The effect of Ce on the high temperature oxidation properties of a Fe ...

The effect of Ce on the high temperature oxidation properties of a Fe–22%Cr steel: microstructural investigation and EELS analysis M. Sattari*, R. Sachitanand, J. Froitzheim, J. E. Svensson and T. Jonsson The effect of a 10 nm Ce coating layer on long term oxidation behaviour (up to 3000 hours) of a Fe–22%Cr ferritic stainless steel for solid oxide fuel cell (SOFC) interconnect application is investigated. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analysis showed segregation of Ce to the grain boundaries of Cr,Mn spinel layer adjacent to the scale-gas interface. The findings in this study are in line with the grain boundary blocking model for the Ce effect mechanism. However, segregation of Ce was observed at the grain boundaries of the (Cr,Mn) spinel in the vicinity of the scale-gas interface. No evidence of Ce segregation was found in the grain boundaries of the chromia layer, neither any Ce rich particle was observed in the chromia layer after longer exposure times. Keywords: Reactive elements effect (REE), Ce coating, High temperature oxidation, Ferritic stainless steel, Solid oxide fuel cell (SOFC), Interconnect

This article is part of a special issue on Microscopy of Oxidation 9

Introduction Ferritic stainless steels are promising candidate materials for interconnects in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (600–800uC). These steels form a Cr based oxide scale, when exposed to elevated temperatures, which provides good protection against oxidation and corrosion and at the same time has good electrical conductivity. However, problems such as volatility of chromium oxide scales and degradation of electronic conductivity during high temperature operation need to be addressed in order to improve the performance of the SOFC.1,2 One way to improve the oxidation resistance of high temperature alloys is to add reactive elements such as Ce, Y and Hf. The main effect related to reactive elements, that is the focus of the present study, is reduction in growth rate of the oxide scale although there are other beneficial effects such as enhanced adhesion of the oxide scale to the substrate, which increases the resistance to spallation during thermal cycling. La, Ce and Y are commonly reported to be the most effective reactive elements.2,3 There are several proposed theories trying to explain the mechanism(s) of the reactive elements effect in improving oxidation properties of chromia forming alloys, the most important of them can be named as modification of the transient stage of oxidation, doping of

Energy and Materials, Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemiva¨ gen 10, Gothenburg SE-412 96, Sweden *Corresponding author, email [email protected]

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ß W. S. Maney & Son Ltd. 2015 Received 16 April 2014; accepted 18 September 2014 DOI 10.1179/0960340914Z.00000000084

the oxide, interface poisoning, and blocking diffusion through the oxide scale.3,4 However, all of these theories fail to fully explain the observed behaviour to some extent and there is still lack of solid experimental evidence. Intensive research programs and studies on ferritic stainless steel for SOFC applications have been started in the past few years to understand and improve the effect of nano-layer coatings of reactive elements such as Ce, Co, La, etc. on the corrosion and oxidation resistance.1,2,5–9 Fontana et al.2 studied the effect of metal organic chemical vapour deposition (MOCVD) coatings (100– 200 nm thick) of La2O3, Nd2O3 and Y2O3 on the oxidation and conductivity of three different ferritic stainless steels, namely: Crofer 22 APU, AL 453 and Haynes 230. They reported reduced oxide growth rates and improved scale adhesion to the substrate in the coated samples. Furthermore, they found that the La2O3 coated Crofer 22 APU shows the best electrical conductivity. Also they found no segregation of reactive element at the oxide scale grain boundaries (after 100 h exposure at 800uC), a phenomenon which is reported by other researchers10 and is believed to be one of the mechanisms responsible for the improved oxidation resistance in reactive element coated alloys. In another study Fontana et al.11 investigated the long term exposure (.2 years) in air at 800uC of Crofer 22 APU coated with La2O3 and Y2O3 using metal organic chemical vapour deposition (MOCVD) technique. They reported improved oxidation resistance, slower oxide growth rate and reduced electrical resistivity in the coated samples compared to the un-coated material. Seo et al.12 investigated a Fe–22Cr–0?5Mn alloy and the effect of three different reactive element coatings, namely Y, Ce and La on its oxidation behaviour in air at

