THE INFLUENCE OF INHIBITORS ON MAGNESIUM ANODES
EFFICIENCY
Ostanina T. N., Ovsyannikova A. N., Rudoi V. M., Zaikov Y.P.
Magnesium and its alloys have a wide application as sacrificial anodes in
protecting of steel constructions against corrosion and as anodes, used in
manufacturing of chemical sources of a current. However, magnesium is largely liable
to spontaneous dissolution which is observed even in the conditions of anode
polarization [1]. The unproductive losses caused by spontaneous dissolution reduce
useful life of magnesium anode materials. Structure of environment in which anodes
are used has a significant influence on the self-dissolution process. So the efficiency
of magnesium protectors in sulfate solutions is 45-50%, and it decreases to 24-25 %
in more aggressive electrolytes, which contain chloride-ions. One of the ways of
cutting the metals losses is the use of corrosion inhibitor. When chasing the inhibitors,
it is necessary to take into account the mechanism of the processes proceeding during
anode dissolution of magnesium.
In water solutions in addition to magnesium oxidation process
Mg 0  2e  Mg 2
(1)
there proceeds the self-dissolution on the mechanism of electrochemical corrosion
under anodic current
2e
Mg 0  2Н 2 О 

Mg (OH) 2  Н 2 ,
(2)
which can be accompanied by formation of intermediate compound – magnesium
hydride (II) [2]
4e
2Mg 0  2Н 2 О 

Mg (OH) 2  MgН 2 . (3)
Magnesium hydride is an unstable compound. It is capable to be oxidized under the
influence of anode current (4), to be exposed to hydrolysis (5), or to participate in the
corrosion process (6)
MgH 2  2e  Mg 2  H 2 ,
(4)
(5)
MgН 2  2Н 2 О  Mg(ОН) 2  2Н 2 ,
2e
MgН 2  2Н 2 О 

Mg (ОН) 2  2Н 2 .
(6)
Accumulation of magnesium hydride on a metal surface takes place when the speed
of the formation reaction (3) exceeds the speeds of oxidation reactions (4-6).
Hydrogen implantating into the structure of magnesium with formation of MgH2 on
grains borders of metal causes the decreasing of mechanical strength and promotes
decomposition of small particles of magnesium into solution [3].
Recently we have shown [4], that with the growth of anode current and raising
of intensity of anode dissolution there is a development of an active surface of metal
that leads to increasing of magnesium self-dissolution speed on mechanism of
electrochemical corrosion and by means of decomposition of small particles. Small
particles of magnesium, getting into solution, are exposed to dissolution with
hydrogen evolution (2). All these processes result in increasing of volume of escaping
hydrogen with growth of an anode current, so-called the negative differential effect.
The purpose of the present work was to research the influence of BTA and
EDTA inhibitors in solutions with various composition at anode polarization of
magnesium.
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EXPERIMENTAL PART
As an object of research, the alloy of magnesium containing 0,45 mass % of
calcium and 0,01 mass % of manganese has been chosen.
Investigations were carried out in solutions 0,5mol/l NaCl (A) and 0,17mol/l
Na2SO4 + 0,017mol/l NaCl (B) with the рН being equal to 8. The A solution is a
standard electrolyte for carrying out the corrosion tests. The choice of B solution
composition is caused by the fact that an activator layer, into which the protector is
placed for increasing the environment electroconductivity, contains sodium sulfate.
Adding a small amount of chlorine anions into the solution promotes increasing of
inhibitor protective action owing to synergistic action. The solutions tested differ in
aggressiveness, but they have an equal ionic force. The content of BTA and EDTA
inhibitors varied from 0,0025 to 0,05 mol/l.
Researches of corrosion behavior of magnesium at anode polarization were
carried out within 2 hours under galvanostatic conditions with current density iа =
20A/m2. Cylindrical samples (S = 10cm2) of tested alloy were subjected to these tests.
