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Electrochemical and surface analysis of the Fe-Cr-Ru system in non-oxidizing acid solutions
Applied Surface North-Holland
Science 44
ELECTROCHEMICAL IN NON-OXIDIZING
(1990)7-15
AND SURFACE ANALYSIS ACID SOLUTIONS
OF THE Fe-Cr-Ru
SYSTEM
SC. TJONG Council for Mineral Received
15 August
Technology, Private Bag X 3015, Randburg 2125, Rep. of South Afrrca 1989; accepted
for publication
19 October
1989
The effect of ruthenium addition on the spontaneous passivation behaviour of Fe-40Cr alloy in 0.5M H2S0, and 0.5M HCl acid solutions has been studied. Auger and XPS techniques were also used to investigate the surface chemistries of the spontaneously passivated film. Electrochemical measurements indicate that the Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys exhibit spontaneous passivation upon exposing them in both hydrochloric and sulphuric acid solutions from 25 to 85 o C. However, the transition time for spontaneous passivation reduces dramatically with an increase in the ruthenium content and solution temperature. Furthermore, this transition time also decreases for the investigated alloys exposed in a less aggressive sulphuric acid solution. AES results show that ruthenium and chromium are enriched in the spontaneous passive films formed on the Fe-40Cr-O.lRu alloy in both hydrochloric and sulphuric acid solutions at 25 o C, and also in the spontaneous passive film formed on the Fe40Cr-0.2Ru alloy in hydrochloric acid solution at 25OC. AES does not detect the presence of ruthenium in the spontaneous passive film formed on the Fe-40Crr0.2Ru alloy in sulphuric acid solution. However, XPS analysis shows that ruthenium and chromium are incorporated into the spontaneous passive films formed on the Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys in both hydrochloric and sulphuric acid solutions as Ru“+ and Cr3+ species.
1. Introduction It is well known that Fe-Cr alloys are characterized by an active-passive transition in nonoxidizing hydrochloric and sulphuric acids. These alloys can be easily transformed from a state of active corrosion into the passive state by alloying them with small amounts of platinum group metals (PGM’s). Such alloys are therefore spontaneously passivated in non-oxidizing environments in the presence of PGM’s. The spontaneous passivation results in a significant reduction of the corrosion rate in non-oxidizing acids. Furthermore, the maintenance of the spontaneous passivation on Fe-Cr alloys in non-oxidizing acid environments depends critically on the chromium content. It has been reported that the Fe-Cr alloys containing 40 wt% Cr and 0.2 wt% Pd exhibited the highest rate of self-passivation and high corrosion resistance in boiling sulphuric acid solution of up to 50% concentration [1,2]. The Fe-40Cr-0.2Pd alloy has superior corrosion resistance to the Hastelloy alloys grade A, B and C [2]. Therefore, the Fe0169.4332/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
40Cr-PGM alloy system has excellent potential for application in the chemical industry, provided that the cost were competitive. In terms of economic considerations, ruthenium costs about one seventh as much as platinum, and half as much as palladium [3]. Significantly, small additions of ruthenium to Fee40Cr alloys would be more cost effective than cathodic additions of palladium. Greene et al. have assessed the effect of the additions of various PGM’s on the corrosion behaviour of chromium in non-oxidizing acids [4]. Their results indicate that the effectiveness of the various alloy additions to chromium exposed in sulphuric and hydrochloric acid solutions generally decreases in the order: iridium, rhodium, ruthenium, platinum, palladium and osmium. According to Streicher’s report. Fe-28Cr-4Mo alloys with small additions of ruthenium exhibited greater pitting resistance in halide media than those alloyed with palladium or platinum [S]. Recently, Chernova et al. reported that the addition of ruthenium is more effective than palladium in
enhancing the corrosion resistance of Fe-24Cr 0.2Ti in non-oxidizing acids and sodium chloride solution [6]. From the literature cited above [ 1.2.4-61, these workers having mainly concentrated on the conventional immersion tests to investigate the corrosion behaviour of Fe-Cr alloys containing PGM additions. Little information is available on the nature of the spontaneous passive film formed on the Fe-Cr-PGM alloy system in non-oxidizing acids. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used extensively for the purpose of identifying the elemental constituents in the passive film formed on the Fe-Cr alloys [7-111. XPS can further provide chemical bonding information on the passivating species. From these reports it can be concluded that the passive film on Fe-Cr alloys with Cr content 2 1X wt’% consists of chromium oxyhydroxide [lO,ll] with the bound water in the form of OH [ll]. On the other hand, it was shown by Tjong [12] using the AES technique that the film formed spontaneously on Fe-40Cr-0.2Pd alloys in OSM HCI solution is enriched in palladium relative to chromium. The enrichment of Pd in the spontaneous passive film permits a low overvoltage for hydrogen evolution, and thereby promotes passivation of the Fe-40Cr-0.2Pd alloy. On the basis of electron microscopic observations. Tomashov et al. [13] reported that large particles of Pd with the sizes of the order of - 1 pm tend to accumulate on the surface of a 25Cr-O.lPd alloy during spontaneous passivation. However. smaller Pd particles with a size of 200&600 A were accumulated on the surface of chromium alloy containing 0.2% and 0.5% of palladium addition. Large Pd particles are less efficient in promoting spontaneous passivation than are smaller particles
Fe-4ocr Fe-4OCr-0.1 Ru Fe-4OCr-0.2Ru
0.01 i 0.02 < 0.02
0.01 < 0.01 < 0.01
[Ii]. This paper reports on an investigation of the electrochemical and surface characteristics of the passive film formed on the Fe-40Cr -0.1 Ru and Fe_4OCr-0.2Ru alloys in both 0.5M HC‘I and 0.5M H,SO, solutions by means of the electrochemical. AES and XPS technique.
2. Experimental The Fe+40Cr. Fe+40Cr-0.1 Ru and Fe-40C‘r 0.2Ru alloys were prepared by vacuum induction melting and subsequently cast into ingots of length - 100 mm and diameter of - 20 mm. The chemical compositions of the alloys investigated are listed in table 1. Specimens of 11 mm diameter and 2 mm thickness were cut from the ingots. All the specimens were solution-treated at 1200 o C’ for 2 h. followed by quenching in water. These specimens were ground with Sic paper down to X00 grit. and were further polished with diamond paste down to 1 pm. They were ultrasonically cleaned in distilled water. Electrochemical measurements were performed in a nitrogen-purged 0.5M HCI and 0.5M H,SO, acid solutions using a Princeton Applied Research 350 corrosion measurement systern. This system has a built-in microprocessor which automatically displayed the electrode potential. A standard glass cell with five bottlenecks housing the working electrode. a saturated calomel reference electrode and graphite counter electrodes was used for the electrochemical studies. Each sample was immersed in the electrolyte and then held cathodically at -800 mV versus SCE for 5 min. It is expected that such a treatment would reduce the air-formed oxide on the sample. Following the cathodic treatment. the variation of the potential with time was recorded.
