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A surface analytical examination of passive layers on CuNi alloys: Part I. Alkaline solution
CorrosionScience,Vol. 38, No. 6, pp. X35-851,1996 Copyright 0 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0010-938X/96 615.00+0.00
PII: SOOlO-938X(96)00170-0
A SURFACE ANALYTICAL EXAMINATION OF PASSIVE LAYERS ON Cu/Ni ALLOYS: PART I. ALKALINE SOLUTION P. DRUSKA,”
H.-H. STREHBLOW”*
and S. GOLLEDGEb
“Heinrich-Heine-Universitlt Dusseldorf, D-40225 Diisseldorf, Germany bUniversity of Washington, Seattle, WA 98195-1750, U.S.A. electrochemical formation of passive layers on Cu-20Ni, Cu-5ONi and Ni-2OCu in various electrolytes has been examined with X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). In this paper the results of investigations in alkaline solution (1 .ON NaOH) are discussed. On the surface of Cu/Ni alloys first an Ni(OH)* layer is formed. Potential and time resolved measurements yield similar results for the structure of passive layers. At short times and low potentials the rapidly growing Ni(OH)2 leads to an accumulation of Cu at the metal surface which causes the formation of an oxide layer underneath, consisting mainly of CuO. At more positive potentials NiO enters the oxide sublayer. CuzO appears to a minor extent at the metal/oxide interface during the initial stages of CuO formation and is no longer detected when CuO grows to a larger thickness at sufficiently positive potentials. In the potential range of the beginning of transpassive behaviour the top Ni(OH)z layer is oxidized to NiOOH. Parallel to the NiOOH formation Cu(OH)2 is incorporated into the hydroxide overlayer. This transpassive oxidation causes a shift of the work function to larger values as determined by UPS and to the same shift of all XPS signals of oxide constituents to smaller binding energies similar to previous observations for pure Ni and Fe/Ni alloys. The passive layer on Cu/Ni alloys reflects the properties of both metal components. Copyright 0 1996 Elsevier Science Ltd. Abstract-The
INTRODUCTION Cu/Ni alloys are highly resistant to corrosion in aggressive aqueous electrolytes at normal and elevated temperatures. They are used for heat exchangers and for parts exposed to sea water, e.g. in the ship building industry, because of their resistance to localized corrosion. Various publications deal with their corrosion properties in sea water or NaCl-containing electrolytes’-5 and at elevated temperatures.6 These alloys are also widely used as catalysts for hydrogenation and dehydrogenation.738 Although technologically very important, very little is known about the protective passive layers and the films which form in the transpassive range. The chemical composition and structure of these layers have therefore been systematically examined. In this paper results of films formed in 1 M NaOH are presented. Studies in acidic electrolytes will be published in a following paper. Figure 1 depicts the potentiodynamic polarization curves in 1 M NaOH of all investigated alloys for a better understanding of the surface analytical examinations. They show the main characteristics of pure Ni with a small anodic peak at E = - 0.5V indicating the passivation of this alloy component and a peak at E = + 0.7 V which is characteristic for its transpassive behaviour. In the range E = 0.0-0.25 V the anodic peaks of Cu related to the formation of Cu(1) oxide and Cu(I1) oxide are detected. The size of these anodic peaks varies *To whom correspondence should be addressed. Manuscript received 11 September 1995. 835
836
P. Druska.
p
H.-H. Strehblow
and S. Golledge
1
500 mA/cm 2 ‘/
I* ,,._,-_.____-~_______ _ _______~_~
Cu20Ni ,~~
.._Jj -._- ._ ._, ,.i __~ ._ ,,__
i
i:
i
with copper content. thus giving ;I first indication ot’ the alloys characteristics determined by the specific properties of their constituents.
EXPERIMENTAL
Specinzen
which are
METHOD
preparcition
The surface analytical studies were performed with an Escalab 200 X (Fisons/VG Instruments) equipped with an analyser and preparation chamber, a fast entry lock and an attached electrochemical chamber containing a small electrolyte vessel of about 2 cm3 volume. All electrolytes were Ar purged and introduced via appropriate channels. The electrochemical chamber can be filled with purified Ar and evacuated. Thus a specimen preparation and transfer without any air contact was realized. Appropriate electrical channels and an electrolytic contact to a reference electrode permitted potentiostatic polarization. A detailed description of the UHV system its performance and the preparation of Cu/Ni alloys for surface analytical investigations is given in detail elsewhere.9~” The CuiNi alloys were melted from pure metals. mechanically treated and heat-treated to obtain void-free single-phase materials. Three alloys with 20, 50 and 80 wt% Ni (Cup 20Ni, CupSONi, Ni-2OCu) were studied. The surface was polished with 1 pm diamond spray, ultrasonically cleaned in ethanol. introduced into the spectrometer, and finally
Passive layers on Cu/Ni alloys-Part
I
837
sputtered with argon. Subsequently the sample was transferred to the electrochemical chamber and contacted with its circular front plane to the electrolyte surface. The selected electrode potential was applied for the passivation time of choice. After passivation the surface was rinsed with pure water, remaining adherent water was blown off with a jet of argon and the specimen was transferred to the UHV of the analyser chamber. The solution was prepared with analytically pure NaOH and deionized water (Millipore water purification system). All potentials were measured relative to a Hg/HgO/l M NaOH electrode (E = 0.14 V) and are given relative to the standard hydrogen electrode (SHE). Surface analysis
XPS analysis was performed with non-monochromatized Al Kcc radiation (1486.6 eV) with an input power of 300 W and a constant pass energy of 20 eV for the spherical sector energy analyser (Marc II, Fisons/VG Instruments). For ISS depth profiles the ion source (EX05, Fisons/VG Instruments) was run with a Ne beam of 3 keV primary energy. For ISS spectra 30 nA were applied to 14.85 mm2 surface area (2. lo- l5 A/m2) and for sputtering 300 nA to 75.19 mm2 (4. lo-l5 A/m’). Thus the crater effect of depth profiles was avoided. The energy analyser was run with 200 eV pass energy. Details about the data acquisition and evaluation are described elsewhere.’ ’ UPS measurements were performed with the He I line (E = 21.22 eV) of a UV source (UVS 10/35, Leybold Heraeus) with a discharge current of 90 mA, a pass energy of 3 eV and a negative specimen bias of 10 V. This bias ensures the total acceptance of the secondary electrons. The energy difference AE of the UP spectrum between the secondary cut-off and the Fermi edge yields the work function e4 according to the relationship AE = hv - e4. Data evaluation
The XPS spectra were evaluated on the basis of well-characterized standards. The preparation of Ni and Cu standards has been described in detail elsewhere.“~i3 The XPS spectra of the different standards are the basis for further evaluations. After background correction according to Shirley14 the standard spectra are described with Gauss/Lorenzians with a tail function to take care of the asymmetry of the XPS signals. For the evaluation of the nickel species the 2P,,, and for oxygen the 1S peaks were used. For the evaluation of the contribution of the various Cu species the Cu XPS signals required a special procedure. Cu metal and Cu(1) species cannot be distinguished by the Cu 2P3,2 signal because of a negligibly small chemical shift. For this purpose the X-ray induced Cu L3MM Auger signal (XAS) was used. This signal has been taken previously for quantitative determination of Cu20 on a Cu metal substrate.’ 5 Peaks were deconvoluted into the contributions of the different species on the basis of the standard spectra. For this purpose the characteristic data of the standard signals, especially the relative sizes of their peaks were kept constant and their total size was varied to meet the actual signal. The integrated intensities of these partial signals of the components were used for further evaluation. A similar evaluation was applied to ISS spectra. The width of the signal is determined by the isotopic composition of the probe gas Ne as well as the target atoms and the relative abundance of these isotopes. Thus the signal of pure standards was composed for each isotopic combination by a simple Gaussian with a minor Lorenzian contribution. The evaluation procedures of the XPS and ISS spectra are described in more detail elsewhere.’ ’
