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AES study of the effect of segregated sulfur on the oxidation of an Fe-6at%Si alloy surface
Applied Surface Science 35 (1988-89) North-Holland, Amsterdam
219
219-226
AES STUDY OF THE EFFECT OF SEGREGATED SULFUR ON THE OXIDATION OF AN Fe-Bat%Si ALLOY SURFACE M. ES-SOUNI GKSS-Research
Center, Max-Planck
Strasse, 2054 Geesthacht, Fed. Rep. of Germany
and A. MOSSER Laboratoire de Cristaliographie, 67000 Strarbourg, France
UA 795 du CNRS, Institut le Bel, 4 Rue B. Pascal,
Received 29 April 1988; accepted for publication 16 August 1988
The effect of segregated sulfur on the oxidation of an Fe-6atBSi alloy surface, at room temperature and an oxygen pressure of lo- s Torr, is investigated using Auger electron spectroscopy. Sulfur is found to decrease drastically the oxidation rate of Fe. The results obtained are discussed in terms of the interactions between iron, oxygen and sulfur at the surface, and are compared to those obtained with sulfur covered surfaces of Cr-bearing steels found in the literature. During subsequent vacuum annealing at 45O“C. Fe is reduced and the surface is enriched with Si oxide. It is shown that for higher temperatures, Si oxide aggregates and segregated sulfur probably share the surface.
1. Introduction Sulfur is one of the most frequently encountered impurity elements in metals and alloys. During heat treatments at sufficiently high temperatures dissolved sulfur segregates to grain boundaries and free surfaces owing to its high segregation energy [l-3]. It is now well known that this phenomenon influences dramatically the behaviour of materials in mechanical and chemical environments, e.g. in catalysis, corrosion or wear (see for example the review article by Oudar [4]). Sulfur has also been reported to influence the oxidation rate of Fe [5,6], stainless steels [6,7] and Ni-Cr [8] alloys and also the adherence of protective oxides and passive layers [9,10]. In the present work the effect of segregated sulfur on the oxidation of an Fe-6atSSi alloy surface at room temperature and on the surface composition after subsequent vacuum annealing is investigated using Auger electron spectroscopy. Attention is paid to the interactions Fe-O-S and Fe-0-S-Si at surfaces. The results obtained are compared to those obtained with sulfur 0169-4332/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
220
M. Es-Sounr, A. Mosser / Effect of segrepted
S on oxidamn
covered surfaces of Cr-bearing alloys. It is shown oxidation of Fe-% and stainless steels differently.
that
of- Fe ~ hat RSI
sulfur
influences
the
2. Experimental The sample was cut from a high purity Fe_6at%Si single crystal with (100) orientation. It was then mechanically polished using alumina grinding paste. The main impurities detected were carbon and sulfur, 60 ppm at. and 3 ppm at. respectively. The sample was mounted on a tantalum sample holder which could be heated up to 1100” C. Temperature was measured with a Pt/ Pt-lO%Rh thermocouple welded on the specimen surface. Auger spectra were obtained in a high vacuum chamber (up to 5 x lo- “’ Torr) with a cylindrical mirror analyser. The analysis conditions were: E, = 2500 V, I = 2.5 PA and a modulation voltage of 1.2 mV peak-to-peak. The initial contaminants were found to be carbon and small amounts of oxygen. They were depleted from the surface with a defocussed 600 eV argon ion beam. The sample was then vacuum annealed at 800” C for 20 h to saturate the surface with segregated sulfur. After cooling the sample to room temperature, oxygen was introduced into the chamber at a pressure of 10 -’ Torr for 30 s. After pumping the system. the different peaks of interest were recorded. The same procedure was repeated until no evolution of the surface was observed. Finally, subsequent vacuum annealing was conducted at temperatures 450, 550, 650 and 72O’C with Auger peaks recorded at each temperature.