0.004 0.002
ix.9 38.5 41.5
u.14 0.25
(70.6 60.5 58 0
The solution temperatures for potential-time response measurements were controlled at 25. 40. 60 and 85 o C. respectively. For the potentiodynamic test, the sample was also cathodically reduced at - 800 mV versus SCE for 5 min. The potentiodynamic scan was automatically initiated at this potential at 1 mV/s and continued until 1200 mV versus SCE. XPS recordings were taken with a VG spectrometer equipped with a non-monochromatized Al Kn (1486.6 eV) source. The Al X-ray source was operated at 11 kV and 20 mA. The base pressure in the spectrometer was better than 1.0 x lo-” Tnrr. The sputtering was performed with an argon ion beam energy of 2 keV, current density of 7 PA/cm’ and with the chamber backfilled to a pressure of 3.5 x lo-’ Torr. Auger analysis was carried out using a Varian UHV chamber equipped with a Physical Electronics model 15-110 A cylindrical mirror analyzer. The chamber was ion pumped to a base pressure of about 10 “’ Torr. A primary electron beam with an accelerating voltage of 5 keV and beam current of 2 PA was used. Sputtering was performed with a 2 keV Ar’ beam with a current density of 7 PA/cm’ and at a pressure of 3.5 X lop5 Torr. Calibration of sputter rate was based on a Ta?O, sample of known thickness. Under these conditions, a sputter rate of - 4 A/min was obtained. Auger depth profiles were obtained by measuring peak-to-peak heights (pph) of the constituent elements in the passive film. The elements S, Cl, Ru. C, 0. Cr and Fe were monitored at 151, 179. 231, 273. 510, 529 and 703 eV, respectively. The spectrum of pure Ru shows five strong Auger peaks located at 37, 150, 200, 231, and 273 eV. respectively [14]. The strongest Auger Ru peak at 273 eV interferes with the principal peak for carbon (273 eV). Furthermore, the Ru(150 eV) peak interferes with the S peak located at 152 eV. Thus the Auger Ru(231 eV) peak was used in this work. The use of the Ru(231 eV) peak would result in a much smaller value of Ru concentration due to the small size of the Ru(231 eV) peak as compared with the strongest peak at 273 eV. Carbon is not shown in the depth profile due to interference with the Ru(273 eV) peak. It is known
---~:e.40C~r-O.l
- ---
Current
Density.
Ku
Fe-40C r.0.2 Ku
‘VA/Cd
Fig. 1. Polarization curves of the Fec4OCr. Fe-4OCr-O.lRu and Fec4OC‘r-O.2Ru alloys in 0.5M HCI solution at 25 ’ C.
that the O(510 eV) peak overlaps with the Cr(529 eV) peak, so that the pph measured for Cr would result in large errors. On the other hand, the background to negative excursion for Cr(529 eV) has been shown to give a corrected Cr content in the Fe-Cr alloys [15,16]. Thus the negative going peak height of the Cr(529 eV) was used in this work.
3. Results and discussion Figs. 1 and 2 show the pntentiodynamic curves of the Fe-40Cr, Fe-40CrO.lRu and Fee40Cr0.2Ru alloys exposed in 0.5M HCI and 0.5M H,SO, acid solutions at 25”C, respectively. From fig. 1, the potentiodynamic curve of the Fe-40Cr alloy exposed in OSM HCl solution is well characterized by an active-passive transition, a higher
800
-------
Fig. 2. Polarization and Fe-4OCr-0.2Ru
Fe.dOCr
Fe-dOCr.O.IKu
curves of the Fe-40Cr. Fe-40Cr-O.lRu alloys in 0.5M H2S0, solution at 25’ C.
-6OOi0
I 100
,
I I / 300 200
rmm.
Fig. 3. Potential-time exposed in OSM HCI tively. For purposes sponse curve for the
responses for the Fe-40Cr-O.lRu alloy solution at 25, 40. 60 and 85 o C, respecof comparison, the potential-time reFe-400 alloy in 0.5M HCI solution at 25 o C is also shown.
critical current density for passivation (I,,) and a higher current density in the passive region (Ir). With the addition of 0.1% Ru to the Fe-40Cr alloy, one can see that the I,, and I, tend to decrease. As the ruthenium content is increased to 0.2%. the active-passive transition in the polarization curve disappears. As a result, the corrosion potential of the Fe-40Cr-0.2Ru alloy exposed in hydrochloric solution tends to move into a more noble direction, i.e. -280 mV versus SCE. On the the potentiodynamic curves of other hand, Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys exposed in OSM H,SO, solution do not exhibit an active-passive transition. This means that the addition of 0.1% Ru to the Fe-40Cr alloy causes spontaneous passivation to occur much more easily in this less aggressive sulphuric acid solution. Fig. 3 shows the potential-time responses for the Fee40Cr-O.lRu alloy exposed in OSM HCI solution at various temperatures after cathodic reduction at - 800 mV. At 25” C. the initial potential of the Fe-40Cr-0.1 Ru alloy was initially located at -564 mV and slowly shifted towards the passive region. This alloy undergoes spontaneous passivation after 550 s as evidenced by a sharp transition to a passivation potential of about - 310 mV. With increasing the temperature of the tested solution, the transition times for spontaneous passivation tend to decrease. At 85°C. it required only 60 s for spontaneous passivation to
I 400
I 500
SEC
Fig. 4. Potential-time responses for the Fe-4OCr-0.2Ru alloy exposed in 0.5M HCI solution at 25. 40. 60 and 85 0 c‘. respectively.
take place. For purposes of comparison, the potential-time response curve of the Fee40Cr alloy is also shown in this figure. The Fe-40Cr alloy does not exhibit spontaneous passivation as the corrosion potential remains in the active region instead of shifting towards the noble direction. As the ruthenium content in the Fe-40Cr alloy is increased to 0.2%, the transition times for spontaneous passivation tend to decrease as shown in fig. 4. On the basis of these results, it appears that the transition time for the occurrence of spontaneous passivation depends critically on both the ruthenium content and solution temperature. Fig. 5 shows the potential-time responses for the Fe-40Cr-0.1 Ru and Fe-40Cr-0.2Ru alloys
"E
I(-’
i
Fe-4OCr-0.1
Ku
. Fe-4OCr-O.PKu
Fig. S. PotentialLtime responses Fe-40Cr-0.2Ru alloys exposed 25°C.
for the Fe--4OCr~O.lRu and in 0.5M H,SO, wlutlon at
11 100
I
I
200 300 ELECTRON
,I.
1
400 500 ENERGY.
