Stainless Steel in 0.5 M H2SO4

TECHNICAL PAPERS
-
ELECTROCHEMICAL SCIENCE AND TECHNOLOGY
The Secondary Passive Film for Type 304
Stainless Steel in 0.5 M H 2SO4
A. Atrens
The Department of Mining, Minerals, and Materials Engineering, The University of Queensland,
Brisbane, Queensland, Australia 4072
B. Baroux* and M. Mantel
Ugine Research Centre, F-73400 Ugine, France
ABSTRACT
An examination has been carried out of the secondary passive film on Type 304 stainless steel in 0.5 M HSO4. The
characterization techniques used were electrochemical (potentiodynamic, potentiostatic, and film reduction experiments)
and surface analytical. A bilayer model for the secondary passive film is proposed. It appears that next to the metal, there
is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied potential.
On top of this modified passive film, the experimental data are consistent with a "porous" corrosion-product film which
adds to the total film thickness but has little influence on the electrochemical response. The composition of the secondary
passive film corresponds most probably to a mixed Fe/Cr oxide/hydroxide enriched in Cr3 +, with a composition similar
to a primary passive film.
Introduction
The aim of this study was to examine the secondary passive film on Type 304 stainless steel in 0.5 M HSO4 in the
secondary passivity potential region, typically at potentials more positive than about 940 mVscE. The primary
interest was in the characterization of the secondary passive film as it might be used in a technological application;
that is after the film had been formed, dried, and exposed
to air. Also of interest were the insights that could be
gained concerning the secondary passive film by a study of
film formation. The literature suggests three different possibilities for surface films in the secondary passive potential range: (i) a modified continuous passive film, (ii) a
porous film, and (iii) a hydroxide gel. Possibility (i) derives
from the passivity literature.'-" Possibilities (ii) and (iii)
derive from the literature on colored stainless steels.35 43
The (primary) passive film on stainless steels has a complex structure'- 3 but can be thought of as a very thin (-1 to
3 nm) film composed of metal oxides/hydroxides and bound
water. For stainless steels, the Cr enrichment in the bulk of
the passive film is evidenced by a Cr°X/(FeOX + Cr °X) ratio
significantly higher than the ratio Crm/(Fem + Crt) characteristic of the alloy 124'11 -13'16-22 There is further enrichment of
Cr in the oxidized state, Cr° X, at the film/solution interface
for films formed in acid conditions.l-.'3 The passive film
grows on the metal surface by an electrochemical mechanism, covers the metal surface as a continuous layer, and
inhibits corrosion by forming a barrier between the corroding metal and the environment. The film has a selfrepair property (under conditions such that the passive
film is stable) because the film formation rate is highest on
a film-free surface (as at a break in the film) and the film
growth rate decreases rapidly with film thickness.4 '0 Thus,
* Electrochemical Society Active Member.
the film can be thought of as having a steady-state thickness, although attainment of the steady-state thickness
can take up to 100 h in acid solutions.4' 0
Both Clayton and Olefjord2 and Macdonald 4 2 2- 6 propose
a bilayer structure for the primary passive film consisting
of an inner barrier layer (consisting mostly of Cr2 O, for
stainless steels') and an outer precipitate layer. This composite structure has been postulated by Sato 28 '29(see also
Ref. 30, 31) to have ion selective properties which can be
important in the formation stages of the passive film. Furthermore, ions from the solution can be incorporated in
the outer precipitate layer 22 23' and provide increased protection against passivity breakdown induced by chloride
ions and consequent pitting corrosion.
Sugimoto and Matsudal s analyzed passive and secondary
passive films using ellipsometry for films formed on FeCr,
alloys in NaSO at pH 2.0 and 6.0. For Fe-Cr,0,, the passive film thickness increased linearly over the whole
potential region from 0.4 to 1.3 VSCE; this behavior is similar to that for pure Fe measured by Sato et al. 4 For FeCr.
(x = 0.15 and 0.20) the primary passive films increased linearly in thickness with increasing potential in the range
200 to 800 mVsce (consistent with the other passivity literature), there was then a transition region in which the film
thickness was approximately constant, and then the secondary passive films increased linearly in thickness with
increasing potential in the range 1000 to 1400 mVsc. The
secondary passive films with average film thicknesses of
2.5 to 3.0 nm were somewhat thicker than the primary
passive films (with average thicknesses 1.5 to 2.0 nm),15
and they had somewhat different optical constants, indicating that there were differences between the passive and
secondary passive films. Similar trends were measured by
Song et al.3 using ellipsometry for type 304 stainless steel
in 0.5 M H2SO 4 at ambient temperature. Song et al." using
x-ray photoelectron spectroscopy (XPS), found high levels
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
3697
3698
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
of Cr 3+ and some Cr6+ in secondary passive films formed in
0.5 M Na2 SO4 . Kirchheim et al.4 used XPS and Ar sputtering to measure concentration profiles for passive and secondary passive films formed on Fe-1OCr in 0.5 M H2 SO4 .