3. Experimental results and discussion 3. I. Segregation
of sulfur
The segregation kinetics of sulfur were monitored at 800 o C using AES. Fig. la shows the variation of the normalized ratio R, = HS/HFec703j as a function of time, where H,, and H, are respectively the peak-to-peak heights of S LMM (at 152 ev> and Fe LMM (at 703 eV) Auger peaks. It can be seen that sulfur increases linearly with time nearly up to saturation. The segregation then proceeds slowly and saturation is achieved after about 12 h. This behaviour is typical for impurities with high segregation energies and can be described by a fi-law as follows [ll]: 0s = [2C”/WJ,,,l where
(W7W7~
M. Es-So@
A. Mosser / Effect of segregated S on oxidation of Fe - 6at %Si
221
a
800 ‘C
' 10
t(h)
>
b
Fig.
d =
1. Segregation kinetics of sulfur at 8CNl’C. (a) Variation of the normalised ratio H,(,,,,/H,,,,,, versus time. (b) Variation of the sulfur coverage 0s versus fi.
R, =
1.433 A is the interplanar sparing, C, is the mole fraction of sulfur, C, is the surface concentration of sulfur, and (C,),,, = 0.5 mole fraction [l] is the surface concentration at saturation. The plot of 8 versus fi (fig. lb) allows the diffusion coefficient of sulfur at 800 o C to be calculated. The calculated value of 2.4 X lo-” cm2/s is close to that determined by Ainslie and Seylbot [12] using a tracer technique. After cooling the sample from 800 o C to RT, the surface was still covered with sulfur; no trace of other impurities was detected. The intensity of the Si LMM peak at 91 eV was almost equal to that of a bombarded surface.
222
M. Es-Souni, A. Mossrr / Effect of segregmed S on oxtdatton of Fe - 6at %SI
3.2. Oxidation
of sulfur saturated surface at RT
Fig. 2a shows the variation of the normalised ratios R, and R,, = versus exposure time in oxygen (1 L = 1 s in lO_ ’ Torr WKW/‘~F,~,,,, 02). A small decrease in the sulfur Auger peak and a corresponding oxygen uptake can be seen at the beginning of the oxidation (300 L). At this stage no oxidation of the surface was detected, e.g. no shape change in the Fe MMM peak at 48 eV. or Si LMM peak at 91 eV was observed. Thus the decrease in the sulfur Auger peak could be attributed to the desorption of small amounts of so,. When the oxidation is pursued (1500 L), the ratio R, continues to decrease and R, to increase. At the same time an Auger peak at 52 eV appears (fig.
a
0.2. 5000
10000
15000
S
Fig. 2. (a) Evolution
LMM
L q
’
OKLL
of oxygen and sulfur normalised ratios with exposure (b) Evolution of the different Auger peaks.
time in oxygen
at RT.
M. Es-Sot&
223
A. Moser / Effect of segregated S on oxidation of Fe - 6ai ‘%Si
2b). The appearance of this peak without the accompanying one at 44 eV characteristic of y-Fe,O, [13], could not be explained by the authors, but it might be attributed to an intermediate Fe oxidation state [14]. The peak at 52 eV continues to grow steadily with increasing 0, exposure. At about 7000 L the peak at 44 eV appears and the Fe MMM peak at 48 eV characteristic of metallic Fe is no longer present, indicating a complete transformation of the surface to y-Fe,O,. In contrast, the sulfur peak has diminished. However, even beyond 15000 L a residual sulfur peak persists. No noticeable variation of the Si LMM peak at 91 eV was observed indicating no participation of Si in the surface reaction. 3.3. Vacuum annealing The temperature of the sample was increased slowly up to 450 o C. After a few minutes at this temperature, Fe is completely reduced (only the Fe MMM peak at 48 eV was observed). At the same time an important increase in silicon oxide at the surface is observed. The recorded structures at 66, 68, 79 and 84 eV and the important shift of the oxygen KLL peak (fig. 3) are characteristic of SiO, and an intermediate SiO, oxide [15]. On the other hand, the height Ho has diminished, probably due to the desorption of physisorbed oxygen and the height Hs has slightly increased. Heating the sample at higher temperatures (550°C) leads to a net increase in silicon oxide, oxygen and sulfur. Further heating at 650°C did induce only a slight change on the surface. At 720“ C silicon oxide decomposes with desorption of oxygen and a strong increase in sulfur is observed. The ratio R, calculated is however inferior to that before oxidation. 0 KLL
S LMM
Fe M M M 23 45 45
720% Xl
“is 4f-
s\r
650°C 55dC
450%
SiO, + SiO I 40
Fig. 3. Evolution
do
of the surface
120
during
I;0
vacuum
annealing
690
ilO
at different
!A0
temperatures.
