Effect of Retained Austenite on Corrosion Behavior of Ultrafine Bainitic Steel in Marine Environment

doi:10.1007/s40195-023-01530-x

Abstract

Abstract:

In this study, the impact of retained austenite on corrosion initiation, propagation, and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformation was investigated using a combination of theoretical calculations and experimental methods. According to the results, the microstructure of ultrafine bainitic steels was composed of parallel arranged bainite ferrite (BF) and retained austenite (RA). The fraction of the block RA was significantly reduced, whereas the amount of the film RA increased through the two-step bainitic transformation. In the initial stage of corrosion, micro-galvanic effects occurred between the multiphases due to the difference in nobility, leading to the selective dissolution of BF sheaves, which is adjacent to the block RA in the one-step bainitic steel. However, uniformly distributed film RA in the two-step bainitic steel acted as a barrier against corrosion propagation. The electrochemical measurements and neutral salt spray tests revealed a relatively lower corrosion rate for the two-step bainitic steel, indicating a higher corrosion resistance than the one-step bainitic steel. The corrosion products layer mainly consisted of α-FeOOH, γ-FeOOH, and Fe₃O₄, which were stable and favorable for the formation of a protective rust layer.

Key words: Ultrafine bainitic steel, Retained austenite, Marine environment, Corrosion behavior, First principle modeling

Xian Zhang, Li Gong, Yanpeng Feng, Zhihui Wang, Miao Yang, Lin Cheng, Jing Liu, Kaiming Wu. Effect of Retained Austenite on Corrosion Behavior of Ultrafine Bainitic Steel in Marine Environment[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(5): 717-731.

Figures/Tables 21

Table 1 Chemical composition of the ultrafine bainitic steel (wt%)

C	Si	Mn	Mo	Cr	Al	Nb + V + Ti	Fe
0.59	1.54	2.00	0.28	1.03	0.04	0.12	Bal.

Table 2 Mechanical properties of ultrafine bainitic steels

	Yield strength (MPa)	Tensile Strength (MPa)	Elongation (%)	Impact toughness (J)	Rockwell hardness (HAC)
One-step bainitic steel	1189	1431	7.6	7.7	51.4
Two-step bainitic steel	1480	1827	8.1	5.3	45.9

Fig. 1 Optical and SEM images of one-step bainitic steel a, b and two-step bainitic steel c, d

Fig. 2 TEM bright field images (left and middle rows), dark field images and electron diffraction patterns (right row) of one-step bainitic steel a-c and two-step bainitic steel d-f

Table 3 Thickness of BF and RA (nm)

	BF	Film RA	Block RA
One-step bainitic steel	226.8 ± 44.8	55.4 ± 16.8	518.7 ± 95.9
Two-step bainitic steel	151.2 ± 70.1	38.8 ± 10.7	-

Fig. 3 XRD diffraction patterns of one-step bainitic steel and two-step bainitic steel

Fig. 4 Surface topography and volta potential distribution of one-step bainitic steel a, b and two-step bainitic steel c, d

Fig. 5 In situ corrosion morphology of one-step bainitic steel a, b and two-step bainitic steel c, d after immersion in 0.5wt% NaCl

Fig. 6 Potentiodynamic polarization curves of one-step and two-step bainitic steels in 0.5 wt% NaCl solution

Table 4 Tafel fitting parameters of the polarization curves

	i₀ (A cm⁻²)	E₀ (V)
One-step bainitic steel	2.85 × 10^-5	− 0.59
Two-step bainitic steel	4.52 × 10^-6	− 0.41

Fig. 7 Nyquist a and Bode b curves of one-step and two-step bainitic steels in 0.5 wt% NaCl

Table 5 Electrochemical parameters deduced by EIS method

	One-step bainitic steel			Two-step bainitic steel
	Fitting value	Error%	χ²	Fitting value	Error%	χ²
R_S (Ω·cm²)	41.82	1.54	1.78 × 10^-3	40.06	1.37	9.73 × 10^-4
Y₀ (S secⁿ cm⁻²⁾	9.84 × 10^-4	2.59		1.01 × 10^-3	1.76
N	0.80	0.98		0.81	0.85
R_ct(Ω·cm²)	886.90	2.79		1420.00	3.87

Fig. 8 Variation of corrosion rate of one-step and two-step bainitic steels with the corrosion time

Fig. 9 XRD diffraction patterns of corrosion products formed on one-step a and two-step b bainitic steels after neutral salt spray test

Fig. 10 Surface morphologies of corrosion products formed on one-step a-d and two-step e-h bainitic steels after neutral salt spray test

Fig. 11 EBSD analysis of one-step bainitic steels a, b and two-step bainitic steel c, d. a and d Optical images, b and e IPF maps, c, f kernel average misorientation (KAM) maps

Fig. 12 Slab models of the three low-indexed crystallographic planes of Fe-FCC

Table 6 Work function and surface energy of Fe-BCC and Fe-FCC

Crystallographic plane	Local potential (eV)	E-fermi (eV)	WF = LP-E-fermi (eV)	Surface energy (J/m²)
Fe-BCC (100)	5.640	1.100	4.540	0.400
Fe-BCC (110)	6.970	1.850	5.120	0.340
Fe-BCC (111)	4.020	- 0.300	4.320	0.410
Fe-FCC (100)	5.670	0.801	4.869	0.214
Fe-FCC (110)	4.400	- 0.031	4.431	0.207
Fe-FCC (111)	6.290	1.101	5.189	0.176

Fig. 13 Schematic of the localized corrosion initiation and evolution process affected by retained austenite of ultrafine bainitic steel

Table 7 Ratio of α-FeOOH to γ*- FeOOH (total amount of γ-FeOOH and Fe3O4) in rust layers according to the XRD results

	Period (h)	γ-FeOOH (%)	α-FeOOH (%)	β-FeOOH (%)	Fe₃O₄ (%)	α/γ*
One-step bainitic steel	6	34.39	1.09	54.69	9.83	0.03
	24	57.26	4.28	9.57	28.89	0.07
	72	70.98	8.76	5.24	15.02	0.12
	168	62.69	15.44	4.33	17.54	0.25
Two-step bainitic steel	6	58.21	4.96	9.19	27.04	0.08
	24	68.21	7.66	7.49	16.64	0.11
	72	69.48	14.36	4.27	11.89	0.21
	168	64.91	20.45	1.04	13.59	0.32

Fig. 14 Cross-sectional morphologies of corrosion products formed on one-step a and two-step b bainitic steels after 168 h neutral salt spray test

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