Platinum-induced oxidation of chromium in O2 at 800 °C

Platinum-induced oxidation of chromium in O2 at 800 °C

Corrosion Science 45 (2003) 2697–2703 www.elsevier.com/locate/corsci Letter Platinum-induced oxidation of chromium in O2 at 800 C G. Hultquist *, E...

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Corrosion Science 45 (2003) 2697–2703 www.elsevier.com/locate/corsci

Letter

Platinum-induced oxidation of chromium in O2 at 800 C G. Hultquist *, E. H€ ornlund, Q. Dong Division of Corrosion Science, Department of Materials Science and Engineering, Royal Institute of Technology, Dr Kristinas V€ ag 51, 100 44 Stockholm, Sweden Received 24 March 2003; accepted 7 May 2003

Abstract The effects of porous Pt on the oxidation of Cr at 800 C have been studied with the 18 OSIMS technique, gas phase analysis and XPS. In oxide areas with Pt a pronounced inward oxygen transport takes place and a substantial oxide growth near the Cr substrate is observed. In oxide grown on areas without Pt the counts of CrO ions in SIMS and the binding energy of O (1s) in XPS depend on the distance from the area with Pt. The experimental observations are believed to be a consequence of a high dissociation efficiency of O2 on areas with Pt in combination with a high diffusivity of O in external and internal oxide surfaces on areas both with and without Pt.  2003 Elsevier Ltd. All rights reserved. Keywords: A. Chromium; A. Platinum; B. SIMS; C. Oxidation; Surface diffusion

1. Introduction The formation of dense and adherent chromium-oxide is essential for many alloys in numerous applications. Oxide grown on ‘‘pure’’ chromium at high temperatures often suffers from spallation but upon removal of hydrogen from the metal substrate the scale adherence can be improved [1]. A more established way to improve the scale adherence is to add oxides of rare earth metals [2,3], but also certain noble metals have a positive effect on oxide scale performance [4]. However, the mechanisms behind the effects of all these additions are not clearly identified and several possible

*

Corresponding author. Tel.: +46-8-790-8208; fax: +46-8-20-8284. E-mail address: [email protected] (G. Hultquist).

0010-938X/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00117-3

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Fig. 1. Oxidised Cr sample with indicated area of porous Pt and analysed positions in SIMS and XPS.

mechanisms have been discussed for a long time [5]. We have pointed out that these additions catalyse the dissociation of oxygen molecules and that this can be a key factor for the mechanistic understanding of their effect of improved oxide scale adherence [6]. In this work the influence of porous platinum on the oxidation of chromium at 800 C in O2 is addressed with the use of mainly the 18 O-SIMS technique.

2. Experimental A 12 mm · 5 mm · 2 mm piece of Cr (Alfa) was mechanically polished to 2400 mesh with SiC-paper and partly sputter coated with a porous Pt film, with a thickness of approximately 20 nm. The partly Pt-coated sample was then heated stepwise in ultra-high vacuum up to 800 C whereby most of the hydrogen in the sample was outgassed. The remaining hydrogen content was thereby 65 wt. ppm. A two-stage oxidation was performed in a closed reaction chamber at 800 C near 20 mbar O2 : the first stage in oxygen with >99% 16 O and the second stage in oxygen with 67% 18 O. The apparatus used in the oxidation has been described earlier [6,7]. Secondary ion mass spectroscopy, SIMS (Csþ source, 10 kV, 200 lm  200 lm sputtered areas with detection from the inner area with £ 70 lm) was used for depth profiling of the oxide formed at different distances from the Pt coated area. The count rates of negative (52 Cr18 O) ions were found after subtraction of the contribution of (54 Cr16 O) ions to the emitted species of mass 70. Monoenergetic Al Ka was used in X-ray photoelectron spectroscopy, XPS. The O (1s) photoelectrons were recorded from different areas, 0.4 mm2 each, on the sample. The sample with indicated analysed positions in SIMS and XPS is shown in Fig. 1.

3. Results and discussion The oxygen uptake by the sample (based on the pressure decrease in the reaction chamber of known volume) and the isotopic composition of the gas are shown in

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Fig. 2. Oxygen uptake in sample (N) and From gas phase analysis.

18

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O/16 O ratio in the O2 gas ( ) during oxidation of sample.

Fig. 2. The gas introduced in the second stage of oxidation was 16;16 O2 and 18;18 O2 with 13% 16;18 O2 . In Fig. 3, the abundance of 16;18 O2 in the second stage is seen to increase as a result of dissociation–diffusion–association of oxygen. The overall rate for this chain of events can be calculated with good accuracy as long as the abundance of the 16;18 O2 molecules is far from statistical equilibrium [6,8]. Two calculated values are shown with triangles in Fig. 3, which indicate a decreasing dissociation rate. This can be interpreted as a result of a decreasing Pt content in the oxide surface since rates in the order of 1000 and 100 lmol O/(cm2 , h) represent pure Pt [7] and pure Cr-oxide [9], respectively. In the second stage of the oxidation the 18 O/16 O ratio in the gas was approximately two as seen in Fig. 2. This ratio is present in the outer part of the oxide (near the gas interface) at all distances from the Pt-area as found from the 52 Cr18 O/52 Cr16 O ratios in



Fig. 3. Fraction of 16;18 O2 in O2 ( ) and calculated O2 -dissociation rate (M) in the second stage of oxidation. From gas phase analysis.

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Fig. 4. Ratios of 52 Cr18 O/52 Cr16 O and 18 O/16 O near the gas interface and near the substrate interface of oxides at different positions on the sample. From SIMS measurements.