,I
I
ml eV
I,
703
Sputter
Time,
Min
Fig. 7. AES depth profile for the Fe-40Cr-0.2Ru alloy passivated spontaneously in 0.5M HCI solution at 25 o C. R-u 100
K”
2co 300 ELECTRON
, 400 5co ENERGY,
I
too eV
I
I
I,
700
Fig. 6. Auger spectra for the Fe-40CrP0.2Ru alloy passivated spont,meously in 0.5M HCI solution at 25°C: (a) before sputtering, (b) after sputtering for 8 min.
exposed in 0.5M H,SO4 solution at 25°C. It can be seen that the transition time for spontaneous passivation is reduced remarkably in the sulphuric acid solution as compared with that in hydrochloric solution. Spontaneous passivation occurs almost immediately for the Fe-40Cr-0.2Ru alloy after the removal of a cathodic potential of -800 mV. Figs. 6a and 6b show typical Auger spectra of the Fe-40Cr-0.2Ru alloy after spontaneous passivation in 0.5M HCl solution at 25 o C. The Auger spectra show the presence of ruthenium in the spontaneously passivated oxide film. The depth profile for the Fe-40Cr-0.2Ru alloy after spontaneous passivation in 0.5M HCl solution is shown in fig. 7. The apparent thickness of the passivated oxide film can be estimated from the AES depth profile when the oxygen amplitude has reduced to half its maximum amplitude, and the sputter rate was assumed to be correlated with that of Ta,O,. Thus the apparent thickness of the spontaneously passivated film is estimated to be 32 A. Figs. 8 and 9 show the Cr/(Cr + Fe + Ru) and Ru/(Cr + Fe + Ru) normalized peak height ratios versu.5 sputtering time for passivated films sponta-
neously formed on the Fee40Cr-O.lRu and Fe-40Cr-0.2Ru alloys in 0.5M HCl solution at 25 ’ C, respectively. It is apparent that enrichment of Cr and Ru occurs in the passivated films spontaneously formed on these two alloys in hydrochloric solution. Tomashov et al. [17] reported that the initial dissolution of Cr into electrolyte from chromium alloys containing PGM additions resulted in the surface diffusion of PGM’s into the surface defect sites of the lattice such as the edges, kinks, corners, etc. This is because the PGM atoms lose their bonds to adjacent Cr atoms and hence become adatoms during active dissolution of the alloy. A significant PGM redistribution associated
2 d 0
Lz
iL 2 0.4 r $0.3 II 5 E 0.2 s 20.1 1 B =
0
10 Sputter
20 lime.
30 Min
Fig. 8. Normalized Auger peak height ratios Cr/(Cr+ Fe + Ru) and Ru/(Cr+ Fe+ Ru) versus sputtering time for the passivated film spontaneously formed on the Fe-40Cr-0.1 Ru alloy in 0.5M HCl solution at 25 o C.
17
,--A Sputter
I ime. Min
A
Auger peak height ratios Cr/(Cr + Fe+ Ku) and Ru/(Cr+ Fe+ Ru) versus sputtering time for the passivated film spontaneously formed on the Fe-40Cr-0.2Ru alloy in 0.5M HCI solution at 25 o C.
with the surface diffusion resulted in the accumulation of PGM’s in the active sites. Consequently. the PGM’s block the defect points in the lattice. and hence the rate of the dissolution of Cr from the chromium alloys in the active sites becomes lower [17]. From figs. 8 and 9. one can see that the normalized ruthenium peak height of the Fe-40Cr-O.lRu alloy is greater than that of the Fe40CrP0.2Ru alloy. This may be explained by the fact that the rate of active dissolution preceding passivation of the FeP40Cr-O.lRu alloy in 0.5M HCI solution is faster than that of the Fe-4OCr0.2Ru alloy. As a result, larger amounts of ruthenium adatoms are accumulated on the surface of FeP40CrP0.1Ru alloy than that of the Fe40Cr-0.2Ru alloy. Figs. 10a and lob show the Auger depth profile of the FeP40-O.lRu and Fee40Cr-0.2Ru alloys spontaneously passivated in 0.5MH,SO, solution at 25” C. From these figures, ruthenium is only incorporated in the passivated film of the Fe 40CrP0.1Ru alloy. It is suggested that the active dissolution rate of Cr from the Fe-40CrP0.2Ru alloy before the onset of passivation is much lower than that of the Fe-40Cr--O.lRu alloy in 0.5M H,SO, solution. Comparing fig. 7 with lob. it is obvious that ruthenium is detected on the film passivated on the FeP40Cr-0.2Ru alloy in 0.5M HCI but not in 0.5 M H,SO, solution. This behaviour can also be expected as larger quantities
1
0
Fig. 9. Normahzed
Cjputter Time.
Win
b_
10 20 Sputter Time. Min
Fig. 10. AES depth profde for the passivated film \pontaneously formed on: (a) Fe-~40C‘rP0.1Ru and (h) Fr-4OC‘r 0.2Ru alloy m 0.5M H ,SO, solution at 2.5 o c‘.
of active dissolution occur in a more aggressive hydrochloric solution than that of the sulphuric acid solution, and hence larger amounts ol ruthenium are accumulated on the surface of FeP40CrP0.2Ru alloy exposed in hydrochloric solution. Fig. 11 shows the Ru 3d,, 1,3, 2 photoelectron spectra for the FeP40CrP0.1Ru alloy spontaneously passivated in 0.5M H,SO, acid solution
278 281 284 287 290 Binding Energy , e\ Fig. Il. Ru 3d spectra recorded from thr Fc+4OC‘rcO.l Ku allc~y paaaivated spontaneously in 0.5M H,SO, wlution at 25OC‘: (1) before sputtermg, (2) after 2 min of sputtermg. (3) after 6 min of sputtering, (4) after 10 min of sputtering.
S.C. TJong / Fe-Cr-Ru
at 25°C. In this figure, a peak at - 280.6 eV before sputtering arises from the Ru4+ state associated with the Ru3d,,, electrons in the surface film. In their XPS study of the surface chemistry of the ruthenium-oxygen system, Rim and Winograd [18] have indicated that the 280.7 eV peak arises from the Ru3d,,, electrons of the RuO, oxide whereas the 281.4 eV peak arises from the electrons of the hydrated RuO, oxide. It Ru 3d5,2 is well known that ruthenium is characterized by high catalytic activity and low overvoltage for oxygen evolution [19]. Furthermore, the pure ruthenium in aqueous electrolytes easily adsorbs oxygen and forms chemically stable oxides [6]. In chromium steel containing ruthenium, Ru tends to be incorporated into the hydroxide or oxide layers formed on the chromium steel [6]. From fig. 11 one can also see that the Ru 3d,,, peak at - 284.9 eV interferes with the Cls peak located at 284.8 eV. Carbon is originated from the hydrocarbon species within the solution or atmosphere. A precise determination of the Ru oxide and hydroxide by peak deconvolution is complicated by the interference of the Cls peak due to the unknown contribution of the Cls line. However, as the Ru3d,,, peak in this work is rather broad (full - 2.5 eV), and exhibiting width at half maximum an asymmetry towards the higher binding energy values, it seems likely that the ruthenium hydroxide may be present in the passivated film spontaneously formed, in addition to the RuO, oxide phase. From fig. 11, the FWHM or Ru 3d,,, peak was reduced to about 1.8 eV, after sputtering for 2 min indicating the removal of at least 1 species by sputtering. The carbon contamination is partially removed by sputtering and hence the intensity of increases. Furthermore, the Ru 3d s,2 Ru 3d,,, peak shifts from 280.6 to 280.0 eV after sputtering. Kim and Winograd have reported that the was located Ru 3d 5,2 peak for metallic ruthenium at 280.0 eV [18]. The appearance of the metallic ruthenium after a short-time of sputtering may be an effect of the ion etching because Ru4+ species is reduced to Rumr’. It is well known that the rare gas sputtering can reduce the oxide phase to the metallic state due to the preferential removal of 02- ions [20]. Further sputtering to 10 min re-
VI non-oxidizing
acrd solutions
Binding
13
Energy,
e\
Fig. 12. Ru3d spectra recorded from the Fe-40Cr-0.2Ru alloy passivated spontaneously in 0.5M H,SO, solution at 25OC: (1) before sputtering, (2) after 2 min of sputtering.
