ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2011, Vol. 47, No. 3, pp. 402–409. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © I.A. Safonov, Yu.Ya. Andreev, E.A. Matvienko, 2011, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2011, Vol. 47, No. 3, pp. 322–329.
INVESTIGATION METHODS
FOR PHYSICOCHEMICAL SYSTEMS
Methods of Measuring Activation
and Passivation Potentials of Fe–Cr Alloys
I. A. Safonov, Yu. Ya. Andreev, and E. A. Matvienko
MISIS National University of Science and Technology, Leninskii pr. 4, Moscow, 119991 Russia
e-mail: Aivan85@yandex.ru
Received December 29, 2009
Abstract—A novel experimental method of estimating the activation potentials of passive layers on metallic
materials is developed taking into account the advantages and drawbacks of conventional methods (switching
off the anodic polarization and reverse potentiodynamic activation–passivation). Theoretical principles of
the proposed method of successive approximations are based on the thermodynamic possibility of forming a
passive layer on the alloy surface. The method involves potentiostatic exposures in the potential range of presumable activation. The change in the current direction with time at a selected potential is considered to be
an indicator of the surface state. The error of measurements is related solely to the accuracy of potentiostat
and is independent of a particular mechanism of the passive layer formation. The method is supported by the
example of Fe–Cr and Ni–Cr alloys, as well as 4541 steel X6CrNiTi18-10, AISI 321. The experimental data
obtained are compared to the results of theoretical calculations of Flade potentials.
DOI: 10.1134/S2070205111030142
INTRODUCTION
EXPERIMENTAL
The development of methods of measuring activation potentials of metals and alloys is very important
for solving the problem of improving the corrosion
resistance of metallic materials and constructing theoretical models that can describe the passivity phenomenon [1–7]. The use of modern techniques presents
new possibilities in studying materials and processes
that take place on the surfaces [8–12]. One can now
observe effects that were unnoticed or previously
neglected because of the low sensitivity of measuring
devices.
A standard three-electrode cell with the separated
spaces filled with the working solution and a saturated
potassium chloride solution respectively was taken for
recording polarization curves. A standard silver-chloride electrode was used as the reference electrode,
while a gold electrode served as the auxiliary electrode.
0.5 M H2SO4 solution was used as the electrolyte.
Electrochemical measurements were carried out with
an IPC 2000 Pro digital potentiostat with a modern
base controlled by a built-in microprocessor connected to a personal computer.
Fe–Cr alloy specimens containing 0–23% chromium were taken as the studied subjects. The working
surface was nearly 0.5 cm2 and was different for each
particular specimen, which was taken into account in
the calculation of current density. Electrolytically pure
chromium and carbonyl iron decarburized in hydrogen flow atmosphere (repeated 2-h annealing at
1100°С) were used when smelting the alloys. The
alloys were produced in a Lay bold Heraues electric
arc furnace by repeated smelting, followed by recrystallizing annealing at T = 830°С for 1 h and quenching
in oil. The purity of alloys was controlled by X-ray fluorescent analysis and spark emission spectroscopy,
which guarantee the purity of alloys in carbon (< 0.01)
and other admixtures.
Before experiments, the specimens were mechanically cleaned with a set of sandpaper starting with
grade no. 80 (coarsest) up to no. 1000 (finest),
degreased with acetone, and washed in distilled water.
Then, they were purified in an ST-405 CTBRAND
ultrasound bath in distilled water for 3 min. Upon
An important theoretical parameter that characterizes the stability of the passive state of metals is Flade
potential EF, which is the equilibrium potential of the
passive-layer (PL) formation (E > EF) and dissolution
(E < EF) [1, 2] corresponding to the following reaction
in the film theory of metal passivity:
mMe + nH 2O ←→ MemO n + 2nH + + 2ne −.