838
P. Druska.
H.-H. Strehblow
EXPERIMENTAL
RESULTS
and S. Golledge
AND DISCUSSION
The results in this paper are based on the layer model presented elsewhere.” In general the passive layer does not have a simple structure. Therefore angular dependent XPS measurements were performed, to serve as a guide in developing a model. On the basis of these examinations one gets to a layer structure as depicted in Fig. 2.” The inner oxide is covered by an outer hydroxide. The lower valent species are in the inner part of each sublayer. This leads to a further subdivision of both the oxide and the hydroxide. The higher valent species appear only at sufficiently positive potentials within the film. The contributions of the different sublayers to the total film thickness depending on the electrode potential are given in the upper part of Figs 4. 8 and 9 for the alloys Cu-20Ni, Cu-5ONi and Ni-20Cu. The potential
dependence of’luyer cornposition
Figure 3 presents the XP spectra of the Cu-5ONi alloy for various passivation potentials. With increasing potential the Cu and Ni metal signals are decreasing and the contributions of signals at higher binding energies are growing due to the formation of oxide and hydroxide. The 0 1s signal at 531 eV binding energy increases due to the formation of a hydroxide layer at low potentials and short passivation times. The formation of a peak at 529 eV electrode potentials E > 0.04 V (SHE) indicates the formation of oxide. The broad tail at higher binding energies, especially for low passivation potentials, is related to water within the hydroxide layer. The oxidation process of Ni(OH)2 to NiOOH in the vicinity of the transpassive potential range at E > 0.5 V is followed by the chemical shift of all XPS signals of oxide components to smaller binding energies. A related observation is the increase of the work function by 0.5 eV obtained from UP spectra related to an appropriate shift of the secondary cut-off of the photoelectrons. Similar observations have been described for pure Ni and Fe/Ni alloys and have been explained with a simple semiconductor mode1.‘2~‘-7~‘h According to this model of the passive layer the increase of the band bending with the electrode potential leads eventually to a crossing of the valence band with the Fermi level. This situation introduces a large number of higher valent states close to the valence band edge which act as p-dopants leading finally to a decrease of the Fermi level. As the binding energies and the work function are given relative to the Fermi level this leads to a shift of both quantities by the same amount but with opposite sign. The formation of NiOOH. i.e. Ni(II1) ions in the transpassive potential range is an alternative more chemical picture to the accumulation of positive holes or of acceptor levels. Figure 7 depicts the increase of the work function by 0.5 eV as obtained from the UP spectra. A d4 Ni(OH)2
cue
NW+),
_
.~ NiO _ ..--
Cu/Ni-Bulk
-0.46 V Fig. 2.
Cu(OH), d3
cue
d2 NiO
cue dl >
cu,o
cu,o
E=
lC~(Ot-0~1
NiOOH
Cu/Ni-Bulk
+0.44
v
hydroxide layer
oxide layer
Cu/Ni-Bulk
+0.99v
The passive layer model of Cu. Ni alloys for three characteristic
potentials
Passive layers on Cu/Ni alloys-Part
I
839
cu 2P&z 4 t1.ll4av
JJ----k to.74ov JJ-I tO.34OV Ik_/--k
t004OV L Q460v
J..-._ A
_JL D
565.0
570.0
575.0
bindingenergy/ eV
580.0
585.0
925.0
930.0 935.0 940.0
086UV
945.0
9!
I.0
tilldingenergy/~V
Fig. 3. XPS and XAS signals of Cu-5ONi passivated in 1 M NaOH for 300 s at six characteristic potentials: (a) Cu L,MM X-ray induced Auger signal; (b) Cu 2Px12XPS signal; (c) Ni 2Pj,z XPS signal; (d) 0 1S XPS signal.
negative bias of 5 V to the specimen ensures the total acceptance of the secondary electrons without any loss which would alter their cut off edge. The energy difference AE between the secondary cut-off edge and the Fermi edge of the metal phase permits the calculation of the work function e4 according to the relationship AE = hv - ec&. Figure 4 depicts the onset of the formation of a Ni(OH)z layer at E = - 0.46 (SHE). At E > - 0.26 V this outer hydroxide is undergrown by an oxide which consists of CuzO/NiO at the metal surface followed by an outer CuO/NiO film. Up to E = 0.06 V this oxide contains only Cu oxides and NiO enters the oxide phase only at more positive potentials. At 0.3 V (SHE) a final thickness of 5.5 nm is reached for the total oxide thickness with no further growth with the potential. The oxidation of the CuzO component of the inner oxide to CuO occurs at E = 0.4 V and of Ni(OH)z to NiOOH is observed at E > 0.6 V which is accompanied by an accumulation of Cu(OH)z within the hydroxide layer. Many details of Fig. 4 are also indicated by the features of the potentiodynamic polarization curve of Cu20Ni of Fig. 1 as the formation of Cu oxides at E > -0.26 V (SHE) and the transpassive oxidation of Ni(OH)* to NiOOH at E > 0.6 V. Although the bulk metal contains only 20% Ni, its concentration is further decreased at the metal surface. This is to be expected, given the preferential incorporation of Ni into the passive layer. To confirm the multi-layer structure of the passive film obtained by XPS measurements
I’
x40
Druska.
H.-H
Strehblow
Ni2Pm
and S. Golledge
01s
.---
v --to.040 4.46oV
Jl.46OV 0.8M)V
0.860v
,~ .___~ 845.0
850.0
~. 855.0
.~~~~~~_ ~_ 860.0
865.0
870.0
525.0
binding energy / eV FIG
I
530.0
535.0
bmding energy
I eV
540.0
t c l,illifiltf ii
IS!5 depth profile4 ~vere performed at characteristic potentials. Figure 5 depicts as an example the change of the 1% signal and its dcconvolution in Cu and Ni contributions during Ne sputtering of a C‘+2ONi specnnen pasaivated in I M NaOH for 300 s at E = 0.89 V. .4t a depth of 0.2 nm a small Cu signal 1s seen In addition to a large Ni signal. Ni is decreasing relative to Cu with depth and ha\ a minimum value at about 7 nm due to its depletion at the metal surface. Figure 6 shows the quantitative evaluation of three depth profiles of specimens passivated at three characteristic potentials together with the XPS data of identically prepared samples evaluated on the basis of the multi-layer structure model. Two main steps are observed. corresponding to the outer Ni(OH)? layer which contains Cu only at sufficiently positive potentials followed b!, the inner oxide where Cu is enriched. The thickness of the oxide increases with the potential. At E = - 0.16 V (SHE) only a shoulder is observed whereas larger steps are observed for higher potentials. The quantitative agreement of the results obtained by the two methods is remarkable. A subdivision of these layers according to the valency of the cations cannot be expected by ISS as this method cannot distinguish between valence states. At the oxide metal interface a slight overshoot indicates the accumulation of the more noble Cu at the metal surface with a slight gradient reaching the bulk composition at about 6 nm depth. The XPS analysis assumes an equally distributed Cu enrichment close to the metal surface over the escape depth of the photoelectrons because this method cannot detect a small gradient over a short distance. Its depth resolution is limited by the escape depth of the photoelectrons and their attenuation by the covering passive film.
Passive layers on Cu/Ni alloys-Part
I
841
6.0 g 5.0 S 4.0 gE 3.0 ‘: 2.0 r - 1.0 0.0
-1.0
4.5
0.0
0.5
1.0
p%n!ME/V
Van
Fig. 4.
v 0.0
a v av
v-v v v-v---qv
The thickness and composition of passive layers on Cu-2ONi passivated in 1 M NaOH for 300 s as a function of the potential.
