Fachikov and Metallurgy, 40, 4, 2005, 319-322
Journal of the University of ChemicalL.Technology
CORROSION RESISTANCE OF SOME METALS IN IODIDE
AND PERCHLORATE MERCURY SOLUTIONS
L. Fachikov
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
Received 28 October 2005
Accepted 11 November 2005
E-mail: [email protected]
ABSTRACT
With the help of gravimetric and polarization methods, the corrosion resistance of different metals and alloys in
iodide and mercury perchlorate electrolytes has been studied. The tests have been carried out upon conditions allowing
both contacts and no contacts of the samples with metallic mercury as well as under the influence of the anodic and the
cathodic polarization of metal surface.
It is established that Mo, Ni and the alloys NiMo79-5 and X5CrNiTi18-9 have high corrosion resistance in iodide
and perchlorate electrolytes in both the cases with contact and without contact with mercury. The corrosion resistance of
metals decreases upon conditions of polarization of the samples and Ti and Mo can be determined as most resistant in
iodide and Ni, X5CrNiTi18-9 and Mo in perchlorate electrolytes.
Keywords: corrosion, corrosion resistance, mercury electrolytes.
INTRODUCTION
The mercury electrolytes are characterized as high
aggressive to metal materials in contact with them. At
the same time the literature data concerning the metal
behavior in these media are quite scarce. For example,
the authors [1] have studied the corrosion resistance of
different metals and alloys in acid perchlorate and neutral mercury-iodide solutions. On the basis of the results obtained, they recommend that in acid electrolytes W and the alloys X5CrNi18-10 and NiCrCu47-25 should be used as current leading terminals of mercury coulombmetres, while the other tested metals
(nickel, iron, platinum and etc.) should be used only in
neutral iodide electrolytes. The cathodic polarization
of metal samples, depending on their nature may increase or decrease their corrosion resistance in mer-
cury electrolytes [2]. The dissolution of metals in solutions containing mercury can be related either to their
direct oxidation from mercury and hydrogen ions or to
the decomposition of the amalgams formed by the interaction of metals with mercury. Mo demonstrates high
corrosion resistance both in mercury – rhodanate solutions and with the alloys X5CrNiTi18-9 and NiMo795 in sulphuric acid media [4]. The main condition for
the resistance of metals is the appropriate correlation
of their electrode potentials with the oxidation - reduction potentials of mercury - electrolytes media.
This work presents the results obtained in the study of
the corrosion behavior of different metals and alloys in iodide and perchlorate mercury electrolytes with contact and
without contact of the experimental samples with mercury as
well as the influence of the cathodic and the anodic polarization upon their resistance in these solutions.
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Journal of the University of Chemical Technology and Metallurgy, 40, 4, 2005
EXPERIMENTAL
The following water solutions of mercury salts
were used as working media:
• Solution No1 - 0.5 M Hg J2; 3.0 M KJ; 2.0 M
KCl
• Solution No2 - 1.0 M Hg2 (ClO4)2; 2.0 M
HClO4
The solutions were prepared with p.a. substances
and monodistilled water. The working volume of the
solutions was 10 ml.
The metallic samples had rectangular shapes and
working areas of 1.0 cm2 .The isolation of the nonworking surface of the samples was prepared with
polyvinylchloride (PVC). The experiments were carried
out with the following metals: Mo, Ni, Ti, X5CrNiTi189, NiMo79-5, CrA120-5.
The corrosion stability of the selected metals was
determined by a gravimetric measurement of the change
of the sample mass during its contact with the medium.
Fig.1 schematically presents the shapes of the glass electrochemical cells, employed in the tests. The cells were
provided with caps for the samples to be mounted and
isolated from the medium.
During the gravimetric studies, cells filled with
the solutions were under thermostatic conditions in a
water bath (20 ± 1oC). The pre-weighed samples were
put in the cells and closed by PVC caps. After a determined time interval the samples were removed, rinsed
with water, dried and a visual test of the treated surface
was carried out. The samples were weighed again and
the corrosion rate was calculated.
During the tests for determination the effect of
the cathodic and the anodic polarization upon the behavior of metals, the two samples (of the same metal)
were mounted in the working compartments of the cell
(Fig.1.) as a cathode and an anode, respectively. The
current with a desired density was supplied by a
galvanostat and after the experiment the samples were
treated in the already described method.
RESULTS AND DISCUSSION
Table 1 presents both the values of the corrosion
rate and the corrosion potential of the metals tested
with contact and without contact with mercury in solution No1. It is evident that all the materials except the
alloy CrA120-5 are stable in this solution in both the
cases with contact and without contact with mercury.
The corrosion rate of molybdenum, nickel and steel
X5CrNiTi18-9 in contact is a little increased, while for
the NiMo79-5 it remains the same. The corrosion potential of all materials tested in solution No1, as a result
of their contact with mercury is removed in negative
direction and its values are nearly equal.
Table 1. Corrosion rate K*,g m-2h-1 and corrosion potentials E, mV( SCE) of the metals tested in a solution
of 0.5 M HgJ2; 3.0 M KJ and 2.0 M KCl
Metal
no a contact with Hg
with a contact with Hg
K
E
K
E
Molybdenum
-2
2.5x10
-320
-2
4.5x10
-353
Nickel
-3
7.0x10
-248
-2
1.0x10
-353
X5CrNiTi18-9
-3
9.0x10
-80
-2
2.0x10
-351
NiMo79-5
-3
5.0x10
-258
-3
4.0x10
-350
CrAl20-5
-2
9.5x10
-150
-1
2.6x10
-297
Note: Time duration of tests: a) without a contact with Hg –
474h; b) with a contact with Hg – 474h.