(1)
In this work, conventional techniques are used to
determine the activation and passivation potentials of
Fe–Cr alloys, i.e.,, switching off the anodic polarization, as well as forward and reverse potentiodynamic
activation. Taking into account their advantages and
drawbacks, we proposed a method of estimating the
critical potential of the passive film stability on metallic materials that most adequately reflects the thermodynamic meaning of the Flade potential. The experimental results are compared to the theoretically calculated Flade potentials of the alloys.
402
METHODS OF MEASURING ACTIVATION
immersing a specimen in the working solution, it was
cathodically treated as follows: a 2-min exposure at a
potential of –980 mV (S.H.E.) accompanied by a
strong hydrogen evolution and a 2-min exposure at a
potential of –380 mV (S.H.E.) accompanied by weak
hydrogen evolution and the final removal of residual
gas bubbles. The examination of the specimen surfaces
at various stages of the experiment was carried out with
an MBC-10 binocular microscope and a HITACHI
TM-1000 scanning electron microscope.
MEASURING TECHNIQUES
Activation by Switching Off Anodic Polarization
The switch-off method was proposed by Flade in
1911 [15] when he studied the electrochemical behavior of iron in sulfuric acid. According to this technique, upon the cathodic treatment at a Ec potential,
the specimen is exposed at the Ep passive-state potential. Upon certain time ∆τ, the polarization is switched
off and the rapid potential change in the negative
direction is followed in time. The horizontal segment
of a Е(τ) potential–time curve, i.e., the temporal delay
before the potential drop, is called the Flade potential
in the literature.
Typical Е(τ) curves on Fe–Cr alloys are shown in
Fig. 1. If we take into account that Flade used the
method on individual iron, we can see that the data
obtained with the method on multicomponent systems may be incorrect; sharply switching the potential
from Ec to Ep eliminates the stage of active alloy dissolution, so that alloy components do not selectively dissolve in the active range and the surface does not
become enriched in a component that is more stable
under particular conditions. Therefore, in the case of
multicomponent systems, it is reasonable to bring the
alloy to the passive state by means of potentiodynamic
scanning followed by exposure at the selected Ep value.
Upon the passivation of a Fe–4 Cr alloy at Ep =
1222 mV for 30 min (Fig. 1a), the horizontal segment
is shorter (cf. curves 1 and 2) when the specimen surface undergoes active dissolution before passivation.
In this case, chromium predominantly dissolves
because its standard potential is much more negative
compared to that of iron. It is important that, in both
cases, the potential of the end of the horizontal segment on Fe–4 Cr is the same and equal to Ea = 580 mV
(S.H.E.). This means that iron oxides play a key role in
the passivation of the alloy.
Studying the effect of the duration and potential of
passivation in the Flade method based on the example
of the Fe–6 Cr alloy showed that a decrease in the passivation duration from 30 to 10 min does not result in
the shortening of the horizontal segment in Е–τ curve,
while an increase in Ep from 822 to 1022 mV leads to
the increase in the time interval before activation by a
factor of three. In all cases, Ea is about 470 mV, which
indicates the similar nature of passive layers grown
403
Potential, mV (S.H.E.)
(а)
1250
1050
850
650
450
1
2
250
50
–150 Axis 1
–350
3300 3350 3400 3400 3500 3550 3600 3650
1750 1800 1850 1900 1950 2000 2050 2100 2150
Axis 2
Time, s
Potential, mV (S.H.E.)
1100
900
700
500
300
1
100
–100
–300
500
1000
(b)
2
3
1500
2000
2500
3000
Time, s
Potential, mV (S.H.E.)
(c)
700
1
600
2
500
400
300
200
100
3
0
–100
–200
–300
500 1000 1500 2000 2500 3000 3500 4000
Time, s
Fig. 1. E–τ curves of activation by switching off anodic
polarization of Fe–Cr alloys: (a) Fe–4 Cr: (1) active dissolution potential + 1222 mV, 30 min (axis 1), and
(2) 1222 mV, 30 min (axis 2); (b) Fe–6 Cr: (1) 822 mV,
10 min, (2) 822 mV, 30 min, and (3) 1022 mV, 30 min; and
(c) Fe–13 Cr: (1, 2, and 3) horizontal potential segments.
under different conditions (Fig. 1b). The character of
the horizontal segment remains nearly the same,
regardless of the exposure at the passive-state potential, but substantially changes depending on the
method of attaining the potential and its value.
