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Characterisation of the passive film on HIPed Stellite 6 alloy using X-ray photoelectron spectroscopy
Materials Science and Engineering A 393 (2005) 91–101
Characterisation of the passive film on HIPed Stellite 6 alloy using X-ray photoelectron spectroscopy U. Malayoglua,1 , A. Nevillea,∗ , G. Beamsonb a
Corrosion and Surface Engineering Research Group, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, Scotland b CLRC Daresbury Laboratory, NCESS, Daresbury, Warrington, Cheshire WA4 4AD, UK Received 19 July 2004; received in revised form 24 September 2004
Abstract In this paper results from the X-ray photoelectron spectroscopy (XPS) analysis of hot isostatically pressed (HIP) Stellite 6 in a 3.5% NaCl liquid medium are reported. The aim of the paper is to determine the composition of the passive film formed at different temperatures and link it to the corrosion properties. It has been shown that the alloy passivates spontaneously in air resulting in the formation of a thin oxide film comprising Cr and Co. Electrochemical oxidation at different temperatures results in the formation of a complex layer, the composition and thickness of which depends on the test temperature. Co was detected in the solution after corrosion; the Co amount increases as the test temperature increases and no Co is found in the passive film after corrosion. © 2004 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Anodic polarisation; Passivity; Corrosion; Stellite 6
1. Introduction The compositional roots of contemporary cobalt-base superalloys stem from the early 1900s when patents covering the cobalt–chromium and cobalt–chromium–tungsten system were issued. Consequently, the Stellite alloys of E. Haynes became important industrial materials for cutlery, machine tools and wear-resistant hardfacing applications [1,2]. The cobalt–chromium–molybdenum casting alloy Vitallium was developed in the 1930s for dental prosthetics, and derivative HS-21 soon became an important material for turbocharger and gas turbine applications during the 1940s. Similarly, wrought cobalt–nickel–chromium alloy S816 was used extensively for both gas turbine blades and vanes during this period. Another key alloy, invented in about 1943 by R.H. ∗ Corresponding author. Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK. Tel.: +44 113 343 6812; fax: +44 113 242 4611. E-mail address: [email protected] (A. Neville). 1 Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK.
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.09.071
Thielemann, was cast cobalt–nickel–chromium–tungsten alloy X-40. This alloy is still used in gas turbine vanes and subsea drill bits, and it has served extensively as a model for newer generations of cobalt-base superalloys. Hot isostatic pressing (HIPing) is rapidly becoming an industry standard as a processing method. Due to increasingly complex engineering shape specifications, higher demands on quality and lowering costs, HIPing has matured to a stage where it is recognized universally and is used on an industrial scale. HIPing requires a high-pressure vessel and consists of applying high isostatic pressure, using an inert gas, to the surface of the piece being processed or on the surface of a can filled with powder. A resistance heater inside the pressure vessel provides the necessary heat for the treatment. The microstructures of Stellite alloys vary considerably with composition, manufacturing process and post treatment. They may either be in the form of hypoeutectic structure consisting of a Co-rich solid solution surrounded by eutectic carbides, or of the hypereutectic type containing large idiomorphic primary chromium-rich carbides and a eutectic [1].
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It is generally acknowledged that the susceptibility of passive metals to localised corrosion (including pitting) and the rate at which this corrosion process occurs are closely related to the ability of the passive film to resist breakdown and to repassivate once corrosion has initiated [2]. The chemical composition of the passive film, its structure, physical properties, coherence and thickness are of paramount importance in the nucleation and propagation of localised corrosion. Investigations into the composition and structure of passive oxide films on stainless steels and other related passive alloys are much more difficult than in the case of iron because the films are thinner, their chemical composition is complicated, and they cannot be reduced cathodically. A major part of the information available on composition and structure of passive films on stainless steels has been obtained with spectroscopic techniques, particularly X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). Other methods such as ion scattering spectroscopy (ISS) and secondary ion mass spectrometry (SIMS) also provide valuable data [3]. The XPS method was used to study the composition and structure of oxide films on iron–chromium alloys. Hashimito et al. [4] used this method to compare the film formed on Fe–30Cr and Fe–30Cr–2Mo alloys during passivation in 1 M KCl. They did not find any substantial differences in the film composition, which was primarily, hydrated chromium oxyhydroxide. XPS and AES methods were also used by Olefjord and Elfstrom to study oxide films on austenitic stainless steels passivated in 0.1 M HCl + 0.4 M NaCl solution [5]. The passive film consisted primarily of chromium oxide. Iron was preferentially dissolved even during passivation. Mo enrichment was observed in high Mo (6%) alloyed steel. Ni was present only to a small extent in the film. The composition of the film is significantly affected by the conditions (e.g. environment, potential, time, temperature) in which the film develops. If the dissolution of a given component of an alloy greatly exceeds the dissolution rate of other elements, the film can be enriched compared to the bulk alloy composition. Preferential dissolution of iron in the Fe–Cr alloy can cause the formation of a chromium-enriched film [5]. Compared to the extensive literature on Fe-based alloys (especially stainless steels) there is very little information available on other alloys which exhibit passivity (e.g. Niand Co-based alloys). Hocking et al. [6] studied the corrosion of Stellite 6 in lithiated high-temperature water used in pressurized water reactors (PWR) coolant circuit. By using XPS they determined the chemical composition at the outer surface of the corrosion layer. McIntyre et al. also studied the formation of the corrosion film on Stellite 6 alloy [7]. According to their work there are two major mechanisms controlling the corrosion behaviour of the alloy during aqueous oxidation. One is ionic migration via solid-state processes, while a second results from dissolution of the cation and its subsequent reprecipitation on the oxide surface. As a result of
their work they concluded that during aqueous exposure in either reducing or mildly oxidizing conditions, a depletion of the cobalt surface composition was observed. This implies that there is a preferential dissolution of cobalt, which had already migrated preferentially to the interface. Only a limited number of studies have focused on the corrosion performance of HIPed Co-based alloys. In this paper XPS has been used to study the composition and structure of the passive film formed on HIPed Stellite 6 alloy in a saline (3.5% NaCl) solution in the as-polished condition and also after accelerated corrosion (anodic polarisation) tests at a range of temperatures.
2. Materials and experimental methods The material investigated in this work is HIPed Stellite 6, which has the chemical composition as shown in Table 1. For electrochemical corrosion tests a wire was soldered on the rear of the sample and it was then embedded in a nonconducting resin. The exposed face was prepared by grinding with 600 and 1200 grit SiC paper followed by diamond polishing with 6-m diamond grit. The surface was rinsed with methanol after polishing and dried with compressed air. In this study DC anodic polarisation tests were carried out to assess the resistance of the material to passivity breakdown and the kinetics of the anodic reactions occurring at the sample surface under specific conditions. This test involved using a three-electrode electrochemical cell, schematically shown in Fig. 1, in which the reference electrode used was saturated calomel (SCE) and the auxiliary electrode was platinum. In anodic polarisation tests the potential was shifted from the free corrosion potential (Ecorr ) in the noble (positive potential) direction at a rate of 15 mV/min. Once a current density of 500 A/cm2 was achieved in the external circuit between the sample and the auxiliary electrode the potential scan was reversed and the potential was scanned in the negative direction to reach Ecorr . Tests were performed in 3.5% NaCl at 20, 30, 40 and 50 ◦ C. Light microscopy and scanning electron microscopy (SEM) with Energy dispersive X-ray analysis (EDX) attachment were used in the initial stages of the programme for characterisation of the materials in the as-received conditions. The microstructure and specifically the size and distribution Table 1 Measured chemical composition of HIPed Stellite 6 Mass% (Co balance) Cr C W Mo Ni Si Fe Mn
29.00 1.15 4.84 0.59 0.85 1.07 0.80 0.75
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Fig. 1. Schematic representation of a three-electrode test cell.
