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Studies of the influence of sulphur on the passivation of nickel by Auger electron spectroscopy and electron spectroscopy for chemical analysis
Materials Science and Engineering, 42 (1980) 191 - 197
191
© Elsevier Sequoia S.A., Lausanne --Printed in the Netherlands
S t u d i e s o f t h e I n f l u e n c e of Sulphur on the Passivation of Nickel by A u g e r E l e c t r o n Spectroscopy and E l e c t r o n S p e c t r o s c o p y for Chemical Analysis*
P. MARCUS and J. OUDAR Ecole Nationale Supdrieure de Chimie de Paris, Universitd Pierre et Marie Curie, Paris (France) I. OLEFJORD Chalmers University of Technology, S-412 96 G6teborg (Sweden)
SUMMARY
During the anodic polarization o f sulphurdoped nickel, selective dissolution o f nickel and surface segregation o f sulphur are observed. These p h e n o m e n a cause a complete inhibition o f passive layer formation when the sulphur content is above a critical concentration. Auger electron spectroscopy and electron spectroscopy for chemical analysis were used to characterize the species f o r m e d on the surface during the sulphur enrichment. The spectra were interpreted by comparison with standard spectra from synthesized compounds: NiS, NizS2, Ni-adsorbed S. A comparison with previous results using radioactive sulphur (35S) allowed us to determine the mechanism o f the sulphur enrichment and o f the consequent inhibition o f passivation by the formation o f an adsorbed sulphur layer and subsequent growth o f a nickel sulphide Ni3S 2 which attains a stationary thickness o f about 30A. 1. INTRODUCTION It is well known t h a t sulphur can change the passivation conditions and can increase the rate of dissolution of metals such as nickel and iron [ 1]. However, until recently the mechanism of this effect was n o t well understood. It has been suggested that the adsorption of sulphur plays a decisive role in this p h e n o m e n o n . Recently straightforward evidence of this role has been given by Oudar and Marcus [2] using a 35S radiotracer and conventional current-potential plots. The *Presented at the International Chalmers Symposium on Surface Problems in Materials Science and Technology, GSteborg, Sweden, June 11 - 13, 1979.
major conclusions of this work concerning the influence that sulphur has on nickel passivation in an acidic solution can be summarized as follows. (1) The adsorbed sulphur increases the rate of dissolution of the metal in the active region of the anodic curve. (2) A total inhibition of the passivation is observed when a critical sulphur concentration exists on the surface at the potential that corresponds normally to the active-to-passive transition of nickel w i t h o u t sulphur. This critical surface concentration is of the o r d e r of one monolayer. The inhibition has b e e n attributed to a blocking effect by sulphur of the sites required for the adsorption of OHanions, the precursor of passive film formation (NiO). (3) This critical surface concentration of sulphur can be obtained on Ni-S alloys by surface segregation due to the selective dissolution of the metal. The corresponding bulk concentration of sulphur has been measured. Depending on the crystallographic orientation of the surface this concentration ranges from 30 × 10 -4 to 50 × 10 -4 wt.% at a potential sweeping rate of 1 V h -1. (4) When the total inhibition of passivation occurs, the metal dissolution takes place through a non-protective film with a remarkably high current density. This film, which will be shown to be the nickel sulphide NisS2, attains a stationary thickness of about t e n monolayers of sulphur (as measured by the radiochemical technique). (5) The same general features are observed on pure nickel in a sulphur-contaminated electrolyte. For Ni-S alloys the mechanism of the inhibition of passivation by sulphur can be illustrated by the model shown in Fig. 1.
192
t
t
t
*
l/ 00
/ S
Ni2+
x i 2+
t
t
s,,,~,.i~
adsorbed
L.t.t.I.d./
dissolution
Nickel Sulphide Ni2+
/
~o
•
•
•
0Q
s t ' g rt~g~t t i Oll o n tilt, s u r f act, blocking effect by s u l p h u r o f tilt, adsorption sitcs f o r OH . I n h i b i t i o n of the passive film formation
S in solid solution
sulphur ,.nr ichmcnt . Formation of nonprt, tcct ire sulphide, film
;i
Fig. 1. A model illustrating the mechanism of the inhibition of passivation by sulphur. The anodic polarization curves of pure nickel, of nickel with a low sulphur c o n t e n t and of Ni-S alloys with a sulphur content above the critical content required for the inhibition of passivation are shown in Fig. 2. The aim of this work was to analyse by Auger electron spectroscopy (AES) and electron spectroscopy for chemical analysis (ESCA) the surface composition of Ni-S alloys after anodic polarization. In this way we attempted to determine the exact nature of the surface film formed at various stages of the anodic metal dissolution in order to confirm previous results obtained from radiochemical analysis and to collect additional information on the phenomenon. AES has been widely used to investigate the metal-gas interface. Considerable knowledge has been obtained by Benard and coworkers [3 - 5] on the adsorption of sulphur
t -¢ .N
|
s/S/SSS .40
//- b /s
.20 /
0 O.S
I I
! 1.5
E V/SHE Fig. 2. Anodic polarization curves of nickel and of Ni-S alloys (0.1 N H2SO4; 1 V h - l ) : curve a, pure (100) Ni; curve b, (100) Ni-S alloy with 0.006 wt.% S; curve c, polycrystalline Ni-S alloy with 0.006 wt.% S.
