THE CORROSION BEHAVIOUR OF NiCu LOW ALLOY STEEL IN A DEAERATED BICARBONATE SOLUTION CONTAINING Cl- IONS
LU Yunfei(), YANG Jingfeng, DONG Junhua, KE Wei
Environmental Corrosion Research Center of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
Cite this article:
LU Yunfei, YANG Jingfeng, DONG Junhua, KE Wei. THE CORROSION BEHAVIOUR OF NiCu LOW ALLOY STEEL IN A DEAERATED BICARBONATE SOLUTION CONTAINING Cl- IONS. Acta Metall Sin, 2015, 51(4): 440-448.
The corrosion behaviour of low alloy steel containing Ni and Cu was studied because it is a promising candidate canister material for the disposal of high-level radioactive waste (HLW) in China. Due to the intensely radioactive nature of HLW, the waste has to be prevented from reaching the biosphere for many tens of thousands of years. Deep geological disposal is now considered to be the most preferable option for isolating HLW and it relies on series of natural and engineered barriers, e.g. a metallic canister. However, as soon as the waste package is settled, groundwater would seep back slowly through the outer barriers and ultimately arrive at the surface of the canister. Accordingly, there comes the groundwater-induced dissolution of the canister and subsequent transport of radionuclides through the barriers. That is to say, the effectiveness of radionuclide retention and isolation depends mostly and finally on the corrosion resistance of metallic canisters in deep groundwater environments. In this work, the test solution is deaerated 0.1 mol/L NaHCO3+0.1 mol/L NaCl, simulating the deep groundwater environment. The evolution of corrosion of NiCu low alloy steel in the test solution was investigated by electrochemical measurements. XRD was used to illustrate the composition of formed corrosion products. SEM was used to observe the electrode surface morphology and the cross section of the rust layer. The electrochemical results showed that low alloy steel has a lower corrosion rate and is less prone to localized corrosion than low carbon steel. In order to understand the mechanism of alloying elements, EDS and EPMA were used to analyse the distribution of alloying elements cross-sectional. XPS and E-pH diagram were used to estimate the possible existence form of alloying elements. By means of EDS and EPMA, it was founded that Ni is concentrated in the inner rust layer while the enrichment of Cu is not so obvious. XRD, XPS and E-pH results indicated that Ni and Cu are existed in the form of NiFe2O4 and CuFeO2 respectively.
Table 1 Chemical compositions of the NiCu low alloy steel and Q235 low carbon steel
Fig.1 Polarisation curves (dashed lines) and evolution curves of open circuit potential (solid lines) of NiCu low alloy steel (a) and low carbon steel[10] (b)
Fig.2 Bode phase plots (a, c) and Bode impedance plots (b, d) of NiCu low alloy steel (a, b) and Q235 low carbon steel (c, d) as a function of immersion time in test solutions
Fig.3 Equivalent circuit for fitting the electrochemical impedance spectra (EIS) data measured in stage I (a) and stage II and III (b) (QHF—capacitance caused by high frequency phse 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, Qpit—capacitance of pitted area, Rpit—resistance of pitted area, Qpassive—capacitance of passive area, Rpassive—resistance of passive area)
Steel
Time
Y0,HF
nHF
Re
Y0,dl
ndl
Rct
Y0,W
d
S·sn·cm-2
W·cm2
S·sn·cm-2
W·cm2
S·s0.5·cm-2
NiCu
1
-
-
17.61
0.0002497
0.8092
3280
0.03069
4
-
-
13.40
0.0005871
0.8940
2234
0.01435
10
-
-
10.13
0.0018340
0.8986
1291
0.02672
Q235
4
2.974×10-8
1
22.47
0.0003375
0.8380
1895
0.02753
10
2.902×10-8
1
22.61
0.0004578
0.8510
2229
0.04776
17
3.365×10-8
1
22.52
0.0006121
0.8304
1962
0.05585
Table 2 Fitted results for EIS data measured during stage I
Fig.4 Low (a) and high (b) magnified surface morphologies of NiCu low alloy steel after immersion for 28 d in the test solution
Steel
Time
Y0,HF
Re
Y0,cp
ncp
Rcp
Y0,pit
npit
Rpit
Y0,passive
npassive
Rpassive
d
S·sn·cm-2
W·cm2
S·sn·cm-2
W·cm2
S·sn·cm-2
W·cm2
S·sn·cm-2
W·cm2
NiCu
16
-
8.882
0.008702
0.6457
2.664
0.002131
0.7883
369.5
0.001638
0.5710
2474
22
-
8.217
0.003152
0.6947
1.322
0.002105
0.8048
667.7
0.001622
0.5637
5547
28
-
7.715
0.003001
0.7126
1.181
0.001952
0.8161
463.5
0.001608
0.5709
4979
Q235
24
2.910×10-8
30.98
0.0001203
0.8086
16.73
0.001795
0.6088
1015
0.0001753
0.5611
6451
32
2.358×10-8
39.29
0.0002296
0.7512
51.99
0.0008616
0.5572
0.1764
0.0003387
0.4057
3412
Table 3 Fitted results for EIS data measured during stages II and III
Fig.5 XRD pattern of the surface layer of NiCu low alloy steel after immersion for 28 d in the test solution
Fig.6 Sectional view of corrosion products and corresponding EDS and EPMA results of NiCu low alloy steel after the immersion test
Fig.7 XPS analysis of chemical states of Ni (a) and Cu (b) elements in the rust layer of NiCu low alloy steel after immersion for 28 d in the test solution
Fig.8 E-pH diagram for the Fe-Ni-Cu-H2O system at 25 ℃ with [Fe2+] = 10-5 mol/L (The shaded aera shows the fluctuation range of open circuit potential and solution pH in stages II and III)
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