Influence of Aging Time on Microstructure and Corrosion Behavior of a Cu-Bearing 17Cr-1Si-0.5Nb Ferritic Heat-Resistant Stainless Steel

doi:10.1007/s40195-020-01049-5

Abstract

Abstract:

17Cr-1Si-0.5Nb-1.2Cu ferritic heat-resistant stainless steel was aged at 750 °C from 10 min to 30 h to simulate time aging and study the microstructural evolution and its effect on corrosion behavior by using optical microscopy, scanning electron microscopy, transmission electron microscopy, potentiodynamic polarization, electrochemical impedance spectroscopy, and the Mott-Schottky approach. Four types of precipitates were discovered, including ε-Cu, NbC, Fe₃Nb₃C, and Fe₂Nb-type Laves phase. The nano-sized ε-Cu phase forms first, and its fraction follows the parabolic law change and is the largest. Compared to NbC and Fe₃Nb₃C particles, the coarsening of the Laves phase is the most pronounced. The aging process is divided into three parts: early-aged (0-5 h), peak-aged (5 h), and over-aged (5-30 h). However, the corrosion resistance is reduced in the early-aged stage of 0-2 h. Further extending the aging time to 30 h, the corrosion resistance is gradually improved. This change may be related to the competitive relationship between the beneficial effects of the Cu-rich phase and the harmful effects of Nb-containing particles. The dissolved Cu on the surface becomes more effective for the suppression of the anodic dissolution by the formation of ionic compounds of chlorine, thereby reducing the deterioration of corrosion resistance caused by Nb-rich precipitation.

Key words: Ferritic stainless steel, Aging, Precipitation, Electrochemical behavior, Passive film

Tong Zhang, Ying Han, Wen Wang, Yang Gao, Ying Song, Xu Ran. Influence of Aging Time on Microstructure and Corrosion Behavior of a Cu-Bearing 17Cr-1Si-0.5Nb Ferritic Heat-Resistant Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(9): 1289-1301.

Figures/Tables 16

Table 1 Chemical composition of 17Cr-1Si-0.5Nb-1.2Cu FHSS (wt%)

C	Si	Mn	P	S	Cr	Nb	Cu	Fe
0.01	0.86	0.25	0.008	0.003	17.07	0.50	1.22	Bal

Fig. 1 Vickers hardness of the studied steel aged at 750 °C for different aging times

Fig. 2 OM micrographs of the studied steel aged at 750 °C for a 0 h (solution-treated status), b 1 h, c 5 h, d 15 h, e 30 h

Fig. 3 SEM micrographs of the studied steel aged at 750 °C for different aging times: a 0 h (solution-treated status), c 2 h, f 15 h, g, h 30 h; and EDS results of the precipitates in the as-solution-treated sample (b) and aged sample (d, e)

Fig. 4 TEM micrographs of the studied steel aged at 750 °C for different aging times: a 10 min, c, f, g 1 h, i 30 h; and SADP of ε-Cu (b), NbC (d), Fe2Nb-Laves phase (e), and Fe3Nb3C (h)

Fig. 5 Variations in area fractions of various precipitates with aging time at 750 °C

Fig. 6 Polarization curves of the studied steel aged at 750 °C for different aging times in a 3.5 wt % NaCl solution

Table 2 Fitting electrochemical parameters for 17Cr-1Si-0.5Nb-1.2Cu FHSS aged at 750 °C for different aging times in a 3.5 wt% NaCl solution

Aging time (h)	i_corr (mA cm^-2)	E_corr (V vs. Ag/AgCl)	E_pit (V vs. Ag/AgCl)
1	0.246	- 1.096	0.014
2	0.297	- 1.100	0.004
5	0.250	- 1.076	0.015
15	0.245	- 1.072	0.017
30	0.222	- 1.056	0.024

Fig. 7 a Nyquist; b Bode plots of the studied steel aged at 750 °C at different times in a 3.5 wt% NaCl solution; c equivalent electrical circuit used to fit the EIS experimental data

Table 3 Fitting parameters corresponding to the equivalent electrical circuit in Fig. 7c

Aging time (h)	R_s (Ωcm^-2)	CPE1 (Ω^-1 cm^-2 sⁿ)	n₁	R_c (Ω cm^-2)	CPE2 (Ω^-1 cm^-2 sⁿ)	n₂	R_et (Ωcm^-2)
1	15.08	4.921 × 10^-5	0.826	1.877 × 10^-4	1.246 × 10^-4	0.864	4041
2	15.34	5.335 × 10^-5	0.880	1.245 × 10^-4	2.205 × 10^-3	0.784	2564
5	15.41	2.931 × 10^-5	0.784	2.081 × 10^-4	2.572 × 10^-4	0.950	9384
15	15.45	2.313 × 10^-5	0.996	4.441 × 10^-4	4.570 × 10^-5	0.863	10,480
30	15.69	1.953 × 10^-5	0.856	6.852 × 10^-4	3.265 × 10^-5	0.895	15,300

Fig. 8 Mott-Schottky curves of the studied steel aged at 750 °C for different aging times in a 3.5 wt% NaCl solution

Table 4 Analysis parameters from the Mott-Schottky curves in Fig. 8

Aging time (h)	R1 (from - 0.9 to - 0.58 V)		R3 (from - 0.36 to 0 V)		E_FB (V)
Aging time (h)	Slope	N (cm³)	Slope	N (cm³)	E_FB (V)
1	- 1.83	4.95 × 10³⁰	15.69	5.76 × 10²⁹	0.11
2	- 1.15	7.88 × 10³⁰	15.11	5.98 × 10²⁹	0.30
5	- 2.94	3.07 × 10³⁰	18.77	4.82 × 10²⁹	- 0.26
15	- 4.39	2.06 × 10³⁰	18.96	4.76 × 10²⁹	- 0.38
30	- 6.09	1.48 × 10³⁰	18.14	4.98 × 10²⁹	- 0.42

Fig. 9 Complete XPS spectra of the passive film of the aged steel formed after immersing for 24 h in a 3.5 wt% NaCl solution

Fig. 10 XPS spectra of the Cr2p3/2 of the aged samples: a 1 h, b 5 h, c 15 h, d 30 h, after immersing in a 3.5 wt% NaCl solution for 24 h

Fig. 11 XPS spectra of the Fe2p3/2 of the aged samples: a 1 h, b 5 h, c 15 h, d 30 h, after immersing in a 3.5 wt% NaCl solution for 24 h

Fig. 12 XPS spectra of the Cu2p and Nb3d of the samples aged for different times after immersing them in a 3.5 wt% NaCl solution for 24 h

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