1 School of Naval Architecture Engineering, Dalian University of Technology, Dalian 116024, China
2 School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
3 School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, China
Cite this article:
Yunze XU, Yi HUANG, Liang YING, Fei YANG, Bing LI, Xiaona WANG. CORROSION BEHAVIOR OF PIPELINE STEEL UNDER DEPOSIT CORROSION AND THE INHIBITION PERFORMANCE OF ORGANIC PHOSPHINE INHIBITOR. Acta Metall Sin, 2016, 52(3): 320-330.
Localized corrosion such as pitting and mesa attack caused by the presence of solid deposits on a metal surface is defined as under deposit corrosion (UDC). UDC is frequently observed in oil and gas transition pipelines where sand, debris biofilm and carbonate deposit are present. Studies have found that the introduction of oxygen would accelerate the galvanic corrosion behavior between the deposit covered area and the area without deposit. Some experiments have been carried out and demonstrated that high concentration inhibitor should be used for the migration of UDC. However, the inhibition effect of the organic phosphonic inhibitor for UDC is rare in the previous studies. In this work, the evaluation of UDC behavior of X65 pipeline steel and the performance of corrosion inhibitor Ethylene Diamine Tetra (Methylene Phosphonic Acid) Sodium (EDTMPS) in the oxygen contained solutions are studied by using polarization dynamic scan (PDS), electrochemical impedance spectra (EIS) and linear polarization resistance (LPR) methods. The galvanic effect caused by the deposit is studied by using wire beam electrode (WBE). The measurement results show that the corrosion rate of deposit-covered electrode is lower than that of bare electrode, but localized corrosion is observed on the deposit-covered steel surface. After 35 mg/L EDTMPS is introduced into the solution, the corrosion rate of the bare steel decreased from 0.17 mm/a to 0.082 mm/a and the corrosion rate of the deposit covered electrode decreased from 0.051 mm/a to 0.026 mm/a. Protective films are observed on both deposit covered steel surface and bare steel surface after EDTMPS added. In the galvanic corrosion monitoring experiment by using WBE, the under deposit area has a lower potential and performs as the anodic area with serious localized corrosion. After 35 mg/L EDTMPS is injected, the average potential begins to decrease. The maximum anodic current density and the total anodic current respectively decrease from 0.21 mA/cm2 and 0.056 mA to 0.078 mA/cm2 and 0.021 mA. The electrochemical measurement results reveal that EDTMPS has an excellent inhibition effect for the corrosion of both bare electrode and deposit covered electrode. The WBE test illustrates that EDTMPS also has an inhibition effect on the galvanic corrosion caused by the covering deposit. However, EDTMPS cannot completely prevent the localized corrosion behavior on WBE surface.
Fund: Supported by National Science and Technology Pillar Program During the Twelfth Five-Year Plan Period (No.2011ZX05056), China Postdoctoral Science Foundation (No.2014M561223) and Fundamental Research Funds for the Central Universities (No DUT15YQ36)
Fig.1 Chemical structure of Ethylene Diamine Tetra (Methylene Phosphonic Acid) Sodium
Fig.2 Schematic of SiO2 applying method on the electrode surface
Fig.3 Photo of wire beam electrode (WBE) (a), schematic of WBE with sand deposit (b) and working principle sketches of WBE (c)
Fig.4 OM images of X65 steel (a) and SiO2 sand particle (b)
Fig.5 Polarization curves of X65 steel with and without EDTMPS injected (a) bare electrode (b) deposit-covered electrode
Electrode
ba
bc
Ecorr V
icorr Acm-2
vcorr mma-1
B
Bare electrode without EDTMPS
89
320
-0.706
1.32×10-5
0.158
30
Bare electrode with EDTMPS
126
174
-0.749
6.78×10-6
0.081
31
Under-deposit electrode without EDTMPS
97
85
-0.743
4.11×10-6
0.049
20
Under-deposit electrode with EDTMPS
262
98
-0.776
2.96×10-6
0.035
31
Table 1 Electrochemical parameters fitted from the polarization curves of each X65 steel electrode
Electrode
Rsol Ωcm2
Rt Ωcm2
n
CPE μΩ-1cm-2sn
Bare electrode without EDTMPS
8.13
1219
0.71
942
Bare electrode with EDTMPS
7.56
2692
0.75
392
Under-deposit electrode without EDTMPS
29.72
4473
0.75
370
Under-deposit electrode with EDTMPS
28.60
8882
0.84
295
Table 2 Fitted parameters of the elements in the equivalent circuit of the EIS measurement results
Fig.6 EIS of X65 steel with and without EDTMPS injected (Zim--imaginary part of the impedance, Zre--real part of the impedance) (a) bare electrode (b) deposit-covered electrode
Fig.7 Equivalent circuit for fitting the EIS measurement results shown in Fig.6 (Rsol--solution resistance, Rt--charge transfer resistance, CPE--constant phase angle element)
Fig.8 Corrosion rate measured by liner polarization resistance (LPR) method
Fig.9 Surface macro morphologies of X65 steel (a) bare electrode without EDTMPS (b) bare electrode with EDTMPS (c) deposit-covered electrode without EDTMPS (d) deposit-covered electrode with EDTMPS
Fig.10 SEM images of X65 steel surface morphologies (a) bare electrode without EDTMPS (b) bare electrode with EDTMPS (c) deposit-covered electrode without EDTMPS (d) deposit-covered electrode with EDTMPS
Fig11 Corrosion potential (a) and galvanic current (b) distribution maps of X65 WBE in the solution without EDTMPS 5 h (E--open circuit potential of each wire electrode; i--galvanic current density of each wire electrode)
Fig 12 Corrosion potential (a, c) and galvanic current (b, d) distribution maps of X65 WBE after EDTMPS introduced into the solution for 5 h (a, b) and 24 h (c, d)
图13 Photo of the WBE after EDTMPS added for 24 h
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