*The value of K, is an arithmetic average of the data obtained
from three samples.
Fig. 1. Schematic presentation of the cells employed in the tests:
1-samples; 2-PVC caps; 3-isolation of the non-working surfaces
of the samples; 4-working solutions; 5-glass cells.
320
According to the results presented in Table 2, the
stability of the metals tested in perchlorate mercury solution can be determined. In these media the stability of
all tested metals was not changed during their contact
with mercury. At the same time the corrosion potentials
are positive, but in contact with mercury, analogous to
L. Fachikov
Table 2. Corrosion rate K, g m-2h-1 and corrosion potentials E, mV( SCE) of the metals tested in a solution
of 1.0 M Hg2(ClO4)2; 2.0 M HClO4
Metal
no contact with Hg
K
Molybdenum
Nickel
X5CrNiTi18-9
NiMo79-5
CrAl20-5
1.0x10
8.3x10
2.0x10
1.0x10
7.0x10
E
-2
407
-2
443
-2
-
-2
-
-2
382
with a contact with Hg
K
3.0x10
4.8x10
3.0x10
1.0x10
2.5x10
E
-2
313
-2
369
-2
375
-2
375
-1
297
Fig. 2. Effect of the current density on the corrosion rate of
molybdenum: A-anodic current; C-cathodic current a) in solution
No1; b) in solution No2.
Note: Time duration of test: a) without a contact with Hg -474
is formed on the surface, which causes interruption of
the electric circuit.
Table 3 presents the results concerning the corsolution No1, they are displaced in a negative direction.
rosion resistance of the other metals according to the
The positive values of the corrosion potential, which the
polarizating current density in the two mercury elecmaterials receive, are related to the high oxidation-retrolytes. The corrosion rate data indicate that for all
duction potential of the perchlorate medium.
polarizating current densities, they are higher than those
Fig. 2 illustrates the effect of the metal surface
obtained at an open electric circuit. Therefore, the corpolarization on the corrosion resistance of molybderosion resistance of the tested materials during the ponum in the two working solutions. It can be seen that
larization of the metal surface (cathodic and anodic) is
the effect of the cathodic polarization is more essential
reduced to a large degree in the two working solutions.
for the two solutions when the corrosion rate increases
The results in Table 3 also show that for some metals
significantly for the tested range of the current density.
the cathodic polarization plays more important role on
The stability of molybdenum in mercury-iodide solutheir corrosion rate (for instance, CrA120-5 in iodide
tion increases with increasing the density of the anodic
solution), while for others the anodic polarization is
current and vice versa, it decreases in mercury perchlomore essential (for instance, X5CrNiTi18-9 in perchlorate. When current densities are higher than those shown
rate solution).
in the figure, a thick blue film with very high resistance
The type of corrosion attack is changed at values
of the polarizating current higher than
those of the tested - from common the
Table 3. Effect of the polarization of metal surface on the corrosion rate
corrosion transfers into local, especially
K, g m-2h-1 of the tested metals
at the anodic polarization of the metal
Sample
Corrosion rate
surface.
Solution ¹1
Solution ¹2
The values, obtained for the corCathodic
Anodic
Cathodic
Anodic
rosion rate of different metals, provide
polarization
polarization
polarization
polarization
a possibility to determine the expedi2
2
2
2
i,mA/cm
i,mA/cm
i,mA/cm
i,mA/cm
ency of their use in solutions contain1
5
10
1
5
10
1
5
10
1
5
10
ing mercury. For example, titanium and
Nickel
4.4 30.4 43.0 3.0 3.2 125.5 0.5 1.49 1.0 2.63 1.53 5.0
molybdenum irrespective of the polarX5CrNiTi18-9 0.23 0.2 1.7 7.0 5.5 80.0 0.3 0.55 1.05 3.25 7.95 10.5 ization have the highest resistance in
CrAl20-5
0.51 5.7 3.0 0.25 0.5 0.61 0.75 1.50 1.01 1.0 3.25 22.0 mercury- iodide solution, while Ni, Mo
Titanium
0.51 1.5 1.7 1.19 1.2 1.71
6.0 3.0
13.5 56.1 and X5CrNiTi18-9 have the highest reh; b) with a contact with Hg – 474h.
321
Journal of the University of Chemical Technology and Metallurgy, 40, 4, 2005
sistance in mercury-perchlorate solution. At the same
time the results presented allow establishing the limits
of the degree of polarization in which each one of the
tested metals is stable.
two solutions. The results obtained allow titanium and
molybdenum to be determined as most resistant in iodide
solutions, while nickel, molybdenum and X5CrNiTi18-9
as most resistant in perchlorate mercury solution.
CONCLUSION
REFERENCES
With the help of gravimetric studies, the corrosion resistance of some metals and alloys in electrolytes containing mercury has been determined. It was
established that:
a) Molybdenum, nickel and the alloys NiMo795 and X5CrNiTi18-9 demonstrated high corrosion resistance in iodide and perchlorate solutions both with
contact and without contact with mercury.
b) The cathodic and the anodic polarization of the
tested metals had strong influence on their stability in the
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Metallov, , 5, 1976, 585, (in Russian).
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Technol. Met.,!#, 2000, 115, (in Bulgarian)
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