on several metal surfaces. Using a 35S radiotracer, Auger signals have been calibrated for the measurement of sulphur coverage from zero to saturation [6]. ESCA has proved very fruitful in analysing passive films formed on metals and alloys in aqueous media [7 - 9]. Since anodic layers are sensitive to air exposure, the use of surface-sensitive techniques such as AES and ESCA requires a transfer of the electrode under a protective gas from the electrolytic cell to the spectrometer. We have previously investigated by ESCA the passive film formed on pure nickel [ 10]. Thus we can compare the anodic layers formed on nickel with and w i t h o u t sulphur. 2. EXPERIMENTAL
2.1. Sample preparation The nickel samples (single crystals and polycrystals 99.999% pure) were polished mechanically and electrochemically. They were annealed in purified H2 at 800 °C. To prepare the Ni-S alloys, clean nickel samples were treated at a high temperature in an H2-H2S mixture of appropriate composition. The experimental conditions werePH:s/PH2 = 6.4 × 10 -4, T = 1130 °C. The samples were air quenched to room temperature. Under these conditions the sulphur content is 0.006 wt.% [11]. The sulphur concentration was controlled by using a SSs radiotracer. This technique enabled us to measure the a m o u n t of sulphur in the bulk and on the surface before and after electrochemical treatments. The Ni-S alloys obtained are homogeneous solid solutions. No sulphide is precipitated within the metallic matrix [ 2]. The NisS 2 samples used for calibration were prepared from clean nickel treated in H2-H2S (PH2s/PH2 = 1.02 × 10 -s) at 380 °C.
193 The adsorbed sulphur layer on (100) Ni used for calibration was prepared by thermal segregation. A sulphur
10 -9 Torr. The sample could be returned to the electrolytic cell under a protective gas in the same way. For the ESCA study the electrolytic cell was directly attached to the spectrometer by a straight-through valve, and an internal handling system allowed the sample to be moved from the cell to the analysis chamber. This equipment has been described elsewhere [12, 13]. The surface analyses were performed in a conventional retarding field analyser Auger spectrometer and in an ESCA-HP 5950 A instrument, with monochromatized A1 Ks radiation.
3. RESULTS 3.1. Sulphur segregation on the surface during anodic polarization Figures 3(b) and 3(c) show ESCA spectra obtained from Ni-S alloys polarized to 0.45 and 0.8 V/SHE. For comparison the spectra recorded from pure nickel passivated at 0.54 V/SHE are also shown (Fig. 3(a)). 3.1.1. Polarization to 0.45 V/SHE The S 2p region exhibits a peak at a binding energy of 162 eV. Assignment of this peak to sulphate which originates from the electrolyte is excluded since sulphur in SO42- produces a peak at a much higher binding energy (about 169 eV). Indeed a sulphur peak at 169 eV is visible in the spectrum of the passive film (Fig. 3(a)). This peak has been shown to be associated with contamination by SO42[ 10]. In addition sulphur cannot be detected on the initial homogeneous Ni-S alloy. Therefore this S 2p peak at 162 eV must originate from sulphur enriched on the surface by anodic dissolution of nickel. This indicates that there is a selective dissolution of nickel and a sulphur segregation on the surface. It should be noted that 0.42 V/SHE corresponds to t h e maximu.m in the polarization curve of pure (100} Ni (Fig. 2). Therefore at the usual passivation potential of pure nickel, sulphur is already enriched on the surface of the Ni-S electrode. The coverage by sulphur will be shown later to be of the order of a monolayer. The Ni 2Psi2 spectrum exhibits a main peak at 852.8 eV, i.e. at the same binding energy as nickel in its metallic state, and the satellite at 858.6 eV is visible. A low signal is also detected
194
Ni2p. I]Nil100)o,,I~s|iwtediIfit *0.S4c,,V A (a)
(SHE)
I
il f
1
s2p ,
~ ,
:
• xcl, 6
----4 - / NI*S alloy polarix*d to *O.t~VISHEI
The Auger spectrum obtained after polarization of the Ni-S alloy to 0.8 V/SHE is shown in Fig. 4. As well as the nickel peak at 62 V, a sulphur peak at 150 V and a carbon peak at 275 V are seen. This Auger analysis demonstrates, in agreement with 35S measurements [2] and with the ESCA results already discussed, that nickel is selectively dissolved and that sulphur is enriched on the surface.
(b) Ni-S OIloy polorized to +0.8 V (SHE)
860 8SS
(c)
S~ 530 290 28S BINDING ENERGY {eV)
1"/0 165 160
Fig. 3. (a) ESCA spectra of pure (100) Ni passivated at 0.54 V/SHE (from ref. 10) and (b), (c) ESCA spectra of Ni-S alloys (0.006 wt.% S) polarized to (b) 0.45 V/SHE and (c) 0.8 V/SHE (0.1 N H2SO4; 1 V h-l).
at 856.4 eV. This peak has been shown to be associated with nickel in Ni(OH)2. The oxygen spectrum exhibits a peak at 531.6 eV and a shoulder at higher binding energy. The former is assigned to oxygen in O H - ions [10] and hence originates from the same species as the nickel peak at 856.4 eV. The shoulder probably corresponds to contamination by CO. This would also account for the carbon peak at 285 eV.
3.1.2. Polarization to 0.8 V/SHE First it is recalled that in sharp contrast with pure nickel, which is passivated at this potential, Ni-S alloys do n o t show passivation behaviour. The dissolution current density is greater than 20 mA cm -2 (Fig. 2), while the residual current density for passivated nickel is less than 10 pA c m -2. The ESCA spectra obtained show features similar to those described above. In addition to the main S 2p peak a low signal is detected at 169 eV, assigned to contamination by SO42- as indicated before. The major observation is that the intensity ratio of S 2p to Ni 2p is increased compared with the situation at 0.45 V/SHE. This reveals that the surface has been further enriched with sulphur by additional polarization from 0.45 to 0.8 V/SHE. The dependence of the intensity ratio on polarization potential and the conclusions that follow are given in Section 3.2.