The profiles of Cr°X/(Fe° x + Crox) were quite similar for
both types of films with the secondary passive film (2.0 nm)
being slightly thicker than the primary passive film
(1.8 nm). Both were enriched in CrOx in the bulk of the film
-1
NIHq
-...
·
t
Tt
-I
ub-
.0
N! I
Experimental
Type 304 stainless steel was used having the following
chemical composition: 17.4% Cr, 8.4% Ni, 0.17% Mo, 1.4%
Mn, 0.49% Si, 0.059% C, 0.15% Cu, 0.12% Co, 0.05% V,
0.013% Sn, 0.024% P, 0.0015% S, 0.05% N, 40 ppm 0. This
was received from the mill in the bright annealed condition'(in-line continuous sheet anneal in a hydrogen atmosphere for about 5 min at 1000°C followed by rapid cooling
in the same atmosphere) which ensures that all species are
in solid solution and produce a bright stainless surface.
The standard specimen preparation was a wet metallurgical polish to 1200 grit with silicon carbide, gentle air dry
and exposure to laboratory air (of ambient humidity) for
about a day (i.e., exposure to laboratory air for at least 8 h).
The electrochemical apparatus' is shown in Fig. 1; this
allowed the specimen to be introduced into the specimen
cell and the specimen surface to be first deaerated by an
equal mixture of H2 and N2, while this gas mixture was
used to deaerate the electrolyte for 1 h in the deaeration
a The electrochemical apparatus is one developed for studies of
passivity and pitting corrosion. In studies of pitting corrosion,
crevice corrosion at the specimen edges is always a concern and
often a practical problem that masks the pitting corrosion that is
being studied. This problem has been overcome4 in a number4 74of
ways. One solution is that adopted by Baroux" and Atrens "
where very careful attention is paid to deaeration of the sample
surface and the deaeration of the test solution. This same
approach was used in the present study. After the specimen is
introduced into the specimen cell, the specimen is exposed to a
flowing gas mixture of equal parts H and N2 . This gas flow
removes oxygen from the specimen cell, removes (lightly bound)
oxygen from the specimen surface and also removes oxygen from
crevices at the specimen edges. Subsequently deaerated electrolyte is introduced into the specimen cell. For pitting studies,
the procedure is useful in preventing crevice corrosion. If a less
rigorous procedure is used, then there could be some oxygen
trapped in crevices at the specimen edge and this could cause initiation of crevice corrosion. In our studies, removal of all oxygen
was also a consideration to ensure low values of residual current
in the reduction experiments. Our procedure was successful in
producing low residual reduction values as indicated by the i values in Table III.
->N,fU,
_
,I.i
Lab
'e
aI i
%
~1.i
I-I
.i
to similar levels with Cr°X/(Fe ° x + CroX) - 0.4. The thick-
ness of the CrOX region was somewhat greater in the secondary passive film. The primary passive film was further
enriched in Crox at the very surface as discussed above.
Coloring of stainless steel was developed 3 5 as a chemical
anodization process. This was modeled 3 6-40 as due to the
chemical precipitation of Fe(OH)3 and Cr2 (SO4 )3, 18H2 0
when their solubilities were exceeded locally at the metal
surface. In contrast to the production of the passive film
by electrochemical oxidation, these corrosion products
were assumed to be produced by a chemical precipitation
process. Porous corrosion products were assumed which
did not influence the corrosion process, and the electrochemical response was modeled on the basis of the diffusion of ionic species in the liquid phase of the pores in the
film. In this context it is worthwhile considering the properties of a hydroxide gel. Metal hydroxides can precipitate
from solution to form a gel; there forms a skeletal threedimensional network of molecules throughout the medium
giving the medium solidlike rigidity although these molecules (i.e., the solid) form a small fraction (-10 to 30%) of
the total volume. Electrochemical corrosion is as for direct
contact of the metal with liquid, as there remains a direct
conduction path through a liquid phase of the gel.
In the present study, secondary passive film formation
was characterized by measuring potentiodynamic polarization curves and by carrying out potentiostatic experiments. Film characterization after film formation was carried out by electrochemical reduction and XPS.
?tfirWfoAg1A&
swa
[email protected]
4 W d(t)
1~- -
ki J-I%
l.-
,,
ssamce
!
ve
fK
Fig. 1. Electrochemical cell.
cell. All solutions were made using analytical grade reactants and deionized water. The deaerated electrolyte was
then introduced into the specimen cell without exposure to
air, and electrochemical measurements were started a
period of 2 to 4 min after electrolyte contact was established with the specimen. Prior experiments have determined that the oxygen concentration in the solution is
around 10 ppb under these conditions. The counterelectrode was a platinum sheet. The reference electrode was
Ag/AgCl. The potentials are reported with respect to a saturated calomel electrode to allow easier comparison with
the literature. The temperature was maintained at 23°C by
the circulation of the thermostated water through the
outer water jacket of both the specimen chamber and the
deaeration cell.
This electrochemical apparatus was used for (i) characterization of film formation by measuring potentiodynamic polarization curves and by carrying out potentiostatic
experiments, and (ii) film characterization after electrochemical film formation by electrochemical reduction.
