Effect of thermal ageing and dissolved gas on corrosion of 308L stainless steel weld metal in simulated PWR primary water

doi:10.1016/j.jmst.2021.05.026

Abstract

Abstract:

The corrosion of unaged and 7000-h thermally aged 308L stainless steel weld metals in simulated PWR primary water under aerated and deaerated conditions was investigated, involving the corrosion of austenite, δ-ferrite and δ-ferrite/austenite phase boundary. The results revealed that thermal ageing for 7000 h had a limited effect on the corrosion behavior of 308L weld metal as it only increased the inner oxide thickness of δ-ferrite slightly under the deaerated condition. No obvious corrosion enhancement of 308L weld metal under the aerated condition was found compared to the deaerated condition regardless of the thermal ageing. Nevertheless, Cr-enrichment on the surface of oxide particles, dissolved regions at the metal/oxide interface and localized corrosion along the δ-ferrite/austenite phase boundary occurred under the aerated condition.

Key words: Stainless steel weld metal, Thermal ageing, Corrosion, High temperature water, TEM

Xiaodong Lin, Qunjia Peng, Yaolei Han, En-Hou Han, Wei Ke. Effect of thermal ageing and dissolved gas on corrosion of 308L stainless steel weld metal in simulated PWR primary water[J]. J. Mater. Sci. Technol., 2022, 96: 308-324.

Figures/Tables 25

Fig. 1. (a) Metallurgical microstructure of 308L stainless steel weld metal showing vermicular δ-ferrite in austenite matrix. (b) TEM image of 308L weld metal showing M23C6 type carbides along the δ-ferrite/austenite phase boundary. The SAED pattern of the M23C6 carbide and austenite is inserted in (b).

Fig. 2. SEM images of the surface morphologies of the oxide scales formed on unaged and 7000-h aged specimens corroded under both aerated and deaerated conditions: (a) unaged, aerated; (b) 7000-h aged, aerated; (c) unaged, deaerated; (d) 7000-h aged, deaerated.

Fig. 3. SEM images of the oxide particles formed on (a) unaged and (b) 7000-h aged specimens corroded under aerated condition, showing that numerous whisker oxides grow on the surface of oxide particles. The whisker oxides are marked by the yellow arrows.

Fig. 4. Cross-sectional TEM observation of the oxide scales formed on δ-ferrite under aerated condition: (a, b) STEM-HAADF images and the corresponding element maps for the (a) unaged and (b) 7000-h aged specimens; (c) composition profiles across the oxide scales on the unaged specimen; (d, e) composition profiles across the oxide scales on the 7000-h aged specimen for different scanning positions. The scanning lines (SL) for the composition profiles are indicated by the yellow dashed arrows in (a) and (b).

Fig. 5. Cr-rich layer on the surface of oxide particles formed on δ-ferrite in the 7000-h aged specimen corroded under aerated condition: (a) STEM-HAADF image; (b) TEM-BF image; (c, d) HRTEM images corresponding to the regions marked by the yellow and red rectangle boxes in (b); (e-h) element maps of O, Fe, Ni and Cr corresponding to (a). The FFT patterns of regions R1 and R2 are inserted in (c).

Fig. 6. Dissolved region in 7000-h aged δ-ferrite under aerated condition: (a) STEM-BF image and the corresponding element maps of O, Fe, Ni and Cr; (b) HRTEM image of the dissolved region marked in (a); (c) FFT pattern of the dissolved region.

Fig. 7. HRTEM images of the oxide scales formed on δ-ferrite in (a) unaged and (b) 7000-h aged specimens corroded under aerated condition. The SAED or FFT patterns corresponding to the oxide particle and inner oxide layer are inserted in the HRTEM images.

Table 1 Chemical compositions (at.%) and structures of the oxide scales formed on δ-ferrite in unaged and 7000-h aged specimens corroded under aerated condition.

Specimen	Position	Fe	Cr	Ni	O	Structure
Unaged	Cr-rich layer	11.30	18.15	0	55.37	FeCr₂O₄
	Outer oxide particle	26.89	1.75	0.59	70.76	Fe₃O₄/FeCr₂O₄
	Inner oxide layer	11.87	19.55	2.55	66.03	Nano FeCr₂O₄
7000-h aged	Cr-rich layer	10.38	18.01	1.84	64.34	FeCr₂O₄
	Outer oxide particle	24.77	3.50	2.04	69.79	Fe₃O₄/FeCr₂O₄
	Inner oxide layer	16.96	16.95	2.83	63.26	Nano FeCr₂O₄

Fig. 8. Cross-sectional TEM observation of the oxide scales formed on δ-ferrite under deaerated condition: (a, b) STEM-HAADF images and the corresponding element maps for the (a) unaged and (b) 7000-h aged specimens; (c, d) composition profiles across the oxide scales on the (c) unaged and (d) 7000-h aged specimens. The scanning lines (SL) for the composition profiles are indicated by the yellow dashed arrows in (a) and (b).

Fig. 9. HRTEM images of the surface of oxide particles formed on δ-ferrite in (a) unaged and (b) 7000-h aged specimens corroded under deaerated condition.

Fig. 10. HRTEM images of the oxide scales formed on δ-ferrite in (a) unaged and (b) 7000-h aged specimens corroded under deaerated condition. The SAED or FFT patterns corresponding to the oxide particle and inner oxide layer are inserted in the HRTEM images.