> EC
(eW
4. Discussion
Though the Fe clean surface is easily oxidised at 10.” Torr 0, [6], a higher oxygen pressure and a long exposure time were needed to oxidise the sulfurcontaminated surface. This result is in agreement with the observation of Ueda and Shimizu [5] and Jardin [6], who found that segregated sulfur hinders the oxidation of Fe single crystals. The former authors found that Fe begins to oxidise only for 0 pressure higher than 10P6 Torr. Following these authors the sulfur acts as a cathodic protection, e.g. sulfur hinders the charge transfer necessary for oxygen adsorption. At higher oxygen pressures oxygen absorbs on surface sites not occupied by sulfur. The process continues by incorporation of oxygen atoms into the adjacent surface layers and their replacement by Fe atoms, which in turn adsorb oxygen, The resulting surface oxide layer explains the attenuation of the sulfur peak amplitude. Ueda and Shimizu [5] did not explain however the persistence of a residual sulfur peak, even after a long exposure time in oxygen. In fact, two mechanisms could be presented to be responsible for the attenuation of the sulfur Auger peak - when excluding the possibility of attenuation by desorption of SO,: (1) Sulfur is confined to the substrate/Fe-oxide interface. (2) Sulfur is diluted in the oxide layer. Considering the first mechanism, with the mean inelastic escape depth of the S LMM Auger peak taken to be about 4-5 A [16]. the amplitude of the sulfur Auger peak should decrease rapidly after the oxidation has taken place. Such behaviour was not observed (fig. la). Besides, supposing the formation of a continous Fe-oxide layer at the surface. and that all sulfur atoms are displaced by the same distance d, the oxide layer thickness d could be calculated from the attenuation of sulfur peak intensity, and is found to be about 2.7 A. Such a result is rather close to the thickness of half a monolayer segregated sulfur [17] but does not make sense for the formation of a y-Fe,O, oxide layer. Thus the persistence of a residual sulfur peak is not compatible with sulfur being dispelled to the interface oxide/substrate. The discussed mechanism is probably occurring for the oxidation of sulfur covered Ni-Cr [8] and Fe-NiiCr [6] surfaces, where sulfur and chromium cosegregate during heat treatment and form a chromium sulfide layer at the surface. In these cases the sulfur Auger peak was observed to decrease rapidly in the early oxidation stages (2 to 10 L). This difference in behaviour is most probably due to the greater affinity of chromium for oxygen than for sulfur, in contrast to Fe which has almost the same affinity for sulfur and oxygen [18]. So in Cr-charged alloys. chromium is readily displaced by oxygen from its sulfide to form an oxide layer at the surface via an exothermal reaction, with sulfur being displaced to the oxide/substrate interface. The second mechanism is the more probable one in our case. The slow progression of the oxidation reaction (result of a weak preferable interaction
M. Es-Souni, A. Moser
/ Effect of segregated S on oxidation of Fe - 6at %Sl
225
Fe-O) and the residual sulfur peak are compatible with sulfur being diluted in the Fe-oxide layer. No indications could however be inferred about the structure or energy of the Auger peaks of the elements involved, to state whether an oxysulfide has been formed or not. More work is needed to clarify this, particularly with X-ray photoelectron spectroscopy, which is a more suitable method for this purpose. Annealing at 450°C leads to the formation of silicon oxide SiO, and an intermediate oxide SiO, by diffusion of silicon from solid solution to the surface and its combination with oxygen. It forms most probably a thin layer of silicon oxide at the surface, surmounting the segregated sulfur. Similar stratification has also been found for the oxidation of a BN covered Fe-Si alloy surface using ESCA grazing emission [19], and could be deduced supposing that the surface energy of compounds is proportional to their heat of formation. The decrease in oxygen Auger signal is attributed to the desorption of physisorbed oxygen and also, to a lesser extent, to diffusion of this element to the adjacent surface layers. Such diffusion is probably due to the attractive effect of dissolved silicon, and was also observed in Fe-Al [20] and Pt-Si [21] alloys. Besides, the diffusion of oxygen is confirmed by annealing at 550°C where an increase in intensity of oxygen and “SiO” signals are observed (fig. 3). This is explicable supposing that diffusion of silicon and oxygen from the bulk to the surface occurs via a cosegregation mechanism (221. An increase in the sulfur Auger peak intensity is also observed. This matches with the supposition that a thickening of Si oxide at the surface results in its precipitation in the form of aggregates, thus making more sulfur “ visible”. This situation is still almost unchanged, when annealing at 650 o C. At 720 o C silicon oxide is thermally decomposed with desorption of oxygen. Only segregated sulfur is still present at the surface, the Auger peak intensity of which is however lower than that before oxidation (R, = 1.25 instead of 1.45) showing that some thermally activated desorption of SO, occurs. Furthermore, no, indications could be inferred concerning the influence of segregated sulfur on the stability of the oxide layer, since silicon oxide formed at a clean surface of the same alloy decomposes also in the range of temperatures indicated [23].