sulted in a reduction of the intensity of the Run=’ 3d,,, signal. A decrease in the Rumrt peak can be expected because this signal is contributed directly from the metallic ruthenium of the bulk alloy. The concentration of ruthenium in the bulk alloy is extremely low (0.14 wt%). and the limit of detection for ruthenium is estimated to be - 0.1 wt% using the XPS technique. Fig. 12 shows the XPS spectra of the Ru 3d5,2.3,2 electrons for the passivated film spontaneously formed on the Fe-40Crr0.2Ru alloy in 0.5M H,SO, solution at 25°C. It is apparent from this figure that the XPS spectrum prior to sputtering shows the presence of
BINDING
ENERGY
, eV
Fig. 13. CrZp,,, spectrum for the passivated film spontaneously on the Fe+40Cr-O.lRu alloy in 0.5M solution at 25 o C.
formed H,SO,
Ku”’ 3d 5 7,3~1 electrons at 280.6 and 284.9 eV. respectively. This Ru ’ + species is also reduced to Ru”‘~ after sputtering for 2 min. As mentioned previously. AES does not detect the presence of ruthenium in the passivated film formed on this alloy in OSM H,SO, solution. This difference can be accounted for by the fact that XPS is more sensitive to the presence of ruthenium than is AES. For the Fe-40Cr-O.lRu and Fee40Cr 0.2Ru alloys exposed to OSM HCI solution. XPS shows the presence of ruthenium in the passivated film formed on these two alloys. Fig. 13 shows the Cr 2p,,,, spectrum for the Fee40Cr-O.lRu exposed in OSM H,SO, solution at 25°C. The Cr2p,,? spectrum prior to sputtering contains a major peak located at - 576.5 eV together with a shoulder located at 573.8 eV. The peak at 576.5 eV is contributed from the Crl+ species whereas the shoulder at 573.8 eV is contributed from the Cr”“’ species. The location of the Cr” and Cr”“’ peaks in this work is in good agreement with that of the Cr’- and Cr’“” peaks reported previously [ll]. Fig. 14 shows the 0 1s spectra for the Fe 40Cr-O.lRu alloy exposed to 0.5M H,SO, solution. The 0 Is spectrum of the passivated film prior to sputtering is very broad with a FWHM of about 4 eV. and it can be resolved at least into two peaks. The binding energy at 530.0 eV is assigned as 0’ bonding (M-O) associated with the Ku4 ’ species whereas the binding energy at
Fig. 14. 015 spectra for the passwated film formed spontaneously on the Fe-4OCr-0.lRu alloy III OSM HzSO, solution at 25 o C: (1) before sputtermg. (2) after 2 min of sputtering, (3) after 6 min of (puttering. (4) after 10 min of sputtering.
532.0 eV is assigned as OH-- bonding (M-OH) associated with Cr”’ or hydrated Ku” ’ species. The 01s spectrum shows a decrease in half-width (- 2.1 eV) and a shift to lower binding energy values after sputtering for 2 min. This implies that the M-OH species are partially removed by sputtering. At this stage, the M-0 bonding becomes predominant. Sputter etching was continued for 10 min until the residual 0 1s spectrum had reached a very small signal. corresponding to an almost complete removal of the oxidized species. The 0 1 s spectra of the Fee40Cr0.2Ru alloy exposed to 0.5M HCI and 0.5M H,SO, solutions exhibit similar characteristics as shown in fig. 14. On the basis of AES and XPS results, it appears that chromium and ruthenium from the Fee4OCr-O.lRu alloy are enriched in the passivating films formed on this alloy in both hydrochloric and sulphuric solutions at 25°C. and the films show oxyhydroxide characteristics.
4. Conclusions (a) Electrochemical measurements indicate that Fe-40Cr-0.1 Ru and Fe-40Cr-0.2Ru alloys exhibit spontaneous passivation in both hydrochloric and sulphuric acid solutions. However. the transition time for spontaneous passivation decreases with increasing ruthenium content and solution temperature. (b) AES measurements show that enrichment of Ku and Cr occurs in the films formed spontaneously in the Fee40Cr-O.lRu alloy in both hydrochloric and sulphuric acid solutions. AES does not detect the presence of Ru in the film formed spontaneously on the Fe-40Cr0.2Ru alloy in aulphuric acid solution. (c) XPS measurements show that ruthenium and chromium are incorporated in the films formed spontaneously on the Fee40Cr0.1 Ru and Fee40C’r--0.2Ru alloys in both hydrochloric and sulphuric acid solutions as Ru4 and Cr7 ’ species.
Acknowledgements This Council
work is published by permission of the for Mineral Technology. The author would
SC.
TJO~I~/ Fe-Cr-Ru
like to thank Professor J.B. Malherbe of the Department of Physics, University of Pretoria for the provision of Auger facilities and Dr. T.E. Derry of the Schonland Research Centre for Nuclear Science, University of the Witwatersrand for the use of XPS facilities.
References [l] G.P. Chernova, N.D. Tomashov and L.A. Chigirinskaja. Corrosion 34 (1978) 445. [2] N.D. Tomashov and G.P. Chernova, Prot. Met. 11 (1975) 319. [3] G.G. Robson. Platinum 1987 (Johnson Matthey, 1987) pp. 49-54. :4] N.D. Greene. C.R. Bishop and M. Stern, J. Electrochem. Sot. 108 (1961) 836. 151 M.A. Streicher. Corrosion 30 (1974) 77. ,6] G.P. Chernova, L.A. Chigirinskaja, N.D. Tomashov and E.A. Nasedkina. Prot. Met. 22 (1986) 704. [7] A.E. Yaniv, J.B. Lumsden and R.W. Staehle. J. Electrothem. Sot. 124 (1977) 490. [8] 1. Olefjord and H. Fischmeister. Corros. Sci. 15 (1975) 697.
in non-oxtdtrrng
acid solutwns
15
[9] K. Asami, K. Hashimoto and S. Shimodaria, Corros. Sci. 16 (1976) 387. [lo] K. Asami. K. Hashimoto and S. Shimodaria, Corros. Sci. 18 (1978) 151. [ll] S.C. Tjong, R.W. Hoffman and E.B. Yeager. J. Electrothem. Sot. 129 (1982) 1662. [12] S.C. Tjong. Surf. Coat. Technol., in press. [13] N.D. Tomashov. G.P. Chernova. L.N. Volkov, A.P. Zakharov and Z.E. Sheshina. Prot. Met. 9 (1974) 289. [14] L.E. Davis. N.D. MacDonald. P.W. Palmberg, G.E. Riach and R.E. Weber. Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie. MN. 1976) p. 135. [15] S.C. Tjong, J. Eldridge and R.W. Hoffman, Appl. Surf. Sci. 14 (1982) 297. [16] W.L.N. Matthews, P.J.K. Paterson and H.K. Wagenfeld, Appl. Surf. Sci. 15 (1983) 281. [17] N.D. Tomashov. G.P. Chernova and E.N. Ustinky. Corrosion 40 (1984) 134. [18] KS. Kim and N. Winograd, J. Catal. 35 (1974) 66. [19] R. Kiitz, H. Lewerenz and S. Stucki, J. Electrochem. Sot. 130 (1983) 825. [20] K. Asami, MS. De Sa and V. Ashworth. Corros. Sci. 26 (1986) 15.