Table 2 Chemical compositions (at.%) and structures of the oxide scales formed on δ-ferrite in unaged and 7000-h aged specimens corroded under deaerated condition.

Specimen	Position	Fe	Cr	Ni	O	Structure
unaged	Outer oxide particle	32.49	0.49	0.48	66.53	Fe₃O₄
unaged	Inner oxide layer	25.06	13.56	2.70	58.61	Nano FeCr₂O₄
7000-h aged	Outer oxide particle	42.06	2.16	2.01	53.75	Fe₃O₄
7000-h aged	Inner oxide layer	19.44	21.24	0	59.32	Nano FeCr₂O₄

Fig. 11. Cross-sectional TEM observation of the oxide scales formed on austenite under aerated condition: (a, b) STEM-HAADF images and the corresponding element maps for the (a) unaged and (b) 7000-h aged specimens; (c, d) composition profiles across the oxide scales on the (c) unaged and (d) 7000-h aged specimens. The scanning lines (SL) for the composition profiles are indicated by the yellow dashed arrows in (a) and (b).

Fig. 12. Dissolved regions in 7000-h aged austenite under aerated condition: (a) TEM-BF image; (b) HRTEM image of the dissolved region marked in (a) and the corresponding FFT pattern of the dissolved region; (c) STEM image and the corresponding element maps of O, Fe, Ni and Cr.

Fig. 13. HRTEM images of the oxide scales formed on austenite in (a) unaged and (b) 7000-h aged specimens corroded under aerated condition. The SAED or FFT patterns corresponding to the oxide particle and inner oxide layer are inserted in the HRTEM images.

Table 3 Chemical compositions (at.%) and structures of the oxide scales formed on austenite in unaged and 7000-h aged specimens corroded under aerated condition.

Specimen	Position	Fe	Cr	Ni	O	Structure
Unaged	Cr-rich layer	12.01	27.34	0	60.65	FeCr₂O₄
	Outer oxide particle	29.78	6.98	3.70	59.54	Fe₃O₄/FeCr₂O₄
	Inner oxide layer	13.77	21.28	4.22	60.74	Nano FeCr₂O₄
7000-h aged	Cr-rich layer	8.25	19.51	1.04	68.57	FeCr₂O₄
	Outer oxide particle	18.97	8.80	1.85	70.32	Fe₃O₄/FeCr₂O₄
	Inner oxide layer	12.48	18.51	3.43	65.53	Nano FeCr₂O₄

Fig. 14. Cross-sectional TEM observation of the oxide scales formed on austenite under deaerated condition: (a, b) STEM-HAADF images and the corresponding element maps for the (a) unaged and (b) 7000-h aged specimens; (c, d) composition profiles across the oxide scales formed on the (c) unaged and (d) 7000-h aged specimens. The scanning lines (SL) for the composition profiles are indicated by the yellow dashed arrows in (a) and (b).

Fig. 15. HRTEM images of the surface of oxide particles formed on austenite in (a) unaged and (b) 7000-h aged specimens corroded under deaerated condition.

Fig. 16. HRTEM images of the oxide scales formed on austenite in (a) unaged and (b) 7000-h aged specimens corroded under deaerated condition. The SAED or FFT patterns corresponding to the oxide particle and inner oxide layer are inserted in the HRTEM images.

Table 4 Chemical compositions (at.%) and structures of the oxide scales formed on austenite in unaged and 7000-h aged specimens corroded under deaerated condition.

Specimen	Position	Fe	Cr	Ni	O	Structure
unaged	Outer oxide particle	26.61	0.35	2.18	70.85	Fe₃O₄
unaged	Inner oxide layer	10.52	14.48	2.74	72.20	Nano FeCr₂O₄/Amorphous
7000-h aged	Outer oxide particle	44.10	0.72	3.88	51.26	Fe₃O₄
7000-h aged	Inner oxide layer	16.40	18.10	3.64	61.85	Nano FeCr₂O₄/Amorphous

Fig. 17. STEM-HAADF images and the corresponding element maps of O, Fe, Ni and Cr of the δ-ferrite/austenite phase boundary regions in (a) unaged and (b) 7000-h aged specimens corroded under aerated condition, showing localized corrosion along the phase boundary. The TEM-BF images of the corrosion region along the phase boundary are inserted in the STEM-HAADF images.

Fig. 18. STEM-HAADF images and the corresponding element maps of O, Fe, Ni and Cr of the δ-ferrite/austenite phase boundary regions in (a) unaged and (b) 7000-h aged specimens corroded under deaerated condition, showing no localized corrosion along the phase boundary. The TEM-BF images of the phase boundary are inserted in the STEM-HAADF images.

Fig. 19. G-phase at the O/M interface of δ-ferrite (deaerated condition and 7000-h aged): (a) HRTEM image; (b-d) FFT patterns corresponding to region A, B and C, respectively; (e) O map; (f) Ni map.

Fig. 20. Schematics of corrosion process of 308L weld metal in simulated PWR primary water under (a) aerated and (b) deaerated conditions.

Fig. 21. Pourbaix diagram of Cr at 300 °C [42,43], in which the red dot corresponds to the ECP and pH of the aerated water in this work.

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