3.2. Evolution of the sulphur enrichment with polarization potential and characterization of the species formed on the surface The extent of the sulphur enrichment was estimated from the intensity ratio of the sulphur and nickel signals. Figure 5 shows the ratio Is 2P//Ni2P as a function of the anodic potential. It is clear that the ratio increases with potential from the dissolution potential to about 0.8 V/SHE and then attains a stationary value (Is 2p/INi 2p ~ 0.03) for higher potentials (up to 1.45 V/SHE). This indicates that the sulphur segregation during anodic dissolution produces a sulphur coverage which increases with potential in the first part of the current-potential curve and is stationary in the second part of the curve. The characterization of the chemical state of sulphur on the surface was carried out by comparison with known species, i.e. with adsorbed sulphur and sulphides. The adsorbed sulphur monolayer was prepared by thermal segregation in a vacuum from sulphur-doped (100) Ni. The
S
Ni
c
E(eV)
Fig. 4. The AES spectrum of Ni-S alloys (0.006 wt.% S) polarized to 0.8 V/SHE (0.1 N H2SO4; 1 V h-X).
195 Ni
2p3n
J~_ NilSz
adllodoed~ll=hur
KrJs
xt,0
Ni3i2
][
[/
~.~o ---'J 862
20 ¢o o
s 2p
,SO /~
.r/
858 854 850 164 BINDING ENERGY(eV)
~
'°°s0 160
Fig. 6. ESCA spectra of Ni3S 2 formed in H2-H2S. 10
r*"
0.5
I
1
J
1.5 E VlSl~
Fig. 5. The intensity ratio of S 2p (of binding energy 162 eV) to Ni 2p (of binding energy 852.8 eV) characterizing sulphur enrichment on the surface during anodic dissolution of sulphur-doped nickel (0.006 wt.% S; 1 V h - l ) . (The arrows indicate the ratios of Is 2p//Ni2p obtained from calibration experiments with adsorbed sulphur (thermal segregation) and bulk Ni3S2.)
sulphur signal from the 2p levels was located at 162 eV. It was found that adsorption of sulphur does n o t cause any shift in the binding energy of Ni 2p3/2 (the main peak is at 852.8 eV and the satellite at 858.6 eV), which indicates a covalent character of the bond. The ratio Is 2p/INi 2p was equal to 0.004. The spectra of Ni3S2 (Fig. 6) exhibit two wellresolved sulphur peaks at 162.2 and 163.2 eV corresponding to S 2Ps/2 and S 2pl/2 respectively. The Ni :2p spectrum shows a peak located at a b o u t the same binding energy as nickel in its metallic state. The possibility of a chemical shift is n o t excluded but further investigation is required before any c o m m e n t can be made. The ratio Is 2p/INi 2p of the intensities was found to be 0.029. The values of the Is 2p/INi 2p ratio obtained from adsorbed sulphur (one monolayer) and from bulk Ni3S2 are given in Fig. 5. It is concluded that the sulphur segregation during anodic polarization produces adsorbed sulphur in the first part of the anodic curve (from the dissolution potential to the potential corresponding to the passivation of pure nickel {0.42 V)). This result is in good agreement with previous observations [2]. (1) From the radiochemical measurements using 35S it was shown that the polarization of Ni-S alloys under the same conditions produces an adsorbed layer of sulphur with a coverage of a b o u t one monolayer.
(2) The calculation of the amount of sulphur freed during the dissolution of sulphurdoped nickel gives the same value, which indicates that in this range of potential the yield of sulphur adsorbed to sulphur made available at the surface is equal to unity. Further enrichm e n t at higher potentials results in the formation of Ni3S2. AES calibration has been performed previously for sulphur adsorption and for the nickel sulphides Ni3S2 and NiS. Experimental details have been reported elsewhere [6, 14]. The ratio of the Auger peak heights of sulphur (150 eV) and nickel (62 eV) obtained on these compounds, as well as the ratio obtained on Ni-S alloy polarized to 0.8 V, are given in Table 1. It is clear that Islr'°ev/INi B2ev for the polarized Ni-8 alloy is almost identical with the ratio obtained with synthetic Ni3S2, whereas NiS can be excluded. Therefore it is concluded, in agreement with the ESCA results already discussed, that the species formed is Ni3S2. The combined use of ESCA and AES allowed us to check whether the sulphide layer is continuous. By ESCA a large area of the sample (5 mm 2) is analysed and the geometrical arrangement of the sample with respect to the TABLE 1 The intensity ratio of low energy Auger signals of sulphur and nickel: a comparison between adsorbed sulphur, bulk sulphides and Ni-S alloy polarized to 0.8 V/SHE
Compound
is150 eV/INi62 eV
Adsorbed sulphur at
0.3
saturation coverage a
NiS a
2.2
NisS2 a Ni-S alloy (0.006 wt.% S) polarized to 0.8 V/SHE (0.1 N H2SO4,~ 1 V h -1)
0.79 0.77
aFrom ref. 14.
196
excitation beam and detector cannot be changed, whereas in the AES system the primary electron beam can be focused on various spots of the sample surface and the sample can be tilted with respect to the electron beam. AES analyses of several areas gave the same ratio of the peak heights of sulphur to nickel within the experimental error ((10%).