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The effect of Ce on oxidation of a Fe–22%Cr steel

1 Top view low loss backscatter SEM micrograph of a as received, b exposed at 850uC for 24 h, c exposed at 850uC for 168 h and d exposed at 850uC for 3000 h Ce coated samples

800uC for short exposure times (100–200 h). Their results showed that segregation of Y happens at the oxide-metal interface while Ce segregates further into the oxide scale and finally La segregates at the oxide scale grain boundaries. The most effective coating in reducing the oxide growth kinetics and electrical resistance was reported to be Y; they proposed hindrance of cation vacancy annihilation at the metal-oxide interface as the active mechanism for this effect. The results of a study by Alman and Jablonski13 on Ce coated Crofer 22 APU showed reduced oxide thickness and oxidation rate as well as decreased electrical resistance. However, they did not propose any mechanism for the effect of Ce coating on the oxidation resistance. More recently Canovic and Froitzheim et al.9 studied the effect of Co, Ce and Co–Ce nano-layer coatings on the oxidation behaviour of Sanergy HT ferritic stainless steel in air with 3%H2O at 850uC. The observation showed significant reduction in Cr volatilisation for the 640 nm Co coated material but not for the 10 nm Ce coated material; on the other hand Co coating layer had almost no effect on the oxidation rate while the 10 nm Ce layer showed to have decreased the oxidation rate. A combination of these two coatings, i.e. 640 nm Co on 10 nm Ce, was shown to exhibit the beneficial effects of both layers. They also suggested that Ce acts as a barrier against outward diffusion of Cr and hence reducing the oxidation rate. However, no further detailed mechanism for the effect of Ce was proposed. The present study investigates the effect of a 10 nm Ce coating* on the long term (up to 3000 h) oxidation

behaviour of a Fe–22%Cr steel in an attempt to correlate the experimental observations to the microstructure of the oxide scale.

Experimental procedure In this study coupons with the dimensions of 15615 mm cut from a 0?2 mm thick Fe–22%Cr stainless steel sheet with a 10 nm Ce coating, applied via PVD process, were oxidized in an atmosphere of airz3%H2O at 850uC. Samples were weighed after exposures. Details of the experimental setup can be found in Ref. 14. Initial condition of the material prior to the experiments was checked by electron energy loss spectroscopy (EELS) analysis of the as received coated material which showed that Ce is in the form of Ce oxide. A Zeiss LEO ULTRA 55 FE-SEM scanning electron microscope operating at an accelerating voltage of 1?2 kV, was used to image the surface of the samples with low loss backscatter imaging technique which gives almost pure Zcontrast. The depth from which the information comes can be tuned to be in the sub-nano meter range, hence it is a very surface sensitive technique.15 Samples for transmission electron microscopy (TEM) were prepared by focused ion beam (FIB) milling and lift-out technique in a FEI Versa 3D DualBeam instrument. Two layers of Pt, first with the help of electron beam and then with the help of ion beam, were deposited on the surface in order to protect the region of interest from ion beam damage during milling. In order to *Equivalent to approximately 80 ppm if added as an alloying element.


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2 a top view low loss backscatter SEM micrograph: black rectangle shows area from which lift-out was made; and b high angle annular dark field (HAADF) STEM cross-section micrograph of sample exposed at 850uC for 3000 h

reduce the amount of ion beam damage and amorphisation, milling of the samples was performed with a gradually decreasing beam current in the following sequence: 2800, 850, 450, 250 and 75 pA at 30 kV and finally 45 pA at 5 kV. Thereafter, Ar ion milling was used to further reduce the amount of damage in the samples employing a Fischione 1010 instrument operating at 1 kV, 3 mA, incident angle of 6u and rocking angle of 25u for about 15 min. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) investigations were carried out in a FEI Titan 80–300 microscope equipped with a Gatan 866 GIF Tridiem energy filter operating at 300 kV.

