Mechanochemical effects in redox reactions with participation of iron

Article abstract of DOI:10.1134/S1070363207030115

The processes of deposition of copper, silver, and lead from their aqueous solutions separately on the concave (contracted) and convex (stretched) sides of a bent steel plate have been investigated. The difference in the deposition rate on the opposite sides gives evidence of the existence of the mechanochemical strain sign effect in redox reactions with the participation of iron. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S1070363207030115

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 3, pp. 398 403.
Pleiades Publishing, Ltd., 2007.
Original Russian Text T.G. Movchan, A.I. Rusanov, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 3, pp. 433 438.
Mechanochemical Effects in Redox Reactions
with Participation of Iron
T. G. Movchan and A. I. Rusanov
Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Akademy of Sciences,
Leninskii pr. 31, Moscow, 117915 Russia
Received December 28, 2006
Abstract The processes of deposition of copper, silver, and lead from their aqueous solutions separately
on the concave (contracted) and convex (stretched) sides of a bent steel plate have been investigated. The
difference in the deposition rate on the opposite sides gives evidence of the existence of the mechanochemical
strain sign effect in redox reactions with the participation of iron.
DOI: 10.1134/S1070363207030115
INTRODUCTION
strain sign effect in redox reactions occurring on the
surface of steel plates at their contact with solutions of
the salts of other metals. The classical example of
such a reaction is the interaction of iron and copper
sulfate. At a contact of iron with a solution of this
salt, the reaction occurs:
It is known for a long time that mechanical stresses
in solids influence the rate of chemical reactions. A
mechanochemical effect of the strain sigh is more
delicate and is manifested, in particular, as a dif-
ference in the rate of a chemical reaction on the op-
posite (concave and convex) sides of an elastically
bent plate, which are different only with respect of
the strain sign. The effect was discovered in 1988 by
the example of the simple dissolution of a monocrys-
talline plate of potassium chloride [1]. In contrast with
the classical theory of elasticity, a thermodynamic
theory of this effect [2, 3] implies the existence of an
initial stress in solids in the absence of external forces.
Since an initial stress is always present in the surface
layer of a solid (as the surface tension), the theory
predicts manifestation of the strain sigh effect for any
chemical reaction with the participation of a solid,
occurring at the solid surface. An experimental con-
firmation have already been obtained at studying cor-
rosion of metals under stress (at dissolving bent steel
plates in solutions of hydrochloric and sulfuric acids)
[4 6], and also dissolving glass and silicon in an
aqueous solution of hydrogen fluoride and potassium
hydroxide, respectively [7]. A difference in the rate
of dissolution on the concave and convex sides of a
bent plate was discovered with the use of various
methods: the weight method in its classical [4] and
modern (with continuous measuring the sample mass
in time) [5, 6, 8] variants, the hydrogen method (by
measuring the volume of hydrogen formed in the
course of a reaction) [5, 6], and optically (by visual
observing the shift of the both opposite boundaries of
a sample through a microscope) [7].
Fe + Cu2+ = Fe2+ + Cu.
Here iron, a reducing agent, donates electrons:
Fe = Fe2+ + 2e.
(1)
(2)
Herewith, the Fe2+ ions are hydrated and pass to
solution. The copper ion, an oxidant, accepts electrons:
Cu2+ + 2e = Cu.
(3)
The reduction of copper to the zero valence should
pass via the stage of formation of a univalent ion of
Cu+:
Cu++ + e
Cu+ + e
Cu+,
(4)
(5)
Cu (deposition),
because the equilibrium potential of the reaction (4)
(0.15 0.17 V) is more negative than the equilibrium
potential of the reaction (3) (0.34 V) [9]. Both the
elementary reactions proceed at the place of contact
of iron with the solution, so that electrons pass di-
rectly from iron to copper ions.