of the carbides were assessed for both materials. The SEM used in this study is equipped with a LaB6 gun, and is capable of operating as a conventional high-vacuum SEM, or under low vacuum in environmental SEM (ESEM) mode. The SEM is fully equipped with a range of secondary electron (SE) and back-scattered electron (BSE) detectors. EDX analyses including light elements are done semi-quantitatively. Corrections for atomic number, absorption and fluorescence (ZAF) are achieved through a virtual standard calibration routine. In XPS, a sample is irradiated with soft X-rays (1–2 keV) and the energies and the intensities of the emitted photoelectrons are measured. The energies provide qualitative information on the elemental and chemical species present in the sample, and the intensities provide quantitative information. The technique is inherently surface sensitive with an analysis depth of 5–10 nm which makes it ideal for the analysis of passive oxide films [8]. XPS data was acquired using the NCESS Scienta ESCA300 spectrometer at CCLRC Daresbury Laboratory. This employs a high-power monochromatised Al K␣ X-
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ray source (hν = 1486.6 eV), high transmission electron optics and multichannel detector [9]. Survey scans and more detailed region scans were run at 300 eV pass energy and 0.8 mm slit width. In order to assess the variation of composition as a function of depth the samples were subjected to argon ion sputtering in the preparation chamber of ESCA300 spectrometer for various times. Curve fitting of the region spectra was performed using the CasaXPS software [10] with a Gaussian–Lorenzian peak profile. After initial setting up of a model for various spectral components within a region spectrum, iteration of the program then optimised the fit. Inductively coupled plasma (ICP) water analysis was performed on the water in which anodic polarisation tests had been conducted using a JY 138 Ultrace, which is a sequential Emission Spectrometer. Calibration samples for Co, Cr and W were analysed prior to starting the water analysis on the extracted samples.
3. Results Fig. 2 shows the SEM images of the as-polished HIPed Stellite 6. The microstructure consists of ␥-Co solid solution (light region) and spheroidal carbides, of average dimension (2 m), which are uniformly distributed in the ␥-Co solid solution. EDX measurements taken from the average areas of the surface (point a) and on the specific carbide (point b) and matrix (point c) of samples are presented in Table 2. The matrix was found to be rich in Co and the carbides rich in Cr although there is evidence to suggest there are other elements in the carbide phase. 3.1. XPS Spectra on the as-polished surface Fig. 3 shows the typical XPS survey spectra from the aspolished HIPed Stellite 6 after argon ion etching for 30 s, for
Fig. 2. (a) SEM and (b) light microscopic images of as-polished HIPed Stellite 6.
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Table 2 Measured chemical composition (wt.%) of the HIPed Stellite on average areas of surface and localized regions using EDAX attachment on ESEM Composition (wt.%)
a b (carbide) c (matrix)
Co
Cr
W
C
Fe
59.10 16.40 60.20
31.20 71.10 26.80
4.30 3.40 4.90
2.60 7.60 0.90
2.90 0.90 2.80
Fig. 5. The change in the C 1s spectrum as a function of etching time on the as-polished sample.
Fig. 3. XPS survey scan of as-polished surface of HIPed Stellite 6 after 0.5 min, etching cycle.
the binding energy range from 0 to 1100 eV. Peaks for Co, Cr, W, O and C are clearly identified. 3.1.1. Evolution of the C 1s peaks on the as-polished surface After 0.5 min of argon ion etching the C1s spectrum of the as-polished surface (Fig. 4) showed two components. The peak at 284.6 eV represents carbon in a graphitic or hydrocarbon environment, probably due to adventitious contamination of the surface, and that at 282.9 eV represents carbon in a carbide environment. After 15 min etching the graphitic/hydrocarbon component is considerably reduced
Fig. 4. Curve fitting of C 1s signal after 0.5 min etching on the as-polished sample.
relative to the carbide component (Fig. 5). Previous XRD work [11] has shown that the carbides in Stellite alloys comprise mainly MC, M7 C3 and M23 C6 components where M represents Co, Cr, W or Mo depending on the particular alloy. 3.1.2. Evolution of the O 1s peaks on the as-polished surface The O 1s spectrum of the as-polished surface (Fig. 6) consists of two components at 530.2 and 531.4 eV, which are assigned to O2− and OH− respectively [12]. As the etching time increased from 0.5 to 15 min, the overall intensity of the O 1s spectrum decreased.
Fig. 6. O 1s spectrum of as-polished Stellite 6 after 30 s etching.
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Fig. 7. The change in the Cr 2p spectrum of as-polished Stellite 6 as a function of etching time.
3.1.3. Evolution of the Cr 2p signals on the as-polished surface The Cr 2p spectrum of the as-polished surface is shown in Fig. 7. The main 2p3/2 component at 574.2 eV represents Cr in metallic and carbide environments (literature binding energies are 574.4 and 574.6 eV respectively [13,14]; hence, the two species are too close in binding energy to be distinguishable). The peak at approximately 577 eV represents Cr3+ in an oxide and/or hydroxide environment [15]. As the argon ion etching time was increased, the relative intensity of the oxide/hydroxide component decreased. 3.1.4. Evolution of the W 4f signals on the as-polished surface A strong tungsten signal was detected after all etching times. W 4f electron binding energies of metallic tungsten were reported in different references as 31 eV [16,17] and 31.4 eV [18]. The peak shown in Fig. 8 was obtained at 31.3 eV. This peak can be generated from both metallic W and/or W-containing carbides as both of them have very close binding energies reported in the literature (W carbides 31.5 eV [15]) and beside this, the XRD and EDX analysis showed the presence of both metallic and W-rich carbides in the microstructure. Also, the oxide peaks WO2 ∼ 33 eV and WO3 ∼ 36 eV, respectively [15], were not detected on the as-
Fig. 8. The W 4F spectrum of as-polished Stellite 6 as a function of etching time.
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Fig. 9. The fitted spectrum of Co 2p3/2 at two different etching times.
polished surface showing that there is no evidence of tungsten oxide formation on the as-polished sample. Another interesting point in Fig. 8 is that the amount of W detected also increased as the etching time increases from 0.5 to 15 min, indicating that W increases deeper into the surface. 3.1.5. Evolution of the Co 2p signals on the as-polished surface The as-polished surface shows a strong cobalt signal, as expected. The Co 2p3/2 peak as Co metal is located at 778.2 eV (Fig. 9) in agreement with literature values of 778.1 and 778.3 eV [12]. The peak at 780.1 eV is the oxide part of the Co 2p3/2 signal, which was reported as either CoO with a binding energy of 780.2 eV or Co2 O3 with a binding energy of 780.0 eV or Co3 O4 which is more stable compared to the other two, with a binding energy of 780.7 eV [15]. 3.2. Nature of the polarisation curves The potentiodynamic anodic polarisation curves for the alloy at 20, 30, 40 and 50 ◦ C in 3.5% NaCl solution are shown in Fig. 10. It can be observed from the curves that at all temperatures the alloy showed a high breakdown potential, indicative of good passive behaviour. On increasing the potential scan, if propagation of corrosion does not occur, it is expected that the current falls rapidly and the curve essentially follows the forward E–i curve. Hence, in this case
Fig. 10. Anodic polarisation curves of HIPed Stellite 6 at four different temperature.
