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Electrochemical and surface characterization of the passive film on Fe-Cr-Pd alloys
Surface and Coatings Technology, 38 (1989) 325
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ELECTROCHEMICAL AND SURFACE CHARACTERIZATION OF THE PASSIVE FILM ON Fe-Cr-Pd ALLOYS SIE CHIN TJONG Council for Mineral Technology, Rand burg (South Africa) (Received February 21, 1989; in revised form April 2, 1989)
Summary Ferritic Fe—4OCr— 0.lPd, Fe—4OCr-- 0.2Pd and palladium-ion-implanted Fe—4OCr alloys were exposed to 0.5 M HC1 solution at various temperatures. Electrochemical measurements show that the Fe—4OCr alloy containing a low concentration of palladium (0.13 wt.%) exhibits spontaneous passivation in 0.5 M HCI solution from 25 to 60 °C, and unstable passivation at 85 °C. However, the Fe—4OCr alloy with 0.23 wt.% Pd content undergoes spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C. Surface implantation of palladium ions at a dose of 2.1 X iO’~ ions cm2 leads to unstable passivation at 85 °C.Thus palladium doses higher than 2.1 X 1015 ions cm2 are needed for the maintenance of spontaneous passivation. Auger electron spectroscopy depth profiling and X-ray photoelectron spectroscopy (XPS) have been used for the characterization of the surface film formed on the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 solution at 25 °C.The AES depth profile shows that the surface film formed spontaneously on the Fe—4OCr— 0.2Pd alloy in 0.5 M HCI solution is enriched in palladium relative to chromium. Furthermore, XPS measurements indicate that the palladium is incorporated in the spontaneous passive film as the Pd2~species. The enrichment of palladium in the spontaneous passive film permits a low overvoltage for hydrogen evolution and thereby promotes passivation of the Fe—4OCr— 0.2Pd alloy. However, the AES depth profile reveals that there is an enrichment of chromium relative to iron in the passive film formed anodically on Fe—4OCr—0.2Pd alloy at 220 mV. The average chromium content [Cr]! ([Cr] + [Fe]) reaches a value of about 0.765 within the passive film and decreases towards the film—substrate interface.
1. Introduction Fe—Cr alloys are considered to be of technological interest owing to the importance of these alloys as the base materials for stainless steels and corrosion-resistant alloys. The corrosion behaviour of Fe—Cr alloys in aqueous solutions has been extensively investigated [1 5]. However, it is -
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generally known that the Fe—Cr alloys are susceptible to rapid corrosion in non-oxidizing environments such as sulphuric and hydrochloric acid solutions. The corrosion resistance of Fe—Cr alloys and stainless steels can be increased by the addition of small amounts of platinum group metals (PGMs) [6 15]. These PGMs are characterized by a low hydrogen overvoltage and good corrosion resistance in acid solutions. The beneficial effect of PGMs is related to the shift of the corrosion potentials of Fe—Cr alloys from the active towards the passive region, thus causing spontaneous passivation. However, it has been reported that the beneficial effect of PGM additions depends on the chromium content [10]. It was found that the Fe—Cr alloys containing 40 wt.% Cr and 0.2 wt.% Pd exhibited the highest rate of selfpassivation and high corrosion resistance in boiling sulphuric acid solution of concentration up to 50% [10, 11]. This alloy has superior corrosion resistance to the Hastelloy alloys grade A, B and C [11]. These workers used the conventional immersion test and electrochemical measurements to determine the corrosion behaviour of Fe—Cr alloys containing the PGM additions. Little information is available on the nature of the passive film formed on Fe—Cr alloys containing the PGM additions in non-oxidizing acids. During the past decade, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used extensively to characterize the thin passive film formed on the metals and alloys in aqueous environments [16 26]. These two surface analytical techniques can give information on the chemical compositions of the passivating species. In combination with rare-gas sputtering, the composition profiles of the passive film as a function of depth can be determined. AES has far better spatial resolution than XPS. However, it is recognized that an electron beam may cause irradiation damage to the passive film, but beam damage is less likely to occur in XPS as most species are stable under X-ray irradiation. XPS is used in preference to AES where chemical bonding information is required. The composition of the passive film formed on the Fe—Cr alloys in neutral borate buffer solution (pH 8.4) has been studied by Tjong et al. [22]. The results showed that the passive film formed on the Fe—Cr alloys with a chromium content of 9 wt.% or more is enriched in chromium relative to iron. The passive film formed on the Fe—Cr alloys in acid solutions is also reported to be enriched in chromium relative to iron [17, 27, 281. The oxygen present in the passive film formed on the Fe—Cr alloys in both acidic and neutral solutions exists in two different binding states as metal—O and metal—OH. This was revealed by XPS [22, 29]. The ratio of metal—OH to metal—O tends to increase as the chromium content in the Fe—Cr alloys increases [22]. Therefore, an increase in the corrosion resistance of the Fe—Cr alloys with high chromium content is due to the protective nature of the hydrated chromium oxy-hydroxide formed [16]. Ion implantation is a useful technique for modifying the chemical and physical properties of a solid. This technique has been used to enhance the aqueous corrosion resistance of the metallic alloys [30, 31]. In the process, -
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a, a, a, a, a,
327 a
ions created in an electrical discharge are accelerated in an electric field, typically through a potential difference in the range 40 150 kV, and allowed to impact on the surface of the target. Ions of kiloelectronvolt energies penetrate the surface layers of the material and lose their energy by excitation of electrons and by elastic collision with the target nuclei. The distribution of ion ranges with depth from the sample surface is approximately gaussian. Ion implantation is a well-established technique in the fabrication of semiconductor devices. However, this technique is emerging as a powerful tool for achieving corrosion-, fatigue- and wearresistant surfaces of metallic alloys [30 36]. In the Fe—Cr—Pd alloy system, protection against non-oxidizing environments is needed only near the surface region, i.e. at the alloy-environment interface, rather than throughout the bulk. Thus ion implantation appears to be a useful surface alloying method in terms of the reduction of palladium consumption. This paper reports an investigation of the electrochemical and surface characteristics of the passive film formed on the Fe—4OCr---Pd alloys in 0.5 M HC1 solution by means of electrochemical, AES and XPS techniques. A preliminary study of the effect of palladium ion implantation on the electrochemical behaviour of the Fe—4OCr alloy was also carried out. -
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2. Experimental procedure The Fe—4OCr, Fe—4OCr--0.lPd and Fe—4OCr—0.2Pd alloys were prepared by melting high purity iron, chromium and palladium in a vacuum induction furnace. The chemical compositions of the alloys investigated are listed in Table 1. The Fe—4OCr--0.lPd and Fe—4OCr—0.2Pd alloys were cast into ingots 150 mm in length and 50 mm in diameter. The Fe—4OCr alloy was cast into larger ingots and subsequently hot-rolled into plates 8 mm thick. Specimens 11 mm in diameter and 4 mm thick were machined from the Fe—4OCr plate, whereas specimens 10 mm in diameter and 2 mm thick were cut from the Fe—4OCr—0.lPd and Fe—4OCr—0.2Pd ingots. All the specimens were annealed at 1200 °Cfor 2 h, followed by quenching in water. These specimens were ground with SiC paper down to 800 grit. For ion implantation, Auger and XPS studies, they were further polished with diamond paste down to 1 pm. TABLE 1 Chemical compositions (wt.%) of the alloys investigated Alloys
Fe—4OCr Fe—4OCr---0.lPd Fe—4OCr—0.2Pd
Elements C
S
Cr
Pd
0.01 <0.01 <0.01
0.01
39.0
—
<0.01
38.3
0.13
O01
379
0.23
Fe Balance Balance Balance
