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An XPS and ISS investigation of passive layers on binary Fe-Al alloys
Corrosion Science,
Vol. 39,
No. 12, pp. 2193-2213, 1997 0 1997 ElsevierScienceLtd
Printed in Great Britain. All rights reserved 001&938X/97517.00+0.00 PII: s0010-938x(97)00103-0
AN XPS AND ISS INVESTIGATION OF PASSIVE LAYERS ON BINARY Fe-Al ALLOYS D. SCHAEPERS
and H.-H. STREHBLOW*
Institut fiir Physikalische Chemie und Elektrochemie, Heinrich-Heine-Universitiit, Dusseldorf, Germany
UniversitltsstraBe
1, D-40225
Abstract-Passive layers have been formed electrochemically under inert conditions on sputter-cleaned Fe-Al alloys. The aluminum content was 8, 15 and 22 at%, which meets the homogeneity region of a-Fe in the Fe-Al phase diagram. The addition of aluminum reduces the active dissolution current density remarkably compared with that of pure iron. X-ray photoelectron spectroscopy (XPS) studies show an accumulation ofaluminum oxide within the center of the passive layer, which is confirmed by elastic-ion scattering spectroscopy (ISS) and XPS depth profiles. Angular-resolved XPS measurements suggest a continuous structural change of the chemical film during its growth. A discussion of the investigations is presented on the basis of a model of oxide growth. 0 1997 Elsevier Science Ltd Keywords: A. Fe-Al alloys, B. potentiostatic,
B. XPS, B. ISS, C. passive films.
INTRODUCTION Fe-Al alloys show an improved scaling property in combination with better weldability and hardening capacity compared with pure iron.’ Stress corrosion cracking by aggressive chemicals and active metal dissolution is reduced by the aluminum additions.’ X-ray photoelectron spectroscopy (XPS) studies of Fe3Al passivated in 0.5 M sulfuric acid showed an enrichment of aluminum oxide within the passive layer.2 The aim of this study was the analysis of passive layers by means of surface analytical methods to get a better understanding of the corrosion behavior of Fe-Al alloys. The investigations are carried out with binary Fe-Al alloys with different aluminum contents after potentiostatic polarization in acidic and alkaline electrolytes with pH values of 2.5 to 13.8. Electrochemical and surface analytical investigations of the passive layers of the abovementioned alloys are of practical interest, owing to their application as low-alloyed structural steels. EXPERIMENTAL
METHOD
Fe-Al alloys containing 4, 8 and 12 wt% (8, 15 and 22 at%) aluminum were manufactured by melting the corresponding amounts of pure iron and aluminum in a vacuum induction furnace. The material was forged into bars of 13 mm diameter. Annealing experiments were carried out with reference to the phase diagram to find the best treatment for the formation of a single-phase alloy. A subsequent annealing for 1 h at *To whom correspondence should be addressed. Manuscript received 22 May 1997. 2193
2194
D. Schaepers
and H.-H. Strehblow
700°C was found to produce the most homogeneous texture of alloys with an a-Fe lattice. The specimens were machined from central parts of the metal bars. The circular front planes of the specimens were polished with diamond paste down to a grain size of 1 urn and they were then cleaned with ethanol in an ultrasonic bath. The freshly prepared oxide layers were studied by XPS and elastic-ion scattering spectroscopy (ISS) in a commercial UHV spectrometer (VG ESCA-LAB 200X). This instrumentation is equipped with a self-designed chamber3 to enable electrochemical preparation under an argon gas atmosphere. It can be pumped separately and allows specimen transfer from the electrolyte to the ultra-high vacuum without any exposure to the laboratory atmosphere. The electrochemical equipment consisted of a potentiostat (Schramm P 1, Dusseldorf), Oregon) and a function generator (Schramm, pulse generators (2663 Tectronics, Dusseldorf). The Hg/Hg$OJO.S M Na2S04 electrode with E=0.64 V served as a reference electrode for ali experiments. All potential values are given with reference to the standard hydrogen electrode (SHE). The solutions were prepared from high-purity chemicals (pro analysi) and de-ionized water (Millipore water purification system) and were de-aerated with argon before use. The specimens were cleaned by argon-ion etching and transferred into the electrochemical preparation chamber. Only the polished front plane was exposed at open circuit conditions to the electrolyte. The potential was pulsed for defined times (10 ms to 300 s) to the value of interest. After passivation, the electrodes were rinsed with de-ionized water, blown dry with a jet of argon, and transferred quickly via the fast-entry lock and the preparation chamber into the analyser chamber. XPS spectra were recorded with Mg K, radiation (300 W, 15 keV) from an Al/Mg twin anode. Except for the angle-resolved measurements, the attenuation angle was adjusted to 30” relative to the surface normal, thereby providing a more surface-sensitive signal. A Penning source (Specs PS IQP 1O/63) was operated with argon (4 kV, 25 uA target current) for sputter cleaning of the specimens and XPS depth profiling. The sputter rate of the Penning source was calibrated to 1.7 nm min ’ with anodically formed TazOs on tantalum according to the method of Young4 and H0fmann.j Quantitative evaluation of spectra was carried out on the basis of standard spectra3’6,7 of oxygen and the alloying components. The quantitative data of the deconvoluted spectra provide more information on the chemical state of the elements and the quantitative composition of the passive layer than the original XPS spectra. Therefore, to reduce the number of figures, original XPS and ISS spectra are not shown in this paper. A detailed presentation of the data evaluation has been given previously.* A scannable filament ion gun (EX-05, VG Scientific) was applied for ISS sputter depth profiles. This ion source has been applied for ISS and sputtering as well. For ISS spectra it was operated with helium (2 kV, 30 nA target current), whereas argon (3 kV, 300 nA target current) was used for sputtering to obtain higher sputter rates. The scanned area was 0.25 cm2 for the measurements and 0.75 cm2 for the sputter steps. The angle between the incident and the scattered ion beam was 90”. The sputter rate of the EX-05 gun was calibrated as mentioned above with a depth profile of TazOs on tantalum. Becuase of the small sputter area, small-area XPS with a higher lateral resolution of 1 mm was used to detect the metal/oxide interface. Data evaluation Deconvolution
of the XPS signals was carried out on the basis of standard
spectra for
Investigation of passive layers on Fe-Al alloys
2195
each oxidation state of iron, aluminum and oxygen, as described previously in detail elsewhere.‘,” Pure argon-sputtered metals served as standards for iron and aluminum metal. Oxide and hydroxide standards were prepared electrochemically. The Fe(III) and 02- standards were prepared by anodic oxidation of sputter-cleaned pure iron at 1.2 V for 300 s in phthalate buffer (pH 5). The Fe(II), OH- and HZ0 standards were prepared by reduction at E= - 0.96 V for 300 s in 1 M NaOH of a previously formed Fe(II1) oxide (0.14 V, 300 s in 1 M NaOH).6 A passivated Fe-Al alloy was used as an aluminum oxide standard. After applying a background correction according to the procedure of Shirley,” the signal of each species was described by one or more Gaussian/Lorentzian (G/L) peaks. Some of these standard signals consisted of more than one G/L peak. The XPS signals of an actual specimen were composed of these standard sets of G/L peaks for the contributing species and, to differentiate between them, a computer program was developed. The form and binding energies of the G/L peaks of the standards were kept constant and only their size was varied to get a close fit of the calculated to the experimental spectra. For these fits the relative size of all G/L peaks belonging to the same standard was kept constant. Thus the contributions of the various species, such as Fe, Fe*+, Fe3+, Al, A13+, O*-, OH- and HzO, to the XPS spectra have been evaluated quantitatively. The areas of these integrated signals were divided by the corresponding photoionization cross-sections,‘2 corrected according to the method of Reilmannt3 From the corrected XPS intensities atomic and cationic fractions were calculated. The oxygen signal was not taken into account for these calculations. Neon ISS spectra are usually corrected with a linear background. However, if 4He+ is used as probe gas, this correction fails for the signal at the highest kinetic energy. All peaks show an exponential tailing of inelastically scattered ions towards lower kinetic energies.14 The largest influence occurs for the signal at the highest kinetic energy (iron). Therefore a Shirley background subtraction was applied in this case. The aluminum and oxygen peaks are influenced by the tailing of the iron signal. Large iron signals change the oxygen and aluminum peak shape and a Shirley background subtraction fails. For that reason the peaks were corrected with a linear background. Sputter-cleaned alloys with known compositions were used for determination of the relative sensitivity factor. From the obtained ISS intensities a relative sensitivity factor Al/Fe of 0.435 was calculated. In previous investigations, differential scattering cross-sections were given for 4He+ ions with 1.5 keV.” From these values a relative sensitivity for Al/Fe of approximately 0.5 can be calculated, which agrees reasonably well with the experimental sensitivity factor. Calibration of the standard spectra and the spectra of actual specimens were evaluated with identical procedures to minimize any systematic deviation of the results.’ EXPERIMENTAL
RESULTS
Electrochemical investigations Potentiodynamic cyclovoltammogramms provide information on the passivation behavior of the alloys. Figures 1 and 2 give examples for the examined alloys in 0.1 M borate buffer (pH 9.3) and 0.005 M H2S04/0.5 M Na2S04 (pH 3.8). Investigations in 1 M NaOH (pH 13.8) yield results similar to those of Fig. 1. In alkaline electrolytes (Fig. 1) the active dissolution peak is seriously suppressed compared with the results in 0.05 M phthalate buffer (pH 5), as mentioned previously.’ The voltammogramms show no significant differences for the different aluminum contents in borate buffer and 1 M NaOH. The electrodes were not embedded in resin for these experiments and no rotation
D. Schaepers and H.-H. Strehblow
2196
-
22 at% Al .
~---- 15 at% Al __. ~__ ~- _ .-. -.... ...---0.5
0.0 E /
Fig. I.