Passivation of Cu-5ONi yields similar results to Cu-20Ni (Fig. 8). Layer formation starts also at E = -0.5V (SHE) with Ni(OH)z formation which is oxidized at E > 0.5V to NiOOH. At E = 0.8 V this oxidation is complete which is not achieved for the Cu-20Ni alloy. Cu(OH)z incorporation starts at slightly more positive potentials and to a much smaller extent. Oxide formation starts also at E = -0.4V. NiO enters at slightly more negative potentials. With a maximum of 2 nm the oxide layer amounts to only half of the thickness of the passive film of Cu-20Ni. Ni depletion at the metal surface leads to a loss of about 50% of the bulk composition i.e. to about 25% Ni content. The results for Ni-20Cu are approaching those of pure Ni due to the high Ni content (Fig. 9). Layer formation starts again at E = -0.5 V (SHE) and the oxidation of Ni(OH)2 to NiOOH occurs in the range 0.4-0.9 V. The oxide layer thickness increases more linearly
842
P. Druska,
H.-H. Strehblow
1200
loo0
1400
and S. Golledge
1600
1800
2000
0.8 0.4 0.0 0.8 0.4 0.0
4nm
0.8 0.4
t
0.8
0.8 0.4 0.0
kinetic energy I eV Fig. 5.
ISS signals
of Cu-2ONipassivated sputter
in 1 M NaOH for 300 sat 0.89 V after seven characteristic times of the depth profile depicted in Fig. 6.
Passive layers on Cu/Ni alloys-Part
I
843
0
0
2
1
4
3
5
sputterdepthd I nm 1.0
I
I
I 0
0.8 3
.= 0.4 8
o”
0.2 0.0
:
a+&&
I
__. _,. _ ,._. _.
.
+0.44 v /
,
4
2
0
,
8
6
sputterd@d/m
0.0
0
”
I”’
2
s’
4
’
“I’
8
”
‘I”
8
”
10
sputterdeplhdlm Fig. 6. ISS depth profile (0) and layer structure according to the quantitative evaluation of XPS data (-) of Cu-2ONi passivated in 1 M NaOH for 300 s at three characteristic potentials.
with the potential in comparison with Cu-rich alloys. This result is closer to that for pure Ni where only NiO changes linearly with the potential whereas the thickness of Ni(OH)2 remains unchanged. The hydroxide layer contains only negligibly small Cu contributions and the Cu concentration of the oxide layer decreases in the range 0.0-l .OV (SHE) to very small values leaving mainly NiO. Also for the N&rich alloy Cu is accumulated at the metal surface. The time dependence of layer composition
The results of time-dependent investigations are similar to those in the potential domain. It is a general observation that lower valent species appear first and will be oxidized later as has been shown for the case of passive Fe and Fe/Cr alloys.173’8Figures 10 and 11 give an
P. Druska.
844
H.-H. Strehblow
and S. Golledge
0.6
0, 0 0
4 ‘; z a”
0.3
-
0.2
i
t i
0
0.1
0.0
c
l--o--o..._~
_o,.
_
-1.5
-1.0
.~
,’
2 ._1
in
0.0
-0.5
0.5
1.0
1.5
potential E/V Fig. 7.
The change
of the work function
deduced
from LJP spectra vs. the passivation
potential
E.
example for Cu-20Ni at two characteristic potentials. one each in the passive and in the transpassive range. The total thickness increases linearly with the logarithm of time indicating the presence of a barrier type of film. The hydroxide layer forms first and is already present within 1 ms. It takes about lo-100 ms to form the first oxide. Cu(1) and Cu(I1) oxides are observed simultaneously within the oxide layer at E = 0.44 V (SHE) (Fig. 10). At E = 0.89 V (SHE) Cu(1) oxide appears only for a short time to a very low level and is oxidized completely to Cu(I1) (Fig. 11). Ni enters the oxide within about 0.1 s at both potentials. At 0.44 V (SHE) the hydroxide consists of Ni(OH)2. At E = 0.89 V Cu(OH)2 enters within 0. l-l s (Fig. 11) and reaches within 10 s the level of Ni(OH)2. The formation of Ni(OH)l within the first 0.1 s and its disappearance within about 100 s are very characteristic. Simultaneously the NiOOH-containing film is formed which incorporates large amounts of Cu(OH)2 up to an equal content of both hydroxides. At E = 0.44 V (SHE) no NiOOH is formed. The metal surface is enriched in Cu which is a consequence of a preferential incorporation of Ni into the passive layer. The results for Cu-5ONi are similar. The lower Cu content prevents its incorporation into the hydroxide layer and only Ni*+ is transferred into the oxide part. In the transpassive range Ni(OH)2 forms within 1 ms to its maximum thickness and is then continuously oxidized to NiOOH which is related to the higher Ni content. The reduction of the passive layer
The reduction of the preformed passive layers at decreasing potentials reverses the oxide formation with electrode potential (Fig. 12). NiOOH is reduced to Ni(OH)* in the range E = 0.9-0.3 V (SHE). Cu20 appears below E = 0.6 V and at E = - 0.7 V most of the oxide has disappeared. The polarization curves of Fig. 1 show a reduction peak in the range E = 0.8-0.5 V where the NiOOH to Ni(OH)z transition occurs. The reduction of Cu oxides and the related disappearance of the oxide layer coincides with the cathodic currents at
Passive layers on Cu/Ni alloys-Part
I
845
7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1 I.0
4.5
0.5
0.0
1.0
potential E / V -1.o il.5 r"","', 0.6 _ tulkauriace
0.5 0.0 I"","",",
1.0
0.2 -
_.,.n__O.O o,o..,o,.. I
i, -1.0
45
0.0
0.5
potential E /V
&
j7 Fig. 8.
oxygen
1
The thickness and composition of passive layers on Cu-SONi passivated in 1 M NaOH for 300 s as a function of the potential.
E < 0.4V (SHE). The lack of a pronounced potential range where this reaction occurs.
second reduction peak agrees with the larger
Model
The formation of the passive layer and the development of its structure with potential and time is the result of a combination of thermodynamic and kinetic factors. At low potentials and short times of potentiostatic passivation the less noble Ni is oxidized and forms Ni(OH)z. According to the Pourbaix diagram this reaction is expected at E < - 0.706 V which agrees reasonably well with the onset of its formation at E = - 0.6 V according to the XPS analysis. The shift of this onset to E = -0.5V with decreasing Ni
846
P. Druska. H.-H. Strebblow and S. Goiledge
-1.0
-0.5
0.0
0.5
1.0
1.5
potent&E/V
hydmxfde lay
0.8;
_
0.6 1
’ ?Ov._c
0.4
[
***ea. 0
55 LgS,___
c
,
CT7VO
•O~O~~*~~-O~*O i
0.2 ,-1.0
-0.5
0.0 0.5 poterh!E/V
1.0
: oxygen Fig. 9.
1.5
-.. 3 cqp?r 0 nickel
j
The thickness and composition of passwe layers on Ni-2OCu passivated BOOs as a function
in
I M NaOH for
of the potential.
within the alloy should be explained with the decrease of the related Ni activity. The more noble Cu remains at the metal surface underneath and is accumulated with the growth of the surface film. At E > -0.4 V and E > -0.2 V (SHE) the formation of CuzO and CuO, and Cu(OH)?, respectively is expected. Indeed, the continuous growth of an oxide layer is found for E > - 0.2 V for all three alloy compositions, which contains mainly Cu(I1) (Figs 4. 8, and 9). This result is found by XPS analysis and agrees also with the anodic features of the polarization curves at E = - 0.2 V. A Cu(1) containing sublayer is detected at E z -0.2 V and remains underneath up to at least 0.8 V which is in agreement with thermodynamic arguments that the lower valent oxide with the more negative Gibbs free energy of formation is in direct contact with the metal. Its oxidation to Cu(I1) is content
Passive layers on Cu/Ni alloys-Part
I
847
6.0
timetls
0.2
l
--.
