Passive Behavior of Corrosion-Resistant Cr-Containing Steel Bars in Simulated High-Alkaline Concrete Pore Solution
WANG Muliang1,2, SUN Yupeng1,2, CHEN Lei1,2, WEI Jie2(), DONG Junhua2()
1 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
WANG Muliang, SUN Yupeng, CHEN Lei, WEI Jie, DONG Junhua. Passive Behavior of Corrosion-Resistant Cr-Containing Steel Bars in Simulated High-Alkaline Concrete Pore Solution. Acta Metall Sin, 2025, 61(6): 929-940.
Reinforced concrete has become the preferred choice for modern building structures owing to its long durability, strong structure, flexible, and diverse designs, wide availability, and low cost. Traditional carbon steel bars are prone to corrosion in marine environments, resulting in problems such as steel bar breakage and concrete cracks, thereby affecting the safety and reliability of marine engineering structures. Therefore, using high-performance corrosion-resistant alloy steel bars can effectively solve the problem of steel corrosion in marine engineering and improve the durability and maintainability of engineering structures. Concrete is a highly alkaline environment when it is free from erosion, and the pH value of its pore solution is 12.5-13.6. When steel bars are exposed to this environment, a stable passive film forms on their surface. This spontaneously formed passive film can keep the steel bars in a passive state, preventing corrosion and considerably extending the service life of reinforced concrete structures. The differences in the composition and structure of the passive film on steel bars represent important reasons for the different corrosion resistance performances of steel bars in concrete. To study the passive behavior of corrosion-resistant rebars (20MnSi steel, 3Cr steel, and 9Cr steel) with different Cr contents (0, 3%, and 9%, mass fraction) in simulated high-alkaline concrete pore solution, electrochemical measurements (including open circuit potential, electrochemical impedance spectroscopy, polarization curve, and Mott-Schottky curve) were used to study the changes in the properties of the passive film on the surface of the rebars over time. XPS was used to analyze the composition and structure of the passive film. The results show that a passive layered film was formed on the surface of the rebars in the simulated high-alkaline concrete pore solution, and the structure, composition, and protective properties of the passive film were closely related to the Cr content and passivation time of the rebars. The passive film of 20MnSi steel was mainly composed of Fe(III) compounds in the outer layer and Fe(II) oxides in the inner layer. The outer layer of the passive film of 3Cr steel and 9Cr steel comprised Fe(III) and Cr(III) oxides and hydroxides, and the inner layer comprised Fe(II) oxides and Cr(III) compounds. The passive films formed by the three types of rebars exhibited n-type semiconductor properties within the potential range of -0.8 to 0.2 V (vs SCE). As the immersion time increased, the defect density in the passive film decreased, leading to decreased corrosion current density of the rebars and improved corrosion resistance. When the Cr content is increased, the point defect density of the passive film decreases. At the same time, the passive film becomes dense, resulting in improved corrosion resistance of the rebars.