Cyclovoltammogramms
0.5
1 .o
v (SHE)
of Fe-Al alloys m borate buffer (pH 9.3), dE/dt = 20 mV s- ’
applied. The fine structure observed on the diagramms arises from oxidation and reduction of hydroxide layers, which are insoluble at these pH values. Some of the hydroxide was formed during the polishing procedure and during air contact. In particular, in 1 M NaOH the current peaks increase with each potentiodynamic scan, suggesting a growing surface oxide which is submitted to oxidation and reduction cycles. Rotating electrodes were used for the studies in the H2S04/Na2S04 (600 rev min- *) resin-coated solution of pH 3.8. The polarization curves at this low pH show higher currents than those in alkaline solutions and pronounced changes are observed between Fe-8 at% Al and specimens of higher aluminum content. The alloy with 8 at% Al shows a large dissolution peak, which grows due to surface roughening with increasing number of cycles. Electrodes with 15 and 22 at% Al show lower anodic currents and an expansion of the passive range close to E = 0.1 V (SHE). The decrease of the anodic current peak at - 0.1 V with increasing number of potentiodynamic scans suggests that a part of the oxide is not reduced or removed, i.e. it remains at the surface and supports repassivation during the next cycle. Owing to the generally lower currents in alkaline solutions, a support of passivation by addition of aluminum is not detectable in polarization curves for this electrolyte. was
XPS investigations Potential-resolved measurements. The specimens were potentiostatically polarized for 300 s at various potentials in 1 M NaOH. The cationic fractions were calculated from the XPS spectra for the different alloys. Figure 3 shows a preferential oxidation of alumimum within the investigated potential range, which yields potential that is almost independent of
Investigation of passive layers on FeAl alloys
a) Fe 8 at.%
“E
400
-
300
-
200
-
100
-
2197
Al
s E \ .-
E / Fig.
2.
V (SHE)
Cyclovoltammogramms of (a) Fe-8 at% Al and (b) Fe-22 at% Al in H$04/Na2S04 (pH3.8),v=6OOrevmin-‘,dE/df=20mVs-’.
aluminum content within the passive layer. The average cationic fraction differs only as a consequence of the bulk aluminum content: 0.18,0.26 and 0.35. At negative potentials the cationic fraction of Fe(II1) starts for all alloys at almost the same value of 0.2, whereas the Fe(I1) cationic fraction in the oxide film decreases with increasing aluminum content from their higher values (0.7, 0.6 and 0.45). Consequently iron oxide is partly replaced by aluminum oxide within the passive layer. According to Vetteri6 and Giihr and Lange,17 passive films on iron consist of a mixed Fe(II)/Fe(III) oxide. The replacement of iron by the less noble aluminum yields a more stable layer in the case of Fe-Al alloys. This has been confirmed previously by galvanostatic reduction of passive layers in combination with sputter depth profiles.’
D. Schaepers
2198 0.8
0.0
r
--
and H.-H. Strehblow
Fe 8 at% Al
-0.6
-0.4
-0.2
Fe
-0.4
-0.6
0.0
15
-0.2
Fe 22
-0.4
-0.6
-0.2
at%
0.0
at%
0.0
E /
l
Fig. 3.
Cationic
fractions
AI(III)
a.2
v
0.4
0.6
1 .a
.
Al
0.2
0.8
0.4
0.6
0.8
1.0
0.4
0.6
0.8
1.0
Al
0.2
v (SHE)
Fe(ll)
v
Fe(lll)
taken from XPS spectra of Fe-Al alloys after 300 s passivation NaOH at various potentials.
in 1 M
Time-resolved measurements provide valuable Time-resolved investigations. information on the development of passive layers. Similar processes occur in the time and potential domains. Although the measured XPS intensities are corrected with their appropriate photoionization cross-sections they are still influenced by the spectrometer settings. To avoid any deviations during comparison of different spectra the intensity ratios are plotted, i.e. relative intensities as a function of the logarithm of time/s. Both intensities
Investigation of passive layers on FeAl alloys
2199
are taken from the same spectrum which ensures equal experimental parameter settings. Figure 4 presents the change of the passive layer composition with time. Freshly polished and sputter-cleaned F-22 at% Al specimens were imersed to 1 M NaOH at - 1.O V (SHE) and pulsed for times in the range from microseconds to hours well within the passive potential range [0.8 V (SHE)]. To ensure short passivation times the counter electrode was switched off after passivation via a relay by a pulse generator. During passivation at 0.8 V (SHE) the Fe(II)/Fe ratio [Fig. 4(a)] increases to an intermediate Fe(I1) maximum at a passivation time of 1 s, followed by a steep decrease and a final plateau value for longer passivation times owing to its oxidation to Fe(II1). The OH/Fe ratio [Fig. 5(e)] shows a similar shape with an OH maximum at 1 s-but not as pronounced as in Fig. 4(a). Fe(III)/ Fe(I1) and Al(III)/Fe(II) [Figs 4(e) and 4(a)] are increase steadily to values of 12 and 7. Figure 4(e) suggests an Al(III)/Fe(III) ratio of the oxide film close to 2 for t> 1 s. These details show that an Fe(I1) film containing aluminum hydroxide is formed initially, with later oxidation of Fe(I1) and further Fe(II1) formation for t > 1 s. These oxidations occur in parallel with a decrease in the water content of the layer with a change from a hydroxide to an oxide. In phthalate buffer an Fe(I1) maximum was found for the same conditions after a passivation time of 10 s.9 Angle-resolved XPS measurements
Although X-rays penetrate a specimen up to some few urn, the information depth of XPS amounts to only some few nm owing to the small escape depth, h, of the photoelectrons. For kinetic photoelectron energies greater than 150 eV, h might be B is 0.054 nm [eV_“‘] for elements calculated for each signal according to’* 1 = B&. and 0.096 nrn [eV-“*I for inorganic compounds. h was calculated for the signals investigated here to be: iron iron oxide aluminum aluminum oxide oxygen
1.26 nm 2.24 nm 1.86 nm 3.3 nm 2.58 nm
The minimum attenuation of photoelectrons is achieved if they leave perpendicular to the specimen surface. Their travel distance increases proportionally to 1jcos 6 with the takeoff angle, 0, relative to the surface normal. The contribution to the XPS intensity of a species at depth d decreases with exp( - d/h cos 0). Therefore the XPS signal of a species close to the surface is enhanced relative to the contributions from species at deeper positions if 8 is increased. Angular-resolved XPS studies may therefore be used to detect a sublayer structure at a specimen surface and provide information about the outer or inner position of layer components. Thus angular-dependent XPS studies are often applied to obtain nondestructive depth profiles. Above the Flade potential the passive layer on iron consists of Fe20s at the oxide/ electrolyte interface and Fes04 at the metal/oxide interface.13*‘4. These details have been confirmed by angle-resolved XPS investigations of passive layers on pure iron passivated in phthalate buffer at pH 5.09,” and in 1 M NaOH.6 The outer part of the passive film consists in this case mainly of Fe(III), whereas the inner part is formed by an oxide containing both Fe(I1) and Fe(II1) ions. As described previously,’ this sublayer structure is not found for
D. Schaepers
2200
4
Fe(ll)
/
and H.-H. Strehblow
b)
Fe
OH /
Fe
3oY 26 0 .-
._0 +
.
20 .
0
I
;
._c
z
15
c; ._c
10 5
+p -2-l
log
4
Fe(lll)
t /
/
s
12
log
t /
s
/
Fe(ll)
AI(III)
Fe(ll)
14,
4
0
3
4
I
AI(III)
/
Fe(lll)
l
0.4 -2-l
Fig. 4.
.
1
0
12
log
t /
.
’ 3
4
s
XPS intensity ratios (corrected by photoionization passivation time of Fe-22 at% Al passivated
cross-sections) as a function at 0.8 V in 1 M NaOH.
of the
Investigationof passivelayerson Fe-AI alloys
2201
passive layers on Fe-Al alloys formed in phthalate buffer (pH 5.0). According to XPS studies it has a chemically more homogeneous structure of a mixed oxide or oxide compound containing Fe(II), Fe(II1) and, remarkably enriched, Al(II1) ions. This film structure is also detected for passivation in 1 M NaOH. Some indication of an in-depth separation of the different oxide species is found only for negative potentials or short passivation times. As an example, Fig. 5 shows relative intensities of angle-resolved XPS investigations of Fe-22 at% Al after 300 s passivation at -0.2 V in 1 M NaOH. According to its thermodynamical properties, aluminum is preferentially oxidized during the initial state of oxide growth. Thus an aluminum-rich oxide is directly formed on top of the alloy. Further oxidation leads to an oxide with less aluminum below this initial film, owing to its depletion at the metal surface. This process yields an oxide structure with strong aluminum enrichment at its top. As a consequence, the intensity ratios Al(III)/Fe(II) and Al(III)/Fe(II) + Fe(II1) increase with take-off angle, 0. The AI(III)/Fe(III) ratio does not show such a pronounced tendency, because Fe(II1) is located preferentially in the outer oxide parts-and thus at the same position as Al(II1). At more positive potentials additional oxide starts to grow, penetrating the aluminum-rich, initially formed oxide. Thus further oxide growth occurs at the oxide/electrolyte interface. This leads to an inner aluminum-rich part of the oxide layer. In both oxide parts-above and below the aluminum-rich partaluminum is still enriched compared with the bulk content. This detailed information is obtained from sputter depth profiles. Although there is a large gradient of the aluminum concentration, an angle dependence of the XPS cation intensity ratios Fe (oxide)/Al (oxide) cannot be found. This result is a direct consequence of the sandwich structure of the passive layer, with less aluminum-rich parts at the outer and inner positions and the aluminumrichest part in between. Owing to this structure and, in addition, the accumulation of aluminum even in the inner and outer positions of the passive layer relative to the bulk composition, XPS averages the Al profile. Thus XPS studies are missing some of these details which may be observed with other methods. XPS sputter depth profiles
Figure 6 shows XPS sputter profiles of alloys passivated in 1 M NaOH for 300 s at 0.6 V, well within the passive range. The evaluation of these XPS spectra provides cationic fractions as depicted in Figs 6(a) and 6(c), where the metals and their oxidized species are plotted as a function of sputter time. Because of preferential sputtering of oxygen, Fe(II1) is reduced to Fe(I1). Therefore a separation between Fe(II1) and Fe(I1) is questionable and the sum of both cations is presented as FeOx. Although the profiles yield a large gradient of the aluminum distribution, also a general aluminum enrichment with respect to the bulk composition is found at any depth. At the oxide surface the aluminum cationic fraction starts to depend on the bulk content (0.08,0.15 and 0.22) with xAl’O.12, 0.2 and 0.3. For each profile these fractions increase with sputter time, go through a maximum at xAl = 0.2, 0.4 and 0.5 in the center of the passive layer, and decrease until the bulk concentration is reached. Figures 6(d) and 6(e) show the atomic fractions, summarizing the contributions of al1 oxidation states for each metal. These diagrams depict in particular the aluminum profile with its maximum in the center of the layer. Taking the inner inflection point of the aluminum oxide signal as a measure of the oxide/metal interface, the thickness of the passive layer amounts to 2.5 nm for Fe-8 at% Al and 3 nm for the other alloys. The oxygen signal is not included in these calculations of the atomic fractions and therefore is not shown.
