Electrochemical Corrosion of Amorphous Alloys with Participation of Oxygen Compounds
M.O. Kovbuz , O.M. Bilyk and K.R. Gorbachevska
Ivan Franko State University of Lviv, Lviv, Ukraine
email: dmytrakh@Vision.IPM.Lviv.UA
Abstract.
The relationship between chemical composition,
crystalline level and reactivity of a number of amorphous alloys
has been established in comparison with crystalline with steels.
The stability of the amorphous alloys in the aggressive environments
was estimated on the basis of values of ratio of anodic and cathodic
processes rates, corrosion potentials and exchange currents. The
kinetics parameters of electrochemical reactions with participation
of oxygen compounds on the amorphous metal surfaces have been
obtained. Catalytic activity of the contact and external sides
of the amorphous alloys tape has been investigated and kinetic
parameters of the process have been determined firstly. The influence
of preheating, magnetic field, mechanic strain on the chemical
surface activity of amorphous alloys has been investigated.
The use of the amorphous alloys (AA) in engineering assumes their work in aggressive chemical environments, constant and variable magnetic fields, mechanical loading, and various temperature modes [1,2]. All these factors are the activators of corrosion processes. The thermodynamic instability and excessive surface energy also determine their catalytic activity in various chemical processes, especially in the electrochemical [3].
Surface characteristics
The specificity of electrodic processes on the AA surface in comparison with crystal steels is observed [4]. Steels, despite of alloying additions, which lower electrochemical dissolution rate, remain more active than AA with close composition in identical conditions (Fig. 1).
External and contact surfaces of the amorphous alloys tape can
differ both by their crystalline degree (Tabl.1), and local inhomogeneity
of chemical composition.
Table 1.
Estimation crystalline of surfaces of a tape AA.
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Surface of the tape formed in the direct contact with a cooling element (contact surface, k) is less crystalline, than the external side (e), which could be seen from value of j (where j=d/m). The shapes of voltammograms of both sides of amorphous tape are substantially different. The higher the dispersivity of the contact side of the tape is the higher corrosion stability of the surface independently of the composition.
The study by the differential method for determination of iron ions, elaborated in our laboratory [5], revealed, that the compounds of iron (II) are formed in the first anodic region, whereas compounds of iron (III) - in the second region. The observed maximum of cathodic current (-1.00 V) corresponds to the reduction of iron (III) ions.
Fig.2. Cyclic voltammograms AA 02/1 in NaCl solutions of various concentration
(v = 20 mV/sec):0.1 mole/l (1,1');0.5 mole/l (2,2'),1.5 mole/l
(3,3'); the 1st cycle (1,2,3), 10th cycle (1',2',3'.
It has been observed that the potential the oxidation current
at - (0.60
0.40)V and Taffel coefficient
are increased practically at all investigated NaCl concentration
(0.1
1.5 mole/l) after some time from the
start of cyclic scanning. At the highest NaCl concentration the
pre-wave of the anodic reaction intermediates, i.e. compound of
iron (II), is observed whereas the potentials of all following
stages are shifted into cathodic field (Tabl.2). The metal dissolution
process becomes more active [6].
Table 2
Electrochemical parameters of corrosion AA 02/1 in NaCl solutions (v=20 mV/sec).
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The study of the influence of potential scanning rate on the shapes
of cyclic voltammograms of AA 02/1 in solutions with various NaCl
concentration has shown, that at low rates (5mV/sec) the intermediate
products of anodic process practically are not identified. Ten-fold
increase of rate (50 mV/sec) allows to register intermediates
of oxidation processes. Thus, separate stages of oxidation-reduction
process are estimated and the intermediates transformation of
the AA surface dissolution process is determined to be of high
velocity.
Influence of hydrogen peroxide H2O2 on
the AA surface dissolution.
Potential of corrosion depends on the ratio of anodic-cathodic reactions speeds, as well as on the concentration of active particles, which promote metal oxidation. In this case such particles are H2O2 and the products of its decomposition, which intensify the corrosion. Then the following equation of corrosion potential Ecor may be written:
In the oxidation reaction Fe - 2e = Fe2+ :
here n=2; kc and ka are the constant of
cathodic and anodic processes rate, 
is
hydrogen peroxide activity.
So, the nonlinearity of Ecor=f(
)
dependence can be explained by the change of the ratio
of cathodic and anodic currents, which are characetristical for
the process rate. In the case of the rise in the corrosion potentials
the cathodic reaction prevails:
The anodic current may be calculated:
where n is the transfer factor
of anodic process.
The activity of hydroxide-ions is directly dependent on the constant of anodic process velocity.
The influence of hydrogen peroxide when its concentration increases
from 
mole/l to 
mole/l is different too (Tabl.3).
Table 3.
Parameters of anodic polarization of AA 02/1 in a 3 % NaCl
solution with the hydrogen peroxide additives.
 ,
mole/l | ,
sec |  , mole/l
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Compared to the corrosion parameters for the case of oxygen dissolved
in 3% NaCl solution, the corrosion processes when 3.5×10-4
mole/l H2O2 is dissolved in the same environment
are more active, thus shifting the corrosion potential by 0,2V
into the cathodic field and increasing corrosion current. The
ten-times increase of H2O2 concentration
results in decelerating of the surface processes. It is likely
to be caused by passivation of the surface. Further competitive
action of surface oxygen compounds and Cl-ions in a solution activates
iron dissolution processes, as wel as the decomposition of hydrogen
peroxide on alloy surface. The latter process at 
mole/l H2O2 concentration partially proceeds
out of a limits of electrode layer in homogeneous area, where
the speed of reaction becomes independ of a further increase of
the H2O2 concentration.