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Table 3 Breakdown potentials and maximum current densities Temperature (◦ C)
Breakdown potential (V)
Max. current density (A/cm2 )
20 30 40 50
0.71 0.68 0.61 0.40
500 500 500 2270
an imax exceeding 500 A/cm2 indicates that the degree of propagation is higher. The breakdown potentials (Eb ) and the maximum current values reached during the test (imax ) are summarised in Table 3. Light microscopy images taken after polarisation tests (Fig. 11), clearly show there is a film formed on the Co-rich matrix. 3.3. Analysis of the surface after anodic polarisation tests using XPS The chemical states of the alloy components in the near surface region were identified by XPS. Co 2p, Cr 2p, W 4f, C
1s and O 1s peaks were analysed for the anodically polarised surfaces. Also, the Cl 2p region was investigated to determine if any chloride ions are present in the passive film, since it was previously reported for stainless steels that the passive film can contain some Cl− ions in the structure [5]. Firstly, the surfaces were analysed without etching with argon and then after 30 s and 5 min etching. This enabled the changes in the chemical composition of the passive film on the polarised samples as a function of depth to be accessed. 3.3.1. Evolution of C 1s after polarisation tests XPS analysis of the Stellite 6 surface after polarisation testing and before argon ion etching revealed C 1s signals with a number of components, indicating the presence of metal carbides and adventitious carbon contamination. Curve fitting (Fig. 12a) showed that C 1s components were present with binding energies of 283.2, 284.6–285.1, 286.2 and 288.9 eV. The peak at 283.2 eV, corresponding to carbide, was also found on the as-polished sample. The 284.6–285.1 eV component represents carbon atoms in graphitic/hydrocarbon
Fig. 11. Comparison of the as-polished surface with anodically polarised surfaces at different temperatures.
Fig. 12. (a) Curve fitting for the C 1s peak of un-etched surface at 18 ◦ C, (b) change in C 1s as a function of temperature, (c) the change in the C 1s carbide peak as a function of temperature after 5 min etching cycle.
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Fig. 13. (a) The O 1s signal obtained without etching the surface after anodic polarisation at 20 ◦ C, (b) the change in the O 1s signal compared with as-polished 15 min argon etched surface.
environments and the 286.2 and 288.9 eV components represent carbon bonded to oxygen atoms (C O and O C O, respectively [19]). As the polarisation test temperature was increased the intensity of the C O, O C O and carbide components on the un-etched surface decreased relative to that of the graphite/hydrocarbon component (Fig. 12b). For all the samples argon ion etching caused the intensity of the carbide component to increase relative to that of the graphite/hydrocarbon component and the O C O component to disappear almost entirely (Fig. 12c). 3.3.2. Evaluation of O 1s after polarisation tests The O 1s spectrum of the anodically polarised surface without argon etching is shown in Fig. 13a. The peak at 531.7 eV represents the OH− as stated earlier in Section 3.1.2. As the etching time increased to 5 min (Fig. 13b), the O 1s
spectrum showed two components. The peak at 530.8 eV represents oxide and 531.7 eV represents hydroxide. This shows that the outer layer of the oxide film is rich in hydroxide and as the etching time increased the oxide component is enhanced. 3.3.3. Evaluation of Cr 2p after polarisation tests A strong Cr 2p peak was detected for all test temperatures as expected when previous results on passive film on stainless steel are considered [20]. The analysis of the surfaces after anodic polarisation showed two doublets at each test temperature. Curve fitting (Fig. 14a) for the sample anodically polarised at 20 ◦ C with no etching showed that Cr 2p components were present with binding energies of 574.4 and 577.8 eV. As stated in Section 3.1.3, the 574.4 eV component corresponds to metallic Cr and the 577.8 eV component represents the oxide, hydroxide or the mixture of the two. In the
Fig. 14. Cr 2p spectra: (a) after polarisation tests at 20 ◦ C, without argon bombardment curve fit components shown for metal/carbide (1) oxide/hydroxide environments (2), (b) as-polished and after-polarisation test at different temperatures without argon bombardment.
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Table 4 XPS peak positions for various components and their concentrations in the passive film Temperature (◦ C)
Etching time
Element
20
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
30
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
40
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
50
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
Peak (eV)
Concentration (%)
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 36
– 88.3 11.7 100
Co Cr2 O3 Cr WO3 WO2
778.4 576.6 574.2 36 34.2
100 81.6 18.4 90.6 9.4
Co Cr2 O3 Cr WO2 W
778.2 577.2 574.3 34.2 31.6
100 37.4 62.6 5.3 94.7
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.9
– 98.4 1.6 100
No Co detected Cr2 O3 Cr WO3 WO2
– 576.9 574.4 36 34.3
– 81.6 18.4 96.2 3.8
Co Cr2 O3 Cr WO3 WO2 W
778.5 576.6 574.8 36 35 31.8
100 84.4 15.6 44.4 1.7 53.9
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.8
– 98.2 1.8 100
No Co detected Cr2 O3 Cr WO3 WO2
– 576.8 574.5 35.9 34.3
– 92.6 7.4 90.1 9.9
Co Cr2 O3 Cr WO3 W
778.3 576.6 574.3 35.9 31.8
100 63.6 36.4 34.7 65.3
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.7
– 99 1 100
No Co detected Cr2 O3 Cr WO3 WO2
– 577 574.3 35.8 34.7
– 69.7 30.3 95 5
Co Cr2 O3 Cr WO3 W
778.6 576.6 574.2 36 31.8
100 80 20 35.5 47.5
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literature Cr2 O3 was reported to be at the binding energy of 576.6 eV and hydroxide, Cr(OH)3 , at 577.5 eV [19], and so the fitted curve 2 in Fig. 14 is likely to contain both components at different concentrations. This is consistent with what is known about stainlesfs steels where the passive film is richer in OH− at the outside [20]. Fig. 14b shows the change of the Cr 2p signals at different test temperatures compared with the as-polished sample. Table 4 summarises the composition of the passive film and the concentration of the oxide and metal at different etching times and different temperatures. 3.3.4. Evaluation of W 4f after polarisation tests Fig. 15 shows an example of the W 4f spectrum for the unetched surface after the anodic polarisation test at 20 ◦ C. The peak occurs at the binding energy of 36 eV. It was reported that for the oxide measured values for the W 4f7/2 spin orbit doublet range from about 31 eV binding energy for metallic tungsten and to about 36 eV for W6+ in WO3 [20,21]. From this the peak can be identified as originating from the oxide state of tungsten. It has been shown for tungsten that it has oxides with compositions corresponding to a spectrum of oxidation states, between +4 and +6, the latter being more thermodynamically stable. There are a number of possible oxidation reactions of tungsten associated with the formation of tungsten oxide as given below. W + 2H2 O = WO2 + 4H+ + 4e− ,
(1)
WO2 + H2 O = W2 O5 + 2H+ + 2e− ,
(2)
W2 O5 + H2 O = 2WO3 + 2H+ + 2e− .