Fig. 4. However, slightly higher ratios are obtained for 18 O/16 O. This might be due to surface water in air exposure upon transfer to the SIMS apparatus. The results in Fig. 4 imply that some oxide growth has taken place near the gas interface at all positions on the sample by metal transport from the substrate. The corresponding 52 Cr18 O/ 52 Cr16 O and 18 O/16 O values near the substrate interface (at a sputter depth where 52 Cr18 O has decreased to approximately 10% of its maximum) are also shown in Fig. 4. The ratios in the Pt-area are high, which means that Pt promotes oxide growth near the substrate by oxygen transport through the oxide. In Fig. 5, counts of 195 Pt and 52 Cr18 O are shown vs. distance from the Pt-area. These counts have been obtained by integrating the count rate over sputter time (depth), until the count rate of 52 Cr18 O has decreased to 1% of its highest value. By combining the data in Fig. 5, a relation between counts of 52 Cr18 O and counts of 195 Pt is shown in Fig. 6. From the results in Figs. 5 and 6, a strong enhancement of the 52 Cr18 O counts in the Pt-area is found. Also some enhancement of the 52 Cr18 O counts is observed adjacent to the Pt-area, but not at distances exceeding 4–5 mm. 18 O was present only in the second stage of the oxidation (Fig. 2). Therefore a depth profile of 18 O-containing ions in SIMS will mimic the relative distribution in depth of oxide growth. The in-depth distribution of 52 Cr18 O from four positions on the sample is shown in Fig. 7. In all four profiles the depth has been set to unity where the count rate has decreased to 0.5% of its highest value. Also the highest count rate has been set to unity in all four profiles. It is found that: • a pronounced oxide growth takes place near the substrate on the area with Pt; • a small, but significant, growth takes place near the substrate in oxide positions 0.1 and 2.9 mm from Pt; • a virtually zero oxide growth takes place near the substrate in oxide positioned 4.8 mm from the Pt-area.

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Fig. 5. Counts of 52 Cr18 O and 195 Pt at different positions on the sample. The counts have been obtained by integration of count rate over sputter time until the count rate of 52 Cr18 O has decreased to 1% of its maximum. From SIMS measurements.

Fig. 6. Counts of

52

Cr18 O plotted vs. counts of

195

Pt. Data from Fig. 5.

XPS gives information from the uppermost few nm of solid surfaces. A higher binding energy peak of O (1s) in Fig. 8 is present both in the Pt-area and 4.3 mm from the Pt-area compared with the energy at a distance of 6.1 mm from the Pt-area. Hence, a major influence of Pt on the uppermost oxide surface is present as far as 4–6 mm from the Pt-area, whereas near the chromium substrate only a minor influence of Pt is present outside the Pt-area (Figs. 4 and 7). These observations strongly point at a fast surface diffusion of dissociated oxygen, On , primarily in the external surface of the oxide, but also in internal oxide surfaces including grain boundaries. This scenario can be explained by a ‘‘spill-over’’ of On from the Pt-area, which acts as a generator for On . Such a spill-over effect of adsorbed species is actually a known phenomenon in the field of heterogeneous catalyses where oxide

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Fig. 7. Normalised count rates of 52 Cr18 O vs. normalised oxide depth from four positions on the sample. From SIMS measurements.

Fig. 8. O (1s) photoelectron peak from three positions on the sample. From XPS measurements.

supported noble metal islands are used. In that case the spill-over effect is predicated on rapid surface diffusion [10]. 4. Summary and conclusions The influence of porous Pt on the oxidation of pure Cr in 20 mbar O2 at 800 C was studied with the 18 O-SIMS technique, gas phase analysis and XPS. Results from the measured emissions of negative (CrO) ions in SIMS, 18 O content in the gas phase and O (1s) photoelectrons in XPS at different oxide depth and positions from an area with porous Pt show that:

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1. The area with Pt acts as a generator for dissociated oxygen, and a mm-ranged influence of Pt is identified which can be explained by a fast diffusion of atomic and/ or ionic oxygen in the external surface of the oxide. 2. A main effect of a Pt-addition on the mechanisms of oxide growth is an increased transport of dissociated oxygen, which predominately takes place on external and internal oxide surfaces. This leads to increased oxide growth near the chromium substrate, which is favourable for the adherence of the oxide. Acknowledgements Financial support from KME (G. Hultquist), HTC (E. H€ ornlund) and SFS (Q. Dong) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

B. Tveten, G. Hultquist, T. Norby, Oxid. Met. 51 (1996) 221. F.A. Golightly, F.H. Stott, G.C. Wood, Oxid. Met. 10 (1976) 163. R.J. Hussey, M.J. Graham, Oxid. Met. 45 (1996) 349. E.J. Felton, Oxid. Met. 10 (1976) 23. D.L. Douglass, P. Kofstad, A. Rahmel, G.C. Wood, Oxid. Met. 45 (1996) 529. G. Hultquist, B. Tveten, E. H€ ornlund, M. Limb€ack, R. Haugsrud, Oxid. Met. 56 (2001) 313. kermark, G. Hultquist, L. Gr T. A asj€ o, J. Trace Microprobe Tech. 14 (1996) 377. E. H€ ornlund, Appl. Surf. Sci. 199 (2002) 195. G. Hultquist, B. Tveten, E. H€ ornlund, Oxid. Met. 54 (2000) 1. G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, John Wiley & Sons, INC, 1994, p. 345.