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Electrochemical measurements were performed in a nitrogen-purged 0.5 M HC1 solution using a Princeton Applied Research 350 corrosion measurement system. This system has a built-in microprocessor which automatically displays the electrode potential. The electrochemical cell consists of a working electrode, a calomel reference electrode and graphite counterelectrodes. The specimen was reduced cathodically at —800 mV for 5 mm. After cathodic reduction, the applied potential was removed, and the specimen was allowed to passivate spontaneously. The variation of the potential with time was recorded. The solution temperatures for potential— time response measurements were controlled at 25 °C, 40 °C, 60 °Cand 85 °Crespectively. The specimens for Auger and XPS studies were also reduced cathodically at —800 mV for 5 mm in order to remove the air-formed oxides. After cathodic reduction, the specimens were allowed to passivate spontaneously. For the purpose of comparison, an anodic film was also formed on the Fe—4OCr---0.2Pd alloy in 0.5 M HC1 solution by rapidly stepping the potential from —-800 mV to an anodic potential of 220 mV. This anodic potential was maintained for 40 mm. After passivation, the specimens were rinsed with distilled water and dried in a nitrogen gas stream. They were subsequently transferred into the Auger or X-ray photoelectron spectrometer. Ion implantation was carried out in a Varian 200-20A2F ion implanter. A beam of palladium ions generated by the ion source is accelerated towards the Fe—4OCr specimens at an energy of 100 keV. Palladium doses of 1.4 X 1014 and 2.1 X 1015 ions cm2 were implanted. Beam sweeping was employed to obtain a uniform dose distribution over the whole implanted area. The current density applied during the implantation was 0.05 pA cm2. The palladium doses of 1.4 X iO’~ and 2.1 X 10’s ions cm~2 approximately corresponded to palladium concentrations of 0.2 at.% and 3 at.% respectively. AES measurements were carried out in a Varian UHV chamber equipped with a cylindrical mirror analyser (PHI, model 15-11OA). The chamber was ion pumped to a base pressure of about 10~b0Torr. A primary electron beam with an energy of S keV and a current of 2 pA was used. Sputtering was done with a PHI ion gun (model 04-191) operated at 2 keV, 7 pA cm2 and 3.5 X iO~Torr of argon. The sputter rate was calibrated with a Ta 205 sample of known thickness. Under these conditions, the sputter rate was about 4 A min’. Auger depth profiles were obtained by measuring the peak-to-peak heights of the constituent elements in the passive film: sulphur (151 eV), chlorine (179 eV), carbon (273 eV), palladium (330 eV), oxygen (510 eV), chromium (529 eV) and iron (703 eV). Since the oxygen (510 eV) peak overlaps with the chromium (529 eV) peak, the peak-to-peak heights measured for chromium would result in large errors. However, the background to negative excursion for chromium (529 eV) has been shown to give a corrected chromium content in the Fe—Cr alloys [37, 381. Thus the negative-going peak height of the chromium (529 eV) was used in this work.
a a a a a a
329 a
The XPS study was performed using a VG spectrometer equipped with a 150° spherical sector electron analyser. The base pressure in the spectrometer analyser was better than 1.0 X iO~ Torr. The X-ray photoelectron spectra were taken using an unmonochromatized aluminium anode (1486.6 eV) operated at 11 kV and at 20 mA emission current. The analyser was set to discriminate electrons with a constant pass energy of 50 eV. The spectrometer was calibrated to give a C-is signal at 284.8 eV. The sputtering was performed2 with an argon ion beam of energy 2 keV, a current density of 7 pA cm and with the chamber backfilled to a pressure of 3.5 X 1O~ Torr. The sputter rate is uncalibrated.
3. Results and discussion Figure 1 shows the variation in the corrosion potentials of the Fe—4OCr— 0.lPd alloy with time in 0.5 M HC1 at various temperatures. At 25 °C, the potential of the Fe—4OCr—0.lPd alloy shifts gradually from the initial active potential of —578 mV towards the passive region. After 1000 s, there is a rapid transition to the passivation potential of about —290 mV. This implies that the Fe—4OCr—0.lPd alloy undergoes spontaneous passivation in 0.5 M HCI after 1000 s. The required time for spontaneous passivation is greatly reduced as the solution temperature increases. However, it can be seen that the corrosion potential of the Fe—4OCr--0.lPd alloy tends to exhibit periodic fluctuations in 0.5 M HC1 solution at 85 °C.This behaviour is associated with the unstable state of passivation. Tomashov et al. [39] reported that the periodic oscillations of the corrosion potentials in the chromium-base alloys containing PGM additions are generated by alternating self-passivation and self-activation caused by the pH changes in the solution layer next to the electrode. ~—2OO-
I
0
200
400
600 800 T~ME SEC
1000
P
IT 1200
Fig. 1. Potential—time responses for the Fe—4OCr—0.lPd alloy exposed in 0.5 M HCI solution at various temperatures: A, 25 ‘C; o, 40 ‘C;u, 60 °C;•, 85 °C.
:
330
As the palladium content in the Fe—4OCr alloy is increased to 0.2%, the transition time for spontaneous passivation tends to decrease as shown in Fig. 2. Furthermore, it can be seen that the corrosion potentials of the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 (25 60 °C)are shifting towards a more anodic region after spontaneous passivation. However, the corrosion potential of this alloy shows only a slight increase after spontaneous passivation in 0.5 M HC1 at 85 °C.On the basis of these results, it is obvious that the addition of 0.2% Pd to Fe—4OCr alloy is very effective in promoting spontaneous passivation in 0.5 M HC1 solution from 25 to 60 °C.At a higher temperature (85 °C), this beneficial effect is less pronounced owing to the aggressiveness of the environment. Forty reported that the selective dissolution of the less-noble element (silver) from the Au—Ag alloys in acid solution resulted in the creation of surface vacancies. These surface vacancies assisted the volume diffusion of silver from the bulk alloy to the corroding surface. The electrochemical reaction proceeds with the surface diffusion of the noble element (gold) into an island structure, and this leads to the passivation of Au—Ag alloys [40]. Tomashov et al. also explained the spontaneous passivation of chromium alloys with the PGM additions based on the surface diffusion of the PGMs during selective dissolution [14, 41]. A redistribution of the PGMs associated with the surface diffusion is responsible for the surface enrichment of the PGMs [14, 41]. Figure 3 shows the simulated range distribution of implanted palladium ions in the Fe—4OCr alloy at an acceleration voltage of 100 keV. The implanted ion-range distribution was obtained using program TRIM developed by Biersack and Haggmark [42]. This computer program is based on a Monte Carlo method and simulates the penetration and slowing down -
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~-4DDz ~5-50C
—Boc
a
I 100
I
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I
200
300
400
TIME
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Fig. 2. Potential—time responses for the Fe—4OCr--0.2Pd alloy exposed in 0.5 Pd HCI solution at various temperatures: A, 25 °C;r~,40 °C;u, 60 °C;•, 85 °C.
331
(I)
0
250 500 DEPTH ~\ Fig. 3. Computer-simulated range distribution of implanted palladium ions in Fe—4OCr alloy at an acceleration voltage of 100 keV.
of energetic ions in materials. It can be seen in this figure that the simulated calculation is typically depicted in the form of a histogram. The average penetration distance travelled by the palladium ions (projected range, R~) is about 230 A. Figure 4 shows the potential—time response for the low dose (1.4 X iO’~ions cm2) palladium-implanted Fe—4OCr alloy in 0.5 M HC1 solution at 25 °C.It can be seen that the low dose palladium-implanted Fe—4OCr alloy does not undergo spontaneous passivation as the corrosion potential of the implanted Fe—4OCr alloy shifts towards the active region instead of in the noble direction. This behaviour can be expected as most of the palladium ions are lying deeper at a distance of about 230 A from the alloy surface, as shown in Fig. 3. The potential—time response of high dose palladiumimplanted Fe—4OCr alloy (2.1 X iO’~ ions cm2) in 0.5 M HC1 solution at 25 °Chad similar features to that shown in Fig. 4. Figure 5 shows the potential—time response for the Fe—4OCr alloy implanted with a palladium dose of 2.1 X 1015 ions cm2 in 0.5 M HC1 solution at 85 °C. At this temperature, the dissolution rates of chromium and iron on the alloy surface are faster than those at 25 °C,thus exposing the -500
0
10
I
I
I
I
20 30 40 50 200 TIME MIN Fig. 4. Potential—time response for the Fe—4 OCr alloy implanted with palladium at a dose of 1.4 x 1014 ions cm2 in 0.5 M HC1 solution at 25 °C.