The graphite core served as a counter electrode. Measurements of the potential were
made in relation to silver chloride reference electrode, and then values of potentials
were recalculated to a normal hydrogen electrode scale. Before carrying out the
experiments the working surface was subjected to smoothing and degreasing in a soda
solution. The total velocity of dissolution ( Р  , g/(m2∙h)) was determined by changing
the samples weight. The amount of the metal dissolved under the current, was the
same in all experiences, and its value was calculated according to the Faraday law,
was equal to Р F = 9,06 g/(m2∙h). During the experiment the volume of escaped
hydrogen was measured and the speed of self-dissolution ( PC , g/(m2∙h)) was found.
Weight of the metal which has passed into the solution as a result of decomposition,
was found as a difference:
Р D  Р   (Р F  PC )
(7)
By a technique developed by Nazarov A.P. and Yurasova T.A. [2], the
quantity of magnesium hydride which was on a samples surface after carrying out the
tests at a constant anode current was determined. For this purpose the sample was
subjected to anode polarization in a solution 1mol/l NaOH at the constant potential (Е
=-1,2 V н.в.э.). During the experiment current changes with time and the volume of
escaped hydrogen were fixed. In these conditions magnesium was covered by a
passive film, and the observed anode current (fig. 1) was caused by ionization of
magnesium hydride:
MgH 2  2OH   2e  Mg (OH) 2  H 2
(8)
MgН 2  4ОH   4е  Mg (ОН) 2  2Н 2 O .
(9)
The charge passed and the volume of the hydrogen escaped allowed to
estimate an yield of reactions 8 and 9 and to calculate the amount of magnesium
hydride on metal samples surfaces ( Р H , g/(m2∙h)).
DISCUSSION OF RESULTS
Potentiometric measurements have shown, steady-state potentials of
magnesium in sodium sulfate and sodium chloride solutions to be close in value. That
is the anion nature does not exert an appreciable influence on potential value at equal
ionic force of a solution.
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ia, A/m2
140
1
120
100
2
80
3
60
40
4
20
0
0
1
2
3
4
5
6
T, min
7
Fig. 1. Dependence of anode current density on time at potentiostatic polarization
( Е  1,2V ) of magnesium in a solution 1mol/l NaOH. Samples were previously
subjected to anode polarization (iа = 20A/m2) within 2 hours in a solution 0,5mol/l
NaCl. BTA concentration, mol/l: 1 – 0; 2 – 0,005; 3 – 0,0075; 4 – 0,01.
With adding BTA and EDTA inhibitors into solutions the steady-state
potential was shifted to the negative area. And, the higher concentration of the
addition is, the greater the shift of potential is (tab. 1).
Table 1. Steady-state potentials of magnesium alloy in the tasted solutions at different
inhibitor concentrations
Solution
A
B
Values of steady-state potential of magnesium alloy, V
without
BТА concentration,
EDTA concentration,
inhibitors
mol/l
mol/l
0,005
0,01
0,05
0,005
0,01
0,05
-1,42
-1,49
-1,59
-1,67
-1,59
-1,62
-1,67
-1,47
-1,53
-1,63
-1,69
-1,60
-1,64
-1,73
The steady-state potential reduction may be caused by increasing of hydrogen
overvoltage. The polarization measurements made showed, that in presence of
inhibitors with contents 0,005 mol/l and 0,01 mol/l resistance of cathode reaction
increased in both solutions. However, the process of hydrogen reduction became
easier at 0,05mol/l concentration. As there are passive films on a magnesium surface,
steady-state potential reduction may mean that surface activation takes place in the
presence of inhibitors. As a result dissolution becomes more uniform, and the
probability of pits formation decreases. Indeed, as it will be shown below, more
uniform magnesium dissolution is observed at the presence of inhibitors.
In the absence of inhibitors, rate of magnesium dissolution, calculated on a
weight decrease ( Р  ), was considerably higher in A solution (fig. 2). This can be
explained by the fact that being depassivator chlorine anion promotes increase in a
segment of an active surface of magnesium where self-dissolution processes take
place. Indeed, intensity of hydrogen evolution, which characterizes the rate of
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corrosion processes at anode polarization, was higher in a solution of sodium chloride
in comparison with a sulfate solution (fig. 3).