Science 44
ELECTROCHEMICAL IN NON-OXIDIZING
(1990)7-15
AND SURFACE ANALYSIS ACID SOLUTIONS
OF THE Fe-Cr-Ru
SYSTEM
SC. TJONG Council for Mineral Received
15 August
Technology, Private Bag X 3015, Randburg 2125, Rep. of South Afrrca 1989; accepted
for publication
19 October
1989
The effect of ruthenium addition on the spontaneous passivation behaviour of Fe-40Cr alloy in 0.5M H2S0, and 0.5M HCl acid solutions has been studied. Auger and XPS techniques were also used to investigate the surface chemistries of the spontaneously passivated film. Electrochemical measurements indicate that the Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys exhibit spontaneous passivation upon exposing them in both hydrochloric and sulphuric acid solutions from 25 to 85 o C. However, the transition time for spontaneous passivation reduces dramatically with an increase in the ruthenium content and solution temperature. Furthermore, this transition time also decreases for the investigated alloys exposed in a less aggressive sulphuric acid solution. AES results show that ruthenium and chromium are enriched in the spontaneous passive films formed on the Fe-40Cr-O.lRu alloy in both hydrochloric and sulphuric acid solutions at 25 o C, and also in the spontaneous passive film formed on the Fe40Cr-0.2Ru alloy in hydrochloric acid solution at 25OC. AES does not detect the presence of ruthenium in the spontaneous passive film formed on the Fe-40Crr0.2Ru alloy in sulphuric acid solution. However, XPS analysis shows that ruthenium and chromium are incorporated into the spontaneous passive films formed on the Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys in both hydrochloric and sulphuric acid solutions as Ru“+ and Cr3+ species.
1. Introduction It is well known that Fe-Cr alloys are characterized by an active-passive transition in nonoxidizing hydrochloric and sulphuric acids. These alloys can be easily transformed from a state of active corrosion into the passive state by alloying them with small amounts of platinum group metals (PGM’s). Such alloys are therefore spontaneously passivated in non-oxidizing environments in the presence of PGM’s. The spontaneous passivation results in a significant reduction of the corrosion rate in non-oxidizing acids. Furthermore, the maintenance of the spontaneous passivation on Fe-Cr alloys in non-oxidizing acid environments depends critically on the chromium content. It has been reported that the Fe-Cr alloys containing 40 wt% Cr and 0.2 wt% Pd exhibited the highest rate of self-passivation and high corrosion resistance in boiling sulphuric acid solution of up to 50% concentration [1,2]. The Fe-40Cr-0.2Pd alloy has superior corrosion resistance to the Hastelloy alloys grade A, B and C [2]. Therefore, the Fe0169.4332/90/$03.50 (North-Holland)
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40Cr-PGM alloy system has excellent potential for application in the chemical industry, provided that the cost were competitive. In terms of economic considerations, ruthenium costs about one seventh as much as platinum, and half as much as palladium [3]. Significantly, small additions of ruthenium to Fee40Cr alloys would be more cost effective than cathodic additions of palladium. Greene et al. have assessed the effect of the additions of various PGM’s on the corrosion behaviour of chromium in non-oxidizing acids [4]. Their results indicate that the effectiveness of the various alloy additions to chromium exposed in sulphuric and hydrochloric acid solutions generally decreases in the order: iridium, rhodium, ruthenium, platinum, palladium and osmium. According to Streicher’s report. Fe-28Cr-4Mo alloys with small additions of ruthenium exhibited greater pitting resistance in halide media than those alloyed with palladium or platinum [S]. Recently, Chernova et al. reported that the addition of ruthenium is more effective than palladium in
enhancing the corrosion resistance of Fe-24Cr 0.2Ti in non-oxidizing acids and sodium chloride solution [6]. From the literature cited above [ 1.2.4-61, these workers having mainly concentrated on the conventional immersion tests to investigate the corrosion behaviour of Fe-Cr alloys containing PGM additions. Little information is available on the nature of the spontaneous passive film formed on the Fe-Cr-PGM alloy system in non-oxidizing acids. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used extensively for the purpose of identifying the elemental constituents in the passive film formed on the Fe-Cr alloys [7-111. XPS can further provide chemical bonding information on the passivating species. From these reports it can be concluded that the passive film on Fe-Cr alloys with Cr content 2 1X wt’% consists of chromium oxyhydroxide [lO,ll] with the bound water in the form of OH [ll]. On the other hand, it was shown by Tjong [12] using the AES technique that the film formed spontaneously on Fe-40Cr-0.2Pd alloys in OSM HCI solution is enriched in palladium relative to chromium. The enrichment of Pd in the spontaneous passive film permits a low overvoltage for hydrogen evolution, and thereby promotes passivation of the Fe-40Cr-0.2Pd alloy. On the basis of electron microscopic observations. Tomashov et al. [13] reported that large particles of Pd with the sizes of the order of - 1 pm tend to accumulate on the surface of a 25Cr-O.lPd alloy during spontaneous passivation. However. smaller Pd particles with a size of 200&600 A were accumulated on the surface of chromium alloy containing 0.2% and 0.5% of palladium addition. Large Pd particles are less efficient in promoting spontaneous passivation than are smaller particles
Fe-4ocr Fe-4OCr-0.1 Ru Fe-4OCr-0.2Ru
0.01 i 0.02 < 0.02
0.01 < 0.01 < 0.01
[Ii]. This paper reports on an investigation of the electrochemical and surface characteristics of the passive film formed on the Fe-40Cr -0.1 Ru and Fe_4OCr-0.2Ru alloys in both 0.5M HC‘I and 0.5M H,SO, solutions by means of the electrochemical. AES and XPS technique.
2. Experimental The Fe+40Cr. Fe+40Cr-0.1 Ru and Fe-40C‘r 0.2Ru alloys were prepared by vacuum induction melting and subsequently cast into ingots of length - 100 mm and diameter of - 20 mm. The chemical compositions of the alloys investigated are listed in table 1. Specimens of 11 mm diameter and 2 mm thickness were cut from the ingots. All the specimens were solution-treated at 1200 o C’ for 2 h. followed by quenching in water. These specimens were ground with Sic paper down to X00 grit. and were further polished with diamond paste down to 1 pm. They were ultrasonically cleaned in distilled water. Electrochemical measurements were performed in a nitrogen-purged 0.5M HCI and 0.5M H,SO, acid solutions using a Princeton Applied Research 350 corrosion measurement systern. This system has a built-in microprocessor which automatically displayed the electrode potential. A standard glass cell with five bottlenecks housing the working electrode. a saturated calomel reference electrode and graphite counter electrodes was used for the electrochemical studies. Each sample was immersed in the electrolyte and then held cathodically at -800 mV versus SCE for 5 min. It is expected that such a treatment would reduce the air-formed oxide on the sample. Following the cathodic treatment. the variation of the potential with time was recorded.
