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Acta Metall Sin  2016, Vol. 52 Issue (9): 1133-1141    DOI: 10.11900/0412.1961.2015.00641
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STRESS CORROSION CRACKING OF X80 PIPELINE STEEL AT COATING DEFECT IN ACIDIC SOIL
Maocheng YAN(),Shuang YANG,Jin XU,Cheng SUN,Tangqing WU,Changkun YU,Wei KE
Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Maocheng YAN,Shuang YANG,Jin XU,Cheng SUN,Tangqing WU,Changkun YU,Wei KE. STRESS CORROSION CRACKING OF X80 PIPELINE STEEL AT COATING DEFECT IN ACIDIC SOIL. Acta Metall Sin, 2016, 52(9): 1133-1141.

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Abstract  

The three-layer polyethylene (3PE or 3LPE) coatings have been widely used on long-distance high pressure transmission pipelines in China. The 3PE coating tends to remain high insulating after disbonding from pipelines, and block the function of cathoidc protection (CP), similar to PE tape coatings that caused stress corrosion cracking (SCC) failure of pipeline. Disbondment 3PE coatings have been reported worldwide. Because of the high integrity and dielectric strength of 3PE coatings, SCC under disbonded 3PE coating becomes an important issue for integrity management and operation of high pressure pipelines. A great deal of researches have been conducting over the past 20 years to reproduce SCC of high strength low alloy (HSLA) pipeline in laboratory. Most of these studies were conducted in bulk solution condition. The methodology neglects particularity of the thin-layer electrolyte under disbonded coating which has been identified as one of the primary environmental factors related to SCC. In this context, a research project has been initiated on this subject. The overall goal is to systematically investigate corrosion scenarios and mechanochemical interaction of HSLA pipeline steels under disbonded 3PE coating in different soil environments, particularly to further mechanistic understanding the initiation of SCC on pipelines under disbonded coating. In this work, SCC behavior of API X80 pipeline steel under disbonded coating with defect was investigated in acidic soil solution by a crevice cell specially designed for simulating coating disbondment. The crevice cell was equipped with a multi-sample loading frame, through which multi specimens in the crevice cell can be loaded simultaneously. Electrochemical impedance spectroscopy (EIS) was applied to characterize local electrochemical process of the tensile specimens. Local environment parameters (potential and pH) were monitored by microelectrodes. Surface morphology of the corrosion specimens indicate that corrosion intensity of X80 steel decreased over the distance from the opening. Intensive anodic dissolution and microcrack initiation were preferential at the opening defect, whereas corrosion was markedly mitigated under disbonded coating. CO2 content gradient is proposed for the special corrosion scenarios under coating disbondment.

Key words:  soil corrosion      pipeline steel      disbonded 3PE coating      stress corrosion cracking (SCC)      cathodic protection     
Received:  15 December 2015     
Fund: Supported by National Natural Science Foundation of China (No.51131001) and National Research and Development Infrastructure and Facility Development Program of China (No.2005DKA10400)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00641     OR     https://www.ams.org.cn/EN/Y2016/V52/I9/1133

Fig.1  Schematic of the test system combining a crevice cell (a) and top view of the test system showing shielding area, arrangement of steel specimens and measurement ports and the multi-sample loading frame (b) (PMMA—polymethyl methacrylate; RE—reference electrode; CE—counter electrode)
Fig.2  Geometry of tensile specimen (unit: mm, thinkness: 0.5 mm)
Fig.3  Local potential of X80 steel at various positions under coating defect in acidic red soil collected from Yingtan, China
Fig.4  SEM surface morphologies of X80 steel with (a~c) or without (d~f) stress applied at opening defect (a, d) or 50 mm (b, e), 250 mm (c, f) away from the defect in red soil solution
Fig.5  SEM images of tensile X80 steel specimen exposed to coating defect
(a, b) intensive corrosion preferentially along heavily deformed metal in scratches or grounding lines on the surface
(c, d) elliptically shaped microcrack, cavities or defects perpendicular to the loading axis
Fig.6  Nyquist plots (a) and Bode plots (b, c) of the tensile and unstressed X80 steel at coating defect immersed for different times
Fig.7  Equivalent electrical circuit used to fit measured EIS (Rs—solution resistance, Rct—charge transfer resistance, constant phase element Qdl is used as a substitute of the double layer capacitance)
Fig.8  Variation of Rct for the tensile and unstressed specimens of X80 pipeline steel
Fig.9  Polarization curves for X80 steel in red soil solution saturated with 5%CO2+95%N2 (Potential scan rate: 0.166 mV/s, icorr—free corrosion current density)
Fig.10  Schematics show distribution of CO2 content (a) and corrosion rate (b) of pipeline steel under disbonded coating as a function of distance from coating defect
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