0
Since the oxidizing agent, copper, (with
0.337 V) and the reducing agent, iron, (with
=
=
0
0.440 V) are located far from each other in the
The goal of this work was the investigation of the
sequence of standard electrode potentials 0, iron will
398

MECHANOCHEMICAL EFFECTS IN REDOX REACTIONS
399
displace copper from an aqueous solution of its salt
practically at any solution concentration. The effect i
s easily visible due to a change of color of the surface
of a steel plate after the deposition of copper on it.
classical placing a subject on the scale and at hanging
it below the scale. In the first variant, the vessel with
a solution was placed on the scale, and, when sus-
pending the cell with a plate immersed from an in-
dependent support, only the changing mass of the
solution was measured. In the second variant, only
the cell with a plate immersed was suspended from
the balance scale, whereas the solution itself rested on
an independent from the balance basement. In this
case, the mass change referred only to the plate itself.
The registration of mass was performed with a fre-
quency not exceeding three measurements per second.
Each sample was placed in its separate solution pre-
pared in advance.
Moreover, iron possesses the same property in solu-
0
tions of silver salts since
= 0.799 V for silver. The
density difference is also important when working
with the weight method. For this reason, not only
copper sulfate and silver nitrate, but also lead nitrate
solutions were taken for investigation in this work as
a medium. The standard electrode potential for lead
0
(
= 0.126 V) is less different from that for iron,
but the difference is enough for realizing the process.
It is of note that the reactions of deposition of metals
from solutions of their salts are often used for protec-
tive coating. This also attaches a certain practical
importance to this investigation.
The influence of the strain sign was established by
comparing the rates of variation of the metal mass on
the opposite sides of a bent plate as in [4 6]. Since
the plates were slightly different in weight, we plotted,
for the sake of unification, kinetic curves for the ratio
m/m0 where m is the running value of the mass of the
cell with a plate and m0 is a certain m value fixed at
the beginning of the experiment (in 10 to 15 s neces-
sary for the attainment of a stationary state). A similar
ratio M/M0 was introduced for the solution. The mass
variation rate was determined from the slope of the
normalized curve m/m0 = f(t) or M/M0 = f(t), where t
is time. Since weighing was made at the location of
the cell with a plate in the solution, it was natural that
the mass measured included an increment (negative
for m and positive for M) related to the action of the
Archimedes’ buoyancy force. However, the Archi-
medes’ contribution for m and, moreover, for M is
not very essential because of the great difference
between the densities of the plate and the surrounding
solution.
EXPERIMENTAL
Working with steel but not with iron (as it would
be appropriate in accordance with the requirement of
purity of a chemical experiment) was forced by the
condition of elasticity of a specimen as was implied
in the theory formulated [2, 3]. There were used, in
our investigations, polished plates of a rectangular
shape (10 50 mm and 10 75 mm) and of a thick-
ness of 0.3 mm made out of a high-alloy steel (of
brand 05Kh22AG15N8M2F-Sh with the carbon
content about 2%), a carbon steel (of brand U7a with
the carbon content 0.7%), and a low-carbon steel (of
brand C1010 produced by Alabama Specialty Pro-
ducts, Inc., USA, with the carbon content 0.1%).
Following to [4 6], the lacquer XB-784 (a corrosion-
resistant coating) was used for the isolation of one of
the plate sides. The thorough degreasing of samples
with ether was performed before the experiment. So-
lutions of copper sulfate (CuSO4 5H2O) and other
salts were prepared on the base of chemically pure
chemicals. Water was taken as a bidistillate. The ex-
periment was performed at 25 C.
The variation of the plate mass in the course of a
redox reaction was caused by replacing iron with a
heavier metal (for example, with copper according to
the reaction (1)), which naturally led to increasing the
plate mass. Besides copper sulfate, salts with heavier
cations (silver and lead nitrates) were tested for
promoting the effect.
We investigated the process of the metal deposition
on a still plate under stress in a U-shaped cell keeping
a plate in a bent state. The cell construction was
similar to that described in [4 6]. The cell with a plate
was immersed in a vessel with a salt solution of a
given concentration. A mass change of both the plate
and the salt solution occurred in the course of a redox
reaction. The measurement of this change for the plate
and the solution was made separately (in different
experiments) in accordance with our method of con-
tinuous registration of mass [6, 8]. We used a certain
type of the Sartorius electronic balance (with ac-
curacy 0.0001 g) that permitted weighing both at the
RESULTS AND DISCUSSIN
As is known, increasing the carbon content in the
iron alloys secures transition from the plasticity to the
elasticity and even brittleness of the solid phase. As
was already mentioned above, we dealt with three
steel samples differing in the carbon content. Let us
begin with the high-alloy steel. As an example, Fig. 1
shows the curves of the relative variation in time of
the cell with two different bent plates immersed in the
5% solution of copper sulfate. As it is expected, the
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400
MOVCHAN, RUSANOV
m/m0
1.025
1.020
1.015
1.010
1
1.005
1.000
2
0.995
t, s
0
200
400
600
Fig. 1. The time (t) dependence of the relative mass m/m of the cell with a plate at the deposition of copper from the
0
5% copper sulfate solution on the concave (1) and convex (2) sides of bent plates of the high-alloy steel.