5. Conclusions The following conclusions can be drawn: (1) Sulfur segregates strongly to the surface and displaces from the surface. Its segregation kinetics follow a h-law.
all other
elements
(2) Segregated sulfur is found to decrease drastically the oxidation rate of Fe at room temperature, and in the range of oxygen pressure chosen, probably by hindering the charge transfer from Fe necessary for oxygen adsorption. (3) Oxygen absorbs on surface sites not occupied by sulfur. The oxidation continues with dilution of sulfur in the oxide layer. (4) During vacuum annealing at 450” C Fe is reduced and the surface is enriched with SiO, and an intermediate silicon oxide SiO,. Increasing the temperature leads to the precipitation of silicon oxide in the form of aggregates. As a result, segregated sulfur and Si-oxide aggregates share the surface. (5) At about 720” C silicon oxide decomposes thermally and the original S-covered surface is obtained again.
References [I] H.J. Grahke. EM. Peterhen and W. Paulitschke. Tranh. Roy. Sot. (London) A 795 (19X0) 1’8; H.J. Grahke. W. Paulitschke. G. Tauhe and V. Viefhaus. Surface SCI. 63 (1977) 377. [2] M. Es-Soum. Th&e de Doctorat d’Etat Strasbourg (1986). [3] T. Miyahara. K. Stolt. D.A. Reed and H.K. Birnhaum. Scripta Met. 19 (19X5) 117. 141 J. Oudar. Mater. Sci. Eng. 42 (1980) 101. [S] K. Ueda and R. Shimizu, Surface Sci. 43 (1974) 77. [6] C. Jardln. Th&se de Doctorat d’Etat Lyon (19X1 ). [7] R.K. Wild. Corroamn Set. 17 (1977) X7. [X] P. Humhert and A. Moser. Surface Sci. 126 (1983) 70X. [9] D.R. Baer and J.T. Prater. J. Vacuum Sci. Technol. 20 (19X2) 1396. [IO] I’. Marcus and J. Oudar. Mater. Sci. Eng. 42 (1980) 191. [I I] S. Hofman and J. Erlewein. Surface Sci. 77 (197X) 591. [I?] N.G. Ainslie and A. Seylbot. J. Iron Steel Inst. 194 (1960) 341. [13] M. Sro. J.B. Lumsden and R.W. Staehle, Surface Sci. SO (1975) 541. [14] SE. Grew. J.P. Roux and J.M. Blakely. Surface Sci. 120 (1982) 203. [IS] B. Carrierr, J.P. Deville and P. Humbert. J. Microac. Spectrosc. Electron. 10 (19X5) 29: see alo A. Moser. SC. Srivastava and B. C’arriere. Surface SCI. 133 (1983) L441. [ 161M.P. Seah and W.A. Dench. Surface Interfact. Anal. 1 (1979) 2. 1171 G. Lorang, L. Minel. S. Bouquet and J.P. Langeron. Surface SCI. I53 (19X5) 947. [1X] J. Benard. J. Oudar. N. Barbouth, F. Margot and Y. Berthier. Surface Sci. XX (1979) L35. [I91 P. Humbert. M. Es-Souni and A. Moser. Surface Interface Anal. 12 (1988) 21X. [20] N. Saidani. M. Es-Souni and A. Moser, J. Microsc. Spectrosc. Electron. 1 I (1986) 345. 1211 M. Salmeron and G.A. Somorjai, J. Vacuum SCI. Trchnol. 19 (19X1) 722. 1221 M. Guttmann. Surface Sci. 53 (1975) 213. [23] H. Vwfhaus and W. Roasoa. Surface Sci. 141 (19X4) 341.