D. Schaepers and H.-H. Strehblow
2202
b) OH/O
AI(III)/Fe(ll) 0.6 0 .+ ?
s 8 ._
4L .-t
c>
2.5 0 ._ + 0 L
2.0
-G
._c
.. ... 1.
0.5
0
IO
20
30
40
50
60
O/”
d)
AI(III)/Fe(lII)
Fe(ll)/Fe(lll)
l
1.5
, . ,
I
0
10
20
30
40
50
60
O/”
1.0
’
0
10
20
30
40
50
60
O/”
e>
AI(III)/Fe(lI)+(lII)
1.2
-$ .-
.
0.8
0.6 0
Fig. 5.
10
20
Angular-resolved
30
40
intensity
50
60
ratios of Fe-22 at% Al after 300 s passivation -0.2
v
in 1 M NaOH
at
Investigation of passive layers on FeAl alloys
d / nm (Ta205)
a)
0.0
1.0
2.0
3.0
4.0
d / nm Ua205)
d)
5.0
1.0,
1.0,
1
J
2203
0.0
1.0
2.0
3.0
4.0
5.0
I __________..
0.8 ’
0 0
0.6
&
0.6-
.i
0.4
8at%Al -
e,
b)
1.0 -
‘.OI”“‘I 0.8 0.6
15at%Al
0.0
4 ;;
0 ,’
f>
“OI
0.8 0.6
I
L
‘.OI
J
_...__._b”“__.._...________________..
0 0
k
k
._
L
0.4
0 4-J
0.2
u 0.0
0.0
50
0
sputter o
Al
Fig. 6.
l
AlOx
100
time v
200
150
Fe
1 50
100
sputter
[s] v
I
0
FeOx
l
Al
+
AlOx
150
time v
Fe
200
[s] +
FeOx
XPS sputter profiles of FeAI alloys after 300 s passivation in 1 M NaOH at 0.6 V.
Figure 7 presents sputter depth profiles of Fe-22 at% Al specimens after 300 s passivation time in 1 N NaOH at various potentials. At - 0.2 V and 0 V small amounts of oxide are formed as seen by the low FeOx signal and the thin layer thickness of ca. 1.5 nm. The aluminum oxide signal shows a maximum close to the surface at a depth of about 0.5 nm. At more positive potentials additional oxide is formed and the height of the Al(II1) maximum is increased and broadened, being found at 1.5 to 2 nm depth. This result suggests
D. Schaepers
2204
-0.2V
SHE
OV SHE
d /
0
50
100 time
150
0
[s]
nm
/
3.0
50
100
sputter
0
XPS sputter
4.0
time
Al
9.0
150
l
100
d /
nm
1.0
2.0
v
[s]
(Ta20J 3.0
50
sputter
AlOx
time
150
SHE
0
[s]
(-rap51
nm
sputter
(Ta205)
2.0
/
50
0.6V
1 .o
0
Fig. 7.
d
SHE d
0.0
(Tap5)
nm
sputter
0.4V
and H.-H. Strehblow
Fe
profiles of Fe-22 at% Al after 300 s passivation potentials.
.
4.0
100
time
150
[s]
FeOx
time in 1 M NaOH
at various
the layer growth at both interfaces, metal/oxide and oxide/electrolyte. Borate buffer, pH= 9.3. Figure 8 shows as an example, for borzte buffer at pH = 9.3, the change in the cationic fractions with passivation potential for Fe-22 at% Al. The most obvious difference is the larger Al(II1) cationic fraction, 0.6, at low potentials compared with that, 0.35, in NaOH (Fig. 3). No appreciable Fe(II1) is detected at these potentials.
Investigation of passive layers on Fe-Al alloys
7
0.0 -600-400-200
0 E /
l
Fig. 8.
2205
AI(III)
v
200
400
600
800
1000
mV (SHE)
Fe(ll)
v
Fe(lll)
Cationic fractions taken from XF’Sspectra of Fe-22 at% Al after 300 s passivation in borate buffer (pH 9.3) at various potentials.
Owing to increased iron oxide formation the Al(II1) cationic fraction diminishes to a value of 0.3 at more positive potentials. Fe(I1) is gradually replaced by Fe(II1) with increasing potential. The XPS sputter depth profiles in borate (Fig. 9) after passivation for 300 s at 1 V show, for all alloys, a slightly larger maximum of the Al(II1) content in comparison to 1 M NaOH (Figs 6 and 7). Surfate solution, pH = 3.8,2.5. XPS sputter profiles after passivation at 1.4 V (SHE) at a pH value of 3.8 show again oxide layers with a maximum of aluminum oxide enrichment similar to the more alkaline solutions. XPS sputter profiles after passivation at 1 V (SHE) in a solution of pH 2.5 show for 15 at% and especially for 8 at% Al (Fig. 10) cationic fractions of Al(II1) with an accumulation plateau near the surface, with a decrease towards the bulk values in the inner layer parts. The accumulation of aluminum is smaller owing to the better solubility of aluminum oxide in more acidic electrolytes. Figure 11 gives an overview of XPS sputter profiles, comparing the layer composition for different pH values. The enrichment of aluminum reaches its highest value in phthalate buffer at pH = 5. This result is a consequence of the amphoteric properties of Al(II1) oxide with a minimum solubility at pH 6. Passivation for longer times. Figure 12 shows a comparison of sputter profiles of Fe22 at% Al after passivation for 300 s and 24 h at 1 V in sulfate at pH 2.5. It is obvious that during long passivation times the outer layer parts were dissolved, which shifts the maximum of the aluminum profile to the surface. The inflection point of Al(II1) suggests a thinning of the oxide with passivation time due to its slow dissolution. 4He+ ISS sputter depth profiles
Elastic-ion scattering spectroscopy (ISS) permits the evaluation of depth profiles with monolayer resolution. This is an advantage in comparison to XPS which collects spectra
2206
D. Schaepers and H.-H. Strehblow
8 at%
15
Al
d / nm (Ta205) 0.0
1.0
2.0
3.0
d / nm (Ta,OJ I
-;;’
0.8
u t
0.6
G.6
0.4
0.4
0.2
a.2
2 .-
-e 0 *
0.0
0.0 0
50
100
sputter
22
Al
4.0
1.0 ,
0 .-
at%
at%
time
150
100
50
0
[s]
sputter
time
150 [s]
Al
d / nm (Tap51 0.0
1.0
2.0
3.0
4.0
‘.O i
sputter Fig. 9.
time
0
Al
.
v
Fe
.
AlOx FeOx
[s]
XPS sputter profiles of Fe-AI alloys after 300 s passivation time in borate buffer (pH 9.3), at E= 1.0 v.
with an average information
depth of some few nm, which is in the range of the escape depth of the photoelectrons. ISS samples only the mass of the surface atoms and does not provide chemical information. Thus only the atomic fractions of Al [Al+Al(III)] are calculated from the experimental data. The vanishing oxygen signal served as indication for reaching the metal/oxide interface. The specimens were passivated in borate buffer, pH = 9.3, and sulfate electrolyte, pH = 3.8, for 300 s at 1 V (SHE). The atomic fractions of aluminum (Figs 13 and 14) show maxima at a depth of about 1 nm, in agreement with XPS profiles. Because of the higher depth resolution the concentration profile is sharper. This causes a sharper and higher peak of the atomic fractions. The maximum values detected by ISS for 22 at% Al are
Investigation of passive layers on Fc+AI alloys
2207
8at%Al
d / nm (Taps) 0
1.0
2
0
4
z
q0 _-
: ‘Z :
4
2
1.0 _______‘-___‘__~---‘___~_______._~______.~hlllk
0.6 0.4 -
0.4 0.2
I
nn V.”
0
50
sputter
100
time
150
[s]
0.0
I
0
-
50
sputter
100
time
~-I 150
[s]
- FeOx v Fe l Al + AlOx v Fe + FeOx l AlOx 0 Al Fig. 10. XPS sputter profile of Fe-8 at% Al after 300 s passivation time in sulfate solution (PH 2.5) at E= 1 V.
up to xAi= 0.85 in borate buffer (Fig. 13) and XAi= 0.65 in sulfate solution (Fig. 14). In sulfate the peak height is nearly the same for the 15 and 22 at% Al alloys, but the maximum peak for 22 at% Al is broader owing to the higher bulk aluminum content. Similar results have been found from ISS depth profiles of Fe+Al alloys passivated in phthalate buffer of pH 5.O.9 DISCUSSION According to ISS and XPS depth profiles, aluminum is enriched within the passive layers of the FeAl alloys. Angle-resolved XPS investigations give no clear indication of a separation in sublayers of different oxidation states and varying aluminum content, like for passive layers on pure iron.3T6No protective overlayer of pure aluminum oxide was found, which might have been predicted from the thermodynamic data of the alloying elements. The investigations by XPS, ISS and electrochemical depth profiling by galvanostatic reduction,’ however, prove an accumulation of Al(II1) ions within the whole passive layer. A strong Al(II1) cation concentration gradient with a maximum in the center of the layer was found by XPS as well as by ISS depth profiling. Depending on the time and potential of passivation, Fe(I1) may be found in the inner and outer parts of the passive layer. In a previous work, Fe(I1) was found preferentially close to the oxide/electrolyte interface for passivation times z 10 s in phthalate buffer, pH = 5.0.9 Apparently, the passive film on Fe-Al alloys is a mixed (Fe, Al) oxide or oxide compound. The preparation parameters cause changes in the concentration gradients-as shown by XPS and ISS depth profiles. Closer examination of these effects by time-resolved measurements provides information on the growth kinetics and chemical changes of the passive layer with time. Within the oxide oxygen anions form a close-packed structure. In the case of pure iron only Fe(I1) ions are present at low potentials and a brucit structure
2208
and H.-H. Strehblow
D. Schaepers
pH
PH
2.5 d / 0.0
nm
1.0
2.0
b,Os) 3.0
4.0
I
0.0
5.3
1.0
2.0
3.0
4.0
5.0
1 .o
:.O I
0.8
5
bulk
I
b”ik
C.6 0.4 0.2 0.0 0
100
50
sputter
pH
time
150
200
100
50
0
sputter
[s]
pH
9.3
0.0
1.0
2.0
3.0
4.0
0.0
5.0
0
0.8
0.8
u
2 .-v : .A::
1.0
nm 2.0
(TapQ 3.0
4.0
5.0
bulk
__.. . . . . . ..__..._........._..._...... 7 .
0.6
0.6 .-u 0.4
._:
0.4
4-J
i-3 0.2
0.2
no _.-
3.0 0
50
sputter
Fig.