Results Figure 1 shows the top view low loss backscatter scanning electron microscope (SEM) images of the as received sample as well as samples exposed to airz3%H2O atmosphere at 850uC for 24, 168 and 3000 h. Apart from

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some cracks and contamination the as received sample micrograph shows almost no contrast, suggesting a uniform coating covering the entire surface (Fig. 1a). After 24 h exposure the surface is covered with an oxide scale which has a broad distribution of grain size and small bright particles (shown by arrow in Fig. 1b). As it was confirmed by TEM analysis (not shown here) the top oxide layer is a (Cr,Mn) spinel consistent with the oxidation behaviour of Fe–22%Cr steels and the bright particles are Ce rich oxide. After 168 h, the bright particles grow in size and the oxide scale grain size becomes more homogeneous. Finally, after 3000 h the bright particles grow slightly larger in size, however their number density decreases quite drastically and they agglomerate to form clusters (see Fig. 1d). Top view low loss backscatter SEM micrograph together with the corresponding high angle annular high angle annular dark field (HAADF) STEM cross section from the 3000 h sample is presented in Fig. 2.

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3 High angle annular dark field (HAADF) STEM crosssection micrograph of the sample exposed at 850C for 3000 h (higher magnification of the area indicated by black square in Fig. 2b)

The area from which the lift out sample has been taken is marked by a black rectangle in Fig. 2a. As indicated in the HAADF micrograph (Fig. 2b) the oxide scale consists of chromia and Cr,Mn spinel on top. The average thickness of chromia and Cr,Mn spinel layers were measured to be 3?5 and 1?9 m respectively. Also noticeable in Fig. 2b is internal oxidation (Ti oxides) and Laves phase precipitates (Nb rich) close to the metal oxide interface, a typical oxidation behaviour of ferritic stainless steels with low amounts of Ti and Nb as alloying elements. A higher magnification HAADF micrograph of the area marked with a black square in Fig. 2b is depicted in Fig. 3 which shows the topmost oxide layer. This micrograph clearly reveals the Ce oxide particle on top of the Cr,Mn spinel layer. Furthermore, smaller Ce oxide particles, in the range of 10–20 nm, were observed inside the Cr,Mn spinel layer. Most of the observed internal Ce oxide particles were found in the vicinity of the top surface of the spinel layer rather than close to the chromia scale. No evidence of Ce oxide or Ce rich particles was found in the chromia scale. Grain boundaries of the Cr,Mn spinel layer as well as chromia layer were examined for traces of Ce. Figure 4a shows high magnification HAADF micrograph of the area marked with a black rectangle in Fig. 3. The result of an electron energy loss spectroscopy (EELS) line scan (indicated by the red line in Fig. 4a) across the grain boundary is presented in Fig. 4b. The line scan shows presence of Ce as well as Ti in the grain boundary. No evidence of Ce was found in the grain boundaries of chromia layer. Although the statistics might not be sufficient, the current investigation points towards the fact that there is almost no Ce left in the chromia layer after 3000 h.

Discussion The gravimetric data show an approximately four times reduced oxidation rate for the Ce coated samples compared to the uncoated samples. After 3000 h at