0.004 0.002
ix.9 38.5 41.5
u.14 0.25
(70.6 60.5 58 0
The solution temperatures for potential-time response measurements were controlled at 25. 40. 60 and 85 o C. respectively. For the potentiodynamic test, the sample was also cathodically reduced at - 800 mV versus SCE for 5 min. The potentiodynamic scan was automatically initiated at this potential at 1 mV/s and continued until 1200 mV versus SCE. XPS recordings were taken with a VG spectrometer equipped with a non-monochromatized Al Kn (1486.6 eV) source. The Al X-ray source was operated at 11 kV and 20 mA. The base pressure in the spectrometer was better than 1.0 x lo-” Tnrr. The sputtering was performed with an argon ion beam energy of 2 keV, current density of 7 PA/cm’ and with the chamber backfilled to a pressure of 3.5 x lo-’ Torr. Auger analysis was carried out using a Varian UHV chamber equipped with a Physical Electronics model 15-110 A cylindrical mirror analyzer. The chamber was ion pumped to a base pressure of about 10 “’ Torr. A primary electron beam with an accelerating voltage of 5 keV and beam current of 2 PA was used. Sputtering was performed with a 2 keV Ar’ beam with a current density of 7 PA/cm’ and at a pressure of 3.5 X lop5 Torr. Calibration of sputter rate was based on a Ta?O, sample of known thickness. Under these conditions, a sputter rate of - 4 A/min was obtained. Auger depth profiles were obtained by measuring peak-to-peak heights (pph) of the constituent elements in the passive film. The elements S, Cl, Ru. C, 0. Cr and Fe were monitored at 151, 179. 231, 273. 510, 529 and 703 eV, respectively. The spectrum of pure Ru shows five strong Auger peaks located at 37, 150, 200, 231, and 273 eV. respectively [14]. The strongest Auger Ru peak at 273 eV interferes with the principal peak for carbon (273 eV). Furthermore, the Ru(150 eV) peak interferes with the S peak located at 152 eV. Thus the Auger Ru(231 eV) peak was used in this work. The use of the Ru(231 eV) peak would result in a much smaller value of Ru concentration due to the small size of the Ru(231 eV) peak as compared with the strongest peak at 273 eV. Carbon is not shown in the depth profile due to interference with the Ru(273 eV) peak. It is known
---~:e.40C~r-O.l
- ---
Current
Density.
Ku
Fe-40C r.0.2 Ku
‘VA/Cd
Fig. 1. Polarization curves of the Fec4OCr. Fe-4OCr-O.lRu and Fec4OC‘r-O.2Ru alloys in 0.5M HCI solution at 25 ’ C.
that the O(510 eV) peak overlaps with the Cr(529 eV) peak, so that the pph measured for Cr would result in large errors. On the other hand, the background to negative excursion for Cr(529 eV) has been shown to give a corrected Cr content in the Fe-Cr alloys [15,16]. Thus the negative going peak height of the Cr(529 eV) was used in this work.
3. Results and discussion Figs. 1 and 2 show the pntentiodynamic curves of the Fe-40Cr, Fe-40CrO.lRu and Fee40Cr0.2Ru alloys exposed in 0.5M HCI and 0.5M H,SO, acid solutions at 25”C, respectively. From fig. 1, the potentiodynamic curve of the Fe-40Cr alloy exposed in OSM HCl solution is well characterized by an active-passive transition, a higher
800
-------
Fig. 2. Polarization and Fe-4OCr-0.2Ru
Fe.dOCr
Fe-dOCr.O.IKu
curves of the Fe-40Cr. Fe-40Cr-O.lRu alloys in 0.5M H2S0, solution at 25’ C.
-6OOi0
I 100
,
I I / 300 200
rmm.
Fig. 3. Potential-time exposed in OSM HCI tively. For purposes sponse curve for the
responses for the Fe-40Cr-O.lRu alloy solution at 25, 40. 60 and 85 o C, respecof comparison, the potential-time reFe-400 alloy in 0.5M HCI solution at 25 o C is also shown.
critical current density for passivation (I,,) and a higher current density in the passive region (Ir). With the addition of 0.1% Ru to the Fe-40Cr alloy, one can see that the I,, and I, tend to decrease. As the ruthenium content is increased to 0.2%. the active-passive transition in the polarization curve disappears. As a result, the corrosion potential of the Fe-40Cr-0.2Ru alloy exposed in hydrochloric solution tends to move into a more noble direction, i.e. -280 mV versus SCE. On the the potentiodynamic curves of other hand, Fe-40Cr-O.lRu and Fe-40Cr-0.2Ru alloys exposed in OSM H,SO, solution do not exhibit an active-passive transition. This means that the addition of 0.1% Ru to the Fe-40Cr alloy causes spontaneous passivation to occur much more easily in this less aggressive sulphuric acid solution. Fig. 3 shows the potential-time responses for the Fee40Cr-O.lRu alloy exposed in OSM HCI solution at various temperatures after cathodic reduction at - 800 mV. At 25” C. the initial potential of the Fe-40Cr-0.1 Ru alloy was initially located at -564 mV and slowly shifted towards the passive region. This alloy undergoes spontaneous passivation after 550 s as evidenced by a sharp transition to a passivation potential of about - 310 mV. With increasing the temperature of the tested solution, the transition times for spontaneous passivation tend to decrease. At 85°C. it required only 60 s for spontaneous passivation to
I 400
I 500
SEC
Fig. 4. Potential-time responses for the Fe-4OCr-0.2Ru alloy exposed in 0.5M HCI solution at 25. 40. 60 and 85 0 c‘. respectively.
take place. For purposes of comparison, the potential-time response curve of the Fee40Cr alloy is also shown in this figure. The Fe-40Cr alloy does not exhibit spontaneous passivation as the corrosion potential remains in the active region instead of shifting towards the noble direction. As the ruthenium content in the Fe-40Cr alloy is increased to 0.2%, the transition times for spontaneous passivation tend to decrease as shown in fig. 4. On the basis of these results, it appears that the transition time for the occurrence of spontaneous passivation depends critically on both the ruthenium content and solution temperature. Fig. 5 shows the potential-time responses for the Fe-40Cr-0.1 Ru and Fe-40Cr-0.2Ru alloys
"E
I(-’
i
Fe-4OCr-0.1
Ku
. Fe-4OCr-O.PKu
Fig. S. PotentialLtime responses Fe-40Cr-0.2Ru alloys exposed 25°C.
for the Fe--4OCr~O.lRu and in 0.5M H,SO, wlutlon at
11 100
I
I
200 300 ELECTRON
,I.
1
400 500 ENERGY.
