Rust Formation Behavior and Mechanism of Q235 Carbon Steel in 5%NaCl Salt Spray Under Elastic Tensile Stress
LI Qian, LIU Kai, ZHAO Tianliang()
State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
Cite this article:
LI Qian, LIU Kai, ZHAO Tianliang. Rust Formation Behavior and Mechanism of Q235 Carbon Steel in 5%NaCl Salt Spray Under Elastic Tensile Stress. Acta Metall Sin, 2023, 59(6): 829-840.
As a structural steel material, carbon steel bears a certain extent of elastic tensile stress in actual service. Elastic tensile stress on steel is supposed to impact the electrochemical process and corrosion behavior, which may further influence the rusting behavior and the phase composition and structure of the formed rust layer. However, stresses on the steel substrate slightly influence the rust layer of carbon steel because no intrinsic change exists in the corrosion mechanism. Here, a remarkable effect of elastic tensile stress on Q235 carbon steel was found on the phase composition and structure of the rust layer formed in 5%NaCl salt spray. The effect on the rust layer was studied using SEM, XRD, and electrochemical impedance spectroscopy. The neutral salt spray test with four-point bending was used to preform the rust layer of Q235 steel under various stress levels. The results show that the elastic tensile stress accelerates the anodic dissolution, thereby promoting the generation of γ-FeOOH, which occurs faster in the electrolyte than the transformation of γ-FeOOH to α-FeOOH and Fe3O4/γ-Fe2O3 in the solid-liquid interface. Consequently, the mass fraction of γ-FeOOH in the rust layer increases as the stress level increases, whereas the mass fraction of α-FeOOH and Fe3O4/γ-Fe2O3 decreases accordingly. As the stress increases from 0 to 0.95σs (σs is yield strength), the mass fraction of Fe3O4/γ-Fe2O3 decreases from 53% to ~46%, the mass fraction of α-FeOOH decreases from ~30% to ~23%, and the mass fraction of γ-FeOOH increases from less than 17% to ~31%. Meanwhile, the phase composition change decreases the density and increases the thickness of the rust layer. Additionally, the acceleration of the anodic dissolution induced by the elastic tensile stress promotes the growth of the rust layer, which further increases the thickness of the rust layer. The increase in thickness and decrease in compactness of the rust layer jointly enhance the protective capability of the rust layer. The former increases the resistance to the electromigration of ions through the rust layer, and the latter mitigates the occlusion effect under the rust layer.
Fund: National Key Research and Development Program of China(2017YFB0702100);Sailing Program for Young Science and Technology Talents of Shanghai(20YF1412900)
Corresponding Authors:
ZHAO Tianliang, associate professor, Tel:15090998966, E-mail: tlzhao@shu.edu.com
Fig.1 Schematic of four-point bending device (H—distance between the outer fulcrums of the fixture, A—distance between adjacent inner and outer fulcrums, y—sample deflection, t—sample thickness)
Fig.2 Time dependences of the weight loss of Q235 carbon steel under various elastic tensile stress levels in 5%NaCl salt spray (σs—yield strength)
Stress level
a
b
R2
0σs
0.0200
0.9154
0.9987
0.5σs
0.0228
0.9031
0.9981
0.8σs
0.0253
0.8835
0.9962
0.95σs
0.0308
0.8368
0.9898
Table 1 Corrosion kinetics fitting results of Q235 carbon steel with various elastic tensile stress levels in 5%NaCl salt spray
Fig.3 Surface morphologies of Q235 carbon steel under various elastic tensile stress levels after exposed in 5%NaCl salt spray for 1 d (a1-a4), 3 d (b1-b4), 6 d (c1-c4), 9 d (d1-d4), and 15 d (e1-e4) (a1-e1) 0σs (a2-e2) 0.5σs (a3-e3) 0.8σs (a4-e4) 0.95σs
Fig.4 Section morphologies and corresponding element distributions of the rust layers of Q235 carbon steel under various elastic tensile stress levels after exposed in 5%NaCl salt spray for 15 d
Fig.5 Variations of the rust thickness with the elastic tensile stress level for Q235 carbon steel after exposed in 5%NaCl salt spray for 15 d
Fig.6 XRD spectra (a) and phase compositions (b) in the rust layer of Q235 carbon steel under various elastic tensile stress levels after exposed in 5%NaCl salt spray for 15 d
Fig.7 Variations of mass per unit area and density of the rust layer with the elastic tensile stress level for Q235 carbon steel after exposed in 5%NaCl salt spray for 15 d
Fig.8 Variations of the open circuit potential measured under the loading methods A and B with elastic tensile stress level for Q235 carbon steel in 5%NaCl salt spray for 15 d (Method A—during the salt spray test period of 15 d and the electrochemical test process, the sample kept the stress loading state; Method B—no stress was loaded on the sample within 15 d of the salt spray test period, and the stress was loaded on the sample during electrochemical test after the salt spray test)
Fig.9 Nyquist (a, d), Bode-impedance modulus (b, e), and Bode-phase angle (c, f) plots of Q235 carbon steel with loading modes A (a-c) and B (d-f) under various elastic tensile stress levels after exposed in 5%NaCl salt spray for 15 d (Zim—imaginary part of impedance, Zre—real component, |Z|—impedance modulus)
Mode
Stress
Rs
Qrust (Y0)
nrust
Rrust
Qct (Y0)
nct
Rct
W
χ2
level
Ω·cm2
10-2 Ω-1·cm-2·s n
Ω·cm2
10-2 Ω-1·cm-2·s n
Ω·cm2
10-2 Ω-1·cm-2·s0.5
10-4
A
0σs
11.60
1.05
0.52
10.51
0.17
0.68
53.21
6.88
7.21
0.5σs
9.59
0.52
0.49
11.25
0.62
0.60
46.28
6.52
1.91
0.8σs
10.83
0.24
0.43
12.59
1.38
0.62
32.98
5.98
4.61
0.95σs
9.94
0.14
0.37
14.46
2.36
0.57
29.04
5.46
2.01
B
0σs
11.60
1.05
0.52
10.51
0.17
0.68
53.21
6.88
7.21
0.5σs
8.76
0.36
0.51
9.94
1.46
0.54
35.43
6.64
1.78
0.8σs
10.19
0.32
0.48
9.07
2.68
0.48
21.62
6.24
3.16
0.95σs
12.16
0.51
0.47
8.83
2.78
0.44
16.48
6.13
2.08
Table 2 Fitting results for the EIS curves of Q235 carbon steel under loading modes A and B with various elastic tensile stress levels after exposed in 5%NaCl salt spray for 15 d
Fig.10 Equivalent electric circuit of the rust layer interface on Q235 carbon steel after exposed in 5%NaCl salt spray for 15 d
Fig.11 Variations of Rct, Rrust (a) and nrust, W (b) in the equivalent electric circuit with elastic tensile stress levels under loading modes A and B
Fig.12 Variations of r2 / r1 and (r2 + r3) / r1 in the rust layer and the rust layer density with elastic tensile stress level on the steel (r1—mass fraction of γ-FeOOH, r2—mass fraction of α-FeOOH, r3—mass fraction of Fe3O4/γ-Fe2O3)
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