[s]
1.0 [ bulk
-G
time
200
13.8 d /
1.0
150
100
time
150
200
sputter
[s]
time
[s]
- FeOx 8 AlOr 0 Al v Fe I 1. XPS sputter profiles of Fe-AI alloys after 300 s passivation in electrolytes with different pH values, at E= 1 V.
[Fe(OH)2] or a rock salt structure (FeO) is assumed, where only the octahedral positions are filled. For a more protecting layer structure the tetrahedral position should be filled as well. Usually, an inversed spine1 structure is assumed for the stationary situation of the passive layer on pure iron at sufficiently positive potentials, with a gradual change from a composition of Fe304 at the metal surface to Fe203 at the oxide/electrolyte interface. This
Investigation of passive layers on Fe-Al alloys
d- /
300
s 1.0 -w
0
nm
2209
(Ta205) 4
2
6
0.8
::
z 0 ._
c
.-0
0.6 0.4
Y
0.2
sputter
time
d / nm
24
h
2
0
-._
0
sputter
o
Al
.
AlOx
v
(Ta@ 4
100
50
[s]
6
150
200
time [s] Fe
v
FeOx
Fig. 12. Comparison of XPS sputter profiles of Fe-Al alloys after 300 s and 24 h passivation in sulfate solution (pH 2.5) at 1 V.
gradual change is expected in the range of the Flade potential, i.e. the passivation potential of iron in strongly acidic electrolytes. Calculations employing the Gibbs free energy, AG”, values for the electrochemical oxidation of Fe304 to Fez03 confirm the experimental value of this critical potential.‘6”7 In the spine1 structure small Fe(II1) ions (r = 0.064 nm)19 are located in the tetrahedral-coordinated lattice sites of the closed-packed O*- matrix, whereas the larger Fe(I1) ions (r = 0.077 nm)19 occupy the octahedral sites.*’ On the basis of these and previous investigations, a model has been described’ which depicts the observed effects during the process of layer formation (Fig. 15). According to the large difference for AGO of aluminum oxide and iron oxide formation (Fes04: - 1016.2 kJ mol-‘; a-Fe203: -742.8 kJ mol-‘; A1203: - 1691.3 kJ mol-‘),21 a preferential oxidation of aluminum is obvious which causes an aluminum-enriched zone
D. Schaepers and H.-H. Strehblow
2210
(Ta205>
d / nm 0
!
1.o r----T-
5
4
----
-
2
5 ‘, Fe
6 I
7
a I
8 at.% Al
1
l Fe 15 at.% Al y.7 Fe 22 at.% Al bulk
sputter Fig. 13
at the surface
time
/
min
4He j ISS sputter profiles (atomic fractions) of Fe-AI alloys after 300 s passivation in borate buffer (pH 9.3) at I V.
of the oxide film [Fig. 15(a)]. However, there is no chance to form a continuous protecting aluminum oxide film because of a lack of aluminum for a bulk content of only some few per cent. Therefore, the less reactive iron forms additional oxide [Figs 15(b) and 15(c)] which grows into a continuous film incorporating the Al(II1) oxide islands. Further oxide growth involves the transport of O* - and OH- from the electrolyte into the film and cations in the opposite direction [Figs 15(b) and 15(d)]. Therefore new oxide is formed at the oxide/electrolyte and metal/oxide interfaces. Rutherford backscattering spectroscopy studies with Xe markers and radioactive tracer methods22-24 showed that for anodic oxide on aluminum and other valve metals, the transfer coefficients for anions and cations are similar. Time-resolved measurements for the passivation of Fe-Al alloys show that the oxide grows at both interfaces with comparable rates, leading to the central position of the aluminum enrichment within the passive layer [Fig. 15(e)]. Time-resolved investigations and XPS analysis of oxide films formed at low potentials show that A13+ ions are formed initially at low potentials and during short passivation times. Because of their size (r =0.053),18 the Al(II1) ions can replace the Fe(II1) ions in the tetrahedral positions of the anion matrix. Thus the oxide films on iron-aluminum alloys can be transformed at low potentials into the spine1 type lattice structure, even if Fe(II1) is not yet formed. Therefore, one expects a hercynite-like structure especially for a maximum Al(II1) content of xAl = 0.7, which is very close to the composition of hercynite with xA1= 0.66. The formation of this hercynite oxide structure is supported by two facts: the solubility of both oxides in the
2211
Investigation of passive layers on Fe-Al alloys
d / nm (Ta206>
0.6
0
5
IO
15
sputter Fig.
14.
20
25
time
30
/
35
40
45
min
4Hef ISS sputter profiles (atomic fractions) of FeAI alloys after 300 s passivation time in sulfate (pH 3.8) at 1V.
matrix of the other and the enthalpy of oxide formation. a-FeOOH has a solubility for Al(Il1) ions of only up to 33 at%.21 Since the maximum aluminum enrichment is about double this value, the formation of a hercynite-like compound is strongly supported, in which an aluminum substitution of 66 at% is possible. The more negative molar AG” value of formation for FeA1204 (- 1879.7 kJ mol-‘) compared with Fe304 (- 1016.2 kJ mol-‘), a-Fe203 (- 742.8 kJ mol-‘) and A1203 (- 1691.3 kJ mol-‘)2’ is another argument for the formation of this compound. It was assumed that the the pH of the electrolyte has a large influence on the aluminum accumulation and its related effects.‘. These predictions could now be verified by means of the investigations in the more acidic sulfate electrolyte after longer passivation time. The higher dissolution rate of the oxide film caused the aluminum enrichment maximum to shift towards the oxide/electrolyte interface [Figs 15(f) and 15(g)]. Even after 24 h passivation time (Fig. 12) aluminum oxide was found in the outer layer parts, thus still protecting the metal surface.
CONCLUSION Aluminum additions facilitate the passivation of FeAl alloys compared with pure iron especially in acidic solutions, as is shown by the lower anodic current peak of the cyclovoltammogramms in various electrolytes. An aluminum oxide enrichment compared
D. Schaepers and H.-H. Strehblow
2212
oxide
growth at
both interfaces
‘i/////n
elektrolyte
B
Al-oxide enrichment
I
]
Fe/Al-oxide bulk
dissolution
in acidic
solutions
Fig. 15. Schematic model of the passive layer formation on Fe-Al-alloys.
with the bulk concentration
was found in the passive layers formed in electrolytes with pH values in the range 2.5 to 13.8. The Al(II1) concentration shows a gradient with a maximum of enrichment within the center of the passive layer. The initially formed Al(III)-rich film contains a large amount of hydroxide. During longer passivation times and at more positive potentials the layer composition changes into a mixed oxide/hydroxide film with a larger amount of additionally formed iron oxide. Al(II1) is still enriched in the whole layer. A model for passive layer formation on Fe-Al alloys for these conditions was developed. It demonstrates the kinetic effects which lead to the central position of the Al(II1) enrichment maximum within the oxide film. Thermodynamic data support the assumption of an initially formed aluminum hydroxide and strongly suggest the formation of a hercynite-like spine1 structure, especially in the region of the maximum Al(III) enrichment. Acknowledgement-The support of this work by the Deutsche Forschungsgemeinschaft (project Str. 200/9-l, 9-2) is gratefully acknowledged.
Investigation of passive layers on FeAl alloys
2213
REFERENCES 1. Ullmanns Encykliiptidie der technischen Chemie, 3rd edn, Vol. 365. Urban and Schwarzenberg, Miinchen/ Berlin, 1953, p. 3. 2. R. Kirchheim, B. Heine, S. Hofmann and H. Hofslss, Corros. Sci. 31, 573 (1990). 3. S. Haupt, C. Calinski, U. Collisi, H.W. Hoppe, H.-D. Speckmann and H.-H. Strehblow, Surf. Interface Anal. 9, 357 (1986). 4. L. Young, Trans. Faraday Sot. 53(I), 841 (1957). 5. S. Hofmann and J.M. Sanz, Surf. Interface Anal. S(5), 210 (1983). 6. S. Haupt and H.-H. Strehblow, Corros. Sci. 29, 163 (1989).
7. H.W. Hoppe, Ph.D. thesis, Heinrich-Heine-Universitat, Diisseldorf, 1990. 8. P. Druska and H.-H. Strehblow, Surf. Interface Anal. 23, 440 (1995). 9. D. Schaepers and H.-H. Strehblow, J. Electrochem. Sot. 147(7), 2210 (1995). 10. D. Schaepers and H.-H. Strehblow, Surf Interface Anal. 21, 342 (1994). 1 I D.A. Shirley, Phys. Rev. B5, 4709 (1972). 12. J.H. Scofield, J. Electrosc. 8, 129 (1976). 13. R.F. Reilmann, A.M. Sezane and S.T. Manson, J. EIectron Spectrosc. Relat. Phenomena 8, 389 (1979). 14. G.C. Nelson, J. Vat. Sci. Technol. A4(3), 1567 (1986). 15. R.E. Honig and W.L. Harrington, in Proceedings of International Conference on Ion Beam Surface Analysis, IBM, Yorktown Heights, NY, 18-20 June 1973. 16. K.J. Vetter, Z. Elektrochem. 62, 642 (1958). 17. H. Gohr and E. Lange, Naturwiss. 43, 12 (1956). 18. M.P. Seah and W.A. Dench, Surf. Interface Anal. l(l), 2 (1979). 19. IJ. Schwertmann and R.M. Cornell, Iron Oxides in the Laboratory. VCH, Weinheim, 1991. 20. K. Wandelt, Surf. Sci. Rep. 2, 93 (1982). 21. Handbook of Chemistry and Physics, 74th edn. CRC Press, Boca Raton, FL, 1993/94. 22. J.P.S. Pringle, J. Electrochem. Sot. 120, 398 (1973). 23. F. Brown and W.D. Macintosh, J. Electrochem. Sot. 120, 1096 (1973). 24. W.D. Macintosh, F. Brown and H.H. Plattner, J. Elecfrochem. Sot. 121, 1281 (1974).