3.3. Thickness of the sulphide layer The m e t h o d utilized previously [10] for calculation of the thickness of the passive film on nickel from ESCA data cannot be utilized here because the signals from metallic nickel and from nickel in the sulphide were not differentiated because of the extremely small chemical shift. An alternative method is the estimation of the thickness from the ratio of the intensities of the sulphur and nickel signals by comparison with known compounds. From AES the ratio IslSOeV/INi 62ev w a s already the same as for " b u l k " Ni3S 2 after polarization to 0.8 V/SHE. The term " b u l k " applies to a layer thickness such that the secondary electrons from the substrate cannot escape. By varying the incidence angle we can change the sensitivity to the outermost layer of atoms. For angles ranging from 10 to 40 ° the ratio Is z5° eV//Ni62eV was the same within experimental error (< 10%). The escape depth for low kinetic energy secondary electrons is expected to be low (about 10 h ) and it can be concluded that the thickness of the sulphide layer at 0.8 V is greater than 10 A. With ESCA the ratio of " b u l k " sulphide is obtained only after polarization to greater than 1 V/SHE. This is in reasonable agreement with the Auger result ( " b u l k " sulphide at 0.8 V/SHE) since the escape depth of Ni 2p3/2 photoelectrons (of kinetic energy 650 eV) is much larger. On this basis the film thickness can be estimated at about 30 A. Comparison with a previous result brings straightforward confirmation and a more accurate value: measurements using a 85S radiotracer [2] indicate that the a m o u n t of sulphur on the surface above 1 V/SHE is 38 × 10 -8 g cm -2. Because we know from AES analysis and ESCA that the compound formed is N i 3 S 2 w e can calculate the thickness. Assuming a continuous layer a value of 26 £ is obtained.
3.4. Comparison with the passive film on pure nickel Passivation of nickel has been shown to be characterized in the ESCA spectra [10] by an O ls signal at 529.8 eV and an Ni 2P3/2 signal at 854.4 eV (see Fig. 3(a)). These peaks were assigned to oxygen and nickel in NiO. It is clear that these peaks do n o t exist in the spectra of Ni-S alloys polarized to potentials above the normal passivation potential of pure nickel (see Figs. 3(b) and 3(c)). This gives evidence that the formation of NiO is precluded on Ni-S alloys. In addition the sulphur peak at 162 eV observed in the spectrum of Ni-S after polarization does not exist in the spectrum of the passive film on pure nickel. Therefore the inhibition of passivation revealed by the current-potential curve (Fig. 2) is directly related to the fact t h a t NiO cannot grow on sulphur or sulphidecovered surfaces. 4. DISCUSSION
The current-potential curves revealed that when sulphur is alloyed to nickel above a critical concentration the active-to-passive transition, characteristic of nickel, is n o t observed. The ESCA investigation indicates that NiO, which was shown to be the passivating compound, is n o t formed on Ni-S alloys. Instead, AES and ESCA show that sulphur is enriched on the surface by selective dissolution of the metal. Segregation of sulphur in the first part of the current-potential curve corresponding to the active region of pure nickel produces a nearly complete monolayer of adsorbed sulphur. The presence of this sulphur monolayer at the normal passivation potential for pure nickel precludes the growth of NiO. A peak corresponding to OH- (O ls at 531.6 eV) was detected in the ESCA spectra (Fig. 3). This indicates that OH- ions can still be adsorbed on the sulphur-covered nickel surface but in this case passivity is n o t attained. From the results on pure nickel it has been suggested that two adjacent OH- ions are required to produce a layer of NiO [ 10]. When a complete or nearly complete sulphur monolayer exists on the surface, OHions can still be adsorbed to some extent but the situation with two adjacent OH- ions seems no longer possible. Consequently the inhibition of the passive film formation will
197
be due to a dilution effect by sulphur of the sites for the adsorption of OH- ions, which precludes the recombination of two OH- ions to yield NiO. Comparable dilution effects are often encountered in catalytic reactions on metallic alloys.
5. CONCLUSIONS
The surface analysis of Ni-S alloys after anodic polarization confirms entirely the previous results and hypotheses. In addition the poisoning effect of adsorbed sulphur on the adsorption of OH- anions, the precursor of passive film formation, has been further clarified. Finally the nature and the thickness of the non-protective sulphide film which grows after the sulphur adsorption and precludes the formation of NiO have been determined.
ACKNOWLEDGMENT
Financial support from the D616gation G6n6rale ~ la Recherche Scientifique et Technique is gratefully acknowledged.
REFERENCES 1 J. Oudar and P. Marcus, Appl. Surf. Sci., 3 (1979) 48, refs. 1 - 14. 2 J. Oudar and P. Marcus, Appl. Surf. Sci., 3 (1979) 48. 3 J. Benard, J. Oudar and P. Cabane-Brouty, Surf. Sc£, 3 (1965) 359. 4 J. L. Domange and J. Oudar, Surf. Sci., 11 (1968) 124. 5 M. Perdereau and J. Oudar, Surf. Sci., 20 {1970) 80. 6 M. Perdereau,Surf. Sci., 24 (1971) 239. 7 I. Olefjord and H. Fischmeister, Corros. Sci., 15 (1975) 697. 8 I. Olefjord and B.-O. Elfstr6m, in T. P. Hoar (ed.), Proc. 6th Eur. Congr. on Metallic Corrosion, London, September 1977, Society of Chemical Industry, London, 1977, p. 21. 9 I. Olefjord, in J. Larinkari (ed.), Proc. 8th Scand. Corrosion Congr. Helsinki, September 1978, Vol. 1, Helsinki University of Technology, Laboratory of Corrosion Science and Technology, Helsinki, 1978, p. 349. 10 P. Marcus, J. Oudar and I. Olefjord, J. Microsc. Spectrosc. Electron., 4 (1979) 63. 11 N. Barbouth and J. Oudar, C. R. Acad. Sci., 269 (1969) 1618. 12 I. Olefjord, Met. Sci., 9 (1975) 263. 13 I. Olefjord and N. G. Vannerberg, Proc. 7th Scand. Corrosion Congr., p. 434. 14 M. Perdereau, C. R. Acad. Sci., 274 (1972) 448.