m/m0
1.004
1
1.000
0.996
0.992
2
0
50
100
150
200
250
t, s
Fig. 2. The time (t) dependence of the relative mass m/m of the cell with a plate at the deposition of copper from the
0
10% copper sulfate solution on the concave (1) and convex (2) sides of bent plates of the carbon steel.
plate mass grows, the growth being faster at the de-
position of copper on the convex side of a bent plate.
If we interpret the result as a manifestation of the
mechanochemical effect of the strain sign, we have to
consider the surface stress at the metal/solution
boundary as positive according to the theory [2, 3]. In
this case, however, there is an alternative trivial inter-
pretation. As was shown in experiments on corrosion
[5, 6], the process of formation of microcracks de-
velops on the convex side of a bent plate. As a result,
the effective area of the convex surface increases
causing a growth of the amount of copper deposited.
Thus, the interpretation of the experimental result is
not unique in this case.
Herewith, the cell with a plate (immersed in the solu-
tion) was hanged on the balance in the first case,
whereas the solution itself was weighed in the second
case. It is seen from the figures that the result for the
carbon steel is opposite to the above result for the
high-alloy steel. Now the deposition of copper and
lead proceeds faster on the concave side than on the
convex one. This permits the unique interpretation of
the result as the mechanochemical effect of the strain
sign (at a negative value of the surface stress). More-
over, one can say that the mechanochemical effect is
large enough to overbear the effect of cracking on the
convex side. However, Fig. 2 also exhibits a tendency
of decreasing mass on the convex side that can in-
dicate its partial destruction (abscission) at cracking.
If the dominancy of the concave side is caused only
by destructing the convex side, the unambiguity of
Figures 2 and 3 present kinetic curves of the copper
and lead deposition on bent plates of the carbon steel.
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MECHANOCHEMICAL EFFECTS IN REDOX REACTIONS
401
M/M0
1.000
0.998
0.996
2
0.994
1
0.992
0.990
0
50
100
t, s
Fig. 3. The time (t) dependence of the relative mass M/M of the 5.6% lead nitrate solution at the deposition of lead on the
0
concave (1) and convex (2) sides of bent plates of the carbon steel.
M/M0
1.0000
0.9999
0.9998
0.9997
0.9996
50
100
150
200
250
300
350 t, s
Fig. 4. The time (t) dependence of the relative mass M/M of the 10% copper sulfate solution at the deposition of copper on
0
both the sides of an unstrained plate of the low-carbon steel.
interpretation again becomes problematic (concerning
the deposition of copper but not lead). In addition, it
strikes the eye an instability of the run of the curves
m/m0 = f(t) and M/M0 = f(t) in Figs. 2 and 3 with
multiple incidental deviations from a monotonic run
in both directions. This nonuniformity is especially
pronounced in the case of the contact of the plate
convex side with the copper sulfate solution (Fig. 2).
The main cause of such behavior is the surface non-
uniformity that increases at the deformation. It is
visually seen that copper (well distinguishable by
color) is deposited in spots. This means that various
accompanying reactions can occur on different parts
of the surface. If we dealt with pure iron, we could
speak about formation of insoluble iron hydroxide
Fe(OH)2. However, we would have a long list of pos-
sible processes in the presence of alloying additions
in the solid phase and on its surface, in the presence
of ions Cu2+, Pb2+, Fe2+, SO42 , NO3, H+, and OH in
solution and at arising active centers on the surface as
a result of strain. There is no logic to discuss these
processes, the more their background does not prevent
discovering main regularities even in our dirty
experiment.
