Influence of Surface Finishing on High-Temperature Oxidation of AISI Type 444 Ferritic Stainless Steel Used in SOFC Stacks

doi:10.1007/s40195-017-0543-1

Abstract

Abstract:

This research is focused on the study of the samples, approximatively 15 × 30 mm² sized, that were mechanically cut from two sheets (0.4 and 0.2 mm thick, respectively) of AISI 444 Type ferritic stainless steel (FSS) (DIN 1.4521, Eu designation X2CrMoTi18-2); this material was in the ‘as-rolled’ state. Part of these specimens were treated superficially on one side using abrasive SiC papers with different average grit sizes (i.e., 46.2, 30.2, 18.3 μm) and diamond suspension (up to 1 μm) in order to obtain various surface roughness. Both the ‘as-rolled’ and superficially treated samples were then aged in a muffle furnace in static air according to a thermal cycle corresponding to the curing phase of an experimental glass used as sealing in the solid oxide fuel cell stacks. After aging, the chemical compositions and morphological peculiarities of the scale formed depending on the thickness of the samples and their surface state were studied by scanning electron microscopy, energy-dispersive spectroscopy, micro-Raman spectroscopy, bright field light optical microscopy. The obtained results show that all scales formed consist of an inner Cr₂O₃ subscale and an outer (Mn,Cr)₃O₄ spinel layer; the relationship between FSS grain size and scale microstructural features is consistent on the samples with mirror-like surface only; the scale thicknesses on SiC grinded samples are comparable; the scales covering the ‘as-rolled’ surfaces are morphologically similar to those grown on the surfaces finished with the 30.2 and 18.3 μm SiC papers, and their thicknesses show an intermediate situation between the abraded and the mirror-like specimens. The last described characteristics depend mainly on the surface and microstructural peculiarities resulting from the rolling process.

Key words: AISI Type 444, SOFC stacks, Surface roughness‘Microstructure, Surface finishing, High-temperature oxidation

Valeria Bongiorno, Paolo Piccardo, Simone Anelli, Roberto Spotorno. Influence of Surface Finishing on High-Temperature Oxidation of AISI Type 444 Ferritic Stainless Steel Used in SOFC Stacks[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(8): 697-711.

Figures/Tables 25

Table 1 AISI 444 nominal composition (wt%) according to A. P. Steel Srl

C	Si	Mn	Cr	Mo	N	S	P	Ti + Nb	Fe
0.013	0.54	0.24	17.63	1.87	<0.02	<0.02	<0.03	0.48	Bal.

Table 2 Description of studied samples prior to oxidation test

Samples GA-GB	Description of the applied surface treatments	Figures
ISO/FEPA grit designation* (average grit size in μm)**	Description of the applied surface treatments	Figures
‘As rolled’		Open image in new window
P320* (46.2 μm)**	Abraded with a P320 SiC paper	Open image in new window
P500* (30.2 μm)**	Abraded with a P500 SiC paper	Open image in new window
P1000* (18.3 μm)**	Abraded with a P1000 SiC paper	Open image in new window
1 μm	Polished up to 1 μm diamond suspension	Open image in new window

Fig. 1 Applied thermal cycle

Fig. 2 SEM-SE micrographs showing the FSS surface after the finishing with 46.2 μm a, 30.2 μm b, 18.3 μm c abrasive SiC papers

Fig. 3 SEM-SE micrographs showing the FSS surface of the ‘as-rolled’ samples: group GA a, GB b

Fig. 4 SEM-SE micrographs showing the surface of the sample GA 46.2 μm at different magnifications: ×1500 a, ×15,000 b

Fig. 5 SEM-SE micrographs showing the surface of the sample GA 30.2 μm at different magnifications: ×1500 a, ×15,000 b

Fig. 6 SEM-SE micrographs showing the surface of the sample GA 18.3 μm at different magnifications: ×1500 a, ×15,000 b

Fig. 7 SEM-SE micrographs showing the surface of the sample GA 1 μm at different magnifications: ×1500 a, ×15,000 b

Fig. 8 SEM-SE micrographs showing the surface of the sample GA ‘as rolled’ at different magnifications: ×1500 a, ×15,000 b

Fig. 9 SEM-SE micrograph of the sample GA 46.2 μm; spot and areas of EDX analysis