191
© Elsevier Sequoia S.A., Lausanne --Printed in the Netherlands
S t u d i e s o f t h e I n f l u e n c e of Sulphur on the Passivation of Nickel by A u g e r E l e c t r o n Spectroscopy and E l e c t r o n S p e c t r o s c o p y for Chemical Analysis*
P. MARCUS and J. OUDAR Ecole Nationale Supdrieure de Chimie de Paris, Universitd Pierre et Marie Curie, Paris (France) I. OLEFJORD Chalmers University of Technology, S-412 96 G6teborg (Sweden)
SUMMARY
During the anodic polarization o f sulphurdoped nickel, selective dissolution o f nickel and surface segregation o f sulphur are observed. These p h e n o m e n a cause a complete inhibition o f passive layer formation when the sulphur content is above a critical concentration. Auger electron spectroscopy and electron spectroscopy for chemical analysis were used to characterize the species f o r m e d on the surface during the sulphur enrichment. The spectra were interpreted by comparison with standard spectra from synthesized compounds: NiS, NizS2, Ni-adsorbed S. A comparison with previous results using radioactive sulphur (35S) allowed us to determine the mechanism o f the sulphur enrichment and o f the consequent inhibition o f passivation by the formation o f an adsorbed sulphur layer and subsequent growth o f a nickel sulphide Ni3S 2 which attains a stationary thickness o f about 30A. 1. INTRODUCTION It is well known t h a t sulphur can change the passivation conditions and can increase the rate of dissolution of metals such as nickel and iron [ 1]. However, until recently the mechanism of this effect was n o t well understood. It has been suggested that the adsorption of sulphur plays a decisive role in this p h e n o m e n o n . Recently straightforward evidence of this role has been given by Oudar and Marcus [2] using a 35S radiotracer and conventional current-potential plots. The *Presented at the International Chalmers Symposium on Surface Problems in Materials Science and Technology, GSteborg, Sweden, June 11 - 13, 1979.
major conclusions of this work concerning the influence that sulphur has on nickel passivation in an acidic solution can be summarized as follows. (1) The adsorbed sulphur increases the rate of dissolution of the metal in the active region of the anodic curve. (2) A total inhibition of the passivation is observed when a critical sulphur concentration exists on the surface at the potential that corresponds normally to the active-to-passive transition of nickel w i t h o u t sulphur. This critical surface concentration is of the o r d e r of one monolayer. The inhibition has b e e n attributed to a blocking effect by sulphur of the sites required for the adsorption of OHanions, the precursor of passive film formation (NiO). (3) This critical surface concentration of sulphur can be obtained on Ni-S alloys by surface segregation due to the selective dissolution of the metal. The corresponding bulk concentration of sulphur has been measured. Depending on the crystallographic orientation of the surface this concentration ranges from 30 × 10 -4 to 50 × 10 -4 wt.% at a potential sweeping rate of 1 V h -1. (4) When the total inhibition of passivation occurs, the metal dissolution takes place through a non-protective film with a remarkably high current density. This film, which will be shown to be the nickel sulphide NisS2, attains a stationary thickness of about t e n monolayers of sulphur (as measured by the radiochemical technique). (5) The same general features are observed on pure nickel in a sulphur-contaminated electrolyte. For Ni-S alloys the mechanism of the inhibition of passivation by sulphur can be illustrated by the model shown in Fig. 1.
192
t
t
t
*
l/ 00
/ S
Ni2+
x i 2+
t
t
s,,,~,.i~
adsorbed
L.t.t.I.d./
dissolution
Nickel Sulphide Ni2+
/
~o
•
•
•
0Q
s t ' g rt~g~t t i Oll o n tilt, s u r f act, blocking effect by s u l p h u r o f tilt, adsorption sitcs f o r OH . I n h i b i t i o n of the passive film formation
S in solid solution
sulphur ,.nr ichmcnt . Formation of nonprt, tcct ire sulphide, film
;i
Fig. 1. A model illustrating the mechanism of the inhibition of passivation by sulphur. The anodic polarization curves of pure nickel, of nickel with a low sulphur c o n t e n t and of Ni-S alloys with a sulphur content above the critical content required for the inhibition of passivation are shown in Fig. 2. The aim of this work was to analyse by Auger electron spectroscopy (AES) and electron spectroscopy for chemical analysis (ESCA) the surface composition of Ni-S alloys after anodic polarization. In this way we attempted to determine the exact nature of the surface film formed at various stages of the anodic metal dissolution in order to confirm previous results obtained from radiochemical analysis and to collect additional information on the phenomenon. AES has been widely used to investigate the metal-gas interface. Considerable knowledge has been obtained by Benard and coworkers [3 - 5] on the adsorption of sulphur
t -¢ .N
|
s/S/SSS .40
//- b /s
.20 /
0 O.S
I I
! 1.5
E V/SHE Fig. 2. Anodic polarization curves of nickel and of Ni-S alloys (0.1 N H2SO4; 1 V h - l ) : curve a, pure (100) Ni; curve b, (100) Ni-S alloy with 0.006 wt.% S; curve c, polycrystalline Ni-S alloy with 0.006 wt.% S.
