Corrosion Behavior of Ni-20Cr-18W-1Mo Superalloy in Supercritical Water

doi:10.1007/s40195-014-0119-2

Abstract

Abstract:

The corrosion behavior of Ni-20C-18W-1Mo superalloy in supercritical water 500°C/25 MPa for 200 h is investigated using gravimetry, SEM/EDS, XPS, and TEM. The oxide films show a layered structure with Ni rich in the outer layer, and Cr rich in the inner layer, consisting of an outer Ni(OH)₂ and NiO layer, including some Cr(OH)₃, and an inner Cr₂O₃, NiCr₂O₄, and WO₃ layer. Mo elements are not oxidized. The oxide films grow via a mixed mechanism, namely metal dissolution/oxide precipitation mechanism and solid-state growth mechanism. The effects of secondary and primary carbides on the weight-gain trend and oxide formation are discussed.

Key words: Ni-20Cr-18 W-1Mo, Corrosion behavior, Corrosion mechanism, Supercritical water, Oxide film

Zhang Jingqing, Hu Rui, Wang Jian, Li Jinshan. Corrosion Behavior of Ni-20Cr-18W-1Mo Superalloy in Supercritical Water[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(6): 1046-1056.

Figures/Tables 17

Fig. 1 The microstructure of Ni-20Cr-18 W-1Mo superalloy in annealed condition

Fig. 2 Weight gain of Ni-20Cr-18 W-1Mo superalloy exposed to SCW at 500 °C/25 MPa for 200 h as a function of aging temperature for 10 h

Fig. 3 Weight gain of Ni-20Cr-18 W-1Mo superalloy exposed to SCW at 500 °C/25 MPa for 200 h as a function of aging time at 700 °C

Fig. 4 SEM morphologies of the oxide film formed on the surface of the alloy aged at different temperatures for 10 h: a 600 °C, b 700 °C, c 800 °C, d 900 °C, e 1,000 °C, f 1,100 °C

Table 1 EDS results of sites 1 and 2 in Fig. 4d (wt%)

Site	Ni	Cr	W	O
1	20.60	13.50	45.77	20.13
2	66.74	8.78	9.62	14.86

Fig. 5 SEM morphologies of the oxide film on the sample aged at 700 °C for 48 h: a low magnification, b high magnification image of area A in a, c high magnification image of areas from the two sides of twin grain boundaries

Fig. 6 SEM morphologies and EDS line scan analysis of the oxide films on the cross-section with different aging states: a SEM morphologies of the cross-section aged at 600 °C for 10 h, b EDS line scan analysis for the sample in a, c the depth profile of elements distribution of the oxide film analyzed by XPS for the sample in a, d aged at 700 °C for 48 h

Fig. 7 XPS analysis of the SCW exposed sample aged at 900 °C for 10 h: a Ni 2p core level spectra, b Cr 2p core level spectra, c W 4f core level spectra, d Mo 3d core level spectra, e O 1s core level spectra

Table 2 Assignments of Ni peaks in Fig. 7a

Element	Ni		Component
BE (eV)	2p_3/2	2p_1/2
	852.82 ± 0.04	870 ± 0.15	Ni
	857.9 ± 0.25	875.3 ± 0.3	Ni,sat
	854.1 ± 0.25	872 ± 0.3	NiO
	861.9 ± 0.3	880 ± 0.5	NiCr₂O₄/Ni(OH)₂,sat
	856.07	874.37	Ni(OH)₂
	854.9	873.37	NiO/Ni(OH)₂

Table 3 Assignments of Cr peaks in Fig. 7b

Element	Cr		Component
BE(eV)	2p_3/2	2p_1/2
	577.1 ± 0.1	586.9 ± 0.1	Cr(OH)₃
	576.6 ± 0.1	586.5 ± 0.1	Cr₂O₃
	574.2 ± 0.2	584.1 ± 0.3	Cr
	579.9 ± 0.2	589.1 ± 0.2	NiCr₂O₄

Table 4 Assignments of W peaks in Fig. 7c

Element	W		Component
BE (eV)	4f_7/2	4f_5/2
	31.4 ± 0.1	33.6 ± 0.2	W
	35.7 ± 0.3	37.8 ± 0.2	WO₃
	41.9 ± 0.1	44 ± 0.1	Primary M ₆C

Table 5 Assignments of Mo peaks in Fig. 7d

Element	Mo		Component
BE (eV)	3d_5/2	3d_3/2
BE (eV)	230.9 ± 0.1	227.9 ± 0.2	Mo

Table 6 Assignments of O peaks in Fig. 7e

Element	O	Component
BE (eV)	1s
	530.5 ± 0.2	O^2-
	531.4 ± 0.1	OH^-

Fig. 8 The microstructures of the samples aged at 700 °C for different times: a 2 h, b 5 h, c 10 h, d 15 h, e 24 h, f 36 h, g 48 h, h 72 h

Fig. 9 The microstructures of the samples aged at different temperatures for 10 h: a 600 °C, b 800 °C, c 900 °C, d 1,000 °C, e 1,100 °C

Table 7 EDS analysis of primary M 6C carbide and secondary carbide in Fig. 9 (wt%)

Spectrum	Ni	Cr	W	Mo	C
Spectrum1	37.03	47.54	11.70	0.94	2.79
Spectrum2	17.59	18.57	51.26	4.17	8.41
Spectrum3	16.15	9.44	65.65	3.81	4.96
Spectrum4	18.57	11.15	62.16	3.50	4.61
Spectrum5	16.22	9.31	66.23	3.07	5.18

Fig. 10 TEM analysis of secondary carbides in the sample aged at 1,000 °C for 10 h: a BF micrograph of the carbide, b [011] SAD pattern for a

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