219
219-226
AES STUDY OF THE EFFECT OF SEGREGATED SULFUR ON THE OXIDATION OF AN Fe-Bat%Si ALLOY SURFACE M. ES-SOUNI GKSS-Research
Center, Max-Planck
Strasse, 2054 Geesthacht, Fed. Rep. of Germany
and A. MOSSER Laboratoire de Cristaliographie, 67000 Strarbourg, France
UA 795 du CNRS, Institut le Bel, 4 Rue B. Pascal,
Received 29 April 1988; accepted for publication 16 August 1988
The effect of segregated sulfur on the oxidation of an Fe-6atBSi alloy surface, at room temperature and an oxygen pressure of lo- s Torr, is investigated using Auger electron spectroscopy. Sulfur is found to decrease drastically the oxidation rate of Fe. The results obtained are discussed in terms of the interactions between iron, oxygen and sulfur at the surface, and are compared to those obtained with sulfur covered surfaces of Cr-bearing steels found in the literature. During subsequent vacuum annealing at 45O“C. Fe is reduced and the surface is enriched with Si oxide. It is shown that for higher temperatures, Si oxide aggregates and segregated sulfur probably share the surface.
1. Introduction Sulfur is one of the most frequently encountered impurity elements in metals and alloys. During heat treatments at sufficiently high temperatures dissolved sulfur segregates to grain boundaries and free surfaces owing to its high segregation energy [l-3]. It is now well known that this phenomenon influences dramatically the behaviour of materials in mechanical and chemical environments, e.g. in catalysis, corrosion or wear (see for example the review article by Oudar [4]). Sulfur has also been reported to influence the oxidation rate of Fe [5,6], stainless steels [6,7] and Ni-Cr [8] alloys and also the adherence of protective oxides and passive layers [9,10]. In the present work the effect of segregated sulfur on the oxidation of an Fe-6atSSi alloy surface at room temperature and on the surface composition after subsequent vacuum annealing is investigated using Auger electron spectroscopy. Attention is paid to the interactions Fe-O-S and Fe-0-S-Si at surfaces. The results obtained are compared to those obtained with sulfur 0169-4332/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
220
M. Es-Sounr, A. Mosser / Effect of segrepted
S on oxidamn
covered surfaces of Cr-bearing alloys. It is shown oxidation of Fe-% and stainless steels differently.
that
of- Fe ~ hat RSI
sulfur
influences
the
2. Experimental The sample was cut from a high purity Fe_6at%Si single crystal with (100) orientation. It was then mechanically polished using alumina grinding paste. The main impurities detected were carbon and sulfur, 60 ppm at. and 3 ppm at. respectively. The sample was mounted on a tantalum sample holder which could be heated up to 1100” C. Temperature was measured with a Pt/ Pt-lO%Rh thermocouple welded on the specimen surface. Auger spectra were obtained in a high vacuum chamber (up to 5 x lo- “’ Torr) with a cylindrical mirror analyser. The analysis conditions were: E, = 2500 V, I = 2.5 PA and a modulation voltage of 1.2 mV peak-to-peak. The initial contaminants were found to be carbon and small amounts of oxygen. They were depleted from the surface with a defocussed 600 eV argon ion beam. The sample was then vacuum annealed at 800” C for 20 h to saturate the surface with segregated sulfur. After cooling the sample to room temperature, oxygen was introduced into the chamber at a pressure of 10 -’ Torr for 30 s. After pumping the system. the different peaks of interest were recorded. The same procedure was repeated until no evolution of the surface was observed. Finally, subsequent vacuum annealing was conducted at temperatures 450, 550, 650 and 72O’C with Auger peaks recorded at each temperature.
3. Experimental results and discussion 3. I. Segregation
of sulfur
The segregation kinetics of sulfur were monitored at 800 o C using AES. Fig. la shows the variation of the normalized ratio R, = HS/HFec703j as a function of time, where H,, and H, are respectively the peak-to-peak heights of S LMM (at 152 ev> and Fe LMM (at 703 eV) Auger peaks. It can be seen that sulfur increases linearly with time nearly up to saturation. The segregation then proceeds slowly and saturation is achieved after about 12 h. This behaviour is typical for impurities with high segregation energies and can be described by a fi-law as follows [ll]: 0s = [2C”/WJ,,,l where
(W7W7~
M. Es-So@
A. Mosser / Effect of segregated S on oxidation of Fe - 6at %Si
221
a
800 ‘C
' 10
t(h)
>
b
Fig.
d =
1. Segregation kinetics of sulfur at 8CNl’C. (a) Variation of the normalised ratio H,(,,,,/H,,,,,, versus time. (b) Variation of the sulfur coverage 0s versus fi.