(3)
As the surface was etched for 30 s and 5 min, the intensity and the position of the peaks changed. Fig. 15b compares the tungsten peaks as a function of etching time for the surface after anodic polarisation at 20 ◦ C. The quantitative results are also summarised in Table 4. Peak values are in good agree-
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Table 5 Quantification of dissolved elements from ICP after anodic polarisation Temperature (◦ C)
20 30 40 50
Elements detected (ppm) Co
Cr
W
Total
0.114 0.108 0.238 1.62
0.024 0.022 0.055 0.032
0.025 0.09 0.01 0.029
0.163 0.139 0.283 1.681
ment with the reactions written in 1–3. The first step is the formation of WO2 at the metal/electrolyte interface, which is not thermodynamically stable and reacts with H2 O to form a more stable oxide WO3 , which occupies the outer regions. 3.3.5. Evaluation of Co 2p after polarisation tests The Co 2p spectrum on the as-polished Stellite 6 alloy comprised two strong peaks and as the surface was etched the intensity of these two peaks increased. After the anodic polarisation of the alloy, the un-etched surface did not show any trace of Co. This was observed at all test temperatures. At lower temperature after a short etching cycle, the Co peak starts to appear and after 5 min etching Co peaks corresponding to metallic Co were observed (Fig. 16). 3.4. Solution analysis The amount of dissolved elements obtained from ICP solution analysis is summarised in Table 5. The passage of a huge current density is evidence of continual reaction of the metal, to result in film thickening and dissolution into the environment or some combination of the two. In good agreement with the XPS findings which showed that there is no Co in the film, the water analysis showed Co was the main element dissolving into the solution after the tests. The amount of dissolved Co generally increased as the test temperature increases, and the amount of dissolved W and Cr stayed almost constant
Fig. 15. W 4f spectrum of anodically polarised surface at 20 ◦ C: (a) without argon etching, (b) in three different etching times.
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Fig. 16. Comparison of the Co 2p peaks in as-polished and anodically polarised sample: (a) non-etched surface (b) 5 min etched surface.
over that temperature range indicating that their key role was in the maintenance of passivity. The possible routes for Co to reach the solution are diffusion through the chromium oxide and hydroxide (Cr2 O3 , Cr(OH)3 and tungsten oxide (WO3 or WO2 ) film or at localised breakdown of the film. The carbide/matrix boundaries are perhaps the most probable sites given that corrosion was shown to initiate and propagate from these regions [11].
4. Discussion The findings presented in this paper demonstrate that after anodic polarisation of HIPed Stellite 6, the passive film evolves to produce a visible film on the matrix at each temperature studied. It has been proved that the new film consists of metallic species and is free from Co. In XPS if the analytical depth (∼10 nm) is greater than the passive film thickness then a proportion of the signal will be derived from the substrate and is recognisable in analysis of the XPS trace. In this study on as-polished surfaces a signal from the substrate is seen after 30 s etching. The increase in the polarisation test temperature increased the film thickness, as was demonstrated by the fact that the Co 2p metal signals from the substrate did not appear until after 5 min etching. It is also known for passive alloys that there is generally an inverse relationship between the thickness of the film and its protective property [5]. In Table 3, it is shown that as the temperature increases (and the film becomes thicker as confirmed by XPS) the breakdown potential decreases (i.e. protective properties decreases) meaning that the material becomes more susceptible to localised corrosion. For the as-polished samples of Stellite 6 the C 1s spectrum reveals the presence of an overlay of carbonaceous contamination, and the O 1s spectrum the presence of ox-
ide/hydroxide species at the surface. The carbon contamination and oxygen containing species are readily removed by argon etching. The C 1s spectrum also reveals the presence of metal carbide species below the contamination over layer. The Cr 2p spectrum comprises Cr metal, carbide and oxide/hydroxide species and the W 4f spectrum is consistent with the metal but not W oxides. The Co 2p spectrum is consistent with the presence of Co metal and oxide/hydroxide species. The XPS signals of the metal oxide/hydroxide species decrease with argon ion etching; hence, the model for the as-polished surface is of a matrix of metal and metal carbide species with an over-layer of Cr and Co oxide/hydroxide and carbonaceous contamination. The analysis of the surface after anodic polarisation shows a presence of multilayer duplex structure, where the composition changes continually with depth. At the outer surface, the film contains no metallic Co, Cr or W and consists of Cr(OH)3 /Cr2 O3 and WO2 . Immediately beneath this layer, depending on the test temperature, the film is composed of Cr(OH)3 /Cr2 O3 and metallic Cr from the bulk material can be observed. Similarly W was detected in the from of WO3 and metallic W. A schematic representation of the key components of the air formed film and corroded surface is shown in Fig. 17. The Cl− ion content that has been reported for the passive film electrochemically formed on stainless steels [22] was not observed for the Stellite 6 alloy. For all test temperatures there was no evidence of a Cl 2p peak as shown in Fig. 18. The passive films formed on stainless steel have been reported to be few nanometres thick [23]. To determine the thickness of the surface layer some simplifications and assumptions were made; (i) there are at least two layers with different composition and structure on top of the substrate; (ii) composition and structure inside a layer are homogeneous; and (iii) boundaries between layers are discontinuous [24].
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oxidation at different temperatures results in the formation of a complex layer whose composition and thickness depend on the test temperature. Co was detected in the solution after the test and the dissolved Co amount increased as the test temperature increased. After corrosion Co is not part of the passive film which is then dominated by Cr and W oxides.
Acknowledgement Fig. 17. Schematic representation and composition of film formed on the matrix: (a) air formed, (b) after anodic polarisation test.
The authors acknowledge the financial support to UM from Deloro Stellite and Heriot-Watt University.
References
Fig. 18. Cl 2p spectrum measured from the surface showing no evidence of Cl in the passive film.
In this current paper it has been demonstrated that the film has composition variation as a function of depth. However, it is not possible to confirm that the composition inside each layer is homogeneous and therefore thickness calculations are not appropriate. Also, Cumpson [25] showed that XPS data is not accurate enough to provide any statistical significance for models with more than three layers including the substrate.
5. Conclusions The composition, structure and thickness of the air-formed and corrosion-reaction films formed on the HIPed Stellite 6 alloy after anodic polarisation at temperatures 20, 30, 40 and 50 ◦ C in 3.5% NaCl have been studied by X-ray photoelectron spectroscopy. The alloy passivates spontaneously in air resulting in the formation of thin oxide film comprising mainly Cr oxide and some Co oxide. Electrochemical
[1] F.R. Morral, Corros. NACE 25 (1969) 307. [2] K.E. Mohamed, M.M.A. Gad, A.E. Nassef, A.W.A. El-Sayed, Z. Metallkd. 90 (1990) 195. [3] G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2186. [4] K. Hashimoto, K. Asami, K. Teramoto, Corros. Sci. 3 (1979) 165. [5] Z. Szklarska-Smoalowska, Pitting Corrosion of Metals, NACE, 1986. [6] W.H. Hocking, F.W. Stanchell, E. McAlpine, D.H. Lister, Corros. Sci. 25 (1985) 531. [7] N.S. McIntyre, D. Zetaruk, E.V. Murphy, Surf. Interface Anal. 1 (1979) 105. [8] N.S. McIntyre, R.D. Davidson, G. Kim, J.T. Francis, Vacuum 69 (2003) 63. [9] G. Beamson, D. Briggs, S.F. Davies, I.W. Fletcher, D.T. Clark, J. Howard, U. Gelius, B. Wannberg, P. Balzer, Surf. Interface Anal. 15 (1990) 541. [10] CasaXPS VAMAS Processing Software, http://www.casaxps.com. [11] U. Malayoglu, A. Neville, Corros. Sci., in press. [12] I. Milosev, H.-H. Strehblow, Electrochim. Acta 48 (2003) 2767. [13] N. Tabet, I. Allam, R.C. Yin, Appl. Surf. Sci. 195 (2002) 166. [14] X.Y. Li, E. Akiyama, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 39 (1997) 1365. [15] NIST X-ray Photoelectron Spectroscopy Database, http://srdata.nist. gov/xps/. [16] Ch. Linsmeier, J. Wanner, Surf. Sci. 454–456 (2000) 305. [17] L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nordling, J. Phys. Chem. Solids 30 (1969) 1835. [18] F.J. Moulder, P.E. Stickle, E.P. Sobol, K.D. Bonben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1993. [19] J. Luthin, Ch. Linsmeier, Surf. Sci. 454–456 (2000) 78. [20] C.O.A. Olsson, D. Landolt, Electrochim. Acta 48 (2003) 1093. [21] A. Neville, T. Hodgkiess, Br. Corros. J. 35 (2000) 183. [22] Y.J. Kim, Corrosion 55 (1999) 81. [23] A. Gildenpfennig, U. Gramberg, G. Hohlneicher, Corros. Sci. 45 (2003) 575. [24] A. Rossi, B. Elsener, G. Hanner, M. Textor, D. Spence, Surf. Interface Anal. 29 (2000) 460. [25] P.J. Cumpson, J. Electron. Spectrosc. 73 (1995) 25.