The catalysis of H2O2 decomposition by
amorphous alloys.
The opposite sides of amorphous tape display different catalytic
activity, as they have different surface structures [7,8,9]. Basing
on our results we can assume, that the hydrogen peroxide transformation
on the surface of alloy with the high contents of iron and low
degree of crystalline, probably, proceeds accordingly to the following
scheme [10]:
(1)
The kinetic parameters of hydrogen peroxide decomposition, calculated
on the basis of decrease of limiting currents of the original
compound (k1) and on the current growth of iron (III)
(k2), are in same order (Tab. 4).
Table 4.
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The order of reaction is essentially dependent on the original
hydrogen peroxide concentration. So, the order of reaction determined
by means of Vant-Goff method in the hydrogen peroxide concentration
range of (
)
mole/l is equal to 0.7, and at further increase of concentration
upto 
mole/l it drops down to 0.1, that
is characteristic for heterogeneous processes. On the basis of
the equation (1) it was calculated the value Ko, i.e.
the ratio of constants of reaction rates by two mechanisms of
decomposition of hydrogen peroxide molecules adsorbed on the metal
surface. When the initial concentration of H2O2
gets rise, the value Ko decreases, thus confirming
its prevailing single-stage electrochemical decomposition.
However, the calculated values of transformation constant for
hydrogen peroxide based on the Fe (III) currents increase at 
=
mole/l are lower than those obtained
proceeding from the results of cathodic currents. These fact confirm
the spreading of peroxide hydrogen decomposition process beyond
the limits of purely electrochemical reaction by the EC - mechanism.
Probably, OH.-radical interacts with hydrogen peroxide
molecules in the bulk of solution as well according to the scheme:
Influence of heat treatment on corrosion stability of AA.
On heating the composition of the AA surface changes. Owing to structural relaxation and physico-chemical properties of amorphous alloys, as well as corrosion stability change in particular.
However, the influence of annealing on electrochemical corrosion parameters of different AA in the 593-973K temperature interval is ambiguous.
So, annealing of AA 02/1 in interval 573-773K results in the shift of corrosion potential to the cathodic field, whereas the further annealing temperature increase up to 973K provides the shift of potential to the anodic field. The similar changes were observed for 86KGSR AA in the temperature intervals of 573-673K and 673-973K respectively. Such results confirm sufficient dependence of properties of the alloys on their chemical composition.
To estimate the structural changes during annealing, electrochemical
corrosion of both sides of AA tape was investigated. The shape
of voltammograms for the alloys before and after annealing are
more different for the contact side. Probably, such annealing
results in structuring of alloys.
Influence of a magnetic field on the electrochemical characteristics
of AA corrosion.
The exposure of the sample of AA 02/1 to a constant magnetic field (4500 Oe) results in slight shift of corrosion potential to the cathodic field and the decrease of exchange currents value and slope of the Taffel range of the polarization curve. The shape of polarization curve is somewhat changed too, particularly in the range of Fe(III) compounds formation. No distinct passivation intervals at the potentials more above than potential -0.55V are observed even after 5 day magnetization of AA, this confirms the output of iron (III) compounds into the solution.
In the case of 86KGSR the exposure of alloy to a constant magnetic field activates the surface and results in a shift of corrosion potential and active dissolution potential to the cathodic field. At the same time, the height of the current of reduction of surface compounds increases.
Action of alternating magnetic field (50 Hz) appears to be more
destructive. The active processes of crystallization begin after
30 minute exposure (Fig.3).
Fig.3. Change of potential of corrosion AA 02/1 in 3
% NaCl solution (1), exposed in magnetic field: constant (2),
alternating (3,4) during: 30 minutes (3), 60 minutes (4), 5 days
(2).
On the basis of electrochemical parameters it is possible to consider,
that the alternating magnetic field causes the movement of domains
and intensify surface structuring processes on AA tape, that changes
corrosion resistance in 80-100 times faster, than the constant
magnetic field [11].
Influence mechanic loading on electrochemical characteristic
of AA.
Mechanical tension (0.01 mm/sec) of AA substantially influences
on the parameters of electrochemical corrosion. For example, when
the specimen of AA 02/1 is gradualy tensed up to its yield limit,
corrosion potential shifts to the cathodic field the currents,
corresponding to the formation of iron (II) and iron (III) compounds
get increased, at the same time reduction currents become three-fold
higher. No changes were observed after further endurance of the
specimen even under the ultmate stress (
)
for 20 hours (Fig.4).
Kinetics of surface potential change during gradual loading is
sufficiently different. That is, at the moment of specimen destruction
the conditions, necessary for formation of juvenile surface, exist.
After gradual loading up to 
and sustaining
for 20 hours, the oxidation of the surface probably proceeds,
the unhomogeneities appearing during the first stage of the process
are likely to be partially redistributed. The surface of the metal
in this case gets homogenized, so its electerochemical corrosion
parameters coinside with the intial.
Fig.4. Cyclic voltammograms AA 02/1 in 3 % NaCl (1)
gradually loaded (0.01 mm/sec) to 
(2)
and maintained 
during 20 hours (3).
AAC86 have revealed its higher stability to mechanical loading
in the similar tests.
Acknowledgement. This work was supported in part by the
International Soros Science Education Program (ISSEP) through
grants ¹ APU 063050 and PSU 053011.