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332
1 00
—700
i
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30 50 70 90 TIME MIN Fig. 5. Potential—time response for the Fe—4 OCr alloy implanted with palladium at a dose 2 in 0.5 M HC1 solution at 85 °C. of 2.1 x 1015 ions cm
deeper underlying palladium atoms. It can be seen in Fig. 5 that the initial potential of the implanted alloy was initially located at —616 mV and it slowly moved in the positive direction. A sharp transition occurs after immersion in the 0.5 M HC1 solution at 85 °Cfor 35 mm. However, as the implanted palladium ions are not uniformly distributed within the Fe—4OCr alloy, the corrosion potential—time curve tends to exhibit oscillations after spontaneous passivation occurs. This is because the concentration of palladium near the alloy surface is extremely low, and hence it is not sufficient to maintain spontaneous passivation for a longer period of time. As a result, the potential of the implanted alloy tends to move back towards the active region after immersion in 0.5 M HC1 solution for about 40 50 mm. Corrosion proceeds further with the deeper lying palladium ions gradually supplying the specimen surface with this element, leading to the shift of the corrosion potential within the passivation region, which oscillates within this region. From these results, it appears that the Fe—4OCr alloy implanted with a dose of 2.1 X iO’~ions cm2 exhibits unstable passivation in 0.5 M HC1 solution at 85 °C. It is obvious that a dose of 2.1 X iO’~ions cm2 and higher is required to maintain the spontaneous passivation. Further work is required to elucidate this problem. However, it should be noted that the palladium ion beam current density generated from the ion source during implantation was very small, i.e. 0.05 pA cm2, and hence it required about 40 h to reach a dose of 2.1 X 1015 ions cm2. The long times required for implantation would be a limitation if this process were used for commercial applications, and a more intense source would be preferable. It can be concluded from the electrochemical measurements that the Fe—4OCr—0.2Pd alloy exhibits spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C.Thus AES and XPS measurements were performed on the passive film formed on this alloy in 0.5 M HC1 solution at 25 °C.Figures 6(a) and 6(b) show typical Auger spectra of the Fe—4OCr--0.2Pd alloy after spontaneous passivation in 0.5 M HC1 solution at 25 °C.The Auger spectra -
333
100
(a)
0 Pd I I I I 200 300 400 500 600 700 ELECTRON ENERGY eV
Fe
S
.
Cr Cr Fe Fe Fe
Pd
II II 100 200 300 400 500 ELECTRON ENERGY I
(b)
.
I 500 eV
I
700
Fig. 6. AES spectra for the Fe—4OCr—O.2Pd alloy passivated spontaneously in 0.5 M HC1 solution at 25 °C:(a) before sputtering, (b) after sputtering for 6 mm.
show the presence of palladium in the spontaneously passivated oxide film. It can be seen that the most pronounced peak for palladium appears at 330 eV, and the second most pronounced Auger peak for palladium at 279 eV. However, the 279 eV palladium peak interferes with the principal peak for carbon (273 eV). The concentration of the constituent elements of the passive film can be estimated by using the relative elemental sensitivity factors as described by Palmberg et al. [43]. Figure 7 shows the composition depth-profile of the spontaneously passivated film formed on the Fe--4OCr— 0.2Pd alloy in 0.5 M HC1 solution. The apparent thickness of the passivated film can be determined from the depth profile where the oxygen amplitude has reduced to half its maximum amplitude, and the sputter rate was assumed to be correlated with that of Ta2O5. Thus the apparent thickness of the film formed spontaneously is estimated to be 24 A. Figures 8 and 9 show the [Pd],’([Cr] + [Fe] + [Pd]) and [Cr]I([Cr] + [Fe] + [Pd]) concen60
-
50
-
~4O z30 5520
~IO. ~I.
10 20 30 SPUTTER TIME .MIN
Fig. 7. AES composition depth-profile for the Fe—4OCr----0.2Pd alloy passivated spontaneously in 0.5 M HC1 solution at 25 °C: A, iron; •, palladium; s, chromium; •, oxygen; k, chlorine; o, sulphur.
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334
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Fig. 8. Variation in [Pd]/([Cr] + [Fe] + [Pd]) concentration ratio with sputtering time for the spontaneous passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 M HC1 solution at 25 ‘C. Fig. 9. Variation in [Cr]/([Cr] + [Fe] + [Pd]) concentration ratio with sputtering time for the spontaneous passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 HC1 solution at 25 °C.
tration ratios vs. ion etch depth. It is obvious that the passivated film is enriched in palladium relative to chromium. The enrichment of palladium in the passivated film is responsible for the occurrence of spontaneous passivation. It is surprising to see that chromium is not enriched in the spontaneously passivated film (see Fig. 9). It is generally known that the passivated film formed on the Fe—Cr alloy is enriched in chromium relative to iron as described in the introduction. It should be noted that such a film was formed under potentiostatic control in the anodic region. Figures 10 and 11 show the composition depth-profile and normalized [Cr],/([Cr] + [Fe]) concentration ratio as a function of ion etch depth for the anodic film formed potentiostatically at 220 mV. In this case the passivated film formed on the Fe—4OCr—O.2Pd alloy is strongly enriched in chromium. The enrichment of chromium in the passive film is the major factor protecting the Fe—Cr alloys from corrosion. The depth profile in Fig. 10 shows without ambiguity that no palladium is incorporated into the film. Figure 12 shows the X-ray photoelectron spectra of the Pd 3d512, 3/2 electrons for the spontaneous passive film formed on the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 solution. The spectrum before 2~species located at 336.1sputtering eV. Kimclearly et al. shows [441 the presence of the Pd reported that the oxides formed on the palladium foil polarized in 1 N H 2S04 solution lower Pd0 potentials consisted predominantly of the PdO 2~)and ata minor 4~).The Pd2~3d phase (Pd 24~3d phase (Pd 5~2peak is located at 336.3 eV whereas the Pd 5~2 peak is located at 337.9 eV. From Fig. 12, it can be seen that the Pd 3d5~2line shifts from 336.1 to 335.2 eV after sputtering for 2 mm. The peak at 335.2 eV corresponds to metallic palladium [45, 46]. The appearance of metallic palladium may[19, be 47]. an effect 2~species is reduced to pdmet of the ion etching because the Pd
0.8
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10 20 30 10 20 30 SPUTTER TIME MIN SPUTTER TIME MIN Fig. 10. AES composition depth-profile for the anodic oxide film formed on the Fe— 4OCr—O.2Pd alloy in 0.5 M HC1 solution at 220 mV: A, iron; •, carbon; o, chromium;•, oxygen;A, chlorine; o, palladium. Fig. 11. Variation in [Cr]/([Cr] + [Fe]) concentration ratio with sputtering time for the anodic oxide film formed on the Fe—4OCr--O.2 Pd alloy in 0.5 M HC1 solution at 220 mV.
0)
2
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332 334 336 BINDING Fig. 12. The Pd spontaneously in tering.