Vol. 39,
No. 12, pp. 2193-2213, 1997 0 1997 ElsevierScienceLtd
Printed in Great Britain. All rights reserved 001&938X/97517.00+0.00 PII: s0010-938x(97)00103-0
AN XPS AND ISS INVESTIGATION OF PASSIVE LAYERS ON BINARY Fe-Al ALLOYS D. SCHAEPERS
and H.-H. STREHBLOW*
Institut fiir Physikalische Chemie und Elektrochemie, Heinrich-Heine-Universitiit, Dusseldorf, Germany
UniversitltsstraBe
1, D-40225
Abstract-Passive layers have been formed electrochemically under inert conditions on sputter-cleaned Fe-Al alloys. The aluminum content was 8, 15 and 22 at%, which meets the homogeneity region of a-Fe in the Fe-Al phase diagram. The addition of aluminum reduces the active dissolution current density remarkably compared with that of pure iron. X-ray photoelectron spectroscopy (XPS) studies show an accumulation ofaluminum oxide within the center of the passive layer, which is confirmed by elastic-ion scattering spectroscopy (ISS) and XPS depth profiles. Angular-resolved XPS measurements suggest a continuous structural change of the chemical film during its growth. A discussion of the investigations is presented on the basis of a model of oxide growth. 0 1997 Elsevier Science Ltd Keywords: A. Fe-Al alloys, B. potentiostatic,
B. XPS, B. ISS, C. passive films.
INTRODUCTION Fe-Al alloys show an improved scaling property in combination with better weldability and hardening capacity compared with pure iron.’ Stress corrosion cracking by aggressive chemicals and active metal dissolution is reduced by the aluminum additions.’ X-ray photoelectron spectroscopy (XPS) studies of Fe3Al passivated in 0.5 M sulfuric acid showed an enrichment of aluminum oxide within the passive layer.2 The aim of this study was the analysis of passive layers by means of surface analytical methods to get a better understanding of the corrosion behavior of Fe-Al alloys. The investigations are carried out with binary Fe-Al alloys with different aluminum contents after potentiostatic polarization in acidic and alkaline electrolytes with pH values of 2.5 to 13.8. Electrochemical and surface analytical investigations of the passive layers of the abovementioned alloys are of practical interest, owing to their application as low-alloyed structural steels. EXPERIMENTAL
METHOD
Fe-Al alloys containing 4, 8 and 12 wt% (8, 15 and 22 at%) aluminum were manufactured by melting the corresponding amounts of pure iron and aluminum in a vacuum induction furnace. The material was forged into bars of 13 mm diameter. Annealing experiments were carried out with reference to the phase diagram to find the best treatment for the formation of a single-phase alloy. A subsequent annealing for 1 h at *To whom correspondence should be addressed. Manuscript received 22 May 1997. 2193
2194
D. Schaepers
and H.-H. Strehblow
700°C was found to produce the most homogeneous texture of alloys with an a-Fe lattice. The specimens were machined from central parts of the metal bars. The circular front planes of the specimens were polished with diamond paste down to a grain size of 1 urn and they were then cleaned with ethanol in an ultrasonic bath. The freshly prepared oxide layers were studied by XPS and elastic-ion scattering spectroscopy (ISS) in a commercial UHV spectrometer (VG ESCA-LAB 200X). This instrumentation is equipped with a self-designed chamber3 to enable electrochemical preparation under an argon gas atmosphere. It can be pumped separately and allows specimen transfer from the electrolyte to the ultra-high vacuum without any exposure to the laboratory atmosphere. The electrochemical equipment consisted of a potentiostat (Schramm P 1, Dusseldorf), Oregon) and a function generator (Schramm, pulse generators (2663 Tectronics, Dusseldorf). The Hg/Hg$OJO.S M Na2S04 electrode with E=0.64 V served as a reference electrode for ali experiments. All potential values are given with reference to the standard hydrogen electrode (SHE). The solutions were prepared from high-purity chemicals (pro analysi) and de-ionized water (Millipore water purification system) and were de-aerated with argon before use. The specimens were cleaned by argon-ion etching and transferred into the electrochemical preparation chamber. Only the polished front plane was exposed at open circuit conditions to the electrolyte. The potential was pulsed for defined times (10 ms to 300 s) to the value of interest. After passivation, the electrodes were rinsed with de-ionized water, blown dry with a jet of argon, and transferred quickly via the fast-entry lock and the preparation chamber into the analyser chamber. XPS spectra were recorded with Mg K, radiation (300 W, 15 keV) from an Al/Mg twin anode. Except for the angle-resolved measurements, the attenuation angle was adjusted to 30” relative to the surface normal, thereby providing a more surface-sensitive signal. A Penning source (Specs PS IQP 1O/63) was operated with argon (4 kV, 25 uA target current) for sputter cleaning of the specimens and XPS depth profiling. The sputter rate of the Penning source was calibrated to 1.7 nm min ’ with anodically formed TazOs on tantalum according to the method of Young4 and H0fmann.j Quantitative evaluation of spectra was carried out on the basis of standard spectra3’6,7 of oxygen and the alloying components. The quantitative data of the deconvoluted spectra provide more information on the chemical state of the elements and the quantitative composition of the passive layer than the original XPS spectra. Therefore, to reduce the number of figures, original XPS and ISS spectra are not shown in this paper. A detailed presentation of the data evaluation has been given previously.* A scannable filament ion gun (EX-05, VG Scientific) was applied for ISS sputter depth profiles. This ion source has been applied for ISS and sputtering as well. For ISS spectra it was operated with helium (2 kV, 30 nA target current), whereas argon (3 kV, 300 nA target current) was used for sputtering to obtain higher sputter rates. The scanned area was 0.25 cm2 for the measurements and 0.75 cm2 for the sputter steps. The angle between the incident and the scattered ion beam was 90”. The sputter rate of the EX-05 gun was calibrated as mentioned above with a depth profile of TazOs on tantalum. Becuase of the small sputter area, small-area XPS with a higher lateral resolution of 1 mm was used to detect the metal/oxide interface. Data evaluation Deconvolution
of the XPS signals was carried out on the basis of standard
spectra for
Investigation of passive layers on Fe-Al alloys
2195
each oxidation state of iron, aluminum and oxygen, as described previously in detail elsewhere.‘,” Pure argon-sputtered metals served as standards for iron and aluminum metal. Oxide and hydroxide standards were prepared electrochemically. The Fe(III) and 02- standards were prepared by anodic oxidation of sputter-cleaned pure iron at 1.2 V for 300 s in phthalate buffer (pH 5). The Fe(II), OH- and HZ0 standards were prepared by reduction at E= - 0.96 V for 300 s in 1 M NaOH of a previously formed Fe(II1) oxide (0.14 V, 300 s in 1 M NaOH).6 A passivated Fe-Al alloy was used as an aluminum oxide standard. After applying a background correction according to the procedure of Shirley,” the signal of each species was described by one or more Gaussian/Lorentzian (G/L) peaks. Some of these standard signals consisted of more than one G/L peak. The XPS signals of an actual specimen were composed of these standard sets of G/L peaks for the contributing species and, to differentiate between them, a computer program was developed. The form and binding energies of the G/L peaks of the standards were kept constant and only their size was varied to get a close fit of the calculated to the experimental spectra. For these fits the relative size of all G/L peaks belonging to the same standard was kept constant. Thus the contributions of the various species, such as Fe, Fe*+, Fe3+, Al, A13+, O*-, OH- and HzO, to the XPS spectra have been evaluated quantitatively. The areas of these integrated signals were divided by the corresponding photoionization cross-sections,‘2 corrected according to the method of Reilmannt3 From the corrected XPS intensities atomic and cationic fractions were calculated. The oxygen signal was not taken into account for these calculations. Neon ISS spectra are usually corrected with a linear background. However, if 4He+ is used as probe gas, this correction fails for the signal at the highest kinetic energy. All peaks show an exponential tailing of inelastically scattered ions towards lower kinetic energies.14 The largest influence occurs for the signal at the highest kinetic energy (iron). Therefore a Shirley background subtraction was applied in this case. The aluminum and oxygen peaks are influenced by the tailing of the iron signal. Large iron signals change the oxygen and aluminum peak shape and a Shirley background subtraction fails. For that reason the peaks were corrected with a linear background. Sputter-cleaned alloys with known compositions were used for determination of the relative sensitivity factor. From the obtained ISS intensities a relative sensitivity factor Al/Fe of 0.435 was calculated. In previous investigations, differential scattering cross-sections were given for 4He+ ions with 1.5 keV.” From these values a relative sensitivity for Al/Fe of approximately 0.5 can be calculated, which agrees reasonably well with the experimental sensitivity factor. Calibration of the standard spectra and the spectra of actual specimens were evaluated with identical procedures to minimize any systematic deviation of the results.’ EXPERIMENTAL
RESULTS
Electrochemical investigations Potentiodynamic cyclovoltammogramms provide information on the passivation behavior of the alloys. Figures 1 and 2 give examples for the examined alloys in 0.1 M borate buffer (pH 9.3) and 0.005 M H2S04/0.5 M Na2S04 (pH 3.8). Investigations in 1 M NaOH (pH 13.8) yield results similar to those of Fig. 1. In alkaline electrolytes (Fig. 1) the active dissolution peak is seriously suppressed compared with the results in 0.05 M phthalate buffer (pH 5), as mentioned previously.’ The voltammogramms show no significant differences for the different aluminum contents in borate buffer and 1 M NaOH. The electrodes were not embedded in resin for these experiments and no rotation
D. Schaepers and H.-H. Strehblow
2196
-
22 at% Al .
~---- 15 at% Al __. ~__ ~- _ .-. -.... ...---0.5
0.0 E /
Fig. I.