on several metal surfaces. Using a 35S radiotracer, Auger signals have been calibrated for the measurement of sulphur coverage from zero to saturation [6]. ESCA has proved very fruitful in analysing passive films formed on metals and alloys in aqueous media [7 - 9]. Since anodic layers are sensitive to air exposure, the use of surface-sensitive techniques such as AES and ESCA requires a transfer of the electrode under a protective gas from the electrolytic cell to the spectrometer. We have previously investigated by ESCA the passive film formed on pure nickel [ 10]. Thus we can compare the anodic layers formed on nickel with and w i t h o u t sulphur. 2. EXPERIMENTAL
2.1. Sample preparation The nickel samples (single crystals and polycrystals 99.999% pure) were polished mechanically and electrochemically. They were annealed in purified H2 at 800 °C. To prepare the Ni-S alloys, clean nickel samples were treated at a high temperature in an H2-H2S mixture of appropriate composition. The experimental conditions werePH:s/PH2 = 6.4 × 10 -4, T = 1130 °C. The samples were air quenched to room temperature. Under these conditions the sulphur content is 0.006 wt.% [11]. The sulphur concentration was controlled by using a SSs radiotracer. This technique enabled us to measure the a m o u n t of sulphur in the bulk and on the surface before and after electrochemical treatments. The Ni-S alloys obtained are homogeneous solid solutions. No sulphide is precipitated within the metallic matrix [ 2]. The NisS 2 samples used for calibration were prepared from clean nickel treated in H2-H2S (PH2s/PH2 = 1.02 × 10 -s) at 380 °C.
193 The adsorbed sulphur layer on (100) Ni used for calibration was prepared by thermal segregation. A sulphur
10 -9 Torr. The sample could be returned to the electrolytic cell under a protective gas in the same way. For the ESCA study the electrolytic cell was directly attached to the spectrometer by a straight-through valve, and an internal handling system allowed the sample to be moved from the cell to the analysis chamber. This equipment has been described elsewhere [12, 13]. The surface analyses were performed in a conventional retarding field analyser Auger spectrometer and in an ESCA-HP 5950 A instrument, with monochromatized A1 Ks radiation.
3. RESULTS 3.1. Sulphur segregation on the surface during anodic polarization Figures 3(b) and 3(c) show ESCA spectra obtained from Ni-S alloys polarized to 0.45 and 0.8 V/SHE. For comparison the spectra recorded from pure nickel passivated at 0.54 V/SHE are also shown (Fig. 3(a)). 3.1.1. Polarization to 0.45 V/SHE The S 2p region exhibits a peak at a binding energy of 162 eV. Assignment of this peak to sulphate which originates from the electrolyte is excluded since sulphur in SO42- produces a peak at a much higher binding energy (about 169 eV). Indeed a sulphur peak at 169 eV is visible in the spectrum of the passive film (Fig. 3(a)). This peak has been shown to be associated with contamination by SO42[ 10]. In addition sulphur cannot be detected on the initial homogeneous Ni-S alloy. Therefore this S 2p peak at 162 eV must originate from sulphur enriched on the surface by anodic dissolution of nickel. This indicates that there is a selective dissolution of nickel and a sulphur segregation on the surface. It should be noted that 0.42 V/SHE corresponds to t h e maximu.m in the polarization curve of pure (100} Ni (Fig. 2). Therefore at the usual passivation potential of pure nickel, sulphur is already enriched on the surface of the Ni-S electrode. The coverage by sulphur will be shown later to be of the order of a monolayer. The Ni 2Psi2 spectrum exhibits a main peak at 852.8 eV, i.e. at the same binding energy as nickel in its metallic state, and the satellite at 858.6 eV is visible. A low signal is also detected
194
Ni2p. I]Nil100)o,,I~s|iwtediIfit *0.S4c,,V A (a)
(SHE)
I
il f
1
s2p ,
~ ,
:
• xcl, 6
----4 - / NI*S alloy polarix*d to *O.t~VISHEI
The Auger spectrum obtained after polarization of the Ni-S alloy to 0.8 V/SHE is shown in Fig. 4. As well as the nickel peak at 62 V, a sulphur peak at 150 V and a carbon peak at 275 V are seen. This Auger analysis demonstrates, in agreement with 35S measurements [2] and with the ESCA results already discussed, that nickel is selectively dissolved and that sulphur is enriched on the surface.
(b) Ni-S OIloy polorized to +0.8 V (SHE)
860 8SS
(c)
S~ 530 290 28S BINDING ENERGY {eV)
1"/0 165 160
Fig. 3. (a) ESCA spectra of pure (100) Ni passivated at 0.54 V/SHE (from ref. 10) and (b), (c) ESCA spectra of Ni-S alloys (0.006 wt.% S) polarized to (b) 0.45 V/SHE and (c) 0.8 V/SHE (0.1 N H2SO4; 1 V h-l).
at 856.4 eV. This peak has been shown to be associated with nickel in Ni(OH)2. The oxygen spectrum exhibits a peak at 531.6 eV and a shoulder at higher binding energy. The former is assigned to oxygen in O H - ions [10] and hence originates from the same species as the nickel peak at 856.4 eV. The shoulder probably corresponds to contamination by CO. This would also account for the carbon peak at 285 eV.
3.1.2. Polarization to 0.8 V/SHE First it is recalled that in sharp contrast with pure nickel, which is passivated at this potential, Ni-S alloys do n o t show passivation behaviour. The dissolution current density is greater than 20 mA cm -2 (Fig. 2), while the residual current density for passivated nickel is less than 10 pA c m -2. The ESCA spectra obtained show features similar to those described above. In addition to the main S 2p peak a low signal is detected at 169 eV, assigned to contamination by SO42- as indicated before. The major observation is that the intensity ratio of S 2p to Ni 2p is increased compared with the situation at 0.45 V/SHE. This reveals that the surface has been further enriched with sulphur by additional polarization from 0.45 to 0.8 V/SHE. The dependence of the intensity ratio on polarization potential and the conclusions that follow are given in Section 3.2.