1.1
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338 340 342 344 346 ENERGY, eV 3d XPS spectra recorded for the Fe—4OCr—O.2Pd alloy passivated 0.5 M HC1 solution: curve 1, before sputtering; curve 2, 2 mm sput-
Figure 13 shows the XPS 0 (is) spectrum of the film passivated spontaneously in 0.5 M HCI solution prior to sputtering. The oxygen spectrum shows a major peak at 530.0 eV, corresponding to the metal—O
336
-
III~II~
528 531 534 BINDING ENERGY, e V
Fig. 13. The 0 (is) XPS spectrum for the passivated film formed spontaneously on the Fe--4OCr--0.2Pd alloy in 0.5 M HC1 solution at 25 °C.
species. According to Kim et al. [44], the Pd 3P3/2 peak for the metallic and oxidized palladium overlap with the 0 (is) spectrum. The Pd 3P3/2 peak from the PdO oxide appeared at 532.7 eV [44]. It has been shown that the high binding energy peak of the 0 (is) spectrum, corresponding to oxygen in the metal—OH bond in the passive film of Fe—Cr alloys, generally appears at 532.0 532.3 eV [16, 22]. The position of a peak located at 532.5 eV in the 0 (is) spectrum from this work could be attributed either to the Pd 3P3/2 or metal—OH species. Figure 14 shows the Cr 2P3/2 spectra for the Fe—4OCr—0.2Pd alloy experiencing spontaneous passivation. The Cr 2P3/2 spectrum prior to sput-
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I
1~,j
572 574 576 578 580 582 BINDING ENERGY,eV
Fig. 14. The Cr 2P3/2 spectra for the passivated film formed spontaneously on the Fe— 4OCr—O.2Pd alloy in 0.5 Pd HC1 solution at 25 ‘C: curve 1, before sputtering; curve 2, 2 mm sputtering; curve 3, .5 mm sputtering; curve 4, 10 mm sputtering.
: tering contains a major peak centred at 576.5 eV together with a shoulder located at 573.8 eV. The peak at 576.5 eV is associated with the Cr34 species, whereas the shoulder at 573.8 eV is associated with the Crmet species. After sputtering for 2 mm, it can be seen that the intensity of the Crmet peak increases. At the sputtering time of 5 mm, the metallic Crmet peak is predominant, and the Cr3~oxide peak becomes a shoulder. Further sputtering to 10 mm resulted in the appearance of only the ~ peak at 573.8 eV.
4. Conclusions (1) Electrochemical measurements indicate that the Fe—4OCr—0 .2Pd alloy undergoes spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C. However, the Fe—4OCr—O.lPd alloy exhibits spontaneous passivation in this solution only from 25 to 60 °C. (2) The Fe—4OCr alloy implanted with palladium ions at a dose of 2.1 X iO’5 ions cm2 exhibits unstable passivation in 0.5 M HC1 solution at 85 °C.Thus palladium doses higher than 2.1 X iO’~ions cm2 are needed for the maintenance of spontaneous passivation. (3) The AES composition depth-profile reveals that the spontaneously passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 M HC1 solution at 25 C is enriched in palladium relative to chromium. However, the passivated film formed anodically at 220 mV is strongly enriched in chromium. (4) XPS measurements indicate that the palladium is present in the spontaneously passivated film as Pd2~,whereas chromium is present as Cr3~. Acknowledgments This work is published by permission of the Council for Mineral Technology. The author would like to thank Professor J. B. Maiherbe 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 Sciences, University of the Witwatersrand, for arranging ion implantation and the use of XPS facilities.
References 1 2 3 4 5 6
R. P. Frankenthal, J. Electrochem. Soc., 116 (1969) 1646. M. S. El-Basiony and S. Haruyama, Corros. Sci., 16 (1976) 529. M. Keddam, 0. R. Mattos and H. Takenouti, Electrochim. Acta, 31 (1986) 1147. N. R. Sorensen, R. B. Diegle and S. T. Picraun, Corrosion (Houston), 43 (1987) 2. P. A. Molian, Scr. Metall., 16 (1982) 65. N. D. Greene, C. R. Bishop and M. Stern, J. Electrochem. Soc., 108 (1961) 836.
338 7 M. A. Streicher, Platinum Met. Rev., 21(1977) 51. 8 G. Bianchi, G. A. Camona, G. Fiori and F. Mazza, Corros. Sci., 8 (1965) 751. 9 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Platinum Met. Rev., 23 (1979) 143. 10 G. P. Chernova, N. D. Tomashov and L. A. Chigirinskaja, Corrosion (Houston), 34 (1978) 445. 11 N. D. Tomashov and G. P. Chernova, Prot. Met. (USSR), 11(1975) 379. 12 G. P. Chernova, T. A. Fedoseeva, L. P. Kornienko and N. D. Tomashov, Surf Technol., 13 (1981) 241. 13 G. P. Chernova, T. A. Fedoseeva, L. P. Kornienko and N. D. Tomashov, Prot. Met. (USSR), 17(1981) 406. 14 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Corrosion (Houston), 40 (1984) 134. 15 G. P. Chernova, L. A. Chigirinskaja, N. D. Tomashov and E. Nasedkina, Prot. Met. (USSR), 22 (1986) 704. 16 K. Asami, K. Hashimoto and S. Shimodaira, Corros. Sci., 18 (1978) 151. 17 K. Hashimoto and K. Asami, Corros. Sci., 19 (1979) 251. 18 P. Marcus, I. Olefjord and J. Oudar, Corros. Sci., 24 (1984) 269. 19 K. Asami, M. S. De Sa and V. Ashworth, Corros. Sci., 26 (1986) 15. 20 K. Asami and K. Hashimoto, Langmuir, 3 (1987) 897. 21 S. C. Tjong and E. Yeager, J. Electrochem. Soc., 128 (1981) 2251. 22 S. C. Tjong, R. Hoffman and E. B. Yeager, J. Electrochem. Soc., 129 (1982) 1663. 23 S. C. Tjong, Mater. Res. Bull., 17 (1982) 1297. 24 R. Goetz and D. Landolt, Elect rochim. Acta, 29 (1984) 667. 25 A. E. Yaniv, J. B. Lumsden and R. W. Staelhe, J. Electrochem. Soc., 125 (1977) 1204. 26 H. Ogawa, H. Omata and 1. Itoh, Corrosion (Houston), 34 (1978) 52. 27 J. E. Holliday and R. P. Frankentahal, J. Electrochem. Soc., 119 (1972) 1190. 28 V. Mitrovic-Scepanovic, B. MacDougall and M. J. Graham, Corros. Sci., 27 (1987) 239. 29 K. Asami, K. Hashimoto and T. Musumoto, Corros. Sci., 16 (1976) 909. 30 Pd. V. Zeller and J. A. Kargol, Appl. Surf. Sci., 18 (1984) 63. 31 P. D. Hicks and F. P. A. Robinson, Corros. Sci., 24 (1984) 885. 32 G. Dearnaley, Metall. Mater. Technol., 12 (1980) 129. 33 W. S. Sampath, R. Wei and P. J. Wilbur, J. Met., 37(1987) 17. 34 G. Dearnaley, Mater. Sci. Eng., 69 (1985) 139. 35 I. L. Singer, Appi. Surf. Sci., 18 (1984) 28. 36 J. H. Kort, T. Fransen and P. J. Gellings, Appi. Surf. Sci., 25 (1986) 237. 37 S. C. Tjong, J. Eldridge and R. Hoffman, Appi. Surf. Sci., 14 (1982) 297. 38 W. L. N. Matthews, P. J. K. Paterson and H. K. Wagenfeld, Appi. Surf. Sd., 15 (1983) 281. 39 N. D. Tomashov, E. N. Utinski and G. P. Chernova, Electrokhimiya, 17 (1981) 969. 40 A. J. Forty, Gold Bull., 14(1981)25. 41 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Prot. Met. (USSR), 17 (1981) 325. 42 J. P. Biersack and L. G. Haggmark, Nuci. Instrum. Methods, 174 (1980) 257. 43 P. W. Palmberg, L. E. Davis, N. C. MacDonald, G. E. Riach and R. E. Weber, Handbook of Auger Electron Spectroscopy (2nd Edn.), Physical Electronics Industries, Eden Prairie, MN, 1976. 44 K. S. Kim, A. F. Gossmann and N. Winograd, Anal. Chem., 46 (1974) 197. 45 K. Asami, J. Electron Spectrosc. Relat. Phenom., 9 (1976) 469. 46 G. Johanson, J. Hedman, A. Berndtsson, M. Kiasson and R. Nilson, J. Electron Spectrosc. Relat. Phenom., 2 (1973) 295. 47 H. Konno and M. Nagayama, in J. Kruger and R. P. Frankenthal (eds.), Passivity of Metals, Electrochemical Society, Princeton, NJ, 1978, p. 585.