Characterisation of the passive film on HIPed Stellite 6 alloy using X-ray photoelectron spectroscopy U. Malayoglua,1 , A. Nevillea,∗ , G. Beamsonb a
Corrosion and Surface Engineering Research Group, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, Scotland b CLRC Daresbury Laboratory, NCESS, Daresbury, Warrington, Cheshire WA4 4AD, UK Received 19 July 2004; received in revised form 24 September 2004
Abstract In this paper results from the X-ray photoelectron spectroscopy (XPS) analysis of hot isostatically pressed (HIP) Stellite 6 in a 3.5% NaCl liquid medium are reported. The aim of the paper is to determine the composition of the passive film formed at different temperatures and link it to the corrosion properties. It has been shown that the alloy passivates spontaneously in air resulting in the formation of a thin oxide film comprising Cr and Co. Electrochemical oxidation at different temperatures results in the formation of a complex layer, the composition and thickness of which depends on the test temperature. Co was detected in the solution after corrosion; the Co amount increases as the test temperature increases and no Co is found in the passive film after corrosion. © 2004 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Anodic polarisation; Passivity; Corrosion; Stellite 6
1. Introduction The compositional roots of contemporary cobalt-base superalloys stem from the early 1900s when patents covering the cobalt–chromium and cobalt–chromium–tungsten system were issued. Consequently, the Stellite alloys of E. Haynes became important industrial materials for cutlery, machine tools and wear-resistant hardfacing applications [1,2]. The cobalt–chromium–molybdenum casting alloy Vitallium was developed in the 1930s for dental prosthetics, and derivative HS-21 soon became an important material for turbocharger and gas turbine applications during the 1940s. Similarly, wrought cobalt–nickel–chromium alloy S816 was used extensively for both gas turbine blades and vanes during this period. Another key alloy, invented in about 1943 by R.H. ∗ Corresponding author. Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK. Tel.: +44 113 343 6812; fax: +44 113 242 4611. E-mail address: [email protected] (A. Neville). 1 Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK.
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.09.071
Thielemann, was cast cobalt–nickel–chromium–tungsten alloy X-40. This alloy is still used in gas turbine vanes and subsea drill bits, and it has served extensively as a model for newer generations of cobalt-base superalloys. Hot isostatic pressing (HIPing) is rapidly becoming an industry standard as a processing method. Due to increasingly complex engineering shape specifications, higher demands on quality and lowering costs, HIPing has matured to a stage where it is recognized universally and is used on an industrial scale. HIPing requires a high-pressure vessel and consists of applying high isostatic pressure, using an inert gas, to the surface of the piece being processed or on the surface of a can filled with powder. A resistance heater inside the pressure vessel provides the necessary heat for the treatment. The microstructures of Stellite alloys vary considerably with composition, manufacturing process and post treatment. They may either be in the form of hypoeutectic structure consisting of a Co-rich solid solution surrounded by eutectic carbides, or of the hypereutectic type containing large idiomorphic primary chromium-rich carbides and a eutectic [1].
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It is generally acknowledged that the susceptibility of passive metals to localised corrosion (including pitting) and the rate at which this corrosion process occurs are closely related to the ability of the passive film to resist breakdown and to repassivate once corrosion has initiated [2]. The chemical composition of the passive film, its structure, physical properties, coherence and thickness are of paramount importance in the nucleation and propagation of localised corrosion. Investigations into the composition and structure of passive oxide films on stainless steels and other related passive alloys are much more difficult than in the case of iron because the films are thinner, their chemical composition is complicated, and they cannot be reduced cathodically. A major part of the information available on composition and structure of passive films on stainless steels has been obtained with spectroscopic techniques, particularly X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). Other methods such as ion scattering spectroscopy (ISS) and secondary ion mass spectrometry (SIMS) also provide valuable data [3]. The XPS method was used to study the composition and structure of oxide films on iron–chromium alloys. Hashimito et al. [4] used this method to compare the film formed on Fe–30Cr and Fe–30Cr–2Mo alloys during passivation in 1 M KCl. They did not find any substantial differences in the film composition, which was primarily, hydrated chromium oxyhydroxide. XPS and AES methods were also used by Olefjord and Elfstrom to study oxide films on austenitic stainless steels passivated in 0.1 M HCl + 0.4 M NaCl solution [5]. The passive film consisted primarily of chromium oxide. Iron was preferentially dissolved even during passivation. Mo enrichment was observed in high Mo (6%) alloyed steel. Ni was present only to a small extent in the film. The composition of the film is significantly affected by the conditions (e.g. environment, potential, time, temperature) in which the film develops. If the dissolution of a given component of an alloy greatly exceeds the dissolution rate of other elements, the film can be enriched compared to the bulk alloy composition. Preferential dissolution of iron in the Fe–Cr alloy can cause the formation of a chromium-enriched film [5]. Compared to the extensive literature on Fe-based alloys (especially stainless steels) there is very little information available on other alloys which exhibit passivity (e.g. Niand Co-based alloys). Hocking et al. [6] studied the corrosion of Stellite 6 in lithiated high-temperature water used in pressurized water reactors (PWR) coolant circuit. By using XPS they determined the chemical composition at the outer surface of the corrosion layer. McIntyre et al. also studied the formation of the corrosion film on Stellite 6 alloy [7]. According to their work there are two major mechanisms controlling the corrosion behaviour of the alloy during aqueous oxidation. One is ionic migration via solid-state processes, while a second results from dissolution of the cation and its subsequent reprecipitation on the oxide surface. As a result of
their work they concluded that during aqueous exposure in either reducing or mildly oxidizing conditions, a depletion of the cobalt surface composition was observed. This implies that there is a preferential dissolution of cobalt, which had already migrated preferentially to the interface. Only a limited number of studies have focused on the corrosion performance of HIPed Co-based alloys. In this paper XPS has been used to study the composition and structure of the passive film formed on HIPed Stellite 6 alloy in a saline (3.5% NaCl) solution in the as-polished condition and also after accelerated corrosion (anodic polarisation) tests at a range of temperatures.