A rapid overview of the electrochemical behavior relevant to film formation was obtained by measuring the
potentiodynamic polarization curve, in 0.5 M H2 SO4 . For
comparison polarization curves were also measured in 5 M
and 6 M H2 SO4 ; a borate buffer, 0.15 M sodium borate
(Na2 B4 O, · 10 HO) + 0.15 M boric acid (H3BO3 ) at pH 8.4;
and in a phosphate solution (4.7% Zn3(PO,) 2 + 9.3% H3PO 4
which had a pH of 1.47). All polarization curves were
measured starting from a cathodic potential (-800 mVscE)
at the rate of 10 mV/min.
Details of the electrochemical behavior during film formation were obtained from potentiostatic experiments. In
these experiments, a specimen was prepared in the standard manner. Its surface was deaerated. It was exposed for
a few minutes to the deaerated 0.5 M H2SO 4 at the free corrosion potential (and passive film was formed on the specimen surface as shown by the potentiodynamic experiments). Subsequently, the potential was "instantaneously"
applied at the value of interest, and the current recorded
as a function of time.
The secondary passive film after electrochemical film
formation was characterized by electrochemical reduction
at -350 mVscE in fresh deaerated 0.5 M H 2SO,. The reduction conditions were chosen to minimize contributions
from other reduction reactions, including reduction of
species in solution and also hydrogen evolution. b The potential value of -350 mVs,, was chosen because of the low
current densities measured in the polarization curves in
0.5 M H2SO4 . For the reduction experiments reported in
this paper a secondary passive film was first formed by
holding a new specimen prepared in the standard manner
at constant potential in deaerated 0.5 M H2SO4 while the
current was recorded. The solution was drained from the
specimen cell. The specimen, in the specimen holder, was
removed from the specimen cell, washed in distilled water,
dried in a jet of clean air, and reintroduced into the specimen cell where the surface of the specimen was deaerated
b In a series of preliminary experiments, reduction was carried
out immediately after film formation in the same solution. The
experimental results were similar to those reported herein,
although the background current densities were somewhat higher than those measured in the experiments reported in this paper.
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
by the N2 /H, gas mixture for about 1 h. During this hour,
a new 0.5 M H2SO, solution was deaerated in the deaeration cell. After the new deaerated solution had been introduced into the specimen cell by means of the deaeration
gas pressure, the potential of -350 mVscE was applied and
the current recorded as a function of time. For each reduction experiment, there was a new specimen and a new
deaerated solution. Some experiments were carried out to
study the stability of the secondary passive film. These
had the same procedure except that, after film formation,
the specimen was removed from the specimen holder and
subjected to an ultrasonic cleaning bath in an
acetone/alcohol mixture. Subsequently, the specimen was
washed in distilled water, reintroduced into the specimen
cell, its surface was deaerated, and the surface film was
characterized by reduction in a new deaerated 0.5 M
HSO
2 4 solution.
Film characterization after film formation was also carried out using XPS. XPS measurements were carried out to
characterize films formed for (i) 200 s and (ii) 30 min at
1200 mVscE in deaerated 0.5 M HSO 4 (followed in each
case by washing in distilled water and drying); these were
compared with (iii) a passive film on a specimen after the
standard specimen preparation (wet abrasion with 1200
grit, dried in air, and at least 8 h exposure to laboratory
air), and (iv) the residual film on a specimen with secondary passive film formed for 30 min at 1200 mVsc
in
deaerated 0.5 M HSO,, reduced in a new deaerated 0.5 M
HSO
2
4 solution at -350 mVscE, washed in distilled water,
and dried in air. Because major emphasis was on gaining
an understanding of the secondary passive film as it might
be relevant in a technological application of the stainless
steel, all specimens were exposed to the atmosphere during transfer to the XPS apparatus. This transfer arrangement might cause the loss of fragile surface species like
OH and adsorbed water. Furthermore, the fine structure
of a modified passive film may revert to that more characteristic of a passive film. Nevertheless, the XPS measurements should provide valid measurements of chemical
species and film thickness. It is also worthwhile mentioning that the passive film has been shown to be quite robust
and to change surprisingly little with air exposures. 1
XPS analysis was performed using a VG XR3E2 spectrometer employing a Mg K0 (1253.6 eV) monochromatic
x-ray source operated at 15 kV with a power of 300 W.
Typical operating pressures were -30 nPa (2 10 0 Torr).
The spectrometer was calibrated using the following photopeaks: Au 4 f7/2 at 83.8 eV, Cu 3p at 75.2 eV, and Cu 2 p 2/3
at 932.7 eV All binding energies were referenced to the
carbon C-H photopeak at 285.0 eV. The area analyzed was
10x
4 mm. Survey (wide) scan spectra were measured
with a pass energy of 60 eV for all samples to determine
what elements were present in the top 5 nm of the surface.