850uC a mass gain of 1?3 mg/cm2 is recorded for the uncoated sample16 compared to 0?37 mg/cm2 for the Ce coated sample.17 The results are expected and in line with the well established reactive elements effect (REE) in chromia forming alloys. Among the proposed mechanisms for reactive elements effect (REE), grain boundary segregation model is one of the most plausible theories. It is well established that at temperatures below 1000uC chromia scale grows with mainly outward diffusion of Cr cations.18 However, bulk diffusion of Cr cations in Cr2O3 has been shown to be far too slow to be able to explain experimentally observed oxidation rates.18,19 Therefore, Cr cation diffusion is believed to take place through ‘short circuts’, i.e. grain boundaries and dislocations. In the grain boundary segregation model it is hypothesised that reactive elements block the grain boundaries and hence hinder cation diffusion through them. Yurek et al. studied the effect of Y2O3 addition on oxidation of a Ni–Cr alloy at 1000uC. They investigated the oxide scale which was mainly comprised of two layers: NiO layer on top (scalegas interface) and chromia at the bottom (metal-scale interface); they found evidence of Y at the grain boundaries of both NiO layer and at the grain boundaries of chromia using scanning transmission electron microscopy (STEM)/energy dispersive X-ray spectroscopy (EDS).20 Czerwinski and Szpunar19 investigated the effect of sol–gel CeO2 coatings on oxidation of Ni, Cr and Ni based superalloys. They reported a Ce rich region close to the oxide-gas interface in the case of NiO oxide layer formed at 700uC. They also reported presence of Ce at the NiO grain boundaries using energy dispersive X-ray spectroscopy (EDS). Furthermore, they observed grain refinement due to addition of CeO2. Cotell et al.21 studied the effect of Y implantation on oxidation of high purity chromium. They reported presence of Y at chromia grain boundaries as well as Y-rich oxide particles close to the scale-gas interface. Based on their 16O/18O experiments they found that 18O is strongly associated with Y at all depths and concluded that oxygen might diffuse along grain boundaries which are also decorated with Y. All the above mentioned findings were after short term oxidation (maximum 24 h). In the present study we provide evidence for segregation of Ce to the grain boundaries after long term oxidation (3000 h). However, the grain boundary segregation was found in the (Cr,Mn) spinel and close to the scale-gas interface; no evidence of Ce was found in the chromia layer beneath the (Cr,Mn) spinel layer. Furthermore, Ce rich oxide particles were spotted only in the spinel layer close to the scale-gas interface, as well as on top of the scale; no evidence of Ce rich particles was found in the chromia layer (see Fig. 3). Although we see the effect of Ce in reducing the oxidation rate/oxide thickness of the chromia layer, our findings do not conform to the generally accepted belief that reactive elements have to be incorporated in the chromia scale in order to see their effect.22 With most of the Ce concentrated in the spinel and close to the scale-gas interface or on top of the scale, the mechanism of reactive element effect in this case seems to be more complicated than physically blocking the Cr cations outward diffusion paths through grain boundaries. Thicker spinel layer with smaller grain width in the case of Ce-coated sample implies larger grain boundary area which is decorated with Ce close to the scale-gas interface which might act as a cap and hence blocking diffusion of Cr and/or oxygen.


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4 High angle annular dark field (HAADF) STEM cross-section micrograph of the sample exposed at 850uC for 3000 h (zoomed into the area indicated by black square in Fig. 3) and b EELS line scan across the grain boundary (along red line) showing Ce and Ti at the spinel grain boundary

The Ce rich particles on top of the scale change in size and number over time (Fig. 1) which implies diffusion on the surface and/or into the spinel grain boundaries. This observation suggests that these Ce rich particles might indeed act as reservoir for Ce in the grain boundaries in the vicinity of the scale-gas interface.

Conclusions Oxidation properties of a Fe–22%Cr steel coated with a layer of 10 nm Ce was investigated. Segregation of Ce was observed at the grain boundaries of Cr,Mn spinel in the vicinity of the scale-gas interface. No evidence of Ce segregation was found in the grain boundaries of the chromia layer, neither any Ce-rich particle was observed in the chromia layer. The evidence provided in the present study suggests the possibility of Ce blocking diffusion paths for oxygen and Cr, i.e. grain boundaries, in the Cr,Mn spinel layer rather than in the chromia scale. However, more investigation and experimentation is needed to speculate the mechanism of reactive elements effect on oxidation behaviour of ferritic stainless steels.

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