2. Materials and experimental methods The material investigated in this work is HIPed Stellite 6, which has the chemical composition as shown in Table 1. For electrochemical corrosion tests a wire was soldered on the rear of the sample and it was then embedded in a nonconducting resin. The exposed face was prepared by grinding with 600 and 1200 grit SiC paper followed by diamond polishing with 6-m diamond grit. The surface was rinsed with methanol after polishing and dried with compressed air. In this study DC anodic polarisation tests were carried out to assess the resistance of the material to passivity breakdown and the kinetics of the anodic reactions occurring at the sample surface under specific conditions. This test involved using a three-electrode electrochemical cell, schematically shown in Fig. 1, in which the reference electrode used was saturated calomel (SCE) and the auxiliary electrode was platinum. In anodic polarisation tests the potential was shifted from the free corrosion potential (Ecorr ) in the noble (positive potential) direction at a rate of 15 mV/min. Once a current density of 500 A/cm2 was achieved in the external circuit between the sample and the auxiliary electrode the potential scan was reversed and the potential was scanned in the negative direction to reach Ecorr . Tests were performed in 3.5% NaCl at 20, 30, 40 and 50 ◦ C. Light microscopy and scanning electron microscopy (SEM) with Energy dispersive X-ray analysis (EDX) attachment were used in the initial stages of the programme for characterisation of the materials in the as-received conditions. The microstructure and specifically the size and distribution Table 1 Measured chemical composition of HIPed Stellite 6 Mass% (Co balance) Cr C W Mo Ni Si Fe Mn
29.00 1.15 4.84 0.59 0.85 1.07 0.80 0.75
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Fig. 1. Schematic representation of a three-electrode test cell.
of the carbides were assessed for both materials. The SEM used in this study is equipped with a LaB6 gun, and is capable of operating as a conventional high-vacuum SEM, or under low vacuum in environmental SEM (ESEM) mode. The SEM is fully equipped with a range of secondary electron (SE) and back-scattered electron (BSE) detectors. EDX analyses including light elements are done semi-quantitatively. Corrections for atomic number, absorption and fluorescence (ZAF) are achieved through a virtual standard calibration routine. In XPS, a sample is irradiated with soft X-rays (1–2 keV) and the energies and the intensities of the emitted photoelectrons are measured. The energies provide qualitative information on the elemental and chemical species present in the sample, and the intensities provide quantitative information. The technique is inherently surface sensitive with an analysis depth of 5–10 nm which makes it ideal for the analysis of passive oxide films [8]. XPS data was acquired using the NCESS Scienta ESCA300 spectrometer at CCLRC Daresbury Laboratory. This employs a high-power monochromatised Al K␣ X-
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ray source (hν = 1486.6 eV), high transmission electron optics and multichannel detector [9]. Survey scans and more detailed region scans were run at 300 eV pass energy and 0.8 mm slit width. In order to assess the variation of composition as a function of depth the samples were subjected to argon ion sputtering in the preparation chamber of ESCA300 spectrometer for various times. Curve fitting of the region spectra was performed using the CasaXPS software [10] with a Gaussian–Lorenzian peak profile. After initial setting up of a model for various spectral components within a region spectrum, iteration of the program then optimised the fit. Inductively coupled plasma (ICP) water analysis was performed on the water in which anodic polarisation tests had been conducted using a JY 138 Ultrace, which is a sequential Emission Spectrometer. Calibration samples for Co, Cr and W were analysed prior to starting the water analysis on the extracted samples.
3. Results Fig. 2 shows the SEM images of the as-polished HIPed Stellite 6. The microstructure consists of ␥-Co solid solution (light region) and spheroidal carbides, of average dimension (2 m), which are uniformly distributed in the ␥-Co solid solution. EDX measurements taken from the average areas of the surface (point a) and on the specific carbide (point b) and matrix (point c) of samples are presented in Table 2. The matrix was found to be rich in Co and the carbides rich in Cr although there is evidence to suggest there are other elements in the carbide phase. 3.1. XPS Spectra on the as-polished surface Fig. 3 shows the typical XPS survey spectra from the aspolished HIPed Stellite 6 after argon ion etching for 30 s, for
Fig. 2. (a) SEM and (b) light microscopic images of as-polished HIPed Stellite 6.
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Table 2 Measured chemical composition (wt.%) of the HIPed Stellite on average areas of surface and localized regions using EDAX attachment on ESEM Composition (wt.%)
a b (carbide) c (matrix)
Co
Cr
W
C
Fe
59.10 16.40 60.20
31.20 71.10 26.80
4.30 3.40 4.90
2.60 7.60 0.90
2.90 0.90 2.80
Fig. 5. The change in the C 1s spectrum as a function of etching time on the as-polished sample.
Fig. 3. XPS survey scan of as-polished surface of HIPed Stellite 6 after 0.5 min, etching cycle.
the binding energy range from 0 to 1100 eV. Peaks for Co, Cr, W, O and C are clearly identified. 3.1.1. Evolution of the C 1s peaks on the as-polished surface After 0.5 min of argon ion etching the C1s spectrum of the as-polished surface (Fig. 4) showed two components. The peak at 284.6 eV represents carbon in a graphitic or hydrocarbon environment, probably due to adventitious contamination of the surface, and that at 282.9 eV represents carbon in a carbide environment. After 15 min etching the graphitic/hydrocarbon component is considerably reduced
Fig. 4. Curve fitting of C 1s signal after 0.5 min etching on the as-polished sample.
relative to the carbide component (Fig. 5). Previous XRD work [11] has shown that the carbides in Stellite alloys comprise mainly MC, M7 C3 and M23 C6 components where M represents Co, Cr, W or Mo depending on the particular alloy. 3.1.2. Evolution of the O 1s peaks on the as-polished surface The O 1s spectrum of the as-polished surface (Fig. 6) consists of two components at 530.2 and 531.4 eV, which are assigned to O2− and OH− respectively [12]. As the etching time increased from 0.5 to 15 min, the overall intensity of the O 1s spectrum decreased.
Fig. 6. O 1s spectrum of as-polished Stellite 6 after 30 s etching.
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Fig. 7. The change in the Cr 2p spectrum of as-polished Stellite 6 as a function of etching time.
3.1.3. Evolution of the Cr 2p signals on the as-polished surface The Cr 2p spectrum of the as-polished surface is shown in Fig. 7. The main 2p3/2 component at 574.2 eV represents Cr in metallic and carbide environments (literature binding energies are 574.4 and 574.6 eV respectively [13,14]; hence, the two species are too close in binding energy to be distinguishable). The peak at approximately 577 eV represents Cr3+ in an oxide and/or hydroxide environment [15]. As the argon ion etching time was increased, the relative intensity of the oxide/hydroxide component decreased. 3.1.4. Evolution of the W 4f signals on the as-polished surface A strong tungsten signal was detected after all etching times. W 4f electron binding energies of metallic tungsten were reported in different references as 31 eV [16,17] and 31.4 eV [18]. The peak shown in Fig. 8 was obtained at 31.3 eV. This peak can be generated from both metallic W and/or W-containing carbides as both of them have very close binding energies reported in the literature (W carbides 31.5 eV [15]) and beside this, the XRD and EDX analysis showed the presence of both metallic and W-rich carbides in the microstructure. Also, the oxide peaks WO2 ∼ 33 eV and WO3 ∼ 36 eV, respectively [15], were not detected on the as-
Fig. 8. The W 4F spectrum of as-polished Stellite 6 as a function of etching time.