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ELECTROCHEMICAL AND SURFACE CHARACTERIZATION OF THE PASSIVE FILM ON Fe-Cr-Pd ALLOYS SIE CHIN TJONG Council for Mineral Technology, Rand burg (South Africa) (Received February 21, 1989; in revised form April 2, 1989)
Summary Ferritic Fe—4OCr— 0.lPd, Fe—4OCr-- 0.2Pd and palladium-ion-implanted Fe—4OCr alloys were exposed to 0.5 M HC1 solution at various temperatures. Electrochemical measurements show that the Fe—4OCr alloy containing a low concentration of palladium (0.13 wt.%) exhibits spontaneous passivation in 0.5 M HCI solution from 25 to 60 °C, and unstable passivation at 85 °C. However, the Fe—4OCr alloy with 0.23 wt.% Pd content undergoes spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C. Surface implantation of palladium ions at a dose of 2.1 X iO’~ ions cm2 leads to unstable passivation at 85 °C.Thus palladium doses higher than 2.1 X 1015 ions cm2 are needed for the maintenance of spontaneous passivation. Auger electron spectroscopy depth profiling and X-ray photoelectron spectroscopy (XPS) have been used for the characterization of the surface film formed on the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 solution at 25 °C.The AES depth profile shows that the surface film formed spontaneously on the Fe—4OCr— 0.2Pd alloy in 0.5 M HCI solution is enriched in palladium relative to chromium. Furthermore, XPS measurements indicate that the palladium is incorporated in the spontaneous passive film as the Pd2~species. The enrichment of palladium in the spontaneous passive film permits a low overvoltage for hydrogen evolution and thereby promotes passivation of the Fe—4OCr— 0.2Pd alloy. However, the AES depth profile reveals that there is an enrichment of chromium relative to iron in the passive film formed anodically on Fe—4OCr—0.2Pd alloy at 220 mV. The average chromium content [Cr]! ([Cr] + [Fe]) reaches a value of about 0.765 within the passive film and decreases towards the film—substrate interface.
1. Introduction Fe—Cr alloys are considered to be of technological interest owing to the importance of these alloys as the base materials for stainless steels and corrosion-resistant alloys. The corrosion behaviour of Fe—Cr alloys in aqueous solutions has been extensively investigated [1 5]. However, it is -
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generally known that the Fe—Cr alloys are susceptible to rapid corrosion in non-oxidizing environments such as sulphuric and hydrochloric acid solutions. The corrosion resistance of Fe—Cr alloys and stainless steels can be increased by the addition of small amounts of platinum group metals (PGMs) [6 15]. These PGMs are characterized by a low hydrogen overvoltage and good corrosion resistance in acid solutions. The beneficial effect of PGMs is related to the shift of the corrosion potentials of Fe—Cr alloys from the active towards the passive region, thus causing spontaneous passivation. However, it has been reported that the beneficial effect of PGM additions depends on the chromium content [10]. It was found that the Fe—Cr alloys containing 40 wt.% Cr and 0.2 wt.% Pd exhibited the highest rate of selfpassivation and high corrosion resistance in boiling sulphuric acid solution of concentration up to 50% [10, 11]. This alloy has superior corrosion resistance to the Hastelloy alloys grade A, B and C [11]. These workers used the conventional immersion test and electrochemical measurements to determine the corrosion behaviour of Fe—Cr alloys containing the PGM additions. Little information is available on the nature of the passive film formed on Fe—Cr alloys containing the PGM additions in non-oxidizing acids. During the past decade, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used extensively to characterize the thin passive film formed on the metals and alloys in aqueous environments [16 26]. These two surface analytical techniques can give information on the chemical compositions of the passivating species. In combination with rare-gas sputtering, the composition profiles of the passive film as a function of depth can be determined. AES has far better spatial resolution than XPS. However, it is recognized that an electron beam may cause irradiation damage to the passive film, but beam damage is less likely to occur in XPS as most species are stable under X-ray irradiation. XPS is used in preference to AES where chemical bonding information is required. The composition of the passive film formed on the Fe—Cr alloys in neutral borate buffer solution (pH 8.4) has been studied by Tjong et al. [22]. The results showed that the passive film formed on the Fe—Cr alloys with a chromium content of 9 wt.% or more is enriched in chromium relative to iron. The passive film formed on the Fe—Cr alloys in acid solutions is also reported to be enriched in chromium relative to iron [17, 27, 281. The oxygen present in the passive film formed on the Fe—Cr alloys in both acidic and neutral solutions exists in two different binding states as metal—O and metal—OH. This was revealed by XPS [22, 29]. The ratio of metal—OH to metal—O tends to increase as the chromium content in the Fe—Cr alloys increases [22]. Therefore, an increase in the corrosion resistance of the Fe—Cr alloys with high chromium content is due to the protective nature of the hydrated chromium oxy-hydroxide formed [16]. Ion implantation is a useful technique for modifying the chemical and physical properties of a solid. This technique has been used to enhance the aqueous corrosion resistance of the metallic alloys [30, 31]. In the process, -
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a, a, a, a, a,
327 a
ions created in an electrical discharge are accelerated in an electric field, typically through a potential difference in the range 40 150 kV, and allowed to impact on the surface of the target. Ions of kiloelectronvolt energies penetrate the surface layers of the material and lose their energy by excitation of electrons and by elastic collision with the target nuclei. The distribution of ion ranges with depth from the sample surface is approximately gaussian. Ion implantation is a well-established technique in the fabrication of semiconductor devices. However, this technique is emerging as a powerful tool for achieving corrosion-, fatigue- and wearresistant surfaces of metallic alloys [30 36]. In the Fe—Cr—Pd alloy system, protection against non-oxidizing environments is needed only near the surface region, i.e. at the alloy-environment interface, rather than throughout the bulk. Thus ion implantation appears to be a useful surface alloying method in terms of the reduction of palladium consumption. This paper reports an investigation of the electrochemical and surface characteristics of the passive film formed on the Fe—4OCr---Pd alloys in 0.5 M HC1 solution by means of electrochemical, AES and XPS techniques. A preliminary study of the effect of palladium ion implantation on the electrochemical behaviour of the Fe—4OCr alloy was also carried out. -
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2. Experimental procedure The Fe—4OCr, Fe—4OCr--0.lPd and Fe—4OCr—0.2Pd alloys were prepared by melting high purity iron, chromium and palladium in a vacuum induction furnace. The chemical compositions of the alloys investigated are listed in Table 1. The Fe—4OCr--0.lPd and Fe—4OCr—0.2Pd alloys were cast into ingots 150 mm in length and 50 mm in diameter. The Fe—4OCr alloy was cast into larger ingots and subsequently hot-rolled into plates 8 mm thick. Specimens 11 mm in diameter and 4 mm thick were machined from the Fe—4OCr plate, whereas specimens 10 mm in diameter and 2 mm thick were cut from the Fe—4OCr—0.lPd and Fe—4OCr—0.2Pd ingots. All the specimens were annealed at 1200 °Cfor 2 h, followed by quenching in water. These specimens were ground with SiC paper down to 800 grit. For ion implantation, Auger and XPS studies, they were further polished with diamond paste down to 1 pm. TABLE 1 Chemical compositions (wt.%) of the alloys investigated Alloys
Fe—4OCr Fe—4OCr---0.lPd Fe—4OCr—0.2Pd
Elements C
S
Cr
Pd
0.01 <0.01 <0.01
0.01
39.0
—
<0.01
38.3
0.13
O01
379
0.23
Fe Balance Balance Balance