With an increase in the chromium content up to
13%, the shape of E–τ curves changes noticeably.
Even at a short exposure (10 min), the Flade method
reveals the multilayer structure of the passive film
(Fig. 1c), which is the main advantage of the method.
In the E–τ curve, there are three horizontal segments
that correspond to the potential delays. Let us assume
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011
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SAFONOV et al.
Current density, mA/cm2
Current density, mA/cm2
Current density, mA/cm2
(а)
190
180
170
160
cr
150
Ep
140
130
120
110
100
200 250 300 350 400 450 500 550 600 650
Potential, mV (S.H.E.)
350
(b)
Еa
cr
300
Ep
1
250
200
2
1
150
2
100
Еao
50
0
35
30
25
20
15
10
5
0
300 350 400 450 500 550 600 650
Potential, mV (S.H.E.)
700
(c)
1
2
3
1
2
3
–300 –250 –200 –150 –100 –50 0
Potential, mV (S.H.E.)
50
that the end of the segment only looks like an ordinary
inflection point when the E–τ dependence is recorded
with an inertial device. Oscillographic investigation
shows that there is actually two, not one delay [16].
This means that data obtained with a potentiostat are
approximate. A shift of the potential in the segment
toward the more negative values is determined by the
inconstancy of the passive layer composition in the
depth. Close to the base metal, the layer is enriched in
the electrochemically negative chromium. This fact
agrees with numerous data [1–7, 11, 12]. When chromium content is further increased, the length and
slope of the segment increase and, if the steady state of
the alloy is passive in the particular environment (due
to self-passivation), activation does not take place.
Thus, we can conclude that the switch-off method
is well applicable for qualitatively determining the
number of layers on the metal surface and the potential
ranges of their stable existence. In the case of multicomponent systems, it is reasonable to bring the alloy
to the passive state by means of potentiodynamic scanning followed by an exposure at the selected potential.
100
Fig. 2. Polarization curves recorded at potentiodynamic
scanning at a rate of 1 mV/s: (a) forward branch with oscillations on individual iron; (b) forward and reverse
branches on low-chromium alloys with the critical potentials shown: (1) Fe–2 Cr, and (2) Fe–6 Cr; and (c) same on
high-chromium alloys: (1) Fe–10 Cr, (2) Fe–13 Cr, and
(3) Fe–23 Cr. Arrows show scanning direction.
that each oxide phase (film or adsorption layer) has
certain dissolution potential. Then, every time a new
layer begins to dissolve on the passive metal surface,
one should see a delay before the potential drop, i.e., a
horizontal segment in the E–τ dependence. The
length of the segment corresponds to the time interval
in which a particular oxide phase is present on the surface. As soon as the layer disappears or becomes sufficiently porous, the potential drops until the next phase
begins to dissolve or until the steady-state potential of
the metal is reached.
The drawbacks of the method are as follows. In an
E–τ curve recorded with a potentiostat, the segment
of the potential delay is never strictly horizontal.
Sukhotin, who studied the activation of iron, found
Potentiodynamic Activation–Passivation
The method involves successively recording forward and reverse potentiodynamic curves. The beginning of the increase in the current density in the
reverse curve is thought to be related to the Eao potential of the activation onset, while the maximum i value
is thought to be related to the E acr, critical activation
potential, which can be treated in a first approximation as the EF potential. An advantage of the method is
cr
its express character; one can obtain a set of E a values under particular conditions with no long exposures. The high potential scanning rate is an essential
tool when studying cathodic loops on individual iron
[17]. A drawback of the method is the dependence of
the activation potential and critical passivation potentials on the scanning rate [12].