3.2. Evolution of the sulphur enrichment with polarization potential and characterization of the species formed on the surface The extent of the sulphur enrichment was estimated from the intensity ratio of the sulphur and nickel signals. Figure 5 shows the ratio Is 2P//Ni2P as a function of the anodic potential. It is clear that the ratio increases with potential from the dissolution potential to about 0.8 V/SHE and then attains a stationary value (Is 2p/INi 2p ~ 0.03) for higher potentials (up to 1.45 V/SHE). This indicates that the sulphur segregation during anodic dissolution produces a sulphur coverage which increases with potential in the first part of the current-potential curve and is stationary in the second part of the curve. The characterization of the chemical state of sulphur on the surface was carried out by comparison with known species, i.e. with adsorbed sulphur and sulphides. The adsorbed sulphur monolayer was prepared by thermal segregation in a vacuum from sulphur-doped (100) Ni. The
S
Ni
c
E(eV)
Fig. 4. The AES spectrum of Ni-S alloys (0.006 wt.% S) polarized to 0.8 V/SHE (0.1 N H2SO4; 1 V h-X).
195 Ni
2p3n
J~_ NilSz
adllodoed~ll=hur
KrJs
xt,0
Ni3i2
][
[/
~.~o ---'J 862
20 ¢o o
s 2p
,SO /~
.r/
858 854 850 164 BINDING ENERGY(eV)
~
'°°s0 160
Fig. 6. ESCA spectra of Ni3S 2 formed in H2-H2S. 10
r*"
0.5
I
1
J
1.5 E VlSl~
Fig. 5. The intensity ratio of S 2p (of binding energy 162 eV) to Ni 2p (of binding energy 852.8 eV) characterizing sulphur enrichment on the surface during anodic dissolution of sulphur-doped nickel (0.006 wt.% S; 1 V h - l ) . (The arrows indicate the ratios of Is 2p//Ni2p obtained from calibration experiments with adsorbed sulphur (thermal segregation) and bulk Ni3S2.)
sulphur signal from the 2p levels was located at 162 eV. It was found that adsorption of sulphur does n o t cause any shift in the binding energy of Ni 2p3/2 (the main peak is at 852.8 eV and the satellite at 858.6 eV), which indicates a covalent character of the bond. The ratio Is 2p/INi 2p was equal to 0.004. The spectra of Ni3S2 (Fig. 6) exhibit two wellresolved sulphur peaks at 162.2 and 163.2 eV corresponding to S 2Ps/2 and S 2pl/2 respectively. The Ni :2p spectrum shows a peak located at a b o u t the same binding energy as nickel in its metallic state. The possibility of a chemical shift is n o t excluded but further investigation is required before any c o m m e n t can be made. The ratio Is 2p/INi 2p of the intensities was found to be 0.029. The values of the Is 2p/INi 2p ratio obtained from adsorbed sulphur (one monolayer) and from bulk Ni3S2 are given in Fig. 5. It is concluded that the sulphur segregation during anodic polarization produces adsorbed sulphur in the first part of the anodic curve (from the dissolution potential to the potential corresponding to the passivation of pure nickel {0.42 V)). This result is in good agreement with previous observations [2]. (1) From the radiochemical measurements using 35S it was shown that the polarization of Ni-S alloys under the same conditions produces an adsorbed layer of sulphur with a coverage of a b o u t one monolayer.
(2) The calculation of the amount of sulphur freed during the dissolution of sulphurdoped nickel gives the same value, which indicates that in this range of potential the yield of sulphur adsorbed to sulphur made available at the surface is equal to unity. Further enrichm e n t at higher potentials results in the formation of Ni3S2. AES calibration has been performed previously for sulphur adsorption and for the nickel sulphides Ni3S2 and NiS. Experimental details have been reported elsewhere [6, 14]. The ratio of the Auger peak heights of sulphur (150 eV) and nickel (62 eV) obtained on these compounds, as well as the ratio obtained on Ni-S alloy polarized to 0.8 V, are given in Table 1. It is clear that Islr'°ev/INi B2ev for the polarized Ni-8 alloy is almost identical with the ratio obtained with synthetic Ni3S2, whereas NiS can be excluded. Therefore it is concluded, in agreement with the ESCA results already discussed, that the species formed is Ni3S2. The combined use of ESCA and AES allowed us to check whether the sulphide layer is continuous. By ESCA a large area of the sample (5 mm 2) is analysed and the geometrical arrangement of the sample with respect to the TABLE 1 The intensity ratio of low energy Auger signals of sulphur and nickel: a comparison between adsorbed sulphur, bulk sulphides and Ni-S alloy polarized to 0.8 V/SHE
Compound
is150 eV/INi62 eV
Adsorbed sulphur at
0.3
saturation coverage a
NiS a
2.2
NisS2 a Ni-S alloy (0.006 wt.% S) polarized to 0.8 V/SHE (0.1 N H2SO4,~ 1 V h -1)
0.79 0.77
aFrom ref. 14.
196
excitation beam and detector cannot be changed, whereas in the AES system the primary electron beam can be focused on various spots of the sample surface and the sample can be tilted with respect to the electron beam. AES analyses of several areas gave the same ratio of the peak heights of sulphur to nickel within the experimental error ((10%).