R, =
1.433 A is the interplanar sparing, C, is the mole fraction of sulfur, C, is the surface concentration of sulfur, and (C,),,, = 0.5 mole fraction [l] is the surface concentration at saturation. The plot of 8 versus fi (fig. lb) allows the diffusion coefficient of sulfur at 800 o C to be calculated. The calculated value of 2.4 X lo-” cm2/s is close to that determined by Ainslie and Seylbot [12] using a tracer technique. After cooling the sample from 800 o C to RT, the surface was still covered with sulfur; no trace of other impurities was detected. The intensity of the Si LMM peak at 91 eV was almost equal to that of a bombarded surface.
222
M. Es-Souni, A. Mossrr / Effect of segregmed S on oxtdatton of Fe - 6at %SI
3.2. Oxidation
of sulfur saturated surface at RT
Fig. 2a shows the variation of the normalised ratios R, and R,, = versus exposure time in oxygen (1 L = 1 s in lO_ ’ Torr WKW/‘~F,~,,,, 02). A small decrease in the sulfur Auger peak and a corresponding oxygen uptake can be seen at the beginning of the oxidation (300 L). At this stage no oxidation of the surface was detected, e.g. no shape change in the Fe MMM peak at 48 eV. or Si LMM peak at 91 eV was observed. Thus the decrease in the sulfur Auger peak could be attributed to the desorption of small amounts of so,. When the oxidation is pursued (1500 L), the ratio R, continues to decrease and R, to increase. At the same time an Auger peak at 52 eV appears (fig.
a
0.2. 5000
10000
15000
S
Fig. 2. (a) Evolution
LMM
L q
’
OKLL
of oxygen and sulfur normalised ratios with exposure (b) Evolution of the different Auger peaks.
time in oxygen
at RT.
M. Es-Sot&
223
A. Moser / Effect of segregated S on oxidation of Fe - 6ai ‘%Si
2b). The appearance of this peak without the accompanying one at 44 eV characteristic of y-Fe,O, [13], could not be explained by the authors, but it might be attributed to an intermediate Fe oxidation state [14]. The peak at 52 eV continues to grow steadily with increasing 0, exposure. At about 7000 L the peak at 44 eV appears and the Fe MMM peak at 48 eV characteristic of metallic Fe is no longer present, indicating a complete transformation of the surface to y-Fe,O,. In contrast, the sulfur peak has diminished. However, even beyond 15000 L a residual sulfur peak persists. No noticeable variation of the Si LMM peak at 91 eV was observed indicating no participation of Si in the surface reaction. 3.3. Vacuum annealing The temperature of the sample was increased slowly up to 450 o C. After a few minutes at this temperature, Fe is completely reduced (only the Fe MMM peak at 48 eV was observed). At the same time an important increase in silicon oxide at the surface is observed. The recorded structures at 66, 68, 79 and 84 eV and the important shift of the oxygen KLL peak (fig. 3) are characteristic of SiO, and an intermediate SiO, oxide [15]. On the other hand, the height Ho has diminished, probably due to the desorption of physisorbed oxygen and the height Hs has slightly increased. Heating the sample at higher temperatures (550°C) leads to a net increase in silicon oxide, oxygen and sulfur. Further heating at 650°C did induce only a slight change on the surface. At 720“ C silicon oxide decomposes with desorption of oxygen and a strong increase in sulfur is observed. The ratio R, calculated is however inferior to that before oxidation. 0 KLL
S LMM
Fe M M M 23 45 45
720% Xl
“is 4f-
s\r
650°C 55dC
450%
SiO, + SiO I 40
Fig. 3. Evolution
do
of the surface
120
during
I;0
vacuum
annealing
690
ilO
at different
!A0
temperatures.