Mutiplex (narrow) scan spectra were obtained with a pass
energy of 30 eV for all major photopeaks. Angle dependent
analysis at 30 and 90 was done by varying the take-off
angle between the surface and the direction of electron detection. Detailed scans were carried out for Fe, Cr, 0, C,
S,
and Si with take-off angles of 300 and 900. Particular
attention was devoted to the S measurements, because
there was an expectation of sulfate incorporation in the
secondary passive film from the literature on colored
stainless steels 3-4 3
Potentiodynamic Polarization
The polarization curves are presented in Fig. 2. Low
currents were measured in the passive range (-200 to
-950 mVsCE) in all cases. In
5
M H2S0 4 there was active
corrosion followed by passivation. Specimens were selfpassivating in the borate buffer solution, in 0.5 M H2 SO,
and in the phosphate solution.
This is of significance for
the interpretation of the subsequent potentiostatic
experiments; these experiments show that there was a
passive film on the specimen surface after the standard
specimen preparation and exposure for a few minutes to
16.0- -
3699
I
-a- 6MH2S04
--
5M 142S04
borate buffer
-- - 0.5MH2S04
-- phosphate
12.0
a
r
8.0
0
b
.0
4.4.0
W
/
.0~
[i
Si--
/10
0.0
;ry' _
7R- .7
No7,
-, !
4
lr
b
I
I
I
I
'-
-0.6
-0.2
0.2
0.6
1.0
1.4
I
1.8
potential, VsCE
Fig. 2. Potentiodynamic polarization curves to characterize film
formation conditions.
the borate buffer-solution, to the phosphate solution, and
to 0.5 M HSO4 .
For 0.5 M HSO
2
4, within the secondary passivity range,
the current increased to a plateau of about 4.6 mA/cm 2 at
1400 mV. In the more concentrated solutions, i.e., in5 and
6 M HSO
the current densities were in general much
2 4
higher in the secondary passivity region. For the phosphate solution, the current density increased to a plateau
of about 2 mA/cm 2 at 1250 mV The curve measured in the
borate buffer was different from the other curves in that
the current density increased nearly linearly with increasing potential; the "noise" on this curve of potentials above
about 1175 mV might be indicative of oxygen evolution
which is expected at these potentials in the higher pH values of the borate buffer.
The potential E,, at which there was a significant current density increase indicative of the onset of secondary
passivity, was similar for all the solutions used, Table I,
with an average value of 940 mVscE,
and soE,
was
approximately independent of pH over the range 0 to 8.4.
Potentiostatic Experiments
Figure 3 presents the current as a function of time measured at 500, 1000, and 1300 mVscE in the potentiostatic
experiments. In the passive region (at 500 mVsc), the current decreased steadily with time, whereas in the secondary-passivity region, the current became independent
of time after an initial transient; this is consistent with the
literature.',"
Figure
4 presents the initial shapes of the transients in
the secondary-passivity potential range forpotentiostatic
experiments at 1100, 1200, and 1300 mVscE. One curve at
1200 mVcE
was recorded for 200 s, whereas all the other
experiments were of 30 min duration, although Fig. 4 illustrates only the initial parts of the transient where there was
significant change. The repeat experiments show excellent
reproducibility. The transient at 1100 mVsCE displayed a
single minimum similar to that at 1000 mVsCE (see Fig. 3).
The transients at 1200 and 1300 mVscE showed a minimum
followed by a maximum, which was more pronounced for
the curve at 1300 mVscF. The steady-state value of the current, i, was very similar to the current density,
i,
measured in the potentiodynamic experiments. This is recorded
Table . Potential,
E,
Solution
marking the onset of secondary passivity in
different solutions.
Esp, mVsCE
6 M HSO4
5 M HSO
970
0.5 M H2 SO 4
Phosphate
Borate
910
940
960
970
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
3700
Table II.Steady-state current density, i,, measured in potentiostatic
experiments compared with the current density, id,measured in
the potetiodynamic experiments.
10
-
j
B---
-`^I'----1
Potential
E (mV)
'..
Current density (mA/cm')
id
i
C)4
[1 1
*
0.1
©3
r::
ooW
00
I 00
a
001
"0
t)
+ 12
0,001
:-
10
i
* 500 mV
0.0001
time, s
Fig. 3. Potentiostatic curves to characterize film formation for
type 304 stainless steel in deaerated 0.5 M H2SO 4 at 500, 1000,
and 1300 mVscE.
in Table II. The speed of the transient response indicated
that there is a fairly rapid transformation of the passive
film into the secondary passive film (which controls the
electrochemical response) with most of the film formation
completed within 400 s.
