CORROSION BEHAVIOR OF 20SiMn STEEL REBAR IN CARBONATE/BICARBONATE SOLUTIONS WITH THE SAME pH VALUE
CAO Fengting, WEI Jie, DONG Junhua(), KE Wei
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
Cite this article:
CAO Fengting, WEI Jie, DONG Junhua, KE Wei. CORROSION BEHAVIOR OF 20SiMn STEEL REBAR IN CARBONATE/BICARBONATE SOLUTIONS WITH THE SAME pH VALUE. Acta Metall Sin, 2014, 50(6): 674-684.
The aggressive ions, such as Cl- and SO42-, as well as the carbonation caused by CO2 from the air are two main reasons for the depassivation of steel rebar in reinforcement concrete. Under normal conditions, the pH value of concrete pore solution is taken as the criterion for determining whether the corrosion of steel occurs or not. However, carbonation process results not only in the decrease of the pH value of concrete pore solution, but also in the accession of HCO3- and CO32-. It is demonstrated that these two ions are able to influence the corrosion behaviors of steel rebar. Additionally, the failure of reinforcement concrete is a time consuming process, so the corrosion evolution laws of steel at the presence of HCO3- and CO32- is necessary to study systemically. Nevertheless, little relative work has been done so far. In this work, the electrochemical behavior of 20SiMn steel in three different content carbonate buffer solutions (0.01, 0.05 and 0.5 mol/L) was studied using electrochemical techniques (polarization curves, free corrosion potential measurements, EIS, Mott-Schottcky (MS) curves and cycle voltage curves) and surface analysis techniques (SEM and in situ Raman spectroscopy), compared with that in NaOH solution (0.437×10-3 mol/L ). These four solutions are of the same pH value 10.64. The results indicated that 20SiMn steel was in active corrosion state in NaOH solution and low content carbonate solution, while it was in passive state in high content carbonate solutions. In NaOH solution, 20SiMn steel was destroyed by uniform corrosion and the corrosion products were a-Fe2O3 and g-FeOOH, transformed from Fe(OH)2. In 0.01 mol/L carbonate solution, 20SiMn steel was destroyed by localized corrosion, and the final products were a-Fe2O3 and b-FeOOH, developed from the intermediate products GRs (green rusts). The passive film formed on 20SiMn steel was more resistive in 0.05 mol/L carbonate solution than that in 0.5 mol/L due to the formation of soluble complex anion Fe(CO3)22- in latter solution. There was a maximum corrosion resistance of the passive film with the increase of carbonate content.
Fig.1 Micro-corrosion morphologies of 20SiMn steel immersed for 7 d in 0.437×10-3 mol/L NaOH solution (a), 0.01 mol/L (b), 0.05 mol/L (c) and 0.5 mol/L (d) carbonate buffer solutions
Fig.2 In situ Raman spectra of 20SiMn steel immersed for 7 d in 0.437×10-3 mol/L NaOH (a) and 0.01 mol/L carbonate buffer solution (b)
Fig.3 Micro-corrosion morphologies of 20SiMn steel (a) and after removal of corrosion products immersed in 0.01 mol/L carbonate buffer solution for 10 min (b), 30 min (c), 1 h (d) and 7 d (e), and in 0.437×10-3 mol/L NaOH solution for 7 d (f) (Insets show the corresponding enlarged images)
Fig.4 Potentiodynamic polarization curves of 20SiMn steel in 0.437×10-3 mol/L NaOH, and 0.01, 0.05 and 0.5 mol/L carbonate buffer solutions
