EFFECTS OF SO42- ON THE CORROSION BEHAVIOR OF NiCu LOW ALLOY STEEL IN DEAERATED BICARBONATE SOLUTIONS
Yunfei LU1,2,Junhua DONG1(),Wei KE1
1 Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 2 Wuhan the Second Ship Design and Research Institute, Wuhan 430064
Cite this article:
Yunfei LU,Junhua DONG,Wei KE. EFFECTS OF SO42- ON THE CORROSION BEHAVIOR OF NiCu LOW ALLOY STEEL IN DEAERATED BICARBONATE SOLUTIONS. Acta Metall Sin, 2015, 51(9): 1067-1076.
High level radioactive waste (HLW) is an extremely dangerous by-product of the global nuclear industry. Due to its intensely radioactive nature and ultra long half-life, HLW has to be safely managed and disposed for thousands of years, isolated from the biosphere. Deep geological repository (DGR) is considered to be the most feasible option worldwide because of its operability, stability, durability, environmental protection and so on. Basically, DGR relies on a multibarrier system and it consists of metallic canisters, backfill materials and a stable geologic formation. Since radionuclides could be moved into the biosphere by action of groundwater, both the geologic formation and backfill materials have to be of very low hydraulic permeability and metal canisters have to be corrosion resistant and prevent contact between the groundwater and the radioactive waste for as long as possible. Low carbon steel has been selected and studied as a candidate canister material in many countries because its long industrial experience, high-strength, low cost and it is less prone to localized corrosion than materials that passivity, but its larger corrosion rate may also set an insuperable barrier for the practical application. Recently, our studies revealed that NiCu low alloy steel is a more promising candidate for the canister material compared with the popular one, low carbon steel, since the former performs a more acceptable corrosion rate without increasing much cost and has better resistance to localized corrosion in environments with high concentration of Cl-. In this work, effects of SO42-, another ubiquitous species in deep groundwater, on the corrosion behavior of NiCu low alloy steel during immersion in simulated deep groundwater environments were investigated by in situ electrochemical measurements and surface analysis techniques. Results show that the addition of SO42- can promote the substrate dissolution during the initial stage of immersion. In the later stage, SO42- weakens the protectiveness of formed films and consequently, active dissolution prevails on the electrode surface rather than the prepassivation. Concentrated SO42- and HCO3- can both promote the formation of Fe6(OH)12CO3. The main components of corrosion products are a-FeOOH, Fe3O4 and Fe6(OH)12CO3, and uniform corrosion is observed.
Fig.1 Polarization curves of NiCu low alloy steel in the deaerated 0.05 mol/L (a) and 0.10 mol/L (b) bicarbonate solutions containing different concentrations of SO42- and without SO42-[15,16](Ecorr—corrosion potential, P1—the first oxidation peak, P2—the second oxidation peak, P3—the third oxidation peak)
Fig.2 Evolution curves of open circuit potential of NiCu low alloy steel in the deaerated 0.05 mol/L (a) and 0.10 mol/L (b) bicarbonate solutions with or without[15,16] 0.10 mol/L SO42-
Fig.3 Bode phase plots (a, c) and Bode impedance plots (b, d) of NiCu low alloy steel for the first (a, b) and third (c, d) stages in the deaerated 0.05 mol/L bicarbonate solution containing 0.10 mol/L SO42-
Fig.4 Bode phase plots (a, c) and Bode impedance plots (b, d) of NiCu low alloy steel for the first (a, b) and third (c, d) stages in the deaerated 0.10 mol/L bicarbonate solution containing 0.10 mol/L SO42-
Fig.5 Equivalent circuits for fitting the EIS data measured in the 0.05 mol/L NaHCO3+0.10 mol/L Na2SO4 (a)and 0.10 mol/L NaHCO3+0.10 mol/L Na2SO4 (b) solutions (QHF—capacitance caused by high frequency phase shift, Re—electrolyte resistance, Qdl—double layer capacitance, Rct—charge transfer resistance, W—Warburg impedance, Qcp—capacitance of precipitated corrosion products layer, Rcp—resistance of precipitated corrosion products layer)
Time
Y0,HF
nHF
Re
Y0,dl
ndl
Rct
Y0,W
d
Ssncm-2
Wcm2
Ssncm-2
Wcm2
Ss0.5cm-2
1
-
-
18.24
0.0002459
0.8338
5188
0.006392
5
1.728×10-7
1
17.84
0.0010690
0.9170
4126
-
9
3.684×10-6
0.7569
18.38
0.0032880
0.8994
3830
0.012470
15
2.367×10-5
0.6325
18.28
0.0065300
0.9053
3072
0.003444
21
4.490×10-5
0.5914
17.97
0.0097040
0.9041
2590
0.003690
27
1.906×10-5
0.6445
17.49
0.0138600
0.8896
3648
0.010770
32
3.000×10-5
0.6158
17.92
0.0174700
0.8708
4204
-
44
3.243×10-5
0.6009
21.60
0.0060670
0.8241
1339
0.006091
50
2.503×10-5
0.6203
22.26
0.0064910
0.8261
1486
0.007765
Table 1 Fitted results for EIS data measured in the deaerated 0.05 mol/L NaHCO3+0.10 mol/L Na2SO4 solution
Fig.6 Evolution of the Rct obtained from EIS data as a function of immersion time in the deaerated 0.05 mol/L (a) and 0.10 mol/L (b) bicarbonate solutions with or without[15,16] 0.10 mol/L SO42-
Time
Re
Y0,cp
ncp
Rcp
Y0,dl
ndl
Rct
Y0,W
d
Wcm2
Ssncm-2
Wcm2
Ssncm-2
Wcm2
Ss0.5cm-2
1
14.58
-
-
-
0.0002781
0.8658
2538
0.012750
4
15.66
-
-
-
0.0011160
0.9163
4236
0.003927
8
14.58
-
-
-
0.0027680
0.8409
3408
-
14
15.32
0.02028
0.6764
27.89
0.0059360
0.9072
4110
-
20
14.72
0.01620
0.6958
41.75
0.0083580
0.8713
3752
-
26
15.05
0.02627
0.9692
55.75
0.0071720
0.8090
3780
-
31
14.89
0.02941
0.9916
65.46
0.0099230
0.8147
4038
-
37
17.34
0.03457
1
107.80
0.0114000
0.8218
5219
-
43
16.42
0.03906
1
104.70
0.0121500
0.8159
4094
-
Table 2 Fitted results for EIS data measured in the deaerated 0.10 mol/L NaHCO3+0.10 mol/L Na2SO4 solution
Fig.7 XRD spectra of the surface layer of NiCu low alloy steel after immersion tests
Fig.8 Low (a, c) and high (b, d) magnified surface morphologies of low alloy steel after immersion tests in 0.05 mol/L NaHCO3+0.10 mol/L Na2SO4 solution for 56 d (a, b) and 0.10 mol/L NaHCO3+0.10 mol/L Na2SO4 solution for 43 d (c, d)
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