1oJ JO4 10”
* IO”
m loo
.
0 . .
1 i
10' IO2 lo3 IO4
timet/s
0 CopPer I nickel / v oxygen j I----l
Fig. 10. The time dependence of the thickness and composition of the passive layer of Cu-2ONi passivated in 1 M NaOH for 300 s at E = 0.44 V:
found by XPS and the anodic peak at E = 0.2 V (Figs 1,4, 8, 9) in the polarization curve. At E > 0.5V the formation of NiOOH is expected in agreement with the XPS results and the anodic peak at E = 0.7V in the polarization curve. The reduction of NiOOH at 0.7 V and of Cu(I1) at E = -0.3V and Cu(1) at E = -0.5V are in reasonable agreement with the thermodynamic data. In conclusion, the formation of the various species and the chemical multilayer structure of the passive layer can be interpreted qualitatively on the basis of the thermodynamic data for the pure metals and their oxides and hydroxides, i.e. the related Pourbaix diagrams, although the influence of the composition of the metal and the layers and the related activities of their components cannot be ignored. The quantitative data on the composition and thickness of the sublayers, however,
848
P. Druska.
H.-H. Strehblow
and S. Golledge
,
bulk&ace
i
1 0.6 I 0.4 i 0.8
a l
0.2 : __~
a
_ _____+
10 -3 10
-2 IO”
10°
timetis
IO’ lo2 lo3 ~~~_____ ‘2 copper l nickel
’
. Oxygen .L FIN.
1I. The time dependence
of the thickness and composmon of the passive layer on Cu-?ONi passivated in 1 M NaOH for 300 s at E = 0.89 V.
involves also the influence of the kinetics as the transfer rates at the phase boundaries and the transfer through the sublayers by migration and diffusion. The solubility of the oxides with each other and the formation of mixed oxides or compound oxides and hydroxides is another parameter. All these details are not sufficiently known and further studies are required to determine the atomic, besides the chemical, structure which requires, however, sophisticated analytical methods. The layers are only a few nanometres thick and in most cases relatively disordered, so that conventional diffraction methods will fail. Therefore diffraction or EXAFS studies with high intensity X-ray sources of storage rings are required. First results have been obtained.” A detailed knowledge about the structure and defect structure may give information on the transport properties of the various species across the passive film and its sublayers.
Passive layers on Cu/Ni alloys-Part
I
849
6.0 5.0 E ; 8
4.0 -
2 2
3.0 L
*
2.0 1.0 0.0 -
tI,...I,,,.I....I.,,.I 1.0
0.5
0.0
-0.5
-1.0
pcWME/V
0.4 1 0.2
1
.--:---..r _*
0.4 1 0.2
_
d ,.......d.: _~ l c ., * 1.0 0.5
..$I .._ l ... .*
,... ---.
--.- 0... .* ---0-4 c*- 0,..n:.....+O. .._ n -0.5 -1.0 0.0
@mtialE/V
Fig. 12. The thickness and composition of passive layer on Cu-2ONi passivated in 1 M NaOH for 300 s at 0.89 V and subsequently reduced gradually for 300 s at potentials down to E = -0.76 V.
For a rather complicated system of two metallic components which are present even in various oxidation states within the film one may give only a qualitative model for the formation of the passive layer as depicted in Fig. 13. Film formation starts with a Ni(OH)* layer leaving behind at the metal surface a Cu enriched zone. At E > - 0.1V (SHE) the Cu enrichment forces also Cu ions to enter the passive layer forming a duplex oxide layer with an inner CuzO and an outer CuO part. The consumption of Cu at the metal surface allows
850
P. Druska.
Fig. 13.
H.-H. Strehblow
and S. Golledge
A model for the development of a multilayer structure of passive films on Cu-Ni during anodic oxidation at sufficiently positive potentials.
alloys
Ni to enter the oxide layer as well. Cu(1) is oxidized to Cu(II) after about 1 s at the oxide surface and is transferred to the hydroxide overlayer. Finally the outer hydroxide is oxidized at E > 0.6 V to NiOOH which may be reduced again to Ni(OH)2 for E < 0.6 V (SHE). Deviation of these results may be obtained in acidic electrolytes. Passive layers on pure Cu are not stable and therefore not protecting for these conditions. Large current densities especially for high Cu content of the alloys indicates that a high Ni content of the passive film is essential for its protecting properties. Thus the layer should contain less Cu ions unless they are trapped in a Ni oxide matrix. CONCLUSION Oxide formation in 1 M NaOH on Cu/Ni alloys reflects the specific properties of both elements. At negative potentials or short times of passivation Ni(OH)2 is formed first. This process leads to an increase of Cu at the metal surface, which causes finally the formation of an oxide layer consisting mainly of Cu oxides. This in turn permits the incorporation of NiO into the already existing inner oxide layer. The depletion of Ni at the metal surface remains independent of the potential and time for the later stages of oxidation. Cu forms first CuzO. At sufficiently long times and positive potentials of passivation CuzO is oxidized completely to CUO/CU(OH)~. This behaviour is different from pure Cu where a duplex Cu2O/CuO, Cu(OH)* structure is observed up to the potentials of the start of transpassive behaviour. Ni(OH)l is oxidized to NiOOH as has been found for pure Ni and other Ni alloys. This oxidation is facilitated with increasing Ni content within the alloy. Also Cu(OH)* is incorporated into the hydroxide overlayer. For E > 0.6 V and Cu-20Ni two-thirds of Ni(OH)z is oxidized whereas for Cu-5ONi and Ni-20Cu the total amount has been changed to NiOOH. Oxide reduction occurs with a sequence opposite to its formation. The changes with potential are similar to those with time. This investigation shows that the electrochemical processes during oxide formation and reduction may be studied with surface analysis. The related changes may be followed qualitatively and quantitatively by combining electrochemical preparation methods and surface analytical methods. Thus the application of both disciplines give a much better insight into the structure and the chemical properties and reactivity of passive layers. Not
Passive layers on Cu/Ni alloys-Part
I
851
many systems have been studied in detail with surface analytical methods with a wellcharacterized electrochemical specimen preparation with a systematic variation of the relevant parameters. Investigations of a similar kind should be performed with other binary and later with more complex alloys to learn not only about the stationary composition of surface films but also about their changes, their formation kinetics and their reactivity. This will help to improve the interpretation of electrochemical studies and will provide a better understanding of the passivating properties of thin oxide films on alloy surfaces. Acknowledgemenfs-The support of this work by the Deutsche Forschungsgemeinschaft gratefully acknowledged.
Project Str 200/S is
REFERENCES 1 2. 3. 4. 5. 6. 7. 8. 9. IO. 11. 12. 13. 14. 15. 16. 17. 18. 19.
D. E. Dobb, J. P. Storvick and G. K. Pagenkopf, Corros. Sci. 26,525 (1986). F. P. Ijsseling, L. J. P. Drolenga and B. H. Kolster, Br. Corros. J. 17, 162 (1982). D. D. MacDonald, B. C. Syrett and S. S. Wing, Corrosion 34, 289 (1978). K. D. Etird, Corrosion 31, 77 (1975). H. P. Dhar, R. E. White, G. Burnell, L. R. Comwell, R. B. Griffin and R. Darby, Corrosion 41, 317 (1985). N. S. McIntyre, T. E. Rummery, M. G. Cook and D. Owen, J. Electrocbem. Sot. 123, 1164 (1976). J. H. Long, J. C. W. Frazer and E. Ott, .I. Am. Chem. Sot. 56, 1101 (1934). J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Cutal. 24, 283 (1972). S. Haupt, C. Calinski, U. Collisi, H.-W. Hoppe, H. D. Speckmann and H.-H. Strehblow, Surf. Inrerf. Anal. 9, 357 (1986). S. Haupt, U. Collisi, H. D. Speckmann and H.-H. Strehblow, J. Electroanal. Chem. 194, 179 (1987).