Table 1 Nominal compositions of 20MnSi, 3Cr, and 9Cr steels
Fig.1 SEM images of three kinds of steel bars after annealing heat treatment (a) 20MnSi steel (b) 3Cr steel (c) 9Cr steel
Fig.2 Open circuit potential (Eocp) of 20MnSi, 3Cr, and 9Cr steels with immersion time in simulated concrete pore solution (SCE—saturated calomel electrode)
Fig.3 Polarization curves of three steels in simulated concrete pore solution for different immersion time (E—potential, i—current density)
Fig.4 Corrosion current density (icorr) of 20MnSi, 3Cr, and 9Cr steels in simulated concrete pore solution for different immersion time
Fig.5 Bode-impedance modulus (|Z|) plots (a1-a3) and Bode-phase angle plots (b1-b3) of 20MnSi (a1, b1), 3Cr (a2, b2), and 9Cr (a3, b3) steels in simulated solution for different immersion time
Fig.6 Equivalent electrical circuit model for EIS data fitting (Rs—solution resistance, Rc—polarization resistance of cathodic oxygen reduction, Qc—cathode oxygen reduction capacitance, Ra—charge transfer resistance, Qa—double layer capacitance)
Steel
Time
h
Rs
Ω·cm2
Qc-Y0
10-3 Ω-1·cm2·S-n
nc
Rc
Ω·cm2
Qa-Y0
10-3 Ω-1·cm2·S-n
na
Ra
Ω·cm2
χ2
20MnSi
0.5
3.66
0.26
0.94
1994
0.180
0.84
28530
1.71 × 10-3
12
2.14
0.21
0.90
2083
0.200
0.84
39500
2.63 × 10-3
24
2.45
0.12
0.96
2467
0.066
0.89
90720
3.04 × 10-3
72
2.18
0.22
0.92
2654
0.066
0.93
208200
9.20 × 10-4
168
2.30
0.19
0.93
4246
0.065
0.94
623600
8.13 × 10-4
3Cr
0.5
2.33
0.16
0.92
3563
0.085
0.93
31360
6.99 × 10-4
12
2.22
0.16
0.92
3958
0.082
0.96
163500
5.50 × 10-4
24
2.22
0.20
0.85
4212
0.059
0.95
391700
1.28 × 10-3
72
2.19
0.17
0.92
5314
0.066
0.93
619500
8.77 × 10-4
168
2.23
0.16
0.94
6838
0.061
0.96
940600
4.46 × 10-4
9Cr
0.5
2.41
0.15
1.00
3104
0.072
0.90
48360
9.64 × 10-4
12
2.35
0.16
0.89
4819
0.055
0.93
642700
7.73 × 10-4
24
1.81
0.18
0.93
5966
0.047
0.94
817900
4.06 × 10-4
72
2.34
0.31
1.00
7037
0.047
0.94
1149000
9.31 × 10-4
168
2.23
0.21
0.93
8154
0.048
0.95
1458000
3.23 × 10-4
Table 2 Equivalent circuit parameters of 20MnSi, 3Cr, and 9Cr steels after different immersion time
Fig.7 Evolutions of Rc (a) and Ra (b) of 20MnSi, 3Cr, and 9Cr steels with time
Fig.8 Mott-Schottky (MS) curves of three steels in simulated concrete pore solution for different immersion time (Csc—capacitance of space charge layer) (a) 20MnSi steel (b) 3Cr steel (c) 9Cr steel
Time
h
20MnSi
3Cr
9Cr
R1
R2
R1
R2
R1
R2
0.5
4.88
5.20
3.62
5.18
3.57
3.48
12
4.52
5.06
3.46
3.89
2.45
3.04
24
4.11
4.42
3.13
3.48
2.32
2.94
72
3.39
3.90
2.92
3.29
1.94
2.57
168
2.54
3.25
2.16
2.48
1.81
2.27
Table 3 Point defect densities of 20MnSi, 3Cr, and 9Cr steels in simulated concrete pore solution after different immersion time
Fig.9 Detailed XPS of the surface passive film formed on the 20MnSi (a), 3Cr (b, d) and 9Cr (c, e) steels with different sputter depths after immersion of 168 h
Fig.10 Depth trend of element contents of three steel specimens (a) 20MnSi steel (b) 3Cr steel (c) 9Cr steel
Fig.11 Distribution of different components with sputter depth within the passive film of 20MnSi (a, b), 3Cr (c, d) and 9Cr (e, f) after 24 h (a, c, e) and 168 h (b, d, f) immersion in simulated concrete pore solutions
Fig.12 Schematics of growth mechanism of the passive film formed on three steels in simulated concrete pore solution (a) initial growth stage of 20MnSi steel passive film (b) stable stage of 20MnSi steel passive film (c) initial growth stage of passive film for 3Cr and 9Cr steels (d) stable stage of passive film for 3Cr and 9Cr steels
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