> EC
(eW
4. Discussion
Though the Fe clean surface is easily oxidised at 10.” Torr 0, [6], a higher oxygen pressure and a long exposure time were needed to oxidise the sulfurcontaminated surface. This result is in agreement with the observation of Ueda and Shimizu [5] and Jardin [6], who found that segregated sulfur hinders the oxidation of Fe single crystals. The former authors found that Fe begins to oxidise only for 0 pressure higher than 10P6 Torr. Following these authors the sulfur acts as a cathodic protection, e.g. sulfur hinders the charge transfer necessary for oxygen adsorption. At higher oxygen pressures oxygen absorbs on surface sites not occupied by sulfur. The process continues by incorporation of oxygen atoms into the adjacent surface layers and their replacement by Fe atoms, which in turn adsorb oxygen, The resulting surface oxide layer explains the attenuation of the sulfur peak amplitude. Ueda and Shimizu [5] did not explain however the persistence of a residual sulfur peak, even after a long exposure time in oxygen. In fact, two mechanisms could be presented to be responsible for the attenuation of the sulfur Auger peak - when excluding the possibility of attenuation by desorption of SO,: (1) Sulfur is confined to the substrate/Fe-oxide interface. (2) Sulfur is diluted in the oxide layer. Considering the first mechanism, with the mean inelastic escape depth of the S LMM Auger peak taken to be about 4-5 A [16]. the amplitude of the sulfur Auger peak should decrease rapidly after the oxidation has taken place. Such behaviour was not observed (fig. la). Besides, supposing the formation of a continous Fe-oxide layer at the surface. and that all sulfur atoms are displaced by the same distance d, the oxide layer thickness d could be calculated from the attenuation of sulfur peak intensity, and is found to be about 2.7 A. Such a result is rather close to the thickness of half a monolayer segregated sulfur [17] but does not make sense for the formation of a y-Fe,O, oxide layer. Thus the persistence of a residual sulfur peak is not compatible with sulfur being dispelled to the interface oxide/substrate. The discussed mechanism is probably occurring for the oxidation of sulfur covered Ni-Cr [8] and Fe-NiiCr [6] surfaces, where sulfur and chromium cosegregate during heat treatment and form a chromium sulfide layer at the surface. In these cases the sulfur Auger peak was observed to decrease rapidly in the early oxidation stages (2 to 10 L). This difference in behaviour is most probably due to the greater affinity of chromium for oxygen than for sulfur, in contrast to Fe which has almost the same affinity for sulfur and oxygen [18]. So in Cr-charged alloys. chromium is readily displaced by oxygen from its sulfide to form an oxide layer at the surface via an exothermal reaction, with sulfur being displaced to the oxide/substrate interface. The second mechanism is the more probable one in our case. The slow progression of the oxidation reaction (result of a weak preferable interaction
M. Es-Souni, A. Moser
/ Effect of segregated S on oxidation of Fe - 6at %Sl
225
Fe-O) and the residual sulfur peak are compatible with sulfur being diluted in the Fe-oxide layer. No indications could however be inferred about the structure or energy of the Auger peaks of the elements involved, to state whether an oxysulfide has been formed or not. More work is needed to clarify this, particularly with X-ray photoelectron spectroscopy, which is a more suitable method for this purpose. Annealing at 450°C leads to the formation of silicon oxide SiO, and an intermediate oxide SiO, by diffusion of silicon from solid solution to the surface and its combination with oxygen. It forms most probably a thin layer of silicon oxide at the surface, surmounting the segregated sulfur. Similar stratification has also been found for the oxidation of a BN covered Fe-Si alloy surface using ESCA grazing emission [19], and could be deduced supposing that the surface energy of compounds is proportional to their heat of formation. The decrease in oxygen Auger signal is attributed to the desorption of physisorbed oxygen and also, to a lesser extent, to diffusion of this element to the adjacent surface layers. Such diffusion is probably due to the attractive effect of dissolved silicon, and was also observed in Fe-Al [20] and Pt-Si [21] alloys. Besides, the diffusion of oxygen is confirmed by annealing at 550°C where an increase in intensity of oxygen and “SiO” signals are observed (fig. 3). This is explicable supposing that diffusion of silicon and oxygen from the bulk to the surface occurs via a cosegregation mechanism (221. An increase in the sulfur Auger peak intensity is also observed. This matches with the supposition that a thickening of Si oxide at the surface results in its precipitation in the form of aggregates, thus making more sulfur “ visible”. This situation is still almost unchanged, when annealing at 650 o C. At 720 o C silicon oxide is thermally decomposed with desorption of oxygen. Only segregated sulfur is still present at the surface, the Auger peak intensity of which is however lower than that before oxidation (R, = 1.25 instead of 1.45) showing that some thermally activated desorption of SO, occurs. Furthermore, no, indications could be inferred concerning the influence of segregated sulfur on the stability of the oxide layer, since silicon oxide formed at a clean surface of the same alloy decomposes also in the range of temperatures indicated [23].
