1. School of Naval Architecture & Ocean Engineering, Dalian University of Technology, Dalian 116024, China 2. School of Innovation and Entrepreneurship, Dalian University of Technology, Dalian 116024, China 3. School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, China
Cite this article:
Kaiqiang LI, Lujia YANG, Yunze XU, Xiaona WANG, Yi HUANG. Influence of SO42- on the Corrosion Behavior of Q235B Steel Bar in Simulated Pore Solution. Acta Metall Sin, 2019, 55(4): 457-468.
Cl- and SO42- are most common aggressive ions containing in the seawater which may cause the localized corrosion of reinforcement structures. It is found that a protective passive film will form on the steel surface in the concrete pore solution. The localized breakdown of the passive film caused by the aggressive ions and the carbonation are the main reason for the localized corrosion initiation of reinforcements. In the previous studies, it is found that the performances of the SO42- on the rebar corrosion were quite different in different pH value conditions and the test results did not unify. Therefore, the influence of pH value and the SO42- on the corrosion behavior of Q235B carbon steel in the simulated pore solution was studied using anodic polarization, electrochemical impedance spectra (EIS), Mott-Schottky (M-S) and potentiostatic polarization methods. The anodic polarization curves indicate that when the pH value of the simulated pore solution was higher than 11, SO42- had no damage to the passive film. However, once the pH value of the simulated pore solution decreased to 10, a small amount of SO42- can lead to the breakdown of the passive film and induce pitting initiation. EIS and M-S measurement results suggest that the stability of the passive film would decrease with the decreasing of the solution pH. The concentration of the defect would increase in the passive film due to the pH decrease. The stability reduction and the increase of defect concentration both can lead to the passive film become fragile and more easily to be destroyed by SO42-. Through the potentiostatic polarization test in conjunction with SEM observation, it is found that SO42- can inhibit the growth of the passive film during the initial film formation period and lead to the appearance of metastable pitting corrosion under high pH value conditions. In the low pH value conditions, SO42- could accumulate at the defect of the passive film and lead to stable pitting propagate on the steel surface.
4 h pre-passivation in pH=12.6 pore solution without SO42-
12.6
0.1
D
No pre-passivation
10.0
0
E
No pre-passivation
10.0
0.1
F
4 h pre-passivation in pH=10 pore solution without SO42-
10.0
0.1
Table 1 The test groups of the constant voltage polarization measurements
Fig.1 The anodic polarization curves of Q235B steel under different SO42- concentration in the simulated pore solution with pH=12.6 (a), 12.0 (b), 11.0 (c) and 10.0 (d) (i—current density, E—voltage)
Fig.2 The Nyquist plots (a, c, e, g) and Bode plots (b, d, f, h) of Q235B steel in the simulated pore solution with pH=12.6 (a, b), 12.0 (c, d), 11.0 (e, f) and 10.0 (g, h)
Fig.3 Equivalent circuit for EIS measurement results fitting shown in Fig.2 (Rs—solution resistance, Rf—film resistance, CPE—constant phase angle element)
Fig.4 The main fitting results of EIS measurement (Y0—admittance of film capacitance, n—dispersion coefficient, Rf—film resistance, R2—coefficient of determination) (a) fitted Y0 (b) fitted n (c) fitted Rf
Fig.5
pH
/ mV
/ (1021 cm-3)
12.6
-808
2.24
12.0
-685
2.66
11.0
-512
3.04
10.0
-410
3.73
Table 2 M-S fitting results under different pH values simulated pore solution
Fig.6 The current noise variation of the electrodes in the test groups A~C as shown in Table 1 (I—current noise, t—test duration. Insets show the magnified curves)(a) A-1 (b) A-2 (c) B-1 (d) B-2 (e) C-1 (f) C-2
Fig.7 The electrode SEM images of the surface morphology in the groups A~C after test(a) A-1 (b) A-2 (c) B-1 (d) B-2 (e) C-1 (f) C-2
Fig.8 The current noise variation of the electrodes in the test groups D~F as shown in Table 1 (Insets show the magnified curves)(a) D-1 (b) D-2 (c) E-1 (d) E-2 (e) F-1 (f) F-2
Fig.9 The electrodes SEM image of the surface morphology in the groups D~F after test(a) D-1 (b) D-2 (c) E-1 (d) E-2 (e) F-1 (f) F-2
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