Reduction Experiments
Typical reduction curves are presented in Fig. 5 for secondary passive films formed for different periods at
1200 mVscE. For secondary passive films, formed under the
normal conditions, the reduction curves could be interpreted as being composed of three reduction waves. Table III
presents the charge associated with each reduction wave,
Q1, Q2, and Q, as defined in Fig. 6. Table III also records
values of the film thickness, d,, which has been estimated
from"
d = Q V,,ox/3F
1400
4.6
5.6
1300
1200
4.0
3.2, 3.3
1100
1000
1.8, 1.8
0.18, 0.16
4.0, 4.1
3.3, 3.4, 3.3, 3.3,
3.1, 3.3
2.0, 1.9
0.32
where 100% current efficiency is assumed during reduction, it is assumed that all the reduction charge Q is associated with M3+; VOX, the oxide molecular volume is taken
as Vox = 10 cm3/mol which is the average value for iron
oxides.' ° The charge associated with each reduction wave
increased with increasing time of film formation, although
there was considerable data scatter, for example, as shown
by the differences in the two reduction curves for films
formed for 30 min at 1200 mV. This data showed that these
secondary passive films could withstand drying. However,
there was very little charge (0.43 mC/cm2 ) associated with
the reduction curve for the film subjected to the acetone/alcohol ultrasonic bath indicating that most of the
secondary passive film had not survived this treatment.
Low values of residual current, i3 (see Table III) indicated success in choosing reduction conditions that minimized reduction reactions other than film reduction.
Measurement of the residual current, i3 due to other reduction reactions, allowed evaluation of the reduction charge
associated only with film reduction.
A most interesting observation is that the film thickness,
as measured by reduction, continued to increase for film
formation times greater than 400 s, Fig. 7. In this time
period, there was little change in the current transients
measured potentiostatically, see Fig. 4, indicating little
change in the electrochemical response. It is worth stressing that the film thickness measurements using cathodic
reduction are to be considered to underestimate the true
film thickness for two reasons. First, it is widely believed
that Cr,O, is not reduced under these conditions.' 44'45 This
can place considerable limitations on this cathodic reduction technique as a means of measuring the film thickness.
Second, the reduction experiments were carried out after
the specimen had been washed in distilled water and
dried. This specimen handling procedure could remove
[1]
o.-
I --
-I ;
-10.0
-I
_-
-
45
4.0
' -20.0
3.5
-
30 min
t -30.0
'-x-
3.0
2.5
= 2.i
05 2.0
-40.030
I
0.0
Is<
"Gir,0.0
.0.1
U130
mV
-+ 1200 mV
0.3
0.5
1100 mV
0.7
t (ks)
Fig. 4. Potentiostatic curves to characterize film formation for
type 304 stainless steel indeaerated 0.5 M HSO4 at 1100, 1200,
and 1300 mVscE. Transients at 1200 mVs,,,, are for specimens held
at 1200 mVscE for 200 s and 30 min. Transients at 1300 and
1100 mVscE are for repeated experiments.
I
0.5
,I
1.0
I
2.0
1.5
t(ks)
200 s
-l
6min
--
30 min
2.5
3.0
3.5
Fig. 5. Film characterization by electrochemical reduction after
formation of the secondary passive film at the stated times at
1200 mV in0.5 M H2SO4. Reduction was carried out at -350 mVscE
in a fresh deaerated solution of 0.5 M H2SO 4 . All data are for the
standard specimen preparation procedure (which included washing
in distilled water and air exposure) except for data identified with
a full circle which refer to specimens subjected to ultrasonic cleaning inacetone/alcohol, followed by washing indistilled water and
air exposure.
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
3701
Table III.Reduction charge, Q, Q2, and Q3, associated with each reduction wave as defined in Fig. 8.
t
t,
(min)
(s)
i2
(gA/cm 2 )
Q5
t2
(mC/cm')
(s)
3.
9
6
12
15
25
30
30
40
45
60
60
100
68
18
37
50
360
418
536
13.8
16.1
47
24.9
30
17.0
11.0
7.0
919
0.69
1.07
0.15
0.14
0.59
2.73
2.64
2.66
515
22
5.62
i2
(gzA/cm 2 )
09
1
358
270
223
400
716
1300
1688
1054
1990
2704
Q2
(mC/cm2 )
5.4
6.3
8.2
4.7
8.7
6.2
5.0
4.1
5.3
16
loosely adherent material from the surface but could not
add material to the surface.
XPS
In all cases there was a significant amount of C present
on the surface as is often observed with XPS analyses, and
this has been attributed to contamination. The surface C is
characterized in terms of the ratio C/O of carbon to oxygen peak heights, Table IV; the least carbon was observed
for specimen (b) (i.e., the secondary passive film formed
for 30 min at 1200 mVs,C), with C/O = 0.4 compared with
C/O in the range 0.6 to 0.8 for the other samples. This is
consistent with a clean surface being produced by the
"corrosion" at 1200 mVscE. The major species present in
the films were Fe, Cr, and 0; with only trace amounts of
the other elements. This data is consistent with the secondary passive films being a mixed Fe/Cr oxide with significant hydroxide and is not consistent with significant
incorporation of SO4 . The detailed oxygen spectra were
also consistent with this interpretation.