P. Druska and H.-H. Strehblow, Surf Interf. Anal. 23, 440 (1995). H.-W. Hoppe and H.-H. Strehblow, Surf. Interf. Anal. 14, 121 (1989). H.-W. Hoppe and H.-H. Strehblow, Corros. Sci. 31, 167 (1990). D. A. Shirley, Phys. Rev. B 5, 4709 (1972). H. D. Speckmann, S. Haupt and H.-H. Strehblow, Surf. fnterf. Anal. 11, 148 (1988). H.-W. Hoppe and H.-H. Strehblow, Surf. Interf Anal. 16, 271 (1990). S. Haupt and H.-H. Strehblow, Langmuir 3, 873 (1987). S. Haupt and H.-H. Strehblow, Corros. Sci. 20, 163 (1989). H. H. Strehblow and P. Borthen, Proc. 7th Int. Symp. on Passivity, Materials Science Forum, (ed. K. E. Heusler), Vol. 185-188, p. 301. Trans Tech Publications, Switzerland (1995).
PII: SOOlO-938X(96)00170-0
A SURFACE ANALYTICAL EXAMINATION OF PASSIVE LAYERS ON Cu/Ni ALLOYS: PART I. ALKALINE SOLUTION P. DRUSKA,”
H.-H. STREHBLOW”*
and S. GOLLEDGEb
“Heinrich-Heine-Universitlt Dusseldorf, D-40225 Diisseldorf, Germany bUniversity of Washington, Seattle, WA 98195-1750, U.S.A. electrochemical formation of passive layers on Cu-20Ni, Cu-5ONi and Ni-2OCu in various electrolytes has been examined with X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). In this paper the results of investigations in alkaline solution (1 .ON NaOH) are discussed. On the surface of Cu/Ni alloys first an Ni(OH)* layer is formed. Potential and time resolved measurements yield similar results for the structure of passive layers. At short times and low potentials the rapidly growing Ni(OH)2 leads to an accumulation of Cu at the metal surface which causes the formation of an oxide layer underneath, consisting mainly of CuO. At more positive potentials NiO enters the oxide sublayer. CuzO appears to a minor extent at the metal/oxide interface during the initial stages of CuO formation and is no longer detected when CuO grows to a larger thickness at sufficiently positive potentials. In the potential range of the beginning of transpassive behaviour the top Ni(OH)z layer is oxidized to NiOOH. Parallel to the NiOOH formation Cu(OH)2 is incorporated into the hydroxide overlayer. This transpassive oxidation causes a shift of the work function to larger values as determined by UPS and to the same shift of all XPS signals of oxide constituents to smaller binding energies similar to previous observations for pure Ni and Fe/Ni alloys. The passive layer on Cu/Ni alloys reflects the properties of both metal components. Copyright 0 1996 Elsevier Science Ltd. Abstract-The
INTRODUCTION Cu/Ni alloys are highly resistant to corrosion in aggressive aqueous electrolytes at normal and elevated temperatures. They are used for heat exchangers and for parts exposed to sea water, e.g. in the ship building industry, because of their resistance to localized corrosion. Various publications deal with their corrosion properties in sea water or NaCl-containing electrolytes’-5 and at elevated temperatures.6 These alloys are also widely used as catalysts for hydrogenation and dehydrogenation.738 Although technologically very important, very little is known about the protective passive layers and the films which form in the transpassive range. The chemical composition and structure of these layers have therefore been systematically examined. In this paper results of films formed in 1 M NaOH are presented. Studies in acidic electrolytes will be published in a following paper. Figure 1 depicts the potentiodynamic polarization curves in 1 M NaOH of all investigated alloys for a better understanding of the surface analytical examinations. They show the main characteristics of pure Ni with a small anodic peak at E = - 0.5V indicating the passivation of this alloy component and a peak at E = + 0.7 V which is characteristic for its transpassive behaviour. In the range E = 0.0-0.25 V the anodic peaks of Cu related to the formation of Cu(1) oxide and Cu(I1) oxide are detected. The size of these anodic peaks varies *To whom correspondence should be addressed. Manuscript received 11 September 1995. 835
836
P. Druska.
p
H.-H. Strehblow
and S. Golledge
1
500 mA/cm 2 ‘/
I* ,,._,-_.____-~_______ _ _______~_~
Cu20Ni ,~~
.._Jj -._- ._ ._, ,.i __~ ._ ,,__
i
i:
i
with copper content. thus giving ;I first indication ot’ the alloys characteristics determined by the specific properties of their constituents.
EXPERIMENTAL
Specinzen
which are
METHOD
preparcition
The surface analytical studies were performed with an Escalab 200 X (Fisons/VG Instruments) equipped with an analyser and preparation chamber, a fast entry lock and an attached electrochemical chamber containing a small electrolyte vessel of about 2 cm3 volume. All electrolytes were Ar purged and introduced via appropriate channels. The electrochemical chamber can be filled with purified Ar and evacuated. Thus a specimen preparation and transfer without any air contact was realized. Appropriate electrical channels and an electrolytic contact to a reference electrode permitted potentiostatic polarization. A detailed description of the UHV system its performance and the preparation of Cu/Ni alloys for surface analytical investigations is given in detail elsewhere.9~” The CuiNi alloys were melted from pure metals. mechanically treated and heat-treated to obtain void-free single-phase materials. Three alloys with 20, 50 and 80 wt% Ni (Cup 20Ni, CupSONi, Ni-2OCu) were studied. The surface was polished with 1 pm diamond spray, ultrasonically cleaned in ethanol. introduced into the spectrometer, and finally
Passive layers on Cu/Ni alloys-Part
I
837
sputtered with argon. Subsequently the sample was transferred to the electrochemical chamber and contacted with its circular front plane to the electrolyte surface. The selected electrode potential was applied for the passivation time of choice. After passivation the surface was rinsed with pure water, remaining adherent water was blown off with a jet of argon and the specimen was transferred to the UHV of the analyser chamber. The solution was prepared with analytically pure NaOH and deionized water (Millipore water purification system). All potentials were measured relative to a Hg/HgO/l M NaOH electrode (E = 0.14 V) and are given relative to the standard hydrogen electrode (SHE). Surface analysis
XPS analysis was performed with non-monochromatized Al Kcc radiation (1486.6 eV) with an input power of 300 W and a constant pass energy of 20 eV for the spherical sector energy analyser (Marc II, Fisons/VG Instruments). For ISS depth profiles the ion source (EX05, Fisons/VG Instruments) was run with a Ne beam of 3 keV primary energy. For ISS spectra 30 nA were applied to 14.85 mm2 surface area (2. lo- l5 A/m2) and for sputtering 300 nA to 75.19 mm2 (4. lo-l5 A/m’). Thus the crater effect of depth profiles was avoided. The energy analyser was run with 200 eV pass energy. Details about the data acquisition and evaluation are described elsewhere.’ ’ UPS measurements were performed with the He I line (E = 21.22 eV) of a UV source (UVS 10/35, Leybold Heraeus) with a discharge current of 90 mA, a pass energy of 3 eV and a negative specimen bias of 10 V. This bias ensures the total acceptance of the secondary electrons. The energy difference AE of the UP spectrum between the secondary cut-off and the Fermi edge yields the work function e4 according to the relationship AE = hv - e4. Data evaluation
The XPS spectra were evaluated on the basis of well-characterized standards. The preparation of Ni and Cu standards has been described in detail elsewhere.“~i3 The XPS spectra of the different standards are the basis for further evaluations. After background correction according to Shirley14 the standard spectra are described with Gauss/Lorenzians with a tail function to take care of the asymmetry of the XPS signals. For the evaluation of the nickel species the 2P,,, and for oxygen the 1S peaks were used. For the evaluation of the contribution of the various Cu species the Cu XPS signals required a special procedure. Cu metal and Cu(1) species cannot be distinguished by the Cu 2P3,2 signal because of a negligibly small chemical shift. For this purpose the X-ray induced Cu L3MM Auger signal (XAS) was used. This signal has been taken previously for quantitative determination of Cu20 on a Cu metal substrate.’ 5 Peaks were deconvoluted into the contributions of the different species on the basis of the standard spectra. For this purpose the characteristic data of the standard signals, especially the relative sizes of their peaks were kept constant and their total size was varied to meet the actual signal. The integrated intensities of these partial signals of the components were used for further evaluation. A similar evaluation was applied to ISS spectra. The width of the signal is determined by the isotopic composition of the probe gas Ne as well as the target atoms and the relative abundance of these isotopes. Thus the signal of pure standards was composed for each isotopic combination by a simple Gaussian with a minor Lorenzian contribution. The evaluation procedures of the XPS and ISS spectra are described in more detail elsewhere.’ ’