Fig.5 Open circuit potential of 20SiMn steel in 0.437×10-3 mol/L NaOH, and 0.01, 0.05 and 0.5 mol/L carbonate buffer solutions
Fig.6 EISs of 20SiMn steel immersed for 7 d in 0.437×10-3 mol/L NaOH (a), and 0.01 mol/L (b), 0.05 mol/L (c) and 0.5 mol/L (d) carbonate buffer solutions
Fig.7 Fig.7 Equivalent electrical circuit used to fit the EIS results in 0.437×10-3 mol/L NaOH (a), and 0.01 mol/L (b), 0.05 mol/L (a) and 0.5 mol/L (a) carbonate solutions (Rs—solution resistance, Qdl—constant phase element of electric double layer, Rct—charge transfer resistance, ZW—Warburg resistance, Q1—constant phase element of rust layer or passive film layer, R1—resistance of rust layer or passive film layer)
Immerse time d
Rs Ω·cm2
Q1 Ω-1·cm-2·sn
n1
R1 Ω·cm2
Qdl Ω-1·cm-2·sn
nct
Rct Ω·cm2
1
33.19
0.00103
0.8726
52.35
7.025×10-4
0.8473
1103
2
42.55
0.00175
0.9408
36.88
7.125×10-4
0.8522
1454
3
78.33
0.09831
1
263.3
7.515×10-4
0.827
1207
4
90.28
0.00237
0.5975
1620
0.00415
1
312
5
67.82
0.00187
0.6404
1057
0.0057
1
161
6
70.91
0.00124
1
8.396
0.00154
0.7005
1186
7
34.11
1.937×10-8
0.8873
106.2
0.00175
0.6847
1095
Table 1 EIS fitting results of the parameters in 0.437×10-3 mol/L NaOH solution
Immerse time d
Rs Ω·cm2
Qct Ω-1·cm-2·sn
nct
Rct Ω·cm2
W Ω-1·cm-2·s0.5
1
28.65
8.905×10-4
0.7949
2348
0.00955
2
59.61
0.001
0.7961
2027
0.00811
3
68.75
9.561×10-4
0.7811
1776
0.01129
4
58.43
9.039×10-4
0.7756
1965
0.00694
5
75.14
9.633×10-4
0.7520
2186
0.00623
6
56.39
9.285×10-4
0.7552
2280
0.00439
7
64.84
8.534×10-4
0.7465
2371
0.00193
Table 2 EIS fitting results of the parameters in 0.01 mol/L carbonate buffer solution
Fig.8 Relationships between Rct or R1 and immerse time in 0.437×10-3 mol/L NaOH and 0.01 mol/L carbonate buffer solution (a), and 0.05 mol/L and 0.5 mol/L carbonate buffer solutions (b)
Immerse time d
Rs Ω·cm2
Q1 Ω-1·cm-2·sn
n1
R1 Ω·cm2
Qct Ω-1·cm-2·sn
nct
Rct Ω·cm2
1
29.71
4.796×10-5
0.9682
1.004×106
1.034×10-4
0.8553
24940
2
35.20
4.731×10-5
0.9819
1.145×106
8.242×10-5
0.8585
36300
3
30.27
4.414×10-5
0.9718
1.256×106
8.412×10-5
0.8653
36490
4
27.96
4.642×10-5
0.9641
1.078×106
8.349×10-5
0.8648
34270
5
26.59
4.457×10-5
0.9794
1.196×106
7.351×10-5
0.8660
44470
6
33.17
4.354×10-5
0.9792
1.236×106
7.237×10-5
0.8653
46890
7
24.01
4.182×10-5
0.9758
1.254×106
7.383×10-5
0.8678
45270
Table 3 EIS fitting results of the parameters in 0.05 mol/L carbonate buffer solution
Immerse time d
Rs Ω·cm2
Q1 Ω-1·cm-2·sn
n1
R1 Ω·cm2
Qct Ω-1·cm-2·sn
nct
Rct Ω·cm2
1
3.222
2.339×10-4
0.8779
57790
9.592×10-5
0.9111
3975
2
4.804
2.320×10-4
0.8754
66880
9.332×10-5
0.9016
5297
3
5.499
2.218×10-4
0.8802
72840
9.025×10-5
0.9006
6201
4
3.539
2.133×10-4
0.8883
75630
8.630×10-5
0.9058
6585
5
10.86
8.927×10-5
0.9358
2.946×105
1.225×10-4
0.9140
1.310×104
6
10.53
7.277×10-5
0.9365
4.733×105
1.384×10-4
0.9088
1.257×104
7
10.03
7.713×10-5
0.9354
4.102×105
1.335×10-4
0.9146
1.270×104
Table 4 EIS fitting results of the parameters in 0.5 mol/L carbonate buffer solution
Fig.9 Mott-Schottcky (MS) plots of 20SiMn steel immersed for 7 d in 0.05 and 0.5 mol/L carbonate buffer solutions
Fig.10 Cyclic voltammograms for the 20SiMn steel in 0.437×10-3 mol/L NaOH (a), and 0.01 mol/L (b), 0.05 mol/L (c) and 0.5 mol/L (d) carbonate buffer solutions
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