,I
I
ml eV
I,
703
Sputter
Time,
Min
Fig. 7. AES depth profile for the Fe-40Cr-0.2Ru alloy passivated spontaneously in 0.5M HCI solution at 25 o C. R-u 100
K”
2co 300 ELECTRON
, 400 5co ENERGY,
I
too eV
I
I
I,
700
Fig. 6. Auger spectra for the Fe-40CrP0.2Ru alloy passivated spont,meously in 0.5M HCI solution at 25°C: (a) before sputtering, (b) after sputtering for 8 min.
exposed in 0.5M H,SO4 solution at 25°C. It can be seen that the transition time for spontaneous passivation is reduced remarkably in the sulphuric acid solution as compared with that in hydrochloric solution. Spontaneous passivation occurs almost immediately for the Fe-40Cr-0.2Ru alloy after the removal of a cathodic potential of -800 mV. Figs. 6a and 6b show typical Auger spectra of the Fe-40Cr-0.2Ru alloy after spontaneous passivation in 0.5M HCl solution at 25 o C. The Auger spectra show the presence of ruthenium in the spontaneously passivated oxide film. The depth profile for the Fe-40Cr-0.2Ru alloy after spontaneous passivation in 0.5M HCl solution is shown in fig. 7. The apparent thickness of the passivated oxide film can be estimated from the AES depth profile when the oxygen amplitude has reduced to half its maximum amplitude, and the sputter rate was assumed to be correlated with that of Ta,O,. Thus the apparent thickness of the spontaneously passivated film is estimated to be 32 A. Figs. 8 and 9 show the Cr/(Cr + Fe + Ru) and Ru/(Cr + Fe + Ru) normalized peak height ratios versu.5 sputtering time for passivated films sponta-
neously formed on the Fee40Cr-O.lRu and Fe-40Cr-0.2Ru alloys in 0.5M HCl solution at 25 ’ C, respectively. It is apparent that enrichment of Cr and Ru occurs in the passivated films spontaneously formed on these two alloys in hydrochloric solution. Tomashov et al. [17] reported that the initial dissolution of Cr into electrolyte from chromium alloys containing PGM additions resulted in the surface diffusion of PGM’s into the surface defect sites of the lattice such as the edges, kinks, corners, etc. This is because the PGM atoms lose their bonds to adjacent Cr atoms and hence become adatoms during active dissolution of the alloy. A significant PGM redistribution associated
2 d 0
Lz
iL 2 0.4 r $0.3 II 5 E 0.2 s 20.1 1 B =
0
10 Sputter
20 lime.
30 Min
Fig. 8. Normalized Auger peak height ratios Cr/(Cr+ Fe + Ru) and Ru/(Cr+ Fe+ Ru) versus sputtering time for the passivated film spontaneously formed on the Fe-40Cr-0.1 Ru alloy in 0.5M HCl solution at 25 o C.
17
,--A Sputter
I ime. Min
A
Auger peak height ratios Cr/(Cr + Fe+ Ku) and Ru/(Cr+ Fe+ Ru) versus sputtering time for the passivated film spontaneously formed on the Fe-40Cr-0.2Ru alloy in 0.5M HCI solution at 25 o C.
with the surface diffusion resulted in the accumulation of PGM’s in the active sites. Consequently. the PGM’s block the defect points in the lattice. and hence the rate of the dissolution of Cr from the chromium alloys in the active sites becomes lower [17]. From figs. 8 and 9. one can see that the normalized ruthenium peak height of the Fe-40Cr-O.lRu alloy is greater than that of the Fe40CrP0.2Ru alloy. This may be explained by the fact that the rate of active dissolution preceding passivation of the FeP40Cr-O.lRu alloy in 0.5M HCI solution is faster than that of the Fe-4OCr0.2Ru alloy. As a result, larger amounts of ruthenium adatoms are accumulated on the surface of FeP40CrP0.1Ru alloy than that of the Fe40Cr-0.2Ru alloy. Figs. 10a and lob show the Auger depth profile of the FeP40-O.lRu and Fee40Cr-0.2Ru alloys spontaneously passivated in 0.5MH,SO, solution at 25” C. From these figures, ruthenium is only incorporated in the passivated film of the Fe 40CrP0.1Ru alloy. It is suggested that the active dissolution rate of Cr from the Fe-40CrP0.2Ru alloy before the onset of passivation is much lower than that of the Fe-40Cr--O.lRu alloy in 0.5M H,SO, solution. Comparing fig. 7 with lob. it is obvious that ruthenium is detected on the film passivated on the FeP40Cr-0.2Ru alloy in 0.5M HCI but not in 0.5 M H,SO, solution. This behaviour can also be expected as larger quantities
1
0
Fig. 9. Normahzed
Cjputter Time.
Win
b_
10 20 Sputter Time. Min
Fig. 10. AES depth profde for the passivated film \pontaneously formed on: (a) Fe-~40C‘rP0.1Ru and (h) Fr-4OC‘r 0.2Ru alloy m 0.5M H ,SO, solution at 2.5 o c‘.
of active dissolution occur in a more aggressive hydrochloric solution than that of the sulphuric acid solution, and hence larger amounts ol ruthenium are accumulated on the surface of FeP40CrP0.2Ru alloy exposed in hydrochloric solution. Fig. 11 shows the Ru 3d,, 1,3, 2 photoelectron spectra for the FeP40CrP0.1Ru alloy spontaneously passivated in 0.5M H,SO, acid solution
278 281 284 287 290 Binding Energy , e\ Fig. Il. Ru 3d spectra recorded from thr Fc+4OC‘rcO.l Ku allc~y paaaivated spontaneously in 0.5M H,SO, wlution at 25OC‘: (1) before sputtermg, (2) after 2 min of sputtermg. (3) after 6 min of sputtering, (4) after 10 min of sputtering.
S.C. TJong / Fe-Cr-Ru
at 25°C. In this figure, a peak at - 280.6 eV before sputtering arises from the Ru4+ state associated with the Ru3d,,, electrons in the surface film. In their XPS study of the surface chemistry of the ruthenium-oxygen system, Rim and Winograd [18] have indicated that the 280.7 eV peak arises from the Ru3d,,, electrons of the RuO, oxide whereas the 281.4 eV peak arises from the electrons of the hydrated RuO, oxide. It Ru 3d5,2 is well known that ruthenium is characterized by high catalytic activity and low overvoltage for oxygen evolution [19]. Furthermore, the pure ruthenium in aqueous electrolytes easily adsorbs oxygen and forms chemically stable oxides [6]. In chromium steel containing ruthenium, Ru tends to be incorporated into the hydroxide or oxide layers formed on the chromium steel [6]. From fig. 11 one can also see that the Ru 3d,,, peak at - 284.9 eV interferes with the Cls peak located at 284.8 eV. Carbon is originated from the hydrocarbon species within the solution or atmosphere. A precise determination of the Ru oxide and hydroxide by peak deconvolution is complicated by the interference of the Cls peak due to the unknown contribution of the Cls line. However, as the Ru3d,,, peak in this work is rather broad (full - 2.5 eV), and exhibiting width at half maximum an asymmetry towards the higher binding energy values, it seems likely that the ruthenium hydroxide may be present in the passivated film spontaneously formed, in addition to the RuO, oxide phase. From fig. 11, the FWHM or Ru 3d,,, peak was reduced to about 1.8 eV, after sputtering for 2 min indicating the removal of at least 1 species by sputtering. The carbon contamination is partially removed by sputtering and hence the intensity of increases. Furthermore, the Ru 3d s,2 Ru 3d,,, peak shifts from 280.6 to 280.0 eV after sputtering. Kim and Winograd have reported that the was located Ru 3d 5,2 peak for metallic ruthenium at 280.0 eV [18]. The appearance of the metallic ruthenium after a short-time of sputtering may be an effect of the ion etching because Ru4+ species is reduced to Rumr’. It is well known that the rare gas sputtering can reduce the oxide phase to the metallic state due to the preferential removal of 02- ions [20]. Further sputtering to 10 min re-
VI non-oxidizing
acrd solutions
Binding
13
Energy,
e\
Fig. 12. Ru3d spectra recorded from the Fe-40Cr-0.2Ru alloy passivated spontaneously in 0.5M H,SO, solution at 25OC: (1) before sputtering, (2) after 2 min of sputtering.