838
P. Druska.
H.-H. Strehblow
EXPERIMENTAL
RESULTS
and S. Golledge
AND DISCUSSION
The results in this paper are based on the layer model presented elsewhere.” In general the passive layer does not have a simple structure. Therefore angular dependent XPS measurements were performed, to serve as a guide in developing a model. On the basis of these examinations one gets to a layer structure as depicted in Fig. 2.” The inner oxide is covered by an outer hydroxide. The lower valent species are in the inner part of each sublayer. This leads to a further subdivision of both the oxide and the hydroxide. The higher valent species appear only at sufficiently positive potentials within the film. The contributions of the different sublayers to the total film thickness depending on the electrode potential are given in the upper part of Figs 4. 8 and 9 for the alloys Cu-20Ni, Cu-5ONi and Ni-20Cu. The potential
dependence of’luyer cornposition
Figure 3 presents the XP spectra of the Cu-5ONi alloy for various passivation potentials. With increasing potential the Cu and Ni metal signals are decreasing and the contributions of signals at higher binding energies are growing due to the formation of oxide and hydroxide. The 0 1s signal at 531 eV binding energy increases due to the formation of a hydroxide layer at low potentials and short passivation times. The formation of a peak at 529 eV electrode potentials E > 0.04 V (SHE) indicates the formation of oxide. The broad tail at higher binding energies, especially for low passivation potentials, is related to water within the hydroxide layer. The oxidation process of Ni(OH)2 to NiOOH in the vicinity of the transpassive potential range at E > 0.5 V is followed by the chemical shift of all XPS signals of oxide components to smaller binding energies. A related observation is the increase of the work function by 0.5 eV obtained from UP spectra related to an appropriate shift of the secondary cut-off of the photoelectrons. Similar observations have been described for pure Ni and Fe/Ni alloys and have been explained with a simple semiconductor mode1.‘2~‘-7~‘h According to this model of the passive layer the increase of the band bending with the electrode potential leads eventually to a crossing of the valence band with the Fermi level. This situation introduces a large number of higher valent states close to the valence band edge which act as p-dopants leading finally to a decrease of the Fermi level. As the binding energies and the work function are given relative to the Fermi level this leads to a shift of both quantities by the same amount but with opposite sign. The formation of NiOOH. i.e. Ni(II1) ions in the transpassive potential range is an alternative more chemical picture to the accumulation of positive holes or of acceptor levels. Figure 7 depicts the increase of the work function by 0.5 eV as obtained from the UP spectra. A d4 Ni(OH)2
cue
NW+),
_
.~ NiO _ ..--
Cu/Ni-Bulk
-0.46 V Fig. 2.
Cu(OH), d3
cue
d2 NiO
cue dl >
cu,o
cu,o
E=
lC~(Ot-0~1
NiOOH
Cu/Ni-Bulk
+0.44
v
hydroxide layer
oxide layer
Cu/Ni-Bulk
+0.99v
The passive layer model of Cu. Ni alloys for three characteristic
potentials
Passive layers on Cu/Ni alloys-Part
I
839
cu 2P&z 4 t1.ll4av
JJ----k to.74ov JJ-I tO.34OV Ik_/--k
t004OV L Q460v
J..-._ A
_JL D
565.0
570.0
575.0
bindingenergy/ eV
580.0
585.0
925.0
930.0 935.0 940.0
086UV
945.0
9!
I.0
tilldingenergy/~V
Fig. 3. XPS and XAS signals of Cu-5ONi passivated in 1 M NaOH for 300 s at six characteristic potentials: (a) Cu L,MM X-ray induced Auger signal; (b) Cu 2Px12XPS signal; (c) Ni 2Pj,z XPS signal; (d) 0 1S XPS signal.
negative bias of 5 V to the specimen ensures the total acceptance of the secondary electrons without any loss which would alter their cut off edge. The energy difference AE between the secondary cut-off edge and the Fermi edge of the metal phase permits the calculation of the work function e4 according to the relationship AE = hv - ec&. Figure 4 depicts the onset of the formation of a Ni(OH)z layer at E = - 0.46 (SHE). At E > - 0.26 V this outer hydroxide is undergrown by an oxide which consists of CuzO/NiO at the metal surface followed by an outer CuO/NiO film. Up to E = 0.06 V this oxide contains only Cu oxides and NiO enters the oxide phase only at more positive potentials. At 0.3 V (SHE) a final thickness of 5.5 nm is reached for the total oxide thickness with no further growth with the potential. The oxidation of the CuzO component of the inner oxide to CuO occurs at E = 0.4 V and of Ni(OH)z to NiOOH is observed at E > 0.6 V which is accompanied by an accumulation of Cu(OH)z within the hydroxide layer. Many details of Fig. 4 are also indicated by the features of the potentiodynamic polarization curve of Cu20Ni of Fig. 1 as the formation of Cu oxides at E > -0.26 V (SHE) and the transpassive oxidation of Ni(OH)* to NiOOH at E > 0.6 V. Although the bulk metal contains only 20% Ni, its concentration is further decreased at the metal surface. This is to be expected, given the preferential incorporation of Ni into the passive layer. To confirm the multi-layer structure of the passive film obtained by XPS measurements
I’
x40
Druska.
H.-H
Strehblow
Ni2Pm
and S. Golledge
01s
.---
v --to.040 4.46oV
Jl.46OV 0.8M)V
0.860v
,~ .___~ 845.0
850.0
~. 855.0
.~~~~~~_ ~_ 860.0
865.0
870.0
525.0
binding energy / eV FIG
I
530.0
535.0
bmding energy
I eV
540.0
t c l,illifiltf ii
IS!5 depth profile4 ~vere performed at characteristic potentials. Figure 5 depicts as an example the change of the 1% signal and its dcconvolution in Cu and Ni contributions during Ne sputtering of a C‘+2ONi specnnen pasaivated in I M NaOH for 300 s at E = 0.89 V. .4t a depth of 0.2 nm a small Cu signal 1s seen In addition to a large Ni signal. Ni is decreasing relative to Cu with depth and ha\ a minimum value at about 7 nm due to its depletion at the metal surface. Figure 6 shows the quantitative evaluation of three depth profiles of specimens passivated at three characteristic potentials together with the XPS data of identically prepared samples evaluated on the basis of the multi-layer structure model. Two main steps are observed. corresponding to the outer Ni(OH)? layer which contains Cu only at sufficiently positive potentials followed b!, the inner oxide where Cu is enriched. The thickness of the oxide increases with the potential. At E = - 0.16 V (SHE) only a shoulder is observed whereas larger steps are observed for higher potentials. The quantitative agreement of the results obtained by the two methods is remarkable. A subdivision of these layers according to the valency of the cations cannot be expected by ISS as this method cannot distinguish between valence states. At the oxide metal interface a slight overshoot indicates the accumulation of the more noble Cu at the metal surface with a slight gradient reaching the bulk composition at about 6 nm depth. The XPS analysis assumes an equally distributed Cu enrichment close to the metal surface over the escape depth of the photoelectrons because this method cannot detect a small gradient over a short distance. Its depth resolution is limited by the escape depth of the photoelectrons and their attenuation by the covering passive film.