P , g/(m2∙h)
25
1
20
2
3
15
4
10
0
0,01
0,02
0,03
0,04
0,05
C , mol/l
Fig. .2 Influence of BTA (1, 3) and EDTA (2, 4) concentration on magnesium
dissolution rate at anode polarization (iа = 20А/m2) in A (1, 2) and B (3, 4) solutions.
PС , g/(m2∙h)
12
10
1
8
2
6
3
4
4
2
0
0,01
0,02
0,03
0,04
0,05
C , mol/l
Fig. 3. Influence of concentration БТА (1, 3) and ЭДТА (2, 4) on intensity of
magnesium corrosion selfdissolution at anode polarization in solutions A (1, 2) and B
(3, 4).
With increasing of BТА and EDTA inhibitors concentration in a solution the
intensity of dissolution (fig. 2) and corrosion selfdissolution (fig. 3) changed on a
curve with a minimum. Such kind of dependence of self-dissolution rate on inhibitor
concentration was observed in both tested solutions. At that the area of inhibitor
concentration, where magnesium losses decreased, was wider (from 0,025 to 0,3
mol/l) in a solution of sodium chloride in comparison with inhibitors concentrations in
a sodium sulfate solution (from 0,005 to 0,15 mol/l). Besides, the maximum degree of
protection in a solution of sodium chloride was 54 % in the presence of 0,01 mol/l
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BТА and it was 50 % in the presence of 0,0075 mol/l EDTA, whereas in sodium
sulfate solution it was 46 % and 43 % in the presence of 0,02 mol/l BТА and EDTA,
correspondingly. In calculating the protection degree extent in presence of inhibitors
the total losses of magnesium in process of anode dissolution were taken into account
(fig. 2).
It should be mentioned, that inhibitors tested reduce selfdissolution only in the
limited range of concentration (fig. 3). Under high concentrations of inhibitors the
intensity raising of hydrogen escaping was observed which testifies the increasing of
corrosion selfdissolution rate as compared to solutions without inhibitors.
The range of concentrations, at which the reducing of the corrosion process
rate was observed, and extent of protection degree were the same for both tested
inhibitors in solution of sodium chloride (fig. 3). In a solution of sodium sulfate, the
action of EDTA inhibitor began at a smaller concentration, than that of BTA inhibitor.
However, in both solutions the maximum effect of BТА adding was a little higher,
than from EDTA introduction.
Comparison of the total losses of magnesium at anode polarization ( Р  ) with
rate of dissolution under the influence of a current ( Р F ) and intensity of selfdissolution by the mechanism of electrochemical corrosion ( PC ) has shown, that there
takes place the decomposition of magnesium small particles into a solution
(decomposition process). The estimation of decomposition intensity ( Р D ) according
to the equation (7) has shown, that magnesium anodes are subject to the greatest
mechanical destruction in a solution of sodium chloride without inhibitors (fig. 4).
PD , PH ,
g/(m2∙h)
PD , PH ,
g/(m2∙h)
a
6
b
6
1
5
1
5
4
4
3
3
2
2
2
2
3
1
1
3
4
4
0
0
0
0,01
0,02
0,03
C , mol/l
0
0,01
0,02
0,03
C , mol/l
Fig. 4. Influence of BТА (a) and EDТА (b) concentration on decomposition intensity
(1, 2) and rate of accumulation of magnesium hydride on a samples surface (3, 4) in A
(1,3) and B(2,4) solutions.
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Due to this, after anode dissolution the samples of magnesium had on their
surface deep ulcers, pits and furrows (fig. 5 a). As you can see, after the electrolysis in
sodium sulfate solution, there are numerous small lesions on the surface of samples,
which are uniformly distributed on metal surface (fig. 5 b).
Adding the BTA and EDTA inhibitors into tested solutions decreased the
intensity of decomposition process (fig. 4), that led to a more uniform dissolution of
magnesium and to preservation of a smooth surface of samples after electrolysis (fig.