The effect of the chromium content on the shape of
polarization curves is illustrated in Fig. 2. Spontaneous current oscillations typical of individual iron [4, 5]
in a potential range of 390–490 mV (Fig. 2a) are
caused by the formation of the adsorption layer of FeO
oxides. In fact, the thermodynamic calculation of the
Flade potential of individual iron based on the formation conditions of the adsorbed FeO oxide on the iron
surface provides EF = 432 mV [13], and in Fig. 2a, this
value falls in the first range of current oscillations. The
second potential range of current oscillations in individual iron is close to the critical passivation potential
E pcr (Fig. 2a). A similar phenomenon is typical of Fe–
Cr alloys with low chromium contents (2 and 4%) as
well, while at a chromium concentration of 6%, no
current oscillation is observed during the passivation
(Fig. 2b). Starting from the chromium content of 6%,
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011
METHODS OF MEASURING ACTIVATION
in accordance with the data of [8, 9]. The
dependence on the chromium content in the alloy, which
was obtained in those experiments, is symbate to the
E pcr curve and shifted in the negative direction. In this
cr
case, the E a values are closest to the theoretically
estimated EF potentials of the Fe–Cr alloys [14] (see
below).
With an increase in the chromium content (10–
23%), the shape of polarization curves noticeably
changes (Fig. 2c). The passivation and activation
curves are smoother and there is a substantial hysteresis between the critical passivation and activation
potentials, similarly to the curves recorded on the lowchromium alloys. It is worth noting that activation
does not take place on an alloy containing 23% Cr, and
even lowering the potential scanning rate cannot solve
the problem. Thus, the method of potentiodynamic
passivation–activation
provides a rapid estimation of
cr
E pcr and E a potentials of Fe–Cr alloys with a chromium content below 10% in non-oxidizing environments (0.5 M sulfuric acid, sodium sulfate, etc.).
Method of Successive Approximations
There is still no common concept about the nature
of the passive layer. Many researchers [18–23] believe
that the layer has an adsorption structure because the
protective properties appear sometimes when the layer
thickness is only part of the monolayer. Some others
adhere to the phase nature of the passive layer [2, 4, 5].
The third point of view proposed by Schwabe [24],
which is most popular among corrosionists [7], is as
follows. During the metal passivation, oxygen-containing particles first adsorb on the metal to produce
chemisorption oxide film. Then, with an increase in
the anodic potential, the oxygen content in the layer
increases, the layer becomes thicker, and transforms
into the barrier film. Later, a phase layer can appear
that is porous and cannot provide a protective effect.
Based on various concepts, many variants of the
definition of passivity were proposed, though, according to Tomashov [7], none of them reflects the phenomenon in full. Moreover, the interpretation of Flade
potential varies from the assumption that it equals the
end point of the second horizontal segment in E–τ
curve [5] to the critical passivation potential [7]. The
interpretation of the decrease in the current density
during the exposure in a passive range is by no means
more definite. The authors of [17] attribute it to the
decrease in the passive layer thickness, which is in turn
related to the decrease in defectiveness of the layer
during its formation that causes the decrease in the
ionic flow via the film along its structure defects and
the deceleration of the layer formation. In classical
works [1–4, 7], an assumption about the increase in
the passive layer thickness at the concurrent decrease
(а)
700
600
500
400
300
200
100
0
–100
–200
3
2
1
Potential, V (S.H.E.)
cr
Ea
0
Potential, V (S.H.E.
E pcr shifts in the negative direction (Fig. 3a), which is
405
2 4
700
600
500
400
300
200
100
0
–100
–200
6
8 10 12 14 16 18 20 22 24
Chromium content, at %.
(b)
3
2
1
0
2
4
6
8 10 12 14 16 18 20 22 24
Chromium content, at %.
Fig. 3. Critical potentials of alloys depending on chromium content: (a) (1) Flade potential calculated according to [14], (2) activation potential, and (3) critical passivation potential; and (b) (1) Flade potential calculated
according to [14], (2) activation onset potential determined by means of potentiodynamic scanning, and
(3) activation onset potential determined by method of
successive approximations.
in its ionic conductivity was adopted. Still, only the
existence of the passive layer upon reaching the passivation potential and the corresponding current density
drop are the common facts of all of the theories.