sulted in a reduction of the intensity of the Run=’ 3d,,, signal. A decrease in the Rumrt peak can be expected because this signal is contributed directly from the metallic ruthenium of the bulk alloy. The concentration of ruthenium in the bulk alloy is extremely low (0.14 wt%). and the limit of detection for ruthenium is estimated to be - 0.1 wt% using the XPS technique. Fig. 12 shows the XPS spectra of the Ru 3d5,2.3,2 electrons for the passivated film spontaneously formed on the Fe-40Crr0.2Ru alloy in 0.5M H,SO, solution at 25°C. It is apparent from this figure that the XPS spectrum prior to sputtering shows the presence of
BINDING
ENERGY
, eV
Fig. 13. CrZp,,, spectrum for the passivated film spontaneously on the Fe+40Cr-O.lRu alloy in 0.5M solution at 25 o C.
formed H,SO,
Ku”’ 3d 5 7,3~1 electrons at 280.6 and 284.9 eV. respectively. This Ru ’ + species is also reduced to Ru”‘~ after sputtering for 2 min. As mentioned previously. AES does not detect the presence of ruthenium in the passivated film formed on this alloy in OSM H,SO, solution. This difference can be accounted for by the fact that XPS is more sensitive to the presence of ruthenium than is AES. For the Fe-40Cr-O.lRu and Fee40Cr 0.2Ru alloys exposed to OSM HCI solution. XPS shows the presence of ruthenium in the passivated film formed on these two alloys. Fig. 13 shows the Cr 2p,,,, spectrum for the Fee40Cr-O.lRu exposed in OSM H,SO, solution at 25°C. The Cr2p,,? spectrum prior to sputtering contains a major peak located at - 576.5 eV together with a shoulder located at 573.8 eV. The peak at 576.5 eV is contributed from the Crl+ species whereas the shoulder at 573.8 eV is contributed from the Cr”“’ species. The location of the Cr” and Cr”“’ peaks in this work is in good agreement with that of the Cr’- and Cr’“” peaks reported previously [ll]. Fig. 14 shows the 0 1s spectra for the Fe 40Cr-O.lRu alloy exposed to 0.5M H,SO, solution. The 0 Is spectrum of the passivated film prior to sputtering is very broad with a FWHM of about 4 eV. and it can be resolved at least into two peaks. The binding energy at 530.0 eV is assigned as 0’ bonding (M-O) associated with the Ku4 ’ species whereas the binding energy at
Fig. 14. 015 spectra for the passwated film formed spontaneously on the Fe-4OCr-0.lRu alloy III OSM HzSO, solution at 25 o C: (1) before sputtermg. (2) after 2 min of sputtering, (3) after 6 min of (puttering. (4) after 10 min of sputtering.
532.0 eV is assigned as OH-- bonding (M-OH) associated with Cr”’ or hydrated Ku” ’ species. The 01s spectrum shows a decrease in half-width (- 2.1 eV) and a shift to lower binding energy values after sputtering for 2 min. This implies that the M-OH species are partially removed by sputtering. At this stage, the M-0 bonding becomes predominant. Sputter etching was continued for 10 min until the residual 0 1s spectrum had reached a very small signal. corresponding to an almost complete removal of the oxidized species. The 0 1 s spectra of the Fee40Cr0.2Ru alloy exposed to 0.5M HCI and 0.5M H,SO, solutions exhibit similar characteristics as shown in fig. 14. On the basis of AES and XPS results, it appears that chromium and ruthenium from the Fee4OCr-O.lRu alloy are enriched in the passivating films formed on this alloy in both hydrochloric and sulphuric solutions at 25°C. and the films show oxyhydroxide characteristics.
4. Conclusions (a) Electrochemical measurements indicate that Fe-40Cr-0.1 Ru and Fe-40Cr-0.2Ru alloys exhibit spontaneous passivation in both hydrochloric and sulphuric acid solutions. However. the transition time for spontaneous passivation decreases with increasing ruthenium content and solution temperature. (b) AES measurements show that enrichment of Ku and Cr occurs in the films formed spontaneously in the Fee40Cr-O.lRu alloy in both hydrochloric and sulphuric acid solutions. AES does not detect the presence of Ru in the film formed spontaneously on the Fe-40Cr0.2Ru alloy in aulphuric acid solution. (c) XPS measurements show that ruthenium and chromium are incorporated in the films formed spontaneously on the Fee40Cr0.1 Ru and Fee40C’r--0.2Ru alloys in both hydrochloric and sulphuric acid solutions as Ru4 and Cr7 ’ species.
Acknowledgements This Council
work is published by permission of the for Mineral Technology. The author would
SC.
TJO~I~/ Fe-Cr-Ru
like to thank Professor J.B. Malherbe of the Department of Physics, University of Pretoria for the provision of Auger facilities and Dr. T.E. Derry of the Schonland Research Centre for Nuclear Science, University of the Witwatersrand for the use of XPS facilities.
References [l] G.P. Chernova, N.D. Tomashov and L.A. Chigirinskaja. Corrosion 34 (1978) 445. [2] N.D. Tomashov and G.P. Chernova, Prot. Met. 11 (1975) 319. [3] G.G. Robson. Platinum 1987 (Johnson Matthey, 1987) pp. 49-54. :4] N.D. Greene. C.R. Bishop and M. Stern, J. Electrochem. Sot. 108 (1961) 836. 151 M.A. Streicher. Corrosion 30 (1974) 77. ,6] G.P. Chernova, L.A. Chigirinskaja, N.D. Tomashov and E.A. Nasedkina. Prot. Met. 22 (1986) 704. [7] A.E. Yaniv, J.B. Lumsden and R.W. Staehle. J. Electrothem. Sot. 124 (1977) 490. [8] 1. Olefjord and H. Fischmeister. Corros. Sci. 15 (1975) 697.
in non-oxtdtrrng
acid solutwns
15
[9] K. Asami, K. Hashimoto and S. Shimodaria, Corros. Sci. 16 (1976) 387. [lo] K. Asami. K. Hashimoto and S. Shimodaria, Corros. Sci. 18 (1978) 151. [ll] S.C. Tjong, R.W. Hoffman and E.B. Yeager. J. Electrothem. Sot. 129 (1982) 1662. [12] S.C. Tjong. Surf. Coat. Technol., in press. [13] N.D. Tomashov. G.P. Chernova. L.N. Volkov, A.P. Zakharov and Z.E. Sheshina. Prot. Met. 9 (1974) 289. [14] L.E. Davis. N.D. MacDonald. P.W. Palmberg, G.E. Riach and R.E. Weber. Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie. MN. 1976) p. 135. [15] S.C. Tjong, J. Eldridge and R.W. Hoffman, Appl. Surf. Sci. 14 (1982) 297. [16] W.L.N. Matthews, P.J.K. Paterson and H.K. Wagenfeld, Appl. Surf. Sci. 15 (1983) 281. [17] N.D. Tomashov. G.P. Chernova and E.N. Ustinky. Corrosion 40 (1984) 134. [18] KS. Kim and N. Winograd, J. Catal. 35 (1974) 66. [19] R. Kiitz, H. Lewerenz and S. Stucki, J. Electrochem. Sot. 130 (1983) 825. [20] K. Asami, MS. De Sa and V. Ashworth. Corros. Sci. 26 (1986) 15.