Passive layers on Cu/Ni alloys-Part
I
841
6.0 g 5.0 S 4.0 gE 3.0 ‘: 2.0 r - 1.0 0.0
-1.0
4.5
0.0
0.5
1.0
p%n!ME/V
Van
Fig. 4.
v 0.0
a v av
v-v v v-v---qv
The thickness and composition of passive layers on Cu-2ONi passivated in 1 M NaOH for 300 s as a function of the potential.
Passivation of Cu-5ONi yields similar results to Cu-20Ni (Fig. 8). Layer formation starts also at E = -0.5V (SHE) with Ni(OH)z formation which is oxidized at E > 0.5V to NiOOH. At E = 0.8 V this oxidation is complete which is not achieved for the Cu-20Ni alloy. Cu(OH)z incorporation starts at slightly more positive potentials and to a much smaller extent. Oxide formation starts also at E = -0.4V. NiO enters at slightly more negative potentials. With a maximum of 2 nm the oxide layer amounts to only half of the thickness of the passive film of Cu-20Ni. Ni depletion at the metal surface leads to a loss of about 50% of the bulk composition i.e. to about 25% Ni content. The results for Ni-20Cu are approaching those of pure Ni due to the high Ni content (Fig. 9). Layer formation starts again at E = -0.5 V (SHE) and the oxidation of Ni(OH)2 to NiOOH occurs in the range 0.4-0.9 V. The oxide layer thickness increases more linearly
842
P. Druska,
H.-H. Strehblow
1200
loo0
1400
and S. Golledge
1600
1800
2000
0.8 0.4 0.0 0.8 0.4 0.0
4nm
0.8 0.4
t
0.8
0.8 0.4 0.0
kinetic energy I eV Fig. 5.
ISS signals
of Cu-2ONipassivated sputter
in 1 M NaOH for 300 sat 0.89 V after seven characteristic times of the depth profile depicted in Fig. 6.
Passive layers on Cu/Ni alloys-Part
I
843
0
0
2
1
4
3
5
sputterdepthd I nm 1.0
I
I
I 0
0.8 3
.= 0.4 8
o”
0.2 0.0
:
a+&&
I
__. _,. _ ,._. _.
.
+0.44 v /
,
4
2
0
,
8
6
sputterd@d/m
0.0
0
”
I”’
2
s’
4
’
“I’
8
”
‘I”
8
”
10
sputterdeplhdlm Fig. 6. ISS depth profile (0) and layer structure according to the quantitative evaluation of XPS data (-) of Cu-2ONi passivated in 1 M NaOH for 300 s at three characteristic potentials.
with the potential in comparison with Cu-rich alloys. This result is closer to that for pure Ni where only NiO changes linearly with the potential whereas the thickness of Ni(OH)2 remains unchanged. The hydroxide layer contains only negligibly small Cu contributions and the Cu concentration of the oxide layer decreases in the range 0.0-l .OV (SHE) to very small values leaving mainly NiO. Also for the N&rich alloy Cu is accumulated at the metal surface. The time dependence of layer composition
The results of time-dependent investigations are similar to those in the potential domain. It is a general observation that lower valent species appear first and will be oxidized later as has been shown for the case of passive Fe and Fe/Cr alloys.173’8Figures 10 and 11 give an
P. Druska.
844
H.-H. Strehblow
and S. Golledge
0.6
0, 0 0
4 ‘; z a”
0.3
-
0.2
i
t i
0
0.1
0.0
c
l--o--o..._~
_o,.
_
-1.5
-1.0
.~
,’
2 ._1
in
0.0
-0.5
0.5
1.0
1.5
potential E/V Fig. 7.
The change
of the work function
deduced
from LJP spectra vs. the passivation
potential
E.
example for Cu-20Ni at two characteristic potentials. one each in the passive and in the transpassive range. The total thickness increases linearly with the logarithm of time indicating the presence of a barrier type of film. The hydroxide layer forms first and is already present within 1 ms. It takes about lo-100 ms to form the first oxide. Cu(1) and Cu(I1) oxides are observed simultaneously within the oxide layer at E = 0.44 V (SHE) (Fig. 10). At E = 0.89 V (SHE) Cu(1) oxide appears only for a short time to a very low level and is oxidized completely to Cu(I1) (Fig. 11). Ni enters the oxide within about 0.1 s at both potentials. At 0.44 V (SHE) the hydroxide consists of Ni(OH)2. At E = 0.89 V Cu(OH)2 enters within 0. l-l s (Fig. 11) and reaches within 10 s the level of Ni(OH)2. The formation of Ni(OH)l within the first 0.1 s and its disappearance within about 100 s are very characteristic. Simultaneously the NiOOH-containing film is formed which incorporates large amounts of Cu(OH)2 up to an equal content of both hydroxides. At E = 0.44 V (SHE) no NiOOH is formed. The metal surface is enriched in Cu which is a consequence of a preferential incorporation of Ni into the passive layer. The results for Cu-5ONi are similar. The lower Cu content prevents its incorporation into the hydroxide layer and only Ni*+ is transferred into the oxide part. In the transpassive range Ni(OH)2 forms within 1 ms to its maximum thickness and is then continuously oxidized to NiOOH which is related to the higher Ni content. The reduction of the passive layer
The reduction of the preformed passive layers at decreasing potentials reverses the oxide formation with electrode potential (Fig. 12). NiOOH is reduced to Ni(OH)* in the range E = 0.9-0.3 V (SHE). Cu20 appears below E = 0.6 V and at E = - 0.7 V most of the oxide has disappeared. The polarization curves of Fig. 1 show a reduction peak in the range E = 0.8-0.5 V where the NiOOH to Ni(OH)z transition occurs. The reduction of Cu oxides and the related disappearance of the oxide layer coincides with the cathodic currents at
Passive layers on Cu/Ni alloys-Part
I
845
7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1 I.0
4.5
0.5
0.0
1.0
potential E / V -1.o il.5 r"","', 0.6 _ tulkauriace
0.5 0.0 I"","",",
1.0
0.2 -
_.,.n__O.O o,o..,o,.. I
i, -1.0
45
0.0
0.5
potential E /V
&
j7 Fig. 8.
oxygen
1
The thickness and composition of passive layers on Cu-SONi passivated in 1 M NaOH for 300 s as a function of the potential.
E < 0.4V (SHE). The lack of a pronounced potential range where this reaction occurs.
second reduction peak agrees with the larger
Model
The formation of the passive layer and the development of its structure with potential and time is the result of a combination of thermodynamic and kinetic factors. At low potentials and short times of potentiostatic passivation the less noble Ni is oxidized and forms Ni(OH)z. According to the Pourbaix diagram this reaction is expected at E < - 0.706 V which agrees reasonably well with the onset of its formation at E = - 0.6 V according to the XPS analysis. The shift of this onset to E = -0.5V with decreasing Ni
846
P. Druska. H.-H. Strebblow and S. Goiledge
-1.0
-0.5
0.0
0.5
1.0
1.5
potent&E/V
hydmxfde lay
0.8;
_
0.6 1
’ ?Ov._c
0.4
[
***ea. 0
55 LgS,___
c
,
CT7VO
•O~O~~*~~-O~*O i
0.2 ,-1.0
-0.5
0.0 0.5 poterh!E/V
1.0
: oxygen Fig. 9.