Table 3 Results of EDX analysis (at.%), sample GA 46.2 μm

Spectrum	O	Cr	Mn
Spectrum 1	44	37	19
Spectrum 2	44	48	8
Spectrum 3	28	41	31

Fig. 10 SEM-SE micrograph of the sample GA 1 μm; spots and areas of EDX analysis

Table 4 Results of EDX analysis (at.%), sample GA 1 μm

Spectrum	O	Cr	Mn
Spectrum 1	56	35	9
Spectrum 2	46	35	19
Spectrum 3	46	35	19
Spectrum 4	60	28	12

Fig. 11 SEM-SE micrograph of the sample GA ‘as rolled’; spot and areas of EDX analysis

Table 5 Results of EDX analyses (at.%), sample GA ‘as rolled’

Spectrum	O	Cr	Mn
Spectrum 1	59	33	8
Spectrum 2	59	33	8
Spectrum 3	59	27	14
Spectrum 4	52	32	16
Spectrum 5	51	36	13

Fig. 12 Average size of surface crystals grown on GA samples

Table 6 SEM-BSE micrographs of aged samples from group GA (0.4 mm thick) and GB (0.2 mm thick)

Applied thermal cycle: 10 h/930 °C and 50 h/850 °C (Fig. 1)
GA samples (0.4 mm thick)	GB samples (0.2 mm thick)
Open image in new window	Open image in new window
SEM-BSE micrograph (magnification 10,000×), sample GA 46.2 μm	SEM-BSE micrograph (magnification 10,000×), sample GB 46.2 μm
Open image in new window	Open image in new window
SEM-BSE micrograph (magnification 10,000×), sample GA 30.2 μm	SEM-BSE micrograph (magnification 10,000×), sample GB 30.2 μm
Open image in new window	Open image in new window
SEM-BSE micrograph (magnification 10,000×), sample GA 18.3 μm	SEM-BSE micrograph (magnification 10,000×), sample GB 18.3 μm
Open image in new window	Open image in new window
SEM-BSE micrograph (magnification 10,000×), sample GA 1 μm	SEM-BSE micrograph (magnification 10,000×), sample GB 1 μm
Open image in new window	Open image in new window
SEM-BSE micrograph (magnification 10,000×), sample GA ‘as rolled’	SEM-BSE micrograph (magnification 10,000×), sample GB ‘as rolled’

Table 7 Average TGO thickness (μm) calculated out of a statistical evaluation of the scale thickness in several different locations across the cross sections

Surface treatment	Samples GA	Samples GB
Surface treatment	Average TGO thickness (μm) (SD, μm)	Average TGO thickness (μm) (SD, μm)
46.2 μm^a	1.51 (0.44)	1.50 (0.78)
30.2 μm^a	1.23 (0.39)	1.15 (0.28)
18.3 μm^a	1.49 (0.49)	1.46 (0.52)
1 μm	1.04 (0.19)	1.17 (0.58)
‘As rolled’	1.26 (0.36)	1.42 (0.20)

Fig. 13 Frequency of thickness distribution in function of the surface treatments, samples GA and GB

Table 8 Average grain size (μm) measured along and orthogonally with respect to the rolling direction on both group of samples before and after the oxidation test

	Average grain size (μm) (SD, μm)
	Parallel to elongation	Orthogonally to the rolling direction
GA—before oxidation test (Fig. 14a)	24.70 (1.54)	13.32 (0.67)
GA—after the oxidation test (Fig. 14b)	23.07 (1.78)	19.54 (0.95)
GB—before the oxidation test (Fig. 15a)	Strongly textured material: no grain size measurements were possible
GB—after the oxidation test (Fig. 15b)	9.98 (0.93)	14.35 (0.58)

Fig. 14 LOM-BF micrographs of the ‘as-rolled’ samples belonging to GA group before a and after oxidation test b, etching with aqua regia

Fig. 15 LOM-BF micrographs of the ‘as-rolled’ samples belonging to GB group before a and after oxidation test b, etching with aqua regia

Fig. 16 BSE micrograph of the sample GB 30.2 μm with EDX semiquantitative distribution profiles of elements

Fig. 17 Raman spectrum of the sample GA 30.2 μm

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