Taking into account that it is necessary to know the
stability range of the passive layer when improving the
corrosion resistance of metallic materials based on
their passivability, we focused chiefly on determining
the Eao activation-onset potential that would be closest
to the Flade potential under the selected conditions.
Therefore, we propose a method of estimating Eao of a
passive layer that is independent of the passivation
mechanism and based on the thermodynamic possibility of the passive layer formation on the alloy surface
solely. Here, we assume that the EF Flade potential is a
theoretical value that characterizes the equilibrium
formation–dissolution of a passive layer. In this
cr
respect, E a is significantl closer to it than E pcr .
The graphical estimation of the Flade potential
proposed by Vetter [2] is illustrated by Fig. 4. In the
energetic aspect, it can be treated as dynamic equilibrium (1) between the active surface and the passive
layer as follows:
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011
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SAFONOV et al.
The Eao indicator as the boundary between the
active and passive ranges is assumed to be the change
in the current direction at the selected potential. A
scheme of the method is shown in Fig. 5.
Passive layer
Current density
Current density
Active surface
Time
E < EF
At the first step, the Ep passivation potential corresponding to the smallest passivity current is determined (Fig. 5, curve 5). For this purpose, potentiodynamic polarization curve is recorded up to the repassivation potential range, and Ep value, which is typically
close to the middle part of the passive range, is found.
In Fig. 6, this potential of Fe–6 Cr alloy is denoted as
E p1 and equal to 822 mV.
Time
EF
E > EF Potential
Fig. 4. Estimation of Flade potential according to
Vetter [2].
Current densit
(а)
Potential
Current densit
1
Eao1 < Eao < Eao2
(b)
2
Ep
3
4
5
Time
Fig. 5. Scheme of determining activation onset potential
by successive approximations in (a) current density–
potential coordinates and (b) current density–time coordinates: (1, 2) activation curves; (3) plateau-activation
curve; (4) plateau curve; and (5) current density in the passive state.
Active
Passive
EF
←
→
surface
layer
(2)
[E < E F ⇒ ∆GPL > 0] [ E > E F ⇒ ∆ GPL < 0] ,
where ∆GPL is the Gibbs formation energy of the passive layer [14], EF is the Flade potential, and E is the
electrode potential.
At the second step, Eao potential is determined by
successive approximations. Using the potentiodynamic scanning at a rate of 1 mV/s, the alloy surface is
brought to the passive state below Ep Ⰷ EF. In this
case, the Gibbs formation energy of the passive layer is
always negative irrespectively of the layer composition:
∆GPL < 0 (Eq. (2)). The active–passive transition is
clearly seen in the current–potential coordinates
(Fig. 5a). The specimen is exposed at the Ep potential
until the current becomes constant (about 30 min).
This passive layer has the best protective properties in
the particular alloy–environment system, which is
indicated by the decrease in i to 10–20 µA/cm2. Then,
the potential is abruptly changed to a certain Eao1 value
from the active dissolution range. Now, the ∆GPL sign
is determined by the Eao1 position with respect to EF. If
Eao1 < EF, then ∆GPL > 0 (the film formation is thermodynamically unfavorable), and the current in I–T
curve increases, which indicates the active anodic dissolution of the alloy surface (Fig. 4; Fig. 5b, curve 1).
If Eao2 > EF, then ∆GPL < 0 and the current in I–T curve
increases up to a certain value equal to the steady-state
passivity current at the selected potential and then
remains unchanged for an infinitely long time, which
indicates the stabilization of the passive layer thickness at
the selected potential (Fig. 5b, curve 4). A logical conclusion is that Eao is intermediate between Eao1 and
Eao2 potentials. Therefore, in the next experiment, the
alloy is exposed at Eao3 potential, which is the arithmetic mean of Eao1 and Eao2 values. Repeating this
step, we make the Eao range progressively narrower by
50% each time at a corresponding increase in the total
duration of the experiment. Here, the accuracy of
determining the Eao potential depends on the sensitivity of potentiostat and the Eaoi boundary values (i = 1,
2, 3, etc.) of the range.