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Electrochemical measurements were performed in a nitrogen-purged 0.5 M HC1 solution using a Princeton Applied Research 350 corrosion measurement system. This system has a built-in microprocessor which automatically displays the electrode potential. The electrochemical cell consists of a working electrode, a calomel reference electrode and graphite counterelectrodes. The specimen was reduced cathodically at —800 mV for 5 mm. After cathodic reduction, the applied potential was removed, and the specimen was allowed to passivate spontaneously. The variation of the potential with time was recorded. The solution temperatures for potential— time response measurements were controlled at 25 °C, 40 °C, 60 °Cand 85 °Crespectively. The specimens for Auger and XPS studies were also reduced cathodically at —800 mV for 5 mm in order to remove the air-formed oxides. After cathodic reduction, the specimens were allowed to passivate spontaneously. For the purpose of comparison, an anodic film was also formed on the Fe—4OCr---0.2Pd alloy in 0.5 M HC1 solution by rapidly stepping the potential from —-800 mV to an anodic potential of 220 mV. This anodic potential was maintained for 40 mm. After passivation, the specimens were rinsed with distilled water and dried in a nitrogen gas stream. They were subsequently transferred into the Auger or X-ray photoelectron spectrometer. Ion implantation was carried out in a Varian 200-20A2F ion implanter. A beam of palladium ions generated by the ion source is accelerated towards the Fe—4OCr specimens at an energy of 100 keV. Palladium doses of 1.4 X 1014 and 2.1 X 1015 ions cm2 were implanted. Beam sweeping was employed to obtain a uniform dose distribution over the whole implanted area. The current density applied during the implantation was 0.05 pA cm2. The palladium doses of 1.4 X iO’~ and 2.1 X 10’s ions cm~2 approximately corresponded to palladium concentrations of 0.2 at.% and 3 at.% respectively. AES measurements were carried out in a Varian UHV chamber equipped with a cylindrical mirror analyser (PHI, model 15-11OA). The chamber was ion pumped to a base pressure of about 10~b0Torr. A primary electron beam with an energy of S keV and a current of 2 pA was used. Sputtering was done with a PHI ion gun (model 04-191) operated at 2 keV, 7 pA cm2 and 3.5 X iO~Torr of argon. The sputter rate was calibrated with a Ta 205 sample of known thickness. Under these conditions, the sputter rate was about 4 A min’. Auger depth profiles were obtained by measuring the peak-to-peak heights of the constituent elements in the passive film: sulphur (151 eV), chlorine (179 eV), carbon (273 eV), palladium (330 eV), oxygen (510 eV), chromium (529 eV) and iron (703 eV). Since the oxygen (510 eV) peak overlaps with the chromium (529 eV) peak, the peak-to-peak heights measured for chromium would result in large errors. However, the background to negative excursion for chromium (529 eV) has been shown to give a corrected chromium content in the Fe—Cr alloys [37, 381. Thus the negative-going peak height of the chromium (529 eV) was used in this work.
a a a a a a
329 a
The XPS study was performed using a VG spectrometer equipped with a 150° spherical sector electron analyser. The base pressure in the spectrometer analyser was better than 1.0 X iO~ Torr. The X-ray photoelectron spectra were taken using an unmonochromatized aluminium anode (1486.6 eV) operated at 11 kV and at 20 mA emission current. The analyser was set to discriminate electrons with a constant pass energy of 50 eV. The spectrometer was calibrated to give a C-is signal at 284.8 eV. The sputtering was performed2 with an argon ion beam of energy 2 keV, a current density of 7 pA cm and with the chamber backfilled to a pressure of 3.5 X 1O~ Torr. The sputter rate is uncalibrated.
3. Results and discussion Figure 1 shows the variation in the corrosion potentials of the Fe—4OCr— 0.lPd alloy with time in 0.5 M HC1 at various temperatures. At 25 °C, the potential of the Fe—4OCr—0.lPd alloy shifts gradually from the initial active potential of —578 mV towards the passive region. After 1000 s, there is a rapid transition to the passivation potential of about —290 mV. This implies that the Fe—4OCr—0.lPd alloy undergoes spontaneous passivation in 0.5 M HCI after 1000 s. The required time for spontaneous passivation is greatly reduced as the solution temperature increases. However, it can be seen that the corrosion potential of the Fe—4OCr--0.lPd alloy tends to exhibit periodic fluctuations in 0.5 M HC1 solution at 85 °C.This behaviour is associated with the unstable state of passivation. Tomashov et al. [39] reported that the periodic oscillations of the corrosion potentials in the chromium-base alloys containing PGM additions are generated by alternating self-passivation and self-activation caused by the pH changes in the solution layer next to the electrode. ~—2OO-
I
0
200
400
600 800 T~ME SEC
1000
P
IT 1200
Fig. 1. Potential—time responses for the Fe—4OCr—0.lPd alloy exposed in 0.5 M HCI solution at various temperatures: A, 25 ‘C; o, 40 ‘C;u, 60 °C;•, 85 °C.
:
330
As the palladium content in the Fe—4OCr alloy is increased to 0.2%, the transition time for spontaneous passivation tends to decrease as shown in Fig. 2. Furthermore, it can be seen that the corrosion potentials of the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 (25 60 °C)are shifting towards a more anodic region after spontaneous passivation. However, the corrosion potential of this alloy shows only a slight increase after spontaneous passivation in 0.5 M HC1 at 85 °C.On the basis of these results, it is obvious that the addition of 0.2% Pd to Fe—4OCr alloy is very effective in promoting spontaneous passivation in 0.5 M HC1 solution from 25 to 60 °C.At a higher temperature (85 °C), this beneficial effect is less pronounced owing to the aggressiveness of the environment. Forty reported that the selective dissolution of the less-noble element (silver) from the Au—Ag alloys in acid solution resulted in the creation of surface vacancies. These surface vacancies assisted the volume diffusion of silver from the bulk alloy to the corroding surface. The electrochemical reaction proceeds with the surface diffusion of the noble element (gold) into an island structure, and this leads to the passivation of Au—Ag alloys [40]. Tomashov et al. also explained the spontaneous passivation of chromium alloys with the PGM additions based on the surface diffusion of the PGMs during selective dissolution [14, 41]. A redistribution of the PGMs associated with the surface diffusion is responsible for the surface enrichment of the PGMs [14, 41]. Figure 3 shows the simulated range distribution of implanted palladium ions in the Fe—4OCr alloy at an acceleration voltage of 100 keV. The implanted ion-range distribution was obtained using program TRIM developed by Biersack and Haggmark [42]. This computer program is based on a Monte Carlo method and simulates the penetration and slowing down -
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Fig. 2. Potential—time responses for the Fe—4OCr--0.2Pd alloy exposed in 0.5 Pd HCI solution at various temperatures: A, 25 °C;r~,40 °C;u, 60 °C;•, 85 °C.
331
(I)
0
250 500 DEPTH ~\ Fig. 3. Computer-simulated range distribution of implanted palladium ions in Fe—4OCr alloy at an acceleration voltage of 100 keV.
of energetic ions in materials. It can be seen in this figure that the simulated calculation is typically depicted in the form of a histogram. The average penetration distance travelled by the palladium ions (projected range, R~) is about 230 A. Figure 4 shows the potential—time response for the low dose (1.4 X iO’~ions cm2) palladium-implanted Fe—4OCr alloy in 0.5 M HC1 solution at 25 °C.It can be seen that the low dose palladium-implanted Fe—4OCr alloy does not undergo spontaneous passivation as the corrosion potential of the implanted Fe—4OCr alloy shifts towards the active region instead of in the noble direction. This behaviour can be expected as most of the palladium ions are lying deeper at a distance of about 230 A from the alloy surface, as shown in Fig. 3. The potential—time response of high dose palladiumimplanted Fe—4OCr alloy (2.1 X iO’~ ions cm2) in 0.5 M HC1 solution at 25 °Chad similar features to that shown in Fig. 4. Figure 5 shows the potential—time response for the Fe—4OCr alloy implanted with a palladium dose of 2.1 X 1015 ions cm2 in 0.5 M HC1 solution at 85 °C. At this temperature, the dissolution rates of chromium and iron on the alloy surface are faster than those at 25 °C,thus exposing the -500
0
10
I
I
I
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20 30 40 50 200 TIME MIN Fig. 4. Potential—time response for the Fe—4 OCr alloy implanted with palladium at a dose of 1.4 x 1014 ions cm2 in 0.5 M HC1 solution at 25 °C.
