CHARACTERISTICS AND EVOLUTION OF THIN LAYER ELECTROLYTE ON PIPELINE STEEL UNDER CATHODIC PROTECTION SHIELDING DISBONDED COATING

doi:10.11900/0412.1961.2014.00156

Acta Metall Sin

2014, Vol. 50

Issue (9): 1137-1145 DOI: 10.11900/0412.1961.2014.00156

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CHARACTERISTICS AND EVOLUTION OF THIN LAYER ELECTROLYTE ON PIPELINE STEEL UNDER CATHODIC PROTECTION SHIELDING DISBONDED COATING

YAN Maocheng(

), WANG Jianqiu, HAN En-hou, SUN Cheng, KE Wei

National Engineering Center for Corrosion Control, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016

Cite this article:

YAN Maocheng, WANG Jianqiu, HAN En-hou, SUN Cheng, KE Wei. CHARACTERISTICS AND EVOLUTION OF THIN LAYER ELECTROLYTE ON PIPELINE STEEL UNDER CATHODIC PROTECTION SHIELDING DISBONDED COATING. Acta Metall Sin, 2014, 50(9): 1137-1145.

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Abstract

External corrosion and stress corrosion cracking (SCC) have been observed in a thin layer electrolyte under disbonded coating shielding cathodic protection (CP). In this work, characteristic and evolution of the thin layer electrolyte environment (potential, pH, etc.) on pipeline steel under disbonded coatings were investigated by microelectrode technology in a simulating crevice cell at various CP levels and different atmosphere conditions (CO