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Fig. 9. The fitted spectrum of Co 2p3/2 at two different etching times.
polished surface showing that there is no evidence of tungsten oxide formation on the as-polished sample. Another interesting point in Fig. 8 is that the amount of W detected also increased as the etching time increases from 0.5 to 15 min, indicating that W increases deeper into the surface. 3.1.5. Evolution of the Co 2p signals on the as-polished surface The as-polished surface shows a strong cobalt signal, as expected. The Co 2p3/2 peak as Co metal is located at 778.2 eV (Fig. 9) in agreement with literature values of 778.1 and 778.3 eV [12]. The peak at 780.1 eV is the oxide part of the Co 2p3/2 signal, which was reported as either CoO with a binding energy of 780.2 eV or Co2 O3 with a binding energy of 780.0 eV or Co3 O4 which is more stable compared to the other two, with a binding energy of 780.7 eV [15]. 3.2. Nature of the polarisation curves The potentiodynamic anodic polarisation curves for the alloy at 20, 30, 40 and 50 ◦ C in 3.5% NaCl solution are shown in Fig. 10. It can be observed from the curves that at all temperatures the alloy showed a high breakdown potential, indicative of good passive behaviour. On increasing the potential scan, if propagation of corrosion does not occur, it is expected that the current falls rapidly and the curve essentially follows the forward E–i curve. Hence, in this case
Fig. 10. Anodic polarisation curves of HIPed Stellite 6 at four different temperature.
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Table 3 Breakdown potentials and maximum current densities Temperature (◦ C)
Breakdown potential (V)
Max. current density (A/cm2 )
20 30 40 50
0.71 0.68 0.61 0.40
500 500 500 2270
an imax exceeding 500 A/cm2 indicates that the degree of propagation is higher. The breakdown potentials (Eb ) and the maximum current values reached during the test (imax ) are summarised in Table 3. Light microscopy images taken after polarisation tests (Fig. 11), clearly show there is a film formed on the Co-rich matrix. 3.3. Analysis of the surface after anodic polarisation tests using XPS The chemical states of the alloy components in the near surface region were identified by XPS. Co 2p, Cr 2p, W 4f, C
1s and O 1s peaks were analysed for the anodically polarised surfaces. Also, the Cl 2p region was investigated to determine if any chloride ions are present in the passive film, since it was previously reported for stainless steels that the passive film can contain some Cl− ions in the structure [5]. Firstly, the surfaces were analysed without etching with argon and then after 30 s and 5 min etching. This enabled the changes in the chemical composition of the passive film on the polarised samples as a function of depth to be accessed. 3.3.1. Evolution of C 1s after polarisation tests XPS analysis of the Stellite 6 surface after polarisation testing and before argon ion etching revealed C 1s signals with a number of components, indicating the presence of metal carbides and adventitious carbon contamination. Curve fitting (Fig. 12a) showed that C 1s components were present with binding energies of 283.2, 284.6–285.1, 286.2 and 288.9 eV. The peak at 283.2 eV, corresponding to carbide, was also found on the as-polished sample. The 284.6–285.1 eV component represents carbon atoms in graphitic/hydrocarbon
Fig. 11. Comparison of the as-polished surface with anodically polarised surfaces at different temperatures.
Fig. 12. (a) Curve fitting for the C 1s peak of un-etched surface at 18 ◦ C, (b) change in C 1s as a function of temperature, (c) the change in the C 1s carbide peak as a function of temperature after 5 min etching cycle.
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Fig. 13. (a) The O 1s signal obtained without etching the surface after anodic polarisation at 20 ◦ C, (b) the change in the O 1s signal compared with as-polished 15 min argon etched surface.
environments and the 286.2 and 288.9 eV components represent carbon bonded to oxygen atoms (C O and O C O, respectively [19]). As the polarisation test temperature was increased the intensity of the C O, O C O and carbide components on the un-etched surface decreased relative to that of the graphite/hydrocarbon component (Fig. 12b). For all the samples argon ion etching caused the intensity of the carbide component to increase relative to that of the graphite/hydrocarbon component and the O C O component to disappear almost entirely (Fig. 12c). 3.3.2. Evaluation of O 1s after polarisation tests The O 1s spectrum of the anodically polarised surface without argon etching is shown in Fig. 13a. The peak at 531.7 eV represents the OH− as stated earlier in Section 3.1.2. As the etching time increased to 5 min (Fig. 13b), the O 1s
spectrum showed two components. The peak at 530.8 eV represents oxide and 531.7 eV represents hydroxide. This shows that the outer layer of the oxide film is rich in hydroxide and as the etching time increased the oxide component is enhanced. 3.3.3. Evaluation of Cr 2p after polarisation tests A strong Cr 2p peak was detected for all test temperatures as expected when previous results on passive film on stainless steel are considered [20]. The analysis of the surfaces after anodic polarisation showed two doublets at each test temperature. Curve fitting (Fig. 14a) for the sample anodically polarised at 20 ◦ C with no etching showed that Cr 2p components were present with binding energies of 574.4 and 577.8 eV. As stated in Section 3.1.3, the 574.4 eV component corresponds to metallic Cr and the 577.8 eV component represents the oxide, hydroxide or the mixture of the two. In the
Fig. 14. Cr 2p spectra: (a) after polarisation tests at 20 ◦ C, without argon bombardment curve fit components shown for metal/carbide (1) oxide/hydroxide environments (2), (b) as-polished and after-polarisation test at different temperatures without argon bombardment.
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Table 4 XPS peak positions for various components and their concentrations in the passive film Temperature (◦ C)
Etching time
Element
20
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
30
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
40
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
50
No etching
Co Cr W
0.5 min
Co Cr W
5 min
Co Cr W
Peak (eV)
Concentration (%)
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 36
– 88.3 11.7 100
Co Cr2 O3 Cr WO3 WO2
778.4 576.6 574.2 36 34.2
100 81.6 18.4 90.6 9.4
Co Cr2 O3 Cr WO2 W
778.2 577.2 574.3 34.2 31.6
100 37.4 62.6 5.3 94.7
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.9
– 98.4 1.6 100
No Co detected Cr2 O3 Cr WO3 WO2
– 576.9 574.4 36 34.3
– 81.6 18.4 96.2 3.8
Co Cr2 O3 Cr WO3 WO2 W
778.5 576.6 574.8 36 35 31.8
100 84.4 15.6 44.4 1.7 53.9
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.8
– 98.2 1.8 100
No Co detected Cr2 O3 Cr WO3 WO2
– 576.8 574.5 35.9 34.3
– 92.6 7.4 90.1 9.9
Co Cr2 O3 Cr WO3 W
778.3 576.6 574.3 35.9 31.8
100 63.6 36.4 34.7 65.3
No Co detected Cr(OH)3 /Cr2 O3 Cr WO3
– 577.5 574.3 35.7
– 99 1 100
No Co detected Cr2 O3 Cr WO3 WO2
– 577 574.3 35.8 34.7
– 69.7 30.3 95 5
Co Cr2 O3 Cr WO3 W
778.6 576.6 574.2 36 31.8
100 80 20 35.5 47.5
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literature Cr2 O3 was reported to be at the binding energy of 576.6 eV and hydroxide, Cr(OH)3 , at 577.5 eV [19], and so the fitted curve 2 in Fig. 14 is likely to contain both components at different concentrations. This is consistent with what is known about stainlesfs steels where the passive film is richer in OH− at the outside [20]. Fig. 14b shows the change of the Cr 2p signals at different test temperatures compared with the as-polished sample. Table 4 summarises the composition of the passive film and the concentration of the oxide and metal at different etching times and different temperatures. 3.3.4. Evaluation of W 4f after polarisation tests Fig. 15 shows an example of the W 4f spectrum for the unetched surface after the anodic polarisation test at 20 ◦ C. The peak occurs at the binding energy of 36 eV. It was reported that for the oxide measured values for the W 4f7/2 spin orbit doublet range from about 31 eV binding energy for metallic tungsten and to about 36 eV for W6+ in WO3 [20,21]. From this the peak can be identified as originating from the oxide state of tungsten. It has been shown for tungsten that it has oxides with compositions corresponding to a spectrum of oxidation states, between +4 and +6, the latter being more thermodynamically stable. There are a number of possible oxidation reactions of tungsten associated with the formation of tungsten oxide as given below. W + 2H2 O = WO2 + 4H+ + 4e− ,
(1)
WO2 + H2 O = W2 O5 + 2H+ + 2e− ,
(2)
W2 O5 + H2 O = 2WO3 + 2H+ + 2e− .