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1 00
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i
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30 50 70 90 TIME MIN Fig. 5. Potential—time response for the Fe—4 OCr alloy implanted with palladium at a dose 2 in 0.5 M HC1 solution at 85 °C. of 2.1 x 1015 ions cm
deeper underlying palladium atoms. It can be seen in Fig. 5 that the initial potential of the implanted alloy was initially located at —616 mV and it slowly moved in the positive direction. A sharp transition occurs after immersion in the 0.5 M HC1 solution at 85 °Cfor 35 mm. However, as the implanted palladium ions are not uniformly distributed within the Fe—4OCr alloy, the corrosion potential—time curve tends to exhibit oscillations after spontaneous passivation occurs. This is because the concentration of palladium near the alloy surface is extremely low, and hence it is not sufficient to maintain spontaneous passivation for a longer period of time. As a result, the potential of the implanted alloy tends to move back towards the active region after immersion in 0.5 M HC1 solution for about 40 50 mm. Corrosion proceeds further with the deeper lying palladium ions gradually supplying the specimen surface with this element, leading to the shift of the corrosion potential within the passivation region, which oscillates within this region. From these results, it appears that the Fe—4OCr alloy implanted with a dose of 2.1 X iO’~ions cm2 exhibits unstable passivation in 0.5 M HC1 solution at 85 °C. It is obvious that a dose of 2.1 X iO’~ions cm2 and higher is required to maintain the spontaneous passivation. Further work is required to elucidate this problem. However, it should be noted that the palladium ion beam current density generated from the ion source during implantation was very small, i.e. 0.05 pA cm2, and hence it required about 40 h to reach a dose of 2.1 X 1015 ions cm2. The long times required for implantation would be a limitation if this process were used for commercial applications, and a more intense source would be preferable. It can be concluded from the electrochemical measurements that the Fe—4OCr—0.2Pd alloy exhibits spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C.Thus AES and XPS measurements were performed on the passive film formed on this alloy in 0.5 M HC1 solution at 25 °C.Figures 6(a) and 6(b) show typical Auger spectra of the Fe—4OCr--0.2Pd alloy after spontaneous passivation in 0.5 M HC1 solution at 25 °C.The Auger spectra -
333
100
(a)
0 Pd I I I I 200 300 400 500 600 700 ELECTRON ENERGY eV
Fe
S
.
Cr Cr Fe Fe Fe
Pd
II II 100 200 300 400 500 ELECTRON ENERGY I
(b)
.
I 500 eV
I
700
Fig. 6. AES spectra for the Fe—4OCr—O.2Pd alloy passivated spontaneously in 0.5 M HC1 solution at 25 °C:(a) before sputtering, (b) after sputtering for 6 mm.
show the presence of palladium in the spontaneously passivated oxide film. It can be seen that the most pronounced peak for palladium appears at 330 eV, and the second most pronounced Auger peak for palladium at 279 eV. However, the 279 eV palladium peak interferes with the principal peak for carbon (273 eV). The concentration of the constituent elements of the passive film can be estimated by using the relative elemental sensitivity factors as described by Palmberg et al. [43]. Figure 7 shows the composition depth-profile of the spontaneously passivated film formed on the Fe--4OCr— 0.2Pd alloy in 0.5 M HC1 solution. The apparent thickness of the passivated film can be determined from the depth profile where the oxygen amplitude has reduced to half its maximum amplitude, and the sputter rate was assumed to be correlated with that of Ta2O5. Thus the apparent thickness of the film formed spontaneously is estimated to be 24 A. Figures 8 and 9 show the [Pd],’([Cr] + [Fe] + [Pd]) and [Cr]I([Cr] + [Fe] + [Pd]) concen60
-
50
-
~4O z30 5520
~IO. ~I.
10 20 30 SPUTTER TIME .MIN
Fig. 7. AES composition depth-profile for the Fe—4OCr----0.2Pd alloy passivated spontaneously in 0.5 M HC1 solution at 25 °C: A, iron; •, palladium; s, chromium; •, oxygen; k, chlorine; o, sulphur.
:
334
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Fig. 8. Variation in [Pd]/([Cr] + [Fe] + [Pd]) concentration ratio with sputtering time for the spontaneous passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 M HC1 solution at 25 ‘C. Fig. 9. Variation in [Cr]/([Cr] + [Fe] + [Pd]) concentration ratio with sputtering time for the spontaneous passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 HC1 solution at 25 °C.
tration ratios vs. ion etch depth. It is obvious that the passivated film is enriched in palladium relative to chromium. The enrichment of palladium in the passivated film is responsible for the occurrence of spontaneous passivation. It is surprising to see that chromium is not enriched in the spontaneously passivated film (see Fig. 9). It is generally known that the passivated film formed on the Fe—Cr alloy is enriched in chromium relative to iron as described in the introduction. It should be noted that such a film was formed under potentiostatic control in the anodic region. Figures 10 and 11 show the composition depth-profile and normalized [Cr],/([Cr] + [Fe]) concentration ratio as a function of ion etch depth for the anodic film formed potentiostatically at 220 mV. In this case the passivated film formed on the Fe—4OCr—O.2Pd alloy is strongly enriched in chromium. The enrichment of chromium in the passive film is the major factor protecting the Fe—Cr alloys from corrosion. The depth profile in Fig. 10 shows without ambiguity that no palladium is incorporated into the film. Figure 12 shows the X-ray photoelectron spectra of the Pd 3d512, 3/2 electrons for the spontaneous passive film formed on the Fe—4OCr—0.2Pd alloy in 0.5 M HC1 solution. The spectrum before 2~species located at 336.1sputtering eV. Kimclearly et al. shows [441 the presence of the Pd reported that the oxides formed on the palladium foil polarized in 1 N H 2S04 solution lower Pd0 potentials consisted predominantly of the PdO 2~)and ata minor 4~).The Pd2~3d phase (Pd 24~3d phase (Pd 5~2peak is located at 336.3 eV whereas the Pd 5~2 peak is located at 337.9 eV. From Fig. 12, it can be seen that the Pd 3d5~2line shifts from 336.1 to 335.2 eV after sputtering for 2 mm. The peak at 335.2 eV corresponds to metallic palladium [45, 46]. The appearance of metallic palladium may[19, be 47]. an effect 2~species is reduced to pdmet of the ion etching because the Pd
0.8
-
10 20 30 10 20 30 SPUTTER TIME MIN SPUTTER TIME MIN Fig. 10. AES composition depth-profile for the anodic oxide film formed on the Fe— 4OCr—O.2Pd alloy in 0.5 M HC1 solution at 220 mV: A, iron; •, carbon; o, chromium;•, oxygen;A, chlorine; o, palladium. Fig. 11. Variation in [Cr]/([Cr] + [Fe]) concentration ratio with sputtering time for the anodic oxide film formed on the Fe—4OCr--O.2 Pd alloy in 0.5 M HC1 solution at 220 mV.
0)
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332 334 336 BINDING Fig. 12. The Pd spontaneously in tering.
1.1
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338 340 342 344 346 ENERGY, eV 3d XPS spectra recorded for the Fe—4OCr—O.2Pd alloy passivated 0.5 M HC1 solution: curve 1, before sputtering; curve 2, 2 mm sput-
Figure 13 shows the XPS 0 (is) spectrum of the film passivated spontaneously in 0.5 M HCI solution prior to sputtering. The oxygen spectrum shows a major peak at 530.0 eV, corresponding to the metal—O
336
-
III~II~
528 531 534 BINDING ENERGY, e V
Fig. 13. The 0 (is) XPS spectrum for the passivated film formed spontaneously on the Fe--4OCr--0.2Pd alloy in 0.5 M HC1 solution at 25 °C.
species. According to Kim et al. [44], the Pd 3P3/2 peak for the metallic and oxidized palladium overlap with the 0 (is) spectrum. The Pd 3P3/2 peak from the PdO oxide appeared at 532.7 eV [44]. It has been shown that the high binding energy peak of the 0 (is) spectrum, corresponding to oxygen in the metal—OH bond in the passive film of Fe—Cr alloys, generally appears at 532.0 532.3 eV [16, 22]. The position of a peak located at 532.5 eV in the 0 (is) spectrum from this work could be attributed either to the Pd 3P3/2 or metal—OH species. Figure 14 shows the Cr 2P3/2 spectra for the Fe—4OCr—0.2Pd alloy experiencing spontaneous passivation. The Cr 2P3/2 spectrum prior to sput-
I
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1~,j
572 574 576 578 580 582 BINDING ENERGY,eV
Fig. 14. The Cr 2P3/2 spectra for the passivated film formed spontaneously on the Fe— 4OCr—O.2Pd alloy in 0.5 Pd HC1 solution at 25 ‘C: curve 1, before sputtering; curve 2, 2 mm sputtering; curve 3, .5 mm sputtering; curve 4, 10 mm sputtering.