5. Conclusions The following conclusions can be drawn: (1) Sulfur segregates strongly to the surface and displaces from the surface. Its segregation kinetics follow a h-law.
all other
elements
(2) Segregated sulfur is found to decrease drastically the oxidation rate of Fe at room temperature, and in the range of oxygen pressure chosen, probably by hindering the charge transfer from Fe necessary for oxygen adsorption. (3) Oxygen absorbs on surface sites not occupied by sulfur. The oxidation continues with dilution of sulfur in the oxide layer. (4) During vacuum annealing at 450” C Fe is reduced and the surface is enriched with SiO, and an intermediate silicon oxide SiO,. Increasing the temperature leads to the precipitation of silicon oxide in the form of aggregates. As a result, segregated sulfur and Si-oxide aggregates share the surface. (5) At about 720” C silicon oxide decomposes thermally and the original S-covered surface is obtained again.
References [I] H.J. Grahke. EM. Peterhen and W. Paulitschke. Tranh. Roy. Sot. (London) A 795 (19X0) 1’8; H.J. Grahke. W. Paulitschke. G. Tauhe and V. Viefhaus. Surface SCI. 63 (1977) 377. [2] M. Es-Soum. Th&e de Doctorat d’Etat Strasbourg (1986). [3] T. Miyahara. K. Stolt. D.A. Reed and H.K. Birnhaum. Scripta Met. 19 (19X5) 117. 141 J. Oudar. Mater. Sci. Eng. 42 (1980) 101. [S] K. Ueda and R. Shimizu, Surface Sci. 43 (1974) 77. [6] C. Jardln. Th&se de Doctorat d’Etat Lyon (19X1 ). [7] R.K. Wild. Corroamn Set. 17 (1977) X7. [X] P. Humhert and A. Moser. Surface Sci. 126 (1983) 70X. [9] D.R. Baer and J.T. Prater. J. Vacuum Sci. Technol. 20 (19X2) 1396. [IO] I’. Marcus and J. Oudar. Mater. Sci. Eng. 42 (1980) 191. [I I] S. Hofman and J. Erlewein. Surface Sci. 77 (197X) 591. [I?] N.G. Ainslie and A. Seylbot. J. Iron Steel Inst. 194 (1960) 341. [13] M. Sro. J.B. Lumsden and R.W. Staehle, Surface Sci. SO (1975) 541. [14] SE. Grew. J.P. Roux and J.M. Blakely. Surface Sci. 120 (1982) 203. [IS] B. Carrierr, J.P. Deville and P. Humbert. J. Microac. Spectrosc. Electron. 10 (19X5) 29: see alo A. Moser. SC. Srivastava and B. C’arriere. Surface SCI. 133 (1983) L441. [ 161M.P. Seah and W.A. Dench. Surface Interfact. Anal. 1 (1979) 2. 1171 G. Lorang, L. Minel. S. Bouquet and J.P. Langeron. Surface SCI. I53 (19X5) 947. [1X] J. Benard. J. Oudar. N. Barbouth, F. Margot and Y. Berthier. Surface Sci. XX (1979) L35. [I91 P. Humbert. M. Es-Souni and A. Moser. Surface Interface Anal. 12 (1988) 21X. [20] N. Saidani. M. Es-Souni and A. Moser, J. Microsc. Spectrosc. Electron. 1 I (1986) 345. 1211 M. Salmeron and G.A. Somorjai, J. Vacuum SCI. Trchnol. 19 (19X1) 722. 1221 M. Guttmann. Surface Sci. 53 (1975) 213. [23] H. Vwfhaus and W. Roasoa. Surface Sci. 141 (19X4) 341.