(3)
As the surface was etched for 30 s and 5 min, the intensity and the position of the peaks changed. Fig. 15b compares the tungsten peaks as a function of etching time for the surface after anodic polarisation at 20 ◦ C. The quantitative results are also summarised in Table 4. Peak values are in good agree-
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Table 5 Quantification of dissolved elements from ICP after anodic polarisation Temperature (◦ C)
20 30 40 50
Elements detected (ppm) Co
Cr
W
Total
0.114 0.108 0.238 1.62
0.024 0.022 0.055 0.032
0.025 0.09 0.01 0.029
0.163 0.139 0.283 1.681
ment with the reactions written in 1–3. The first step is the formation of WO2 at the metal/electrolyte interface, which is not thermodynamically stable and reacts with H2 O to form a more stable oxide WO3 , which occupies the outer regions. 3.3.5. Evaluation of Co 2p after polarisation tests The Co 2p spectrum on the as-polished Stellite 6 alloy comprised two strong peaks and as the surface was etched the intensity of these two peaks increased. After the anodic polarisation of the alloy, the un-etched surface did not show any trace of Co. This was observed at all test temperatures. At lower temperature after a short etching cycle, the Co peak starts to appear and after 5 min etching Co peaks corresponding to metallic Co were observed (Fig. 16). 3.4. Solution analysis The amount of dissolved elements obtained from ICP solution analysis is summarised in Table 5. The passage of a huge current density is evidence of continual reaction of the metal, to result in film thickening and dissolution into the environment or some combination of the two. In good agreement with the XPS findings which showed that there is no Co in the film, the water analysis showed Co was the main element dissolving into the solution after the tests. The amount of dissolved Co generally increased as the test temperature increases, and the amount of dissolved W and Cr stayed almost constant
Fig. 15. W 4f spectrum of anodically polarised surface at 20 ◦ C: (a) without argon etching, (b) in three different etching times.
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Fig. 16. Comparison of the Co 2p peaks in as-polished and anodically polarised sample: (a) non-etched surface (b) 5 min etched surface.
over that temperature range indicating that their key role was in the maintenance of passivity. The possible routes for Co to reach the solution are diffusion through the chromium oxide and hydroxide (Cr2 O3 , Cr(OH)3 and tungsten oxide (WO3 or WO2 ) film or at localised breakdown of the film. The carbide/matrix boundaries are perhaps the most probable sites given that corrosion was shown to initiate and propagate from these regions [11].
4. Discussion The findings presented in this paper demonstrate that after anodic polarisation of HIPed Stellite 6, the passive film evolves to produce a visible film on the matrix at each temperature studied. It has been proved that the new film consists of metallic species and is free from Co. In XPS if the analytical depth (∼10 nm) is greater than the passive film thickness then a proportion of the signal will be derived from the substrate and is recognisable in analysis of the XPS trace. In this study on as-polished surfaces a signal from the substrate is seen after 30 s etching. The increase in the polarisation test temperature increased the film thickness, as was demonstrated by the fact that the Co 2p metal signals from the substrate did not appear until after 5 min etching. It is also known for passive alloys that there is generally an inverse relationship between the thickness of the film and its protective property [5]. In Table 3, it is shown that as the temperature increases (and the film becomes thicker as confirmed by XPS) the breakdown potential decreases (i.e. protective properties decreases) meaning that the material becomes more susceptible to localised corrosion. For the as-polished samples of Stellite 6 the C 1s spectrum reveals the presence of an overlay of carbonaceous contamination, and the O 1s spectrum the presence of ox-
ide/hydroxide species at the surface. The carbon contamination and oxygen containing species are readily removed by argon etching. The C 1s spectrum also reveals the presence of metal carbide species below the contamination over layer. The Cr 2p spectrum comprises Cr metal, carbide and oxide/hydroxide species and the W 4f spectrum is consistent with the metal but not W oxides. The Co 2p spectrum is consistent with the presence of Co metal and oxide/hydroxide species. The XPS signals of the metal oxide/hydroxide species decrease with argon ion etching; hence, the model for the as-polished surface is of a matrix of metal and metal carbide species with an over-layer of Cr and Co oxide/hydroxide and carbonaceous contamination. The analysis of the surface after anodic polarisation shows a presence of multilayer duplex structure, where the composition changes continually with depth. At the outer surface, the film contains no metallic Co, Cr or W and consists of Cr(OH)3 /Cr2 O3 and WO2 . Immediately beneath this layer, depending on the test temperature, the film is composed of Cr(OH)3 /Cr2 O3 and metallic Cr from the bulk material can be observed. Similarly W was detected in the from of WO3 and metallic W. A schematic representation of the key components of the air formed film and corroded surface is shown in Fig. 17. The Cl− ion content that has been reported for the passive film electrochemically formed on stainless steels [22] was not observed for the Stellite 6 alloy. For all test temperatures there was no evidence of a Cl 2p peak as shown in Fig. 18. The passive films formed on stainless steel have been reported to be few nanometres thick [23]. To determine the thickness of the surface layer some simplifications and assumptions were made; (i) there are at least two layers with different composition and structure on top of the substrate; (ii) composition and structure inside a layer are homogeneous; and (iii) boundaries between layers are discontinuous [24].
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oxidation at different temperatures results in the formation of a complex layer whose composition and thickness depend on the test temperature. Co was detected in the solution after the test and the dissolved Co amount increased as the test temperature increased. After corrosion Co is not part of the passive film which is then dominated by Cr and W oxides.
Acknowledgement Fig. 17. Schematic representation and composition of film formed on the matrix: (a) air formed, (b) after anodic polarisation test.
The authors acknowledge the financial support to UM from Deloro Stellite and Heriot-Watt University.
References
Fig. 18. Cl 2p spectrum measured from the surface showing no evidence of Cl in the passive film.
In this current paper it has been demonstrated that the film has composition variation as a function of depth. However, it is not possible to confirm that the composition inside each layer is homogeneous and therefore thickness calculations are not appropriate. Also, Cumpson [25] showed that XPS data is not accurate enough to provide any statistical significance for models with more than three layers including the substrate.
5. Conclusions The composition, structure and thickness of the air-formed and corrosion-reaction films formed on the HIPed Stellite 6 alloy after anodic polarisation at temperatures 20, 30, 40 and 50 ◦ C in 3.5% NaCl have been studied by X-ray photoelectron spectroscopy. The alloy passivates spontaneously in air resulting in the formation of thin oxide film comprising mainly Cr oxide and some Co oxide. Electrochemical
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