: tering contains a major peak centred at 576.5 eV together with a shoulder located at 573.8 eV. The peak at 576.5 eV is associated with the Cr34 species, whereas the shoulder at 573.8 eV is associated with the Crmet species. After sputtering for 2 mm, it can be seen that the intensity of the Crmet peak increases. At the sputtering time of 5 mm, the metallic Crmet peak is predominant, and the Cr3~oxide peak becomes a shoulder. Further sputtering to 10 mm resulted in the appearance of only the ~ peak at 573.8 eV.
4. Conclusions (1) Electrochemical measurements indicate that the Fe—4OCr—0 .2Pd alloy undergoes spontaneous passivation in 0.5 M HC1 solution from 25 to 85 °C. However, the Fe—4OCr—O.lPd alloy exhibits spontaneous passivation in this solution only from 25 to 60 °C. (2) The Fe—4OCr alloy implanted with palladium ions at a dose of 2.1 X iO’5 ions cm2 exhibits unstable passivation in 0.5 M HC1 solution at 85 °C.Thus palladium doses higher than 2.1 X iO’~ions cm2 are needed for the maintenance of spontaneous passivation. (3) The AES composition depth-profile reveals that the spontaneously passivated film formed on the Fe—4OCr—O.2Pd alloy in 0.5 M HC1 solution at 25 C is enriched in palladium relative to chromium. However, the passivated film formed anodically at 220 mV is strongly enriched in chromium. (4) XPS measurements indicate that the palladium is present in the spontaneously passivated film as Pd2~,whereas chromium is present as Cr3~. Acknowledgments This work is published by permission of the Council for Mineral Technology. The author would like to thank Professor J. B. Maiherbe 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 Sciences, University of the Witwatersrand, for arranging ion implantation and the use of XPS facilities.
References 1 2 3 4 5 6
R. P. Frankenthal, J. Electrochem. Soc., 116 (1969) 1646. M. S. El-Basiony and S. Haruyama, Corros. Sci., 16 (1976) 529. M. Keddam, 0. R. Mattos and H. Takenouti, Electrochim. Acta, 31 (1986) 1147. N. R. Sorensen, R. B. Diegle and S. T. Picraun, Corrosion (Houston), 43 (1987) 2. P. A. Molian, Scr. Metall., 16 (1982) 65. N. D. Greene, C. R. Bishop and M. Stern, J. Electrochem. Soc., 108 (1961) 836.
338 7 M. A. Streicher, Platinum Met. Rev., 21(1977) 51. 8 G. Bianchi, G. A. Camona, G. Fiori and F. Mazza, Corros. Sci., 8 (1965) 751. 9 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Platinum Met. Rev., 23 (1979) 143. 10 G. P. Chernova, N. D. Tomashov and L. A. Chigirinskaja, Corrosion (Houston), 34 (1978) 445. 11 N. D. Tomashov and G. P. Chernova, Prot. Met. (USSR), 11(1975) 379. 12 G. P. Chernova, T. A. Fedoseeva, L. P. Kornienko and N. D. Tomashov, Surf Technol., 13 (1981) 241. 13 G. P. Chernova, T. A. Fedoseeva, L. P. Kornienko and N. D. Tomashov, Prot. Met. (USSR), 17(1981) 406. 14 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Corrosion (Houston), 40 (1984) 134. 15 G. P. Chernova, L. A. Chigirinskaja, N. D. Tomashov and E. Nasedkina, Prot. Met. (USSR), 22 (1986) 704. 16 K. Asami, K. Hashimoto and S. Shimodaira, Corros. Sci., 18 (1978) 151. 17 K. Hashimoto and K. Asami, Corros. Sci., 19 (1979) 251. 18 P. Marcus, I. Olefjord and J. Oudar, Corros. Sci., 24 (1984) 269. 19 K. Asami, M. S. De Sa and V. Ashworth, Corros. Sci., 26 (1986) 15. 20 K. Asami and K. Hashimoto, Langmuir, 3 (1987) 897. 21 S. C. Tjong and E. Yeager, J. Electrochem. Soc., 128 (1981) 2251. 22 S. C. Tjong, R. Hoffman and E. B. Yeager, J. Electrochem. Soc., 129 (1982) 1663. 23 S. C. Tjong, Mater. Res. Bull., 17 (1982) 1297. 24 R. Goetz and D. Landolt, Elect rochim. Acta, 29 (1984) 667. 25 A. E. Yaniv, J. B. Lumsden and R. W. Staelhe, J. Electrochem. Soc., 125 (1977) 1204. 26 H. Ogawa, H. Omata and 1. Itoh, Corrosion (Houston), 34 (1978) 52. 27 J. E. Holliday and R. P. Frankentahal, J. Electrochem. Soc., 119 (1972) 1190. 28 V. Mitrovic-Scepanovic, B. MacDougall and M. J. Graham, Corros. Sci., 27 (1987) 239. 29 K. Asami, K. Hashimoto and T. Musumoto, Corros. Sci., 16 (1976) 909. 30 Pd. V. Zeller and J. A. Kargol, Appl. Surf. Sci., 18 (1984) 63. 31 P. D. Hicks and F. P. A. Robinson, Corros. Sci., 24 (1984) 885. 32 G. Dearnaley, Metall. Mater. Technol., 12 (1980) 129. 33 W. S. Sampath, R. Wei and P. J. Wilbur, J. Met., 37(1987) 17. 34 G. Dearnaley, Mater. Sci. Eng., 69 (1985) 139. 35 I. L. Singer, Appi. Surf. Sci., 18 (1984) 28. 36 J. H. Kort, T. Fransen and P. J. Gellings, Appi. Surf. Sci., 25 (1986) 237. 37 S. C. Tjong, J. Eldridge and R. Hoffman, Appi. Surf. Sci., 14 (1982) 297. 38 W. L. N. Matthews, P. J. K. Paterson and H. K. Wagenfeld, Appi. Surf. Sd., 15 (1983) 281. 39 N. D. Tomashov, E. N. Utinski and G. P. Chernova, Electrokhimiya, 17 (1981) 969. 40 A. J. Forty, Gold Bull., 14(1981)25. 41 N. D. Tomashov, G. P. Chernova and E. N. Utinski, Prot. Met. (USSR), 17 (1981) 325. 42 J. P. Biersack and L. G. Haggmark, Nuci. Instrum. Methods, 174 (1980) 257. 43 P. W. Palmberg, L. E. Davis, N. C. MacDonald, G. E. Riach and R. E. Weber, Handbook of Auger Electron Spectroscopy (2nd Edn.), Physical Electronics Industries, Eden Prairie, MN, 1976. 44 K. S. Kim, A. F. Gossmann and N. Winograd, Anal. Chem., 46 (1974) 197. 45 K. Asami, J. Electron Spectrosc. Relat. Phenom., 9 (1976) 469. 46 G. Johanson, J. Hedman, A. Berndtsson, M. Kiasson and R. Nilson, J. Electron Spectrosc. Relat. Phenom., 2 (1973) 295. 47 H. Konno and M. Nagayama, in J. Kruger and R. P. Frankenthal (eds.), Passivity of Metals, Electrochemical Society, Princeton, NJ, 1978, p. 585.