It is worth noting that the i–τ dependence can also
have the shape of curve 4 (Fig. 5b) in the case of the
local alloy dissolution, which is determined by the heterogeneity of the metal surface and the passive layer. In
the general case, the imperfect structure of the passive
layer may be caused by distortions of the base metal
structure, namely, grain boundaries; various (metallic
or nonmetallic) inclusions; crystal faces with the orientation less favorable for passivation; or some finer
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011
RESULTS AND DISCUSSION
On Fe–Cr alloys containing 2–23% chromium, hysteresis is pronounced in the curves constructed using the
passivation–activation method (Figs. 2b, 2c). On all
407
1.0
(а)
0.8
0.6
0.4
1
Ep
0.2
0
400 600 800 1000 1200 1400 1600 1800 2000
Potential, mV (S.H.E.)
Current density, mA/cm2
220
(b)
180
140
Activation
100
60
20
–20
1
3
5
7
10
Current density, mA/cm2
defects, such as dislocations or inclusions of foreign
atoms in the metal lattice [7].
At potentials close to Eao, the shape of i–τ curves
changes (Fig. 5b, curves 2, 3). At first, a plateau at
small current values is observed; then, upon a certain
time, the alloy becomes active (Fig. 6b). Scaling up the
curve in the i direction, one can notice that current
increases with time, which indicates the gradual
decrease in the protective properties of the passive
layer (Fig. 6c). The Fe–6 Cr alloy was completely activated (i = 230 mA/cm2) at 502 mV (S.H.E.) upon 17-h
exposure. Occasional current waves, which are especially strong at the 13th hour of exposure, are
explained by the breakdown of the passive layer at sites
where the surface microrelief is uneven. The passive
layer is more strained at convexes where its internal
energy is higher and, hence, the activation potential is
shifted in the positive direction with respect to the
mean surface value. Upon breakdown, the active dissolution of the site begins and leads to the general
smoothening. When the height at the breakdown site
levels off with respect to the neighboring surface, the
potential at the site shifts in the positive direction
under the effect of the neighborhood, the site again
becomes passive and the passive layer is sealed.
The closer the potential of exposure to the Eao value
(on a condition that Eaoi < EF), the longer the complete
dissolution of the passive layer before the active dissolution of the alloy itself and the more substantial the
role played by grain boundaries in the surface activation. For example, at the exposure of Fe–15 Cr alloy
at a potential of –58 mV, no visible activation was
noticed and the current density was only
0.35 mA/cm2, even at 20 h. In this case, the i–τ curve
was oscillating and did not reach a plateau (Fig. 7a).
The examination of the specimen surface after the
experiment with a scanning electron microscope
showed that grain boundaries are strongly etched out
compared to those upon the exposure in a passive
range (Figs. 8b, 7c). Pits can also be seen at sites where
dislocations exit to the surface.
Comparing the data obtained by the method of
successive approximations and the results of potentiodynamic activation reveals the similarity in the potential dependence on the chromium content (Fig. 3).
The Eao values obtained by the former method are
more positive, since in this case, the potential measurements are not affected by the scanning rate. The
advantage of the method developed is as follows: it
takes into account the thermodynamics of the active–
passive transition and, hence, yields an Eao value close
to ЕF. This method enabled us to estimate Eao on Fe–
23 Cr alloy, where all the residual techniques failed.
Current density, mA/cm2
METHODS OF MEASURING ACTIVATION
9
11
Time, h
13
15
17
13
15
17
(c)
8
6
4
2
0
1
3
5
7
9
11
Time, h
Fig. 6. Example of successive approximations on Fe–6 Cr
alloy: (a) selecting passivation potential in passive range;
(b) activation by exposure at potential of 502 mV; and
(c) same scaled up in i direction.