Cyclovoltammogramms
0.5
1 .o
v (SHE)
of Fe-Al alloys m borate buffer (pH 9.3), dE/dt = 20 mV s- ’
applied. The fine structure observed on the diagramms arises from oxidation and reduction of hydroxide layers, which are insoluble at these pH values. Some of the hydroxide was formed during the polishing procedure and during air contact. In particular, in 1 M NaOH the current peaks increase with each potentiodynamic scan, suggesting a growing surface oxide which is submitted to oxidation and reduction cycles. Rotating electrodes were used for the studies in the H2S04/Na2S04 (600 rev min- *) resin-coated solution of pH 3.8. The polarization curves at this low pH show higher currents than those in alkaline solutions and pronounced changes are observed between Fe-8 at% Al and specimens of higher aluminum content. The alloy with 8 at% Al shows a large dissolution peak, which grows due to surface roughening with increasing number of cycles. Electrodes with 15 and 22 at% Al show lower anodic currents and an expansion of the passive range close to E = 0.1 V (SHE). The decrease of the anodic current peak at - 0.1 V with increasing number of potentiodynamic scans suggests that a part of the oxide is not reduced or removed, i.e. it remains at the surface and supports repassivation during the next cycle. Owing to the generally lower currents in alkaline solutions, a support of passivation by addition of aluminum is not detectable in polarization curves for this electrolyte. was
XPS investigations Potential-resolved measurements. The specimens were potentiostatically polarized for 300 s at various potentials in 1 M NaOH. The cationic fractions were calculated from the XPS spectra for the different alloys. Figure 3 shows a preferential oxidation of alumimum within the investigated potential range, which yields potential that is almost independent of
Investigation of passive layers on FeAl alloys
a) Fe 8 at.%
“E
400
-
300
-
200
-
100
-
2197
Al
s E \ .-
E / Fig.
2.
V (SHE)
Cyclovoltammogramms of (a) Fe-8 at% Al and (b) Fe-22 at% Al in H$04/Na2S04 (pH3.8),v=6OOrevmin-‘,dE/df=20mVs-’.
aluminum content within the passive layer. The average cationic fraction differs only as a consequence of the bulk aluminum content: 0.18,0.26 and 0.35. At negative potentials the cationic fraction of Fe(II1) starts for all alloys at almost the same value of 0.2, whereas the Fe(I1) cationic fraction in the oxide film decreases with increasing aluminum content from their higher values (0.7, 0.6 and 0.45). Consequently iron oxide is partly replaced by aluminum oxide within the passive layer. According to Vetteri6 and Giihr and Lange,17 passive films on iron consist of a mixed Fe(II)/Fe(III) oxide. The replacement of iron by the less noble aluminum yields a more stable layer in the case of Fe-Al alloys. This has been confirmed previously by galvanostatic reduction of passive layers in combination with sputter depth profiles.’
D. Schaepers
2198 0.8
0.0
r
--
and H.-H. Strehblow
Fe 8 at% Al
-0.6
-0.4
-0.2
Fe
-0.4
-0.6
0.0
15
-0.2
Fe 22
-0.4
-0.6
-0.2
at%
0.0
at%
0.0
E /
l
Fig. 3.
Cationic
fractions
AI(III)
a.2
v
0.4
0.6
1 .a
.
Al
0.2
0.8
0.4
0.6
0.8
1.0
0.4
0.6
0.8
1.0
Al
0.2
v (SHE)
Fe(ll)
v
Fe(lll)
taken from XPS spectra of Fe-Al alloys after 300 s passivation NaOH at various potentials.
in 1 M
Time-resolved measurements provide valuable Time-resolved investigations. information on the development of passive layers. Similar processes occur in the time and potential domains. Although the measured XPS intensities are corrected with their appropriate photoionization cross-sections they are still influenced by the spectrometer settings. To avoid any deviations during comparison of different spectra the intensity ratios are plotted, i.e. relative intensities as a function of the logarithm of time/s. Both intensities
Investigation of passive layers on FeAl alloys
2199
are taken from the same spectrum which ensures equal experimental parameter settings. Figure 4 presents the change of the passive layer composition with time. Freshly polished and sputter-cleaned F-22 at% Al specimens were imersed to 1 M NaOH at - 1.O V (SHE) and pulsed for times in the range from microseconds to hours well within the passive potential range [0.8 V (SHE)]. To ensure short passivation times the counter electrode was switched off after passivation via a relay by a pulse generator. During passivation at 0.8 V (SHE) the Fe(II)/Fe ratio [Fig. 4(a)] increases to an intermediate Fe(I1) maximum at a passivation time of 1 s, followed by a steep decrease and a final plateau value for longer passivation times owing to its oxidation to Fe(II1). The OH/Fe ratio [Fig. 5(e)] shows a similar shape with an OH maximum at 1 s-but not as pronounced as in Fig. 4(a). Fe(III)/ Fe(I1) and Al(III)/Fe(II) [Figs 4(e) and 4(a)] are increase steadily to values of 12 and 7. Figure 4(e) suggests an Al(III)/Fe(III) ratio of the oxide film close to 2 for t> 1 s. These details show that an Fe(I1) film containing aluminum hydroxide is formed initially, with later oxidation of Fe(I1) and further Fe(II1) formation for t > 1 s. These oxidations occur in parallel with a decrease in the water content of the layer with a change from a hydroxide to an oxide. In phthalate buffer an Fe(I1) maximum was found for the same conditions after a passivation time of 10 s.9 Angle-resolved XPS measurements
Although X-rays penetrate a specimen up to some few urn, the information depth of XPS amounts to only some few nm owing to the small escape depth, h, of the photoelectrons. For kinetic photoelectron energies greater than 150 eV, h might be B is 0.054 nm [eV_“‘] for elements calculated for each signal according to’* 1 = B&. and 0.096 nrn [eV-“*I for inorganic compounds. h was calculated for the signals investigated here to be: iron iron oxide aluminum aluminum oxide oxygen
1.26 nm 2.24 nm 1.86 nm 3.3 nm 2.58 nm
The minimum attenuation of photoelectrons is achieved if they leave perpendicular to the specimen surface. Their travel distance increases proportionally to 1jcos 6 with the takeoff angle, 0, relative to the surface normal. The contribution to the XPS intensity of a species at depth d decreases with exp( - d/h cos 0). Therefore the XPS signal of a species close to the surface is enhanced relative to the contributions from species at deeper positions if 8 is increased. Angular-resolved XPS studies may therefore be used to detect a sublayer structure at a specimen surface and provide information about the outer or inner position of layer components. Thus angular-dependent XPS studies are often applied to obtain nondestructive depth profiles. Above the Flade potential the passive layer on iron consists of Fe20s at the oxide/ electrolyte interface and Fes04 at the metal/oxide interface.13*‘4. These details have been confirmed by angle-resolved XPS investigations of passive layers on pure iron passivated in phthalate buffer at pH 5.09,” and in 1 M NaOH.6 The outer part of the passive film consists in this case mainly of Fe(III), whereas the inner part is formed by an oxide containing both Fe(I1) and Fe(II1) ions. As described previously,’ this sublayer structure is not found for
D. Schaepers
2200
4
Fe(ll)
/
and H.-H. Strehblow
b)
Fe
OH /
Fe
3oY 26 0 .-
._0 +
.
20 .
0
I
;
._c
z
15
c; ._c
10 5
+p -2-l
log
4
Fe(lll)
t /
/
s
12
log
t /
s
/
Fe(ll)
AI(III)
Fe(ll)
14,
4
0
3
4
I
AI(III)
/
Fe(lll)
l
0.4 -2-l
Fig. 4.
.
1
0
12
log
t /
.
’ 3
4
s
XPS intensity ratios (corrected by photoionization passivation time of Fe-22 at% Al passivated
cross-sections) as a function at 0.8 V in 1 M NaOH.
of the
Investigationof passivelayerson Fe-AI alloys
2201
passive layers on Fe-Al alloys formed in phthalate buffer (pH 5.0). According to XPS studies it has a chemically more homogeneous structure of a mixed oxide or oxide compound containing Fe(II), Fe(II1) and, remarkably enriched, Al(II1) ions. This film structure is also detected for passivation in 1 M NaOH. Some indication of an in-depth separation of the different oxide species is found only for negative potentials or short passivation times. As an example, Fig. 5 shows relative intensities of angle-resolved XPS investigations of Fe-22 at% Al after 300 s passivation at -0.2 V in 1 M NaOH. According to its thermodynamical properties, aluminum is preferentially oxidized during the initial state of oxide growth. Thus an aluminum-rich oxide is directly formed on top of the alloy. Further oxidation leads to an oxide with less aluminum below this initial film, owing to its depletion at the metal surface. This process yields an oxide structure with strong aluminum enrichment at its top. As a consequence, the intensity ratios Al(III)/Fe(II) and Al(III)/Fe(II) + Fe(II1) increase with take-off angle, 0. The AI(III)/Fe(III) ratio does not show such a pronounced tendency, because Fe(II1) is located preferentially in the outer oxide parts-and thus at the same position as Al(II1). At more positive potentials additional oxide starts to grow, penetrating the aluminum-rich, initially formed oxide. Thus further oxide growth occurs at the oxide/electrolyte interface. This leads to an inner aluminum-rich part of the oxide layer. In both oxide parts-above and below the aluminum-rich partaluminum is still enriched compared with the bulk content. This detailed information is obtained from sputter depth profiles. Although there is a large gradient of the aluminum concentration, an angle dependence of the XPS cation intensity ratios Fe (oxide)/Al (oxide) cannot be found. This result is a direct consequence of the sandwich structure of the passive layer, with less aluminum-rich parts at the outer and inner positions and the aluminumrichest part in between. Owing to this structure and, in addition, the accumulation of aluminum even in the inner and outer positions of the passive layer relative to the bulk composition, XPS averages the Al profile. Thus XPS studies are missing some of these details which may be observed with other methods. XPS sputter depth profiles
Figure 6 shows XPS sputter profiles of alloys passivated in 1 M NaOH for 300 s at 0.6 V, well within the passive range. The evaluation of these XPS spectra provides cationic fractions as depicted in Figs 6(a) and 6(c), where the metals and their oxidized species are plotted as a function of sputter time. Because of preferential sputtering of oxygen, Fe(II1) is reduced to Fe(I1). Therefore a separation between Fe(II1) and Fe(I1) is questionable and the sum of both cations is presented as FeOx. Although the profiles yield a large gradient of the aluminum distribution, also a general aluminum enrichment with respect to the bulk composition is found at any depth. At the oxide surface the aluminum cationic fraction starts to depend on the bulk content (0.08,0.15 and 0.22) with xAl’O.12, 0.2 and 0.3. For each profile these fractions increase with sputter time, go through a maximum at xAl = 0.2, 0.4 and 0.5 in the center of the passive layer, and decrease until the bulk concentration is reached. Figures 6(d) and 6(e) show the atomic fractions, summarizing the contributions of al1 oxidation states for each metal. These diagrams depict in particular the aluminum profile with its maximum in the center of the layer. Taking the inner inflection point of the aluminum oxide signal as a measure of the oxide/metal interface, the thickness of the passive layer amounts to 2.5 nm for Fe-8 at% Al and 3 nm for the other alloys. The oxygen signal is not included in these calculations of the atomic fractions and therefore is not shown.