3.3. Thickness of the sulphide layer The m e t h o d utilized previously [10] for calculation of the thickness of the passive film on nickel from ESCA data cannot be utilized here because the signals from metallic nickel and from nickel in the sulphide were not differentiated because of the extremely small chemical shift. An alternative method is the estimation of the thickness from the ratio of the intensities of the sulphur and nickel signals by comparison with known compounds. From AES the ratio IslSOeV/INi 62ev w a s already the same as for " b u l k " Ni3S 2 after polarization to 0.8 V/SHE. The term " b u l k " applies to a layer thickness such that the secondary electrons from the substrate cannot escape. By varying the incidence angle we can change the sensitivity to the outermost layer of atoms. For angles ranging from 10 to 40 ° the ratio Is z5° eV//Ni62eV was the same within experimental error (< 10%). The escape depth for low kinetic energy secondary electrons is expected to be low (about 10 h ) and it can be concluded that the thickness of the sulphide layer at 0.8 V is greater than 10 A. With ESCA the ratio of " b u l k " sulphide is obtained only after polarization to greater than 1 V/SHE. This is in reasonable agreement with the Auger result ( " b u l k " sulphide at 0.8 V/SHE) since the escape depth of Ni 2p3/2 photoelectrons (of kinetic energy 650 eV) is much larger. On this basis the film thickness can be estimated at about 30 A. Comparison with a previous result brings straightforward confirmation and a more accurate value: measurements using a 85S radiotracer [2] indicate that the a m o u n t of sulphur on the surface above 1 V/SHE is 38 × 10 -8 g cm -2. Because we know from AES analysis and ESCA that the compound formed is N i 3 S 2 w e can calculate the thickness. Assuming a continuous layer a value of 26 £ is obtained.
3.4. Comparison with the passive film on pure nickel Passivation of nickel has been shown to be characterized in the ESCA spectra [10] by an O ls signal at 529.8 eV and an Ni 2P3/2 signal at 854.4 eV (see Fig. 3(a)). These peaks were assigned to oxygen and nickel in NiO. It is clear that these peaks do n o t exist in the spectra of Ni-S alloys polarized to potentials above the normal passivation potential of pure nickel (see Figs. 3(b) and 3(c)). This gives evidence that the formation of NiO is precluded on Ni-S alloys. In addition the sulphur peak at 162 eV observed in the spectrum of Ni-S after polarization does not exist in the spectrum of the passive film on pure nickel. Therefore the inhibition of passivation revealed by the current-potential curve (Fig. 2) is directly related to the fact t h a t NiO cannot grow on sulphur or sulphidecovered surfaces. 4. DISCUSSION
The current-potential curves revealed that when sulphur is alloyed to nickel above a critical concentration the active-to-passive transition, characteristic of nickel, is n o t observed. The ESCA investigation indicates that NiO, which was shown to be the passivating compound, is n o t formed on Ni-S alloys. Instead, AES and ESCA show that sulphur is enriched on the surface by selective dissolution of the metal. Segregation of sulphur in the first part of the current-potential curve corresponding to the active region of pure nickel produces a nearly complete monolayer of adsorbed sulphur. The presence of this sulphur monolayer at the normal passivation potential for pure nickel precludes the growth of NiO. A peak corresponding to OH- (O ls at 531.6 eV) was detected in the ESCA spectra (Fig. 3). This indicates that OH- ions can still be adsorbed on the sulphur-covered nickel surface but in this case passivity is n o t attained. From the results on pure nickel it has been suggested that two adjacent OH- ions are required to produce a layer of NiO [ 10]. When a complete or nearly complete sulphur monolayer exists on the surface, OHions can still be adsorbed to some extent but the situation with two adjacent OH- ions seems no longer possible. Consequently the inhibition of the passive film formation will
197
be due to a dilution effect by sulphur of the sites for the adsorption of OH- ions, which precludes the recombination of two OH- ions to yield NiO. Comparable dilution effects are often encountered in catalytic reactions on metallic alloys.
5. CONCLUSIONS
The surface analysis of Ni-S alloys after anodic polarization confirms entirely the previous results and hypotheses. In addition the poisoning effect of adsorbed sulphur on the adsorption of OH- anions, the precursor of passive film formation, has been further clarified. Finally the nature and the thickness of the non-protective sulphide film which grows after the sulphur adsorption and precludes the formation of NiO have been determined.
ACKNOWLEDGMENT
Financial support from the D616gation G6n6rale ~ la Recherche Scientifique et Technique is gratefully acknowledged.
REFERENCES 1 J. Oudar and P. Marcus, Appl. Surf. Sci., 3 (1979) 48, refs. 1 - 14. 2 J. Oudar and P. Marcus, Appl. Surf. Sci., 3 (1979) 48. 3 J. Benard, J. Oudar and P. Cabane-Brouty, Surf. Sc£, 3 (1965) 359. 4 J. L. Domange and J. Oudar, Surf. Sci., 11 (1968) 124. 5 M. Perdereau and J. Oudar, Surf. Sci., 20 {1970) 80. 6 M. Perdereau,Surf. Sci., 24 (1971) 239. 7 I. Olefjord and H. Fischmeister, Corros. Sci., 15 (1975) 697. 8 I. Olefjord and B.-O. Elfstr6m, in T. P. Hoar (ed.), Proc. 6th Eur. Congr. on Metallic Corrosion, London, September 1977, Society of Chemical Industry, London, 1977, p. 21. 9 I. Olefjord, in J. Larinkari (ed.), Proc. 8th Scand. Corrosion Congr. Helsinki, September 1978, Vol. 1, Helsinki University of Technology, Laboratory of Corrosion Science and Technology, Helsinki, 1978, p. 349. 10 P. Marcus, J. Oudar and I. Olefjord, J. Microsc. Spectrosc. Electron., 4 (1979) 63. 11 N. Barbouth and J. Oudar, C. R. Acad. Sci., 269 (1969) 1618. 12 I. Olefjord, Met. Sci., 9 (1975) 263. 13 I. Olefjord and N. G. Vannerberg, Proc. 7th Scand. Corrosion Congr., p. 434. 14 M. Perdereau, C. R. Acad. Sci., 274 (1972) 448.