The Fe spectra are given in Fig. 8; this shows the largest
signal for specimen (b) with similar signals for the other
three specimens. The largest Fe signal for specimen (b)
correlated with the lowest carbon contamination so a
qualitative analysis of the peak heights leads to the conclusion of comparable amounts of Fe signal for all the
specimens (a) to (d). In all cases the main signal was associated with an oxide with a binding energy of about 711 eV
with some signal associated with the metallic state at a
binding energy of 707 eV. The presence of a metallic Fe signal indicates that there are areas of the film that are very
thin (in the range of a few nanometers), that is thinner
than the escape depth of the photoelectrons associated
t2
i3
(VA/cm 2)
Q3
(mC/cm2 )
de
(nm)
9
4611
99
2
11
2.4
2.4
4.1
2.9
5.9
12.4
9.0
5.0
6.5
10.6
1400
1802
1600
1106
2236
3127
5227
3121
7099
5396
2.8
3.2
4.6
2.5
2.5
2.3
3.0
1.4
3.7
8.9
4.9
5.2
7.5
4.5
15.9
22.9
18.4
11.9
15.1
43.2
1.7
1.8
2.6
1.6
5.5
7.9
6.4
4.1
5.2
14.9
with the metallic iron signal. This is consistent with the
literature results of 4,15,32 which show that the secondary
passive film is quite thin, in the range of a few nanometers, but is somewhat thicker than a primary passive film.
The Cr spectra are given in Fig. 9. This also shows the
largest signal for specimen (b) with decreasingly smaller
signals for specimens (a), (d), and (c). The binding energy
3+
in all cases corresponds to Cr , with the presence of no
Cr6 +. Cr3 + is expected to be oxidized to Cr6 + is the potential region of secondary passivity, and so a significant Cr6 +
signal might be expected. Significant Cr 6 + signals were in32
+
deed measured by Song et al. However, Cr is extremely
6
soluble and soluble Cr' on the film surface would have
been removed by the water rinsing procedure used in our
experiments. Furthermore, prior work characterizing
Cr2 O, oxide on stainless steels after oxygen/plasma treatment, has shown that Cr6 + is easily removed by rinsing the
sample with water. Another explanation could have been
the well-known effect of beam reduction of Cr6+ or Cr3 + .
However, there was no residual Cr6+ signal. Although
beam-induced structural damage has been found to result
in chemical alteration of metal oxides, we have not experienced any Cr6 + reduction in other comparable work. 2 '
The Cr enrichment in the secondary passive film can be
analyzed from the ratios of the Cr and Fe signals with 90'
45
(N
E
U
40
35
30
time
tI
t,
(s)
t3
c 25
.
i.
g 20
.
(A
il
U
a)
a'
.
4-
15
4)
0
11
0
0
20
40
60
time, min
Fig. 6. Schematic illustration of the three reduction waves and the
definition of the charges Q,, Q2, and Q3.
Fig. 7. Total reduction charge, Q3, (from Table III), increased as a
function of film formation time at 1200 mV in 0.5 M H2S04.
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
3702
Table IV.XPS peak height ratios.
Specimen
C/O
Crg0/Feg0
Cr30/Fe0,
a
0.66
0.38
0.55
0.83
0.94
0.82
0.31
0.75
0.67
0.71
0.21
0.5
b
c
d
and 30 ° take-off angles: Cr,,/Fe,,0 and Cr30 /Fe30, see
Table IV. The 30 ° take-off angle provided data with a
greater contribution from the outer surface layers of the
passive film, whereas the 90 ° take-off data had a greater
contribution from a greater depth into the passive film.
These data are consistent with the secondary passive film
having a higher Cr enrichment than the primary passive
film consistent with the results of Song et al.32 This observation of Cr enrichment is also consistent with the work of
Kirchheim et al.4 who measured similar Cr°X/(Fe ° x + Cro X)
concentration profiles for passive and secondary passive
films both having Cr°X/(Fe ° x + Cro° ) - 0.4 with the thickness of the Cro° region being somewhat greater in the secondary passive film.
There was a higher Cr enrichment in the film (d) i.e.,
after reduction, consistent with incomplete reduction of
Cr2,O,3 .
Despite the original concerns about specimen preparation and the transfer arrangements to the XPS apparatus,
the consistency of the present results with literature results lends comfort to the reliability of the main conclusions to the drawn from the present results: the secondary
passive film is thin (in the range of a few nanometers) and
has a composition comparable with that of the passive
film. The thinness of the secondary passive film in these
experiments is to be contrasted with the very much thicker film (micrometers in thickness) formed during the coloring of stainless steels. 5 43
Proposed Model
The present work taken together with the literature4 ' 53 2
has led us to propose a bilayer model for the secondary
passive film. The general case is illustrated in Fig. 10,
whereas Fig. 11 illustrates the specific case for the secondary passive film in 0.5 M H2SO4 after washing in distilled water and drying. It appears that, next to the metal,
there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied
Binding eerV (eV)
Fig. 9. The Cr XPS spectra. 900 take-off angle. Specimen preparation as for Fig. 8.
potential. On top of this modified passive film, the experimental data are consistent with a "porous" corrosion product film which adds to the total film thickness but has
little influence on the electrochemical response. The corrosion product film could be in -the form of mounds separated by areas where the modified passive film is in direct
contact with the solution. The overall composition of the
secondary passive film is most probably of a mixed Fe/Cr
oxide/hydroxide enriched in Cr3+ , with a composition
quite similar to a primary passive film.