D. Schaepers and H.-H. Strehblow
2202
b) OH/O
AI(III)/Fe(ll) 0.6 0 .+ ?
s 8 ._
4L .-t
c>
2.5 0 ._ + 0 L
2.0
-G
._c
.. ... 1.
0.5
0
IO
20
30
40
50
60
O/”
d)
AI(III)/Fe(lII)
Fe(ll)/Fe(lll)
l
1.5
, . ,
I
0
10
20
30
40
50
60
O/”
1.0
’
0
10
20
30
40
50
60
O/”
e>
AI(III)/Fe(lI)+(lII)
1.2
-$ .-
.
0.8
0.6 0
Fig. 5.
10
20
Angular-resolved
30
40
intensity
50
60
ratios of Fe-22 at% Al after 300 s passivation -0.2
v
in 1 M NaOH
at
Investigation of passive layers on FeAl alloys
d / nm (Ta205)
a)
0.0
1.0
2.0
3.0
4.0
d / nm Ua205)
d)
5.0
1.0,
1.0,
1
J
2203
0.0
1.0
2.0
3.0
4.0
5.0
I __________..
0.8 ’
0 0
0.6
&
0.6-
.i
0.4
8at%Al -
e,
b)
1.0 -
‘.OI”“‘I 0.8 0.6
15at%Al
0.0
4 ;;
0 ,’
f>
“OI
0.8 0.6
I
L
‘.OI
J
_...__._b”“__.._...________________..
0 0
k
k
._
L
0.4
0 4-J
0.2
u 0.0
0.0
50
0
sputter o
Al
Fig. 6.
l
AlOx
100
time v
200
150
Fe
1 50
100
sputter
[s] v
I
0
FeOx
l
Al
+
AlOx
150
time v
Fe
200
[s] +
FeOx
XPS sputter profiles of FeAI alloys after 300 s passivation in 1 M NaOH at 0.6 V.
Figure 7 presents sputter depth profiles of Fe-22 at% Al specimens after 300 s passivation time in 1 N NaOH at various potentials. At - 0.2 V and 0 V small amounts of oxide are formed as seen by the low FeOx signal and the thin layer thickness of ca. 1.5 nm. The aluminum oxide signal shows a maximum close to the surface at a depth of about 0.5 nm. At more positive potentials additional oxide is formed and the height of the Al(II1) maximum is increased and broadened, being found at 1.5 to 2 nm depth. This result suggests
D. Schaepers
2204
-0.2V
SHE
OV SHE
d /
0
50
100 time
150
0
[s]
nm
/
3.0
50
100
sputter
0
XPS sputter
4.0
time
Al
9.0
150
l
100
d /
nm
1.0
2.0
v
[s]
(Ta20J 3.0
50
sputter
AlOx
time
150
SHE
0
[s]
(-rap51
nm
sputter
(Ta205)
2.0
/
50
0.6V
1 .o
0
Fig. 7.
d
SHE d
0.0
(Tap5)
nm
sputter
0.4V
and H.-H. Strehblow
Fe
profiles of Fe-22 at% Al after 300 s passivation potentials.
.
4.0
100
time
150
[s]
FeOx
time in 1 M NaOH
at various
the layer growth at both interfaces, metal/oxide and oxide/electrolyte. Borate buffer, pH= 9.3. Figure 8 shows as an example, for borzte buffer at pH = 9.3, the change in the cationic fractions with passivation potential for Fe-22 at% Al. The most obvious difference is the larger Al(II1) cationic fraction, 0.6, at low potentials compared with that, 0.35, in NaOH (Fig. 3). No appreciable Fe(II1) is detected at these potentials.
Investigation of passive layers on Fe-Al alloys
7
0.0 -600-400-200
0 E /
l
Fig. 8.
2205
AI(III)
v
200
400
600
800
1000
mV (SHE)
Fe(ll)
v
Fe(lll)
Cationic fractions taken from XF’Sspectra of Fe-22 at% Al after 300 s passivation in borate buffer (pH 9.3) at various potentials.
Owing to increased iron oxide formation the Al(II1) cationic fraction diminishes to a value of 0.3 at more positive potentials. Fe(I1) is gradually replaced by Fe(II1) with increasing potential. The XPS sputter depth profiles in borate (Fig. 9) after passivation for 300 s at 1 V show, for all alloys, a slightly larger maximum of the Al(II1) content in comparison to 1 M NaOH (Figs 6 and 7). Surfate solution, pH = 3.8,2.5. XPS sputter profiles after passivation at 1.4 V (SHE) at a pH value of 3.8 show again oxide layers with a maximum of aluminum oxide enrichment similar to the more alkaline solutions. XPS sputter profiles after passivation at 1 V (SHE) in a solution of pH 2.5 show for 15 at% and especially for 8 at% Al (Fig. 10) cationic fractions of Al(II1) with an accumulation plateau near the surface, with a decrease towards the bulk values in the inner layer parts. The accumulation of aluminum is smaller owing to the better solubility of aluminum oxide in more acidic electrolytes. Figure 11 gives an overview of XPS sputter profiles, comparing the layer composition for different pH values. The enrichment of aluminum reaches its highest value in phthalate buffer at pH = 5. This result is a consequence of the amphoteric properties of Al(II1) oxide with a minimum solubility at pH 6. Passivation for longer times. Figure 12 shows a comparison of sputter profiles of Fe22 at% Al after passivation for 300 s and 24 h at 1 V in sulfate at pH 2.5. It is obvious that during long passivation times the outer layer parts were dissolved, which shifts the maximum of the aluminum profile to the surface. The inflection point of Al(II1) suggests a thinning of the oxide with passivation time due to its slow dissolution. 4He+ ISS sputter depth profiles
Elastic-ion scattering spectroscopy (ISS) permits the evaluation of depth profiles with monolayer resolution. This is an advantage in comparison to XPS which collects spectra
2206
D. Schaepers and H.-H. Strehblow
8 at%
15
Al
d / nm (Ta205) 0.0
1.0
2.0
3.0
d / nm (Ta,OJ I
-;;’
0.8
u t
0.6
G.6
0.4
0.4
0.2
a.2
2 .-
-e 0 *
0.0
0.0 0
50
100
sputter
22
Al
4.0
1.0 ,
0 .-
at%
at%
time
150
100
50
0
[s]
sputter
time
150 [s]
Al
d / nm (Tap51 0.0
1.0
2.0
3.0
4.0
‘.O i
sputter Fig. 9.
time
0
Al
.
v
Fe
.
AlOx FeOx
[s]
XPS sputter profiles of Fe-AI alloys after 300 s passivation time in borate buffer (pH 9.3), at E= 1.0 v.
with an average information
depth of some few nm, which is in the range of the escape depth of the photoelectrons. ISS samples only the mass of the surface atoms and does not provide chemical information. Thus only the atomic fractions of Al [Al+Al(III)] are calculated from the experimental data. The vanishing oxygen signal served as indication for reaching the metal/oxide interface. The specimens were passivated in borate buffer, pH = 9.3, and sulfate electrolyte, pH = 3.8, for 300 s at 1 V (SHE). The atomic fractions of aluminum (Figs 13 and 14) show maxima at a depth of about 1 nm, in agreement with XPS profiles. Because of the higher depth resolution the concentration profile is sharper. This causes a sharper and higher peak of the atomic fractions. The maximum values detected by ISS for 22 at% Al are
Investigation of passive layers on Fc+AI alloys
2207
8at%Al
d / nm (Taps) 0
1.0
2
0
4
z
q0 _-
: ‘Z :
4
2
1.0 _______‘-___‘__~---‘___~_______._~______.~hlllk
0.6 0.4 -
0.4 0.2
I
nn V.”
0
50
sputter
100
time
150
[s]
0.0
I
0
-
50
sputter
100
time
~-I 150
[s]
- FeOx v Fe l Al + AlOx v Fe + FeOx l AlOx 0 Al Fig. 10. XPS sputter profile of Fe-8 at% Al after 300 s passivation time in sulfate solution (PH 2.5) at E= 1 V.
up to xAi= 0.85 in borate buffer (Fig. 13) and XAi= 0.65 in sulfate solution (Fig. 14). In sulfate the peak height is nearly the same for the 15 and 22 at% Al alloys, but the maximum peak for 22 at% Al is broader owing to the higher bulk aluminum content. Similar results have been found from ISS depth profiles of Fe+Al alloys passivated in phthalate buffer of pH 5.O.9 DISCUSSION According to ISS and XPS depth profiles, aluminum is enriched within the passive layers of the FeAl alloys. Angle-resolved XPS investigations give no clear indication of a separation in sublayers of different oxidation states and varying aluminum content, like for passive layers on pure iron.3T6No protective overlayer of pure aluminum oxide was found, which might have been predicted from the thermodynamic data of the alloying elements. The investigations by XPS, ISS and electrochemical depth profiling by galvanostatic reduction,’ however, prove an accumulation of Al(II1) ions within the whole passive layer. A strong Al(II1) cation concentration gradient with a maximum in the center of the layer was found by XPS as well as by ISS depth profiling. Depending on the time and potential of passivation, Fe(I1) may be found in the inner and outer parts of the passive layer. In a previous work, Fe(I1) was found preferentially close to the oxide/electrolyte interface for passivation times z 10 s in phthalate buffer, pH = 5.0.9 Apparently, the passive film on Fe-Al alloys is a mixed (Fe, Al) oxide or oxide compound. The preparation parameters cause changes in the concentration gradients-as shown by XPS and ISS depth profiles. Closer examination of these effects by time-resolved measurements provides information on the growth kinetics and chemical changes of the passive layer with time. Within the oxide oxygen anions form a close-packed structure. In the case of pure iron only Fe(I1) ions are present at low potentials and a brucit structure
2208
and H.-H. Strehblow
D. Schaepers
pH
PH
2.5 d / 0.0
nm
1.0
2.0
b,Os) 3.0
4.0
I
0.0
5.3
1.0
2.0
3.0
4.0
5.0
1 .o
:.O I
0.8
5
bulk
I
b”ik
C.6 0.4 0.2 0.0 0
100
50
sputter
pH
time
150
200
100
50
0
sputter
[s]
pH
9.3
0.0
1.0
2.0
3.0
4.0
0.0
5.0
0
0.8
0.8
u
2 .-v : .A::
1.0
nm 2.0
(TapQ 3.0
4.0
5.0
bulk
__.. . . . . . ..__..._........._..._...... 7 .
0.6
0.6 .-u 0.4
._:
0.4
4-J
i-3 0.2
0.2
no _.-
3.0 0
50
sputter
Fig.