To weaken the above unpleasant secondary phe-
nomena, we undertook experiments with the most
pure (among our samples) low-carbon steel of
American production. Its carbon content is such small
that the steel already possesses certain plasticity. This
lowers the probability of cracking at strain. The ex-
periment with an unstrained plate in 10% copper
sulfate solution exhibited a monotonic decrease of the
solution weight as copper leaved the solution for the
deposition (Fig. 4). The data are presented in Figs. 5
and 6 for the deposition of copper and silver on the
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402
MOVCHAN, RUSANOV
M/M0
1.00001
1.00000
0.99999
0.99998
0.99997
2
0.99996
0.99995
1
0
10
20
30
40
50
60
70 t, s
Fig. 5. The time (t) dependence of the relative mass M/M of the 10% copper sulfate solution at the deposition of copper on
0
the concave (1) and convex (2) sides of bent plates of the low-carbon steel.
M/M0
1.00002
1.00000
2
0.99998
0.99996
0.99994
0.99992
1
0.99990
0
50
100
150
t, s
Fig. 6. The time (t) dependence of the relative mass M/M of the 10% silver nitrate solution at the deposition of silver on the
0
concave (1) and convex (2) sides of bent plates of the low-carbon steel.
sides of bent plates from the copper sulfate and silver
nitrate solutions. There are only few incidental de-
viations. The plot for silver is especially good, and
this is understandable since silver, with its density and
the standard electrode potential, is the most ap-
propriate object for experiments with the redox reac-
tion of its deposition on iron. Both the figures give
evidence of the dominant deposition of copper and
silver on the concave side of bent plates. As was
already noted, this uniquely establishes the strain sign
effect. In this way, the existence of the mechano-
chemical effect of the strain sign in redox reactions
with participation of iron can be considered as proved.
a redox process is determined by its chemical affinity
that is a combination of chemical (electrochemical)
potentials corresponding to the reaction stoichiometry.
For the process of oxidation of iron, Eq. (2), the
chemical affinity is
A =
2
,
e
(6)
Fe
Fe2+
where
state at the sample surface,
potential of iron ions in solution, and
is the chemical potential of iron in the solid
Fe
Fe2+
is the electrochemical
is the elec-
e
trochemical potential of electron. The higher the iron
chemical potential in the solid state, the larger is the
chemical affinity and, therefore, also the rate of iron
dissolution. The chemical affinity for the process of
reduction of copper, Eq. (3), is
We now turn to the explanation of the effect ob-
served. As for any other chemical reaction, the rate of
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MECHANOCHEMICAL EFFECTS IN REDOX REACTIONS
403
Cu2+
e
Cu
A
+ 2
,
(7)
equal in absolute value but opposite in sign strains).
The mechanochemical theory explains the origin of
this effect.
Cu2+
where
is the electrochemical potential of copper
ions in solution and
is the chemical potential of
the reduced copper in Cthue solid state on the specimen
surface. For the total redox reaction (1), the chemical
affinity is given by the expression
ACKNOWLEDGMENTS
The authors are indebted to Dr. A. Bingham for
donating samples of the low-carbon steel. The work
was supported by the Russian Foundation for Basic
Research (grant no. 04-03-33086) and the program
Leading Scientific Schools of Russian Federation
(grant no. NSh-4241.2006.3).
A =
+
.
Cu
(8)
Fe
Cu2+
Fe2+
We now discuss how the stress in steel plates
influences the chemical affinity of the above reactions.
Evidently, an artificially created stress can influence
only the solid part of a system and does not touch
ions located in solution. Since electrons freely move
and easily equilibrate each other inside a metal, we
can assume their chemical potential to be not sub-
jected to the action of a local stress. The electrical
double layer forms at the contact of a metal with
water: iron ions partially pass to solution, and remain-
ing (at the surface on the side of the solid phase) elec-
trons create (due to their mutual repulsion) a negative
stress (a positive two-dimensional pressure) in the
surface monolayer and a negative contribution to the
surface tension of a solid. According to thermody-
namics [2, 3], this leads to an increase in the chemical
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Fe
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Thus, the investigation of continuous variation of
the mass of bent steel plates in aqueous solutions of
salts shows that a redox reaction proceeds with dif-
ferent rates on the opposite sides of a bent plate (with
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 3 2007