The two layer aspect of the secondary passive film was
proposed to explain the measured increase in the film
thickness, as measured by reduction, for film formation
times greater than 400 s, Fig. 7. In this time period, there
was little change in the current transients measured potentiostatically, see, e.g., Fig. 4, indicating little change in
the electrochemical response. The two layers were proposed to have different functions. It was proposed that the
modified passive film controls the electrochemical response; i.e., governs the current for any applied potential.
In contrast, it was proposed that the "porous" corrosion
product film adds to the total film thickness but has little
influence on the electrochemical response.
corrosion product film -
/
'
/
/
/ / / //
///
/modified passive
film/
metal
u
indi energy (ev)
Fig. 8. The Fe XPS spectra. 90 ° take-off angle. Air transfer to
XPS apparatus after following specimen preparation: (a)the secondary passive films formed for 200 s at 1200 mVscE in deaerated
0.5 M H2SO4 followed by washing in distilled water and drying; (b)
the secondary passive films formed for 30 min at 1200 mVc in
deaerated 0.5 M H2SO4 followed by washing in distilled water and
drying; (c)passive film on a specimen after the standard specimen
preparation (wet abrasion with 1200 grit, dried in air, and at least
8 h exposure to laboratory air); and (d)the residual film on a specimen with secondary passive film formed for 30 min at 1200 mVscE in
deaerated 0.5 M H2SO4, reduced in a new deaerated 0.5 M H2SO4
solution at -350 mVsc,, washed indistilled water, and dried inair.
Formed electrochemically,
continuous,
self repairing
of uniform thickness.
Formed by precipitation
irregular,
probably porous gel-like.
Barrier film,
ie controls current.
No influence on current.
Fig. 10. Model proposes a bilayer film for the general case of the
secondary passive film. Next to the metal there is a modified passive film which controls the electrochemical response; i.e., governs
the current for any applied potential. On top, there is a precipitate
film which adds to the total film thickness but has little influence on
the electrochemical response.
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
Fig. 11. For the specific case for the secondary passive film
formed in0.5 M H2S0 4 after washing indistilled water and drying,
it is proposed that the corrosion-product film is in the form of
mounds between which the barrier film is in direct contact with the
solution.
There was a lot of scatter in the data of Fig. 7. This is
consistent with a precipitate film, because precipitation is
a stochastic process and the conditions of precipitation are
expected to be very greatly influenced by such experimental variables as solution velocity at the specimen surface
over which there was little control in the present apparatus. This would also explain the present measurements
which imply secondary passive films somewhat thicker
than those reported in Ref. 4, 15, and 32.
The XPS results indicating that there are areas of the
secondary passive film that are very thin (in the range of a
few nanometers) are to be contrasted with the data from
the reduction experiments that indicated that existence of
a somewhat thicker film that continued to increase in
thickness with increasing time at the secondary passivity
potential, see Table III and Fig. 7. These two contrasting
sets of data can be reconciled with the model given in Fig.
11 for the secondary passive film. Figure 11 shows the corrosion product film in the form of mounds separated by
areas where there is no corrosion product film. The areas
between the mounds would give rise to the XPS metallic
iron signal indicative of very thin areas. The mounds of
corrosion product film could continue to grow in thickness
with increasing time at potential to give rise to higher
reduction charges.
Both the electrochemical reduction experiments and the
XPS measurements were carried out for secondary passive
films prepared in similar ways including electrochemical
film formation, washing in distilled water, and drying.
Thus these two data sets are directly comparable and lead
to the model presented in Fig. 11 for the case of the secondary passive film formed in 0.5 M H2SO4 after washing
in distilled water and drying.
The next question is the relation between the secondary
passive film after washing and drying (as characterized by
the reduction experiments and the XPS measurements)
and the secondary passive film during its formation (as
characterized by the polarization curves and the potentiostatic experiments). It is worth repeating the argument
that the reduction experiments underestimate the true
film thickness. Therefore, a two-layer model is indeed required to reconcile the different trends for film formation
times greater than 400 s identified in the reduction experiments and the current transients.
The form of the corrosion product film could however
depend on the influence on the washing and drying steps.
Two possibilities can be identified: (i) Fig. 11 represents the
secondary passive film during film formation and also during film characterization after washing and drying. This
3703
model is indeed consistent with all the data as argued
above; and (ii) Fig. 10 represents the situation during formation of the secondary passive film. This would require
the modified passive film to control the electrochemical
response and the corrosion product film to the "porous"
and to have negligible influence on charge-transfer. The
subsequent washing and drying procedure changes the form
of the corrosion product film into the form illustrated in
Fig. 11: that is into a discontinuous film as required to be
consistent with the experimental measurements.
It is possible that the washing and drying steps changed
the morphology of the as-formed secondary passive film
into a film as illustrated in Fig. 11. Observations were carried out with optical microscopy and scanning electron
microscopy up to a magnification of 30,000 times. No morphological features could be observed pertaining to the
secondary passive film. This was not surprising considering the scale of Fig. 11. This means that a choice between
the two above possibilities requires detailed observations
of the morphology of the secondary passive film, both
in situ and after drying, using for example a scanning tunneling microscope.