alloys, the critical passivation potential is shifted in the
positive direction with respect to E ac r value. However,
the character of the polarization curves on the lowchromium alloys (2–6% Cr) differs from that of the
curves on the alloys containing more than 10% chromium; a current jump is observed both at the active–
passive and passive–active transitions (Fig. 2b). Thermodynamic calculation of the passive layer composition on the low-chromium alloys provides a very low
Cr2O3 content compared to the main FeO oxide layer
directly adjacent to the Fe–Cr alloy surface [14]. For
this reason, the E ac r potential is close (≤500 mV) to the
Flade potential of individual iron. However, besides
the inner FeO layer slightly enriched in Cr2O3, the
outer layer composed of the higher iron oxides is probably formed on the alloy surfaces. This layer is also
enriched in Cr2O3 oxide to a certain degree. The exist-
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011
Urrent density, mA/cm2
408
SAFONOV et al.
(а)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
(b)
6
10
14
Time, h
18
22
100 µm
determines the potential delay in curve shown in
Fig. 1c, since there is no similar segment in the curves
recorded on the alloys with lower chromium contents
(Figs. 1a, 1b). Judging from the switch-off curve
(Fig. 1c), we can suppose that the passive film still has
a two-layer structure on this alloy. The convergence of
the experimental curves of the passivation and activation onset potentials of the alloys (Fig. 3a, curves 2, 3),
which correspond to the outer layer compositions, and
theoretical curve 1 that corresponds to the Flade
potential of the inner layer indicates the transition
from a two-layer (or possibly three-layer) structure to a
one-layer film. This is confirmed by the data of X-ray
photoelectron spectroscopy of the passive layer
formed on Fe–20 Cr alloy. The analysis revealed only
one layer composed of 80% Cr2O3 and 20% FeO [25].
The enrichment of the inner passive film layer with
Cr2O3 oxide strongly decelerates the potentiodynamic
activation of the alloys with a high chromium content
(10–23%). As is shown by Fig. 2c, this is reflected in
the smooth shape of the polarization curves, as well as
the decrease in the critical passivation and activation
currents. The novel method of successive approximations alone enables one to activate the high-chromium
alloy (23% Cr). Based on the condition of the steady
passive state, Е > EF, we can say that the theoretical
dependence on the chromium content in the alloy is
the boundary that determines the passivity range, in
particular above the curves in Fig. 3. Experimental values obtained using different methods are satisfactorily
close to the theoretical dependence.
CONCLUSIONS
(c)
100 µm
Fig. 7. Activation of Fe–15 Cr alloy along grain boundaries: (a) current density–time dependence and
(b, c) microphotos of surface upon exposure at potential,
mV: (b) –58 (presumable activation) and (c) 670 (passive
range).
ence of the layer is indicated by the potential delays in
a range around Е ≈ 600 mV (Fig. 1a), which is typical
of the passivation of individual iron. The absence of a
horizontal segment corresponding to the thin inner
layer is explained by the low Cr2O3 concentration,
which results in its rapid dissolution upon switching
off the potential.
Thermodynamic analysis of the composition of the
inner passive layer on Fe–13 Cr alloy provides an estimate of 70% Cr2O3. It is this enrichment that probably
1. Passivation and activation potentials of Fe–Cr
alloys are studied using three methods, namely,
switching off the anodic polarization, potentiodynamic passivation–activation, and successive approximations. Critical passivation and activation potentials
obtained by the three methods are in satisfactory
agreement with each other.
2. An experimental method is proposed for determining the activation potentials (the method of successive approximations to the equilibrium Flade
potential) of passive layers formed in metallic materials, which enables one to estimate the activation onset
potential of an alloy more accurately and correctly
than the conventional methods.
3. The obtained experimental dependence of the
activation potential of Fe–Cr alloys satisfactorily
agrees with the results of thermodynamic calculations
[14] of the dependence of Flade potentials of Fe–Cr
alloys on their chromium content.
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