1.5
-.. 3 cqp?r 0 nickel
j
The thickness and composition of passwe layers on Ni-2OCu passivated BOOs as a function
in
I M NaOH for
of the potential.
within the alloy should be explained with the decrease of the related Ni activity. The more noble Cu remains at the metal surface underneath and is accumulated with the growth of the surface film. At E > -0.4 V and E > -0.2 V (SHE) the formation of CuzO and CuO, and Cu(OH)?, respectively is expected. Indeed, the continuous growth of an oxide layer is found for E > - 0.2 V for all three alloy compositions, which contains mainly Cu(I1) (Figs 4. 8, and 9). This result is found by XPS analysis and agrees also with the anodic features of the polarization curves at E = - 0.2 V. A Cu(1) containing sublayer is detected at E z -0.2 V and remains underneath up to at least 0.8 V which is in agreement with thermodynamic arguments that the lower valent oxide with the more negative Gibbs free energy of formation is in direct contact with the metal. Its oxidation to Cu(I1) is content
Passive layers on Cu/Ni alloys-Part
I
847
6.0
timetls
0.2
l
--.
1oJ JO4 10”
* IO”
m loo
.
0 . .
1 i
10' IO2 lo3 IO4
timet/s
0 CopPer I nickel / v oxygen j I----l
Fig. 10. The time dependence of the thickness and composition of the passive layer of Cu-2ONi passivated in 1 M NaOH for 300 s at E = 0.44 V:
found by XPS and the anodic peak at E = 0.2 V (Figs 1,4, 8, 9) in the polarization curve. At E > 0.5V the formation of NiOOH is expected in agreement with the XPS results and the anodic peak at E = 0.7V in the polarization curve. The reduction of NiOOH at 0.7 V and of Cu(I1) at E = -0.3V and Cu(1) at E = -0.5V are in reasonable agreement with the thermodynamic data. In conclusion, the formation of the various species and the chemical multilayer structure of the passive layer can be interpreted qualitatively on the basis of the thermodynamic data for the pure metals and their oxides and hydroxides, i.e. the related Pourbaix diagrams, although the influence of the composition of the metal and the layers and the related activities of their components cannot be ignored. The quantitative data on the composition and thickness of the sublayers, however,
848
P. Druska.
H.-H. Strehblow
and S. Golledge
,
bulk&ace
i
1 0.6 I 0.4 i 0.8
a l
0.2 : __~
a
_ _____+
10 -3 10
-2 IO”
10°
timetis
IO’ lo2 lo3 ~~~_____ ‘2 copper l nickel
’
. Oxygen .L FIN.
1I. The time dependence
of the thickness and composmon of the passive layer on Cu-?ONi passivated in 1 M NaOH for 300 s at E = 0.89 V.
involves also the influence of the kinetics as the transfer rates at the phase boundaries and the transfer through the sublayers by migration and diffusion. The solubility of the oxides with each other and the formation of mixed oxides or compound oxides and hydroxides is another parameter. All these details are not sufficiently known and further studies are required to determine the atomic, besides the chemical, structure which requires, however, sophisticated analytical methods. The layers are only a few nanometres thick and in most cases relatively disordered, so that conventional diffraction methods will fail. Therefore diffraction or EXAFS studies with high intensity X-ray sources of storage rings are required. First results have been obtained.” A detailed knowledge about the structure and defect structure may give information on the transport properties of the various species across the passive film and its sublayers.
Passive layers on Cu/Ni alloys-Part
I
849
6.0 5.0 E ; 8
4.0 -
2 2
3.0 L
*
2.0 1.0 0.0 -
tI,...I,,,.I....I.,,.I 1.0
0.5
0.0
-0.5
-1.0
pcWME/V
0.4 1 0.2
1
.--:---..r _*
0.4 1 0.2
_
d ,.......d.: _~ l c ., * 1.0 0.5
..$I .._ l ... .*
,... ---.
--.- 0... .* ---0-4 c*- 0,..n:.....+O. .._ n -0.5 -1.0 0.0
@mtialE/V
Fig. 12. The thickness and composition of passive layer on Cu-2ONi passivated in 1 M NaOH for 300 s at 0.89 V and subsequently reduced gradually for 300 s at potentials down to E = -0.76 V.
For a rather complicated system of two metallic components which are present even in various oxidation states within the film one may give only a qualitative model for the formation of the passive layer as depicted in Fig. 13. Film formation starts with a Ni(OH)* layer leaving behind at the metal surface a Cu enriched zone. At E > - 0.1V (SHE) the Cu enrichment forces also Cu ions to enter the passive layer forming a duplex oxide layer with an inner CuzO and an outer CuO part. The consumption of Cu at the metal surface allows
850
P. Druska.
Fig. 13.
H.-H. Strehblow
and S. Golledge
A model for the development of a multilayer structure of passive films on Cu-Ni during anodic oxidation at sufficiently positive potentials.
alloys
Ni to enter the oxide layer as well. Cu(1) is oxidized to Cu(II) after about 1 s at the oxide surface and is transferred to the hydroxide overlayer. Finally the outer hydroxide is oxidized at E > 0.6 V to NiOOH which may be reduced again to Ni(OH)2 for E < 0.6 V (SHE). Deviation of these results may be obtained in acidic electrolytes. Passive layers on pure Cu are not stable and therefore not protecting for these conditions. Large current densities especially for high Cu content of the alloys indicates that a high Ni content of the passive film is essential for its protecting properties. Thus the layer should contain less Cu ions unless they are trapped in a Ni oxide matrix. CONCLUSION Oxide formation in 1 M NaOH on Cu/Ni alloys reflects the specific properties of both elements. At negative potentials or short times of passivation Ni(OH)2 is formed first. This process leads to an increase of Cu at the metal surface, which causes finally the formation of an oxide layer consisting mainly of Cu oxides. This in turn permits the incorporation of NiO into the already existing inner oxide layer. The depletion of Ni at the metal surface remains independent of the potential and time for the later stages of oxidation. Cu forms first CuzO. At sufficiently long times and positive potentials of passivation CuzO is oxidized completely to CUO/CU(OH)~. This behaviour is different from pure Cu where a duplex Cu2O/CuO, Cu(OH)* structure is observed up to the potentials of the start of transpassive behaviour. Ni(OH)l is oxidized to NiOOH as has been found for pure Ni and other Ni alloys. This oxidation is facilitated with increasing Ni content within the alloy. Also Cu(OH)* is incorporated into the hydroxide overlayer. For E > 0.6 V and Cu-20Ni two-thirds of Ni(OH)z is oxidized whereas for Cu-5ONi and Ni-20Cu the total amount has been changed to NiOOH. Oxide reduction occurs with a sequence opposite to its formation. The changes with potential are similar to those with time. This investigation shows that the electrochemical processes during oxide formation and reduction may be studied with surface analysis. The related changes may be followed qualitatively and quantitatively by combining electrochemical preparation methods and surface analytical methods. Thus the application of both disciplines give a much better insight into the structure and the chemical properties and reactivity of passive layers. Not
Passive layers on Cu/Ni alloys-Part
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many systems have been studied in detail with surface analytical methods with a wellcharacterized electrochemical specimen preparation with a systematic variation of the relevant parameters. Investigations of a similar kind should be performed with other binary and later with more complex alloys to learn not only about the stationary composition of surface films but also about their changes, their formation kinetics and their reactivity. This will help to improve the interpretation of electrochemical studies and will provide a better understanding of the passivating properties of thin oxide films on alloy surfaces. Acknowledgemenfs-The support of this work by the Deutsche Forschungsgemeinschaft gratefully acknowledged.
Project Str 200/S is
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