This model illustrated in Fig. 10 and 11 gives rise to
some interesting possibilities of interpretation for the general behavior of the current with increasing potential
throughout the whole passivity potential region for which
we can take the behavior in the phosphate solution as a
model, see Fig. 2. Here it should be noted that the measured stationary current densities were in good agreement
with the current densities measured with the potentiodynamic method, Table II, indicating that, in the secondary
passivity region, the potentiodynamic method gives a
measured current density which is a good approximation
to the stationary current density.
For the phosphate, the current density increased rapidly with potential at the start of the secondary passivity
region, the current density peaked and then was approximately constant for increasing potentials up to about
1500 mVscE (see Fig. 2). The transport of current through
the passive film is usually assumed to be by a field assisted migration of cations given by 9'"
i
i exp (BE) with B = aZFs/RT and i = kCv, [2]
where the factor i is proportional to the concentration of
moving species (as given by the vacancy concentration Cv
and a proportionality constant k), ZF is the charge of the
moving ions, s their jump distance, the change-transfer
coefficient, and E is the electric field within the film. The
increase in current density in the initial region of secondary passivity is consistent with an exponential increase in current with potential as governed by Eq. 2 if the
structure of the film and the film thickness remains constant. The present work is consistent with the secondary
passive film having a similar structure to the primary passive film, and Sugimoto and Matsuda"5 measured a constant thickness for the film thickness on stainless steels
between the potential regions of primary and secondary
passivity. The subsequent plateau in the stationary current
density is explicable in the same manner as the constant
stationary current density in the primary passivity potential region; viz. the high field migration model together
with a linear increase of the secondary film thickness with
increasing potential as was observed by Sugimoto and
Matsuda."5 Kirchheim has interpreted the compositional
changes within the (primary) passive film as being caused
by the faster diffusion of Fe in the passive film. It would
be tempting to use a similar explanation for the further
compositional changes as the passive film transforms to
the secondary passive film, particularly as our XPS measurements indicate that these further compositional
changes appear to be associated with a further enrichment
in Cr° X in the secondary passive film.
For 0.5 M H,2SO4 there was a similar shape to the plot
of stationary current density as a function of potential,
see Fig. 2. Thus the interpretation should be essentially
3704
J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc.
as for the phosphate solution. For 5 and 6 M H2 SO4 the
only part of the curve recorded was that corresponding to
the initial rapid increase in current density. For the
borate buffer the oxygen evolution reaction seems to
have influenced the measurements, and this makes the
interpretation very difficult.
The present work did not detect SO, in the secondary
passive film in contrast to the expectations from the literature on colored stainless steels. 5 45 As the colored stainless steel films are known to be quite soft, it is possible
that the corrosion-product films were lost between the
electrochemical cell and the XPS apparatus. However, this
does appear unlikely as our XPS results were in agreement
with the literature. A more plausible explanation is that
the details of corrosion-product film are very dependent
on the specific conditions at the specimen surface. The
scatter in our reduction data point also in this direction.
Precipitation is a stochastic process, which is very dependent of the local flow conditions at the specimen surface;
these flow conditions were not controlled in the present
experimental apparatus. In the present work, the surface
of the stainless steel specimen has been maintained by a
potentiostat in the region of secondary passivity. The use
of a potentiostat ensures that only the anodic reaction
occurs at the specimen surface, with the cathodic reaction
occurring at some distance away at the counter electrode.
In contrast, coloring of stainless steels was developed as a
chemical process where a reduction reaction polarizes the
stainless steel surface into the region of secondary passivity. The presence of the reduction reaction at the specimen
surface can cause significant local pH changes and also
introduces additional chemical species which can then be
incorporated into the corrosion product film.
Conclusions
The secondary passivity potential region occurs for potentials greater than about 940 mV.sc independent of pH
(O < pH < 8.4).
When a passive film was instantaneously polarized in
0.5 M H2SO4 to a potential within the region of secondary
passivity, the duration of the transient response was -400 s
in 0.5 M H2SO4 . This indicates a fairly rapid transformation of the passive film into the secondary passive film.
The secondary passive film thickness in 0.5 M H2 SO4 , as
measured by reduction, continued to change for film formation times greater than 400 s. In this time period, there
was little change in the electrochemical response. This
leads directly to the conclusion that the coulometric reduction experiments have a large contribution from the
outer layers of the corrosion product film. This is the basis
for the bilayer model for the secondary passive film.
The secondary passive film in 0.5 M H2 SO4 is capable of
withstanding drying but could not withstand an acetone/alcohol ultrasonic bath.
The secondary passive film is most probably a mixed
Fe/Cr oxide/hydroxide enriched in Cr 3+, with a composition quite similar to a primary passive film.
Acknowledgment
A.A. wishes to thank the Ugine Research Centre for the
provision of financial support which allowed him to spend
a very stimulating sabbatical in Ugine.
Manuscript submitted June 19, 1996; revised manuscript
received Aug. 4, 1997.
The Centre de Recherche Ugine assisted in meeting the
publication costs of this article.
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