Study of Interference Parameters Variation Regularity and Corrosion Behavior of X80 Steel in Guangdong Soil Under High Voltage Direct Current Interference
Runzhi QIN1, Yanxia DU1(), Minxu LU1, Li OU2, Haiming SUN2
1 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 2 Sinopec Petroleum Engineering Corporation, Dongying 257026, China
Cite this article:
Runzhi QIN, Yanxia DU, Minxu LU, Li OU, Haiming SUN. Study of Interference Parameters Variation Regularity and Corrosion Behavior of X80 Steel in Guangdong Soil Under High Voltage Direct Current Interference. Acta Metall Sin, 2018, 54(6): 886-894.
High voltage direct current transmission (HVDC) systems develop fast in China recently. The ground electrodes of HVDC systems can inject/absorb large amount of DC current into/from soil, introducing DC interference to nearby pipelines. Then the pipe-to-soil potential shifts positively and high corrosion risk may appear. In this work, indoor HVDC simulation experiments were designed and carried out based on the field test results. Under high voltages, the variation regularity of DCdensity and the corrosion behavior of X80 steel in Guangdong soil were studied. The result showed that under 50, 100, 200 and 300 V DC voltages, the DC density of the coupons had the same trend and could be divided into 3 stages. Firstly, the DC density climbed to peak sharply in several seconds. Then, the DC density decreased gradually to steady value in hundreds of seconds. Lastly, the DC density stayed at that level for the rest of time. The local environment was monitored. The results indicated the variation of the DC density was mainly related to the local soil temperature increment, water content decrement and the substantially growth of the soil spread resistance. After the interference, the corrosion rates were measured to be 5.56, 7.85, 10.63 and 7.78 μm/h, respectively. The variation regularity of the corrosion rates was same with the steady values of DC density, but different from the peak values. Furthermore, 3 methods of calculating corrosion rates were studied. The theoretical corrosion rates calculated by integration of DC density curve had the smallest errors compared with the measured values. The method of using steady DC density had bigger errors and using peak DC density led to the biggest errors. Based on the results, the method of predicting HVDC corrosion rate was proposed.
Fig.1 Schematic of high voltage direct current (HVDC) interference and test system (WE—working electrode, RE—reference electrode, CE—auxiliary electrode)
Fig.2 Direct current (DC) density of X80 coupon under 50~300 V (a) and 300 V with 3 stages (b)
Fig.3 Variation of peak value (a) and steady value (b) of current density with interference voltage
Fig.4 Variation of soil temperature at 1 cm away from coupon under HVDC interference
Fig.5 Variation of soil water content at different distance from coupon surface before and after high voltage direct current interference
Fig.6 Variation of spread resistance (Rsoil) before and after high voltage direct current interference
Fig.7 Macro morphologies of X80 steel coupons after high voltage direct current interference at 50 V (a), 100 V (b), 200 V (c) and 300 V (d) for 1 h
Fig.8 Raman spectra of rust after high voltage direct current interference at different voltages
Voltage
Mass loss
Corrosion rate
V
mg
μmh-1
50
3.42
5.56
100
5.90
7.85
200
8.74
10.63
300
5.25
7.78
Table 1 Mass loss and corrosion rate of X80 steel under 50~300 V high voltage direct current interference
Voltage
Current density / (Acm-2)
Error
V
Measured
Calculated
%
50
30.0
32.47
8.23
100
44.8
45.19
0.87
200
68.6
71.27
3.89
300
36.0
36.13
0.36
Table 2 Comparison of measured current density and calculated (by Eq.(4)) current density after HVDC interference
Fig.9 Corrosion rate calculated by current density curve under 300 V high voltage direct current interference
Voltage
Measured
Current density curve
Peak current density
Steady current density
V
corrosion
Corrosion
Error
Corrosion
Error
Corrosion
Error
rate
rate
%
rate
%
rate
%
μmh-1
μmh-1
μmh-1
μmh-1
50
5.56
4.32
22.33
10.59
90.64
3.97
28.51
100
7.85
7.25
7.64
32.16
309.84
5.93
24.40
200
10.63
10.74
0.99
69.24
551.21
9.08
14.57
300
7.78
6.64
14.63
115.01
1378.97
4.77
38.70
Table 3 Corrosion rate and errors calculated by three kinds of current density
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