[s]
1.0 [ bulk
-G
time
200
13.8 d /
1.0
150
100
time
150
200
sputter
[s]
time
[s]
- FeOx 8 AlOr 0 Al v Fe I 1. XPS sputter profiles of Fe-AI alloys after 300 s passivation in electrolytes with different pH values, at E= 1 V.
[Fe(OH)2] or a rock salt structure (FeO) is assumed, where only the octahedral positions are filled. For a more protecting layer structure the tetrahedral position should be filled as well. Usually, an inversed spine1 structure is assumed for the stationary situation of the passive layer on pure iron at sufficiently positive potentials, with a gradual change from a composition of Fe304 at the metal surface to Fe203 at the oxide/electrolyte interface. This
Investigation of passive layers on Fe-Al alloys
d- /
300
s 1.0 -w
0
nm
2209
(Ta205) 4
2
6
0.8
::
z 0 ._
c
.-0
0.6 0.4
Y
0.2
sputter
time
d / nm
24
h
2
0
-._
0
sputter
o
Al
.
AlOx
v
(Ta@ 4
100
50
[s]
6
150
200
time [s] Fe
v
FeOx
Fig. 12. Comparison of XPS sputter profiles of Fe-Al alloys after 300 s and 24 h passivation in sulfate solution (pH 2.5) at 1 V.
gradual change is expected in the range of the Flade potential, i.e. the passivation potential of iron in strongly acidic electrolytes. Calculations employing the Gibbs free energy, AG”, values for the electrochemical oxidation of Fe304 to Fez03 confirm the experimental value of this critical potential.‘6”7 In the spine1 structure small Fe(II1) ions (r = 0.064 nm)19 are located in the tetrahedral-coordinated lattice sites of the closed-packed O*- matrix, whereas the larger Fe(I1) ions (r = 0.077 nm)19 occupy the octahedral sites.*’ On the basis of these and previous investigations, a model has been described’ which depicts the observed effects during the process of layer formation (Fig. 15). According to the large difference for AGO of aluminum oxide and iron oxide formation (Fes04: - 1016.2 kJ mol-‘; a-Fe203: -742.8 kJ mol-‘; A1203: - 1691.3 kJ mol-‘),21 a preferential oxidation of aluminum is obvious which causes an aluminum-enriched zone
D. Schaepers and H.-H. Strehblow
2210
(Ta205>
d / nm 0
!
1.o r----T-
5
4
----
-
2
5 ‘, Fe
6 I
7
a I
8 at.% Al
1
l Fe 15 at.% Al y.7 Fe 22 at.% Al bulk
sputter Fig. 13
at the surface
time
/
min
4He j ISS sputter profiles (atomic fractions) of Fe-AI alloys after 300 s passivation in borate buffer (pH 9.3) at I V.
of the oxide film [Fig. 15(a)]. However, there is no chance to form a continuous protecting aluminum oxide film because of a lack of aluminum for a bulk content of only some few per cent. Therefore, the less reactive iron forms additional oxide [Figs 15(b) and 15(c)] which grows into a continuous film incorporating the Al(II1) oxide islands. Further oxide growth involves the transport of O* - and OH- from the electrolyte into the film and cations in the opposite direction [Figs 15(b) and 15(d)]. Therefore new oxide is formed at the oxide/electrolyte and metal/oxide interfaces. Rutherford backscattering spectroscopy studies with Xe markers and radioactive tracer methods22-24 showed that for anodic oxide on aluminum and other valve metals, the transfer coefficients for anions and cations are similar. Time-resolved measurements for the passivation of Fe-Al alloys show that the oxide grows at both interfaces with comparable rates, leading to the central position of the aluminum enrichment within the passive layer [Fig. 15(e)]. Time-resolved investigations and XPS analysis of oxide films formed at low potentials show that A13+ ions are formed initially at low potentials and during short passivation times. Because of their size (r =0.053),18 the Al(II1) ions can replace the Fe(II1) ions in the tetrahedral positions of the anion matrix. Thus the oxide films on iron-aluminum alloys can be transformed at low potentials into the spine1 type lattice structure, even if Fe(II1) is not yet formed. Therefore, one expects a hercynite-like structure especially for a maximum Al(II1) content of xAl = 0.7, which is very close to the composition of hercynite with xA1= 0.66. The formation of this hercynite oxide structure is supported by two facts: the solubility of both oxides in the
2211
Investigation of passive layers on Fe-Al alloys
d / nm (Ta206>
0.6
0
5
IO
15
sputter Fig.
14.
20
25
time
30
/
35
40
45
min
4Hef ISS sputter profiles (atomic fractions) of FeAI alloys after 300 s passivation time in sulfate (pH 3.8) at 1V.
matrix of the other and the enthalpy of oxide formation. a-FeOOH has a solubility for Al(Il1) ions of only up to 33 at%.21 Since the maximum aluminum enrichment is about double this value, the formation of a hercynite-like compound is strongly supported, in which an aluminum substitution of 66 at% is possible. The more negative molar AG” value of formation for FeA1204 (- 1879.7 kJ mol-‘) compared with Fe304 (- 1016.2 kJ mol-‘), a-Fe203 (- 742.8 kJ mol-‘) and A1203 (- 1691.3 kJ mol-‘)2’ is another argument for the formation of this compound. It was assumed that the the pH of the electrolyte has a large influence on the aluminum accumulation and its related effects.‘. These predictions could now be verified by means of the investigations in the more acidic sulfate electrolyte after longer passivation time. The higher dissolution rate of the oxide film caused the aluminum enrichment maximum to shift towards the oxide/electrolyte interface [Figs 15(f) and 15(g)]. Even after 24 h passivation time (Fig. 12) aluminum oxide was found in the outer layer parts, thus still protecting the metal surface.
CONCLUSION Aluminum additions facilitate the passivation of FeAl alloys compared with pure iron especially in acidic solutions, as is shown by the lower anodic current peak of the cyclovoltammogramms in various electrolytes. An aluminum oxide enrichment compared
D. Schaepers and H.-H. Strehblow
2212
oxide
growth at
both interfaces
‘i/////n
elektrolyte
B
Al-oxide enrichment
I
]
Fe/Al-oxide bulk
dissolution
in acidic
solutions
Fig. 15. Schematic model of the passive layer formation on Fe-Al-alloys.
with the bulk concentration
was found in the passive layers formed in electrolytes with pH values in the range 2.5 to 13.8. The Al(II1) concentration shows a gradient with a maximum of enrichment within the center of the passive layer. The initially formed Al(III)-rich film contains a large amount of hydroxide. During longer passivation times and at more positive potentials the layer composition changes into a mixed oxide/hydroxide film with a larger amount of additionally formed iron oxide. Al(II1) is still enriched in the whole layer. A model for passive layer formation on Fe-Al alloys for these conditions was developed. It demonstrates the kinetic effects which lead to the central position of the Al(II1) enrichment maximum within the oxide film. Thermodynamic data support the assumption of an initially formed aluminum hydroxide and strongly suggest the formation of a hercynite-like spine1 structure, especially in the region of the maximum Al(III) enrichment. Acknowledgement-The support of this work by the Deutsche Forschungsgemeinschaft (project Str. 200/9-l, 9-2) is gratefully acknowledged.
Investigation of passive layers on FeAl alloys
2213
REFERENCES 1. Ullmanns Encykliiptidie der technischen Chemie, 3rd edn, Vol. 365. Urban and Schwarzenberg, Miinchen/ Berlin, 1953, p. 3. 2. R. Kirchheim, B. Heine, S. Hofmann and H. Hofslss, Corros. Sci. 31, 573 (1990). 3. S. Haupt, C. Calinski, U. Collisi, H.W. Hoppe, H.-D. Speckmann and H.-H. Strehblow, Surf. Interface Anal. 9, 357 (1986). 4. L. Young, Trans. Faraday Sot. 53(I), 841 (1957). 5. S. Hofmann and J.M. Sanz, Surf. Interface Anal. S(5), 210 (1983). 6. S. Haupt and H.-H. Strehblow, Corros. Sci. 29, 163 (1989).
7. H.W. Hoppe, Ph.D. thesis, Heinrich-Heine-Universitat, Diisseldorf, 1990. 8. P. Druska and H.-H. Strehblow, Surf. Interface Anal. 23, 440 (1995). 9. D. Schaepers and H.-H. Strehblow, J. Electrochem. Sot. 147(7), 2210 (1995). 10. D. Schaepers and H.-H. Strehblow, Surf Interface Anal. 21, 342 (1994). 1 I D.A. Shirley, Phys. Rev. B5, 4709 (1972). 12. J.H. Scofield, J. Electrosc. 8, 129 (1976). 13. R.F. Reilmann, A.M. Sezane and S.T. Manson, J. EIectron Spectrosc. Relat. Phenomena 8, 389 (1979). 14. G.C. Nelson, J. Vat. Sci. Technol. A4(3), 1567 (1986). 15. R.E. Honig and W.L. Harrington, in Proceedings of International Conference on Ion Beam Surface Analysis, IBM, Yorktown Heights, NY, 18-20 June 1973. 16. K.J. Vetter, Z. Elektrochem. 62, 642 (1958). 17. H. Gohr and E. Lange, Naturwiss. 43, 12 (1956). 18. M.P. Seah and W.A. Dench, Surf. Interface Anal. l(l), 2 (1979). 19. IJ. Schwertmann and R.M. Cornell, Iron Oxides in the Laboratory. VCH, Weinheim, 1991. 20. K. Wandelt, Surf. Sci. Rep. 2, 93 (1982). 21. Handbook of Chemistry and Physics, 74th edn. CRC Press, Boca Raton, FL, 1993/94. 22. J.P.S. Pringle, J. Electrochem. Sot. 120, 398 (1973). 23. F. Brown and W.D. Macintosh, J. Electrochem. Sot. 120, 1096 (1973). 24. W.D. Macintosh, F. Brown and H.H. Plattner, J. Elecfrochem. Sot. 121, 1281 (1974).