5 c, d, e, f). Magnesium losses, caused by particles decomposition into a solution,
sharply decreased with the growth of BTA and EDTA inhibitors concentration in both
solutions. After reaching the inhibitor concentration, at which the maximum reducing
of corrosion process rate was observed, (fig.3), the decomposition, actually, stopped.
With the further increasing of inhibitor concentration, metal decomposition did not
take place.
The analysis of chronoamperograms (fig. 1) obtained under potentiostatic
polarization of samples in sodium hydroxide solution has allowed to determine MgH2
amount on a magnesium surface ( Р H ). The greatest amount of magnesium hydride
was found on the surface of samples which were subjected to dissolution by an
external current in solutions without inhibitors (fig. 4). The increase of BTA and
EDTA inhibitors concentration in both solutions led to decrease in the content of
magnesium hydride on a surface (fig. 4). Comparison of curves course both
Р H  t and Р D  t allowed to make a conclusion, that magnesium losses caused by
decomposition, depend on maintenance of MgH2 on a surface.
Hydrogen saturation of magnesium in sodium chloride and sodium sulfate
solutions was identical; however decomposition intensity in the A solution was almost
2 times higher than this index in the B solution. Obviously, the process of mechanical
decomposition depends on solution aggressiveness. In presence of chlorine anions
there is an intensive and non-uniform dissolution of magnesium, with the formation of
deep ulcers and pits which leads to mechanical destruction of metal.
Thus, the researches carried out allowed to state, that optimum concentration
of BTA and EDTA inhibitors in sodium chloride solution is 0,2 mol/l, and in a
solution of sodium sulphate is 0,0075 mol/l for EDTA and 0,01 mol/l - for BТА. On
pictures (fig. 5 е, f) it is clearly seen, that samples which were exposed to anode
dissolution in solutions at optimum concentration of inhibitors, have an even, smooth
surface. When inhibitor concentration becomes above optimum, braking degree of
corrosion process of anodes destruction decreases, but character of dissolution
remains uniform as decomposition of small particles does not take place.
CONCLUSIONS
1. It is stated, that BTA and EDTA inhibitors slow down the process of
magnesium selfdissolution at anodic polarization both in a solution 0,5mol/l NaCl,
and in a solution 0,17 mol/l Na2SO4 + 0,017mol/l NaCl.
2. The range of concentrations, where it is observed the decreasing of
magnesium corrosion selfsolution rate of in anodic polarization is wider in sodium
chloride solution.
3. With addition of investigated inhibitors into solution magnesium losses are
noticeably reduced, as a result of decomposition of small particles, it promotes the
uniform dissolution of samples and the preservation of smooth surface.
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4. Decomposition intensity in solution of sodium chloride is noticeably higher
than the one in solution of sodium sulfate, also it depends on hydrogen saturation
(presence of magnesium hydride on the surface).
a
b
c
d
e
f
Fig. 5. Appearance of samples after anodic dissolution (iа=20А/м2) within 2 hours in
A (a, c, e) and B (b, d, f) solutions. The concentration of inhibitors: a, b – is not
present; c – 0,02mol/l BTA; d –0,075mol/l BTA; e – 0,01mol/l EDTA; f – 0,02 mol/l
EDTA.
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The present work has been done with the financial support of innovative
educational program of the USTU-UPI.
REFERENCES
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dissolution of metals’. Itogi nayki. Electrochimia 1971 7 5-64.
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magnesium dissolution at the presence of complexing agent’. Zaschita metallov 1995
31 (2) 139-144.
3. Nazarov А.P., Lisovsky А.P., Mihailovsky Yu.N.: ‘Formation of MgH2 in
electrochemical dissolution of magnesium in aqueous electrolytes’. Zaschita metallov
1989 25 (5) 760-765.
4. Kozlova A.N., Ostanina T.N., Rudoi V.M., and others: ‘Corrosion and
anode dissolution of magnesium alloys in presence of inhibitor’. Zaschita metallov
2008 44 (6) (Paper is being in publication)
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