important role in the protection of magnesium alloys, as cast.
The authors wish to acknowledge PROCAT Bolzano for providing the anodes and
sharing the information on anodes production, ERASMUS project officers for allowing the
partnership between University of Trento and UMIST Manchester. We thank Dr R. K.
Fagbagyi and L. Benedetti for the essential support during the experimental work.
 Metals Handbook, 9
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Materials and Corrosion, vol. 50, (1999) pp. 2-6.
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 G. R. Hoey, M. Cohen,
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 ASTM B 296, Annual Book of ASTM, B 296, vol. 2.02, 1990
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Al Mg Mn Zn Si
5.5-6.5 89-92 <0.15 2.4-3.5 <0.05 <50 <60 <10 5-15
Table 1 - Chemical composition, wt.%, of AZ63 alloy.
Components Composition Electrochemical Parameters
170mg/l pH 7.4
50mg/l Hardness 20°F
210mg/l Conductivity 590µS/cm
Table 2 - Composition and electrochemical properties of the artificial water
Anode efficiency %
] 1 week 2 weeks 4 weeks
0.05 11.67 14.69 15.84
0.39 41.73 48.71 49.23
1 50.39 56.83 58.28
Table 3 - Anode efficiency evaluated with weight loss tests as a function of the applied currents and
Anode efficiency %
] 1 week 2 weeks 4 weeks
0.05 37.92 40.42 47.19
0.39 70.37 70.37 72.93
1 72.63 72.63 76.53
Table 4 - Anode efficiency evaluated with hydrogen evolution tests as a function of the applied current
and exposure time
E % after heat treatment E % before heat treatment
] 2 weeks 2 weeks
0.05 10.47 14.69
0.39 40.00 48.71
1 52.60 56.83
Table 5: Efficiency of anodes before and after heat treatment
evaluated by weight loss tests
E % after heat treatment E % before heat treatment
] 2 weeks 2 weeks
0.05 28.29 40.42
0.39 52.42 70.37
1 64.37 72.63
Table 6: Efficiency of anodes before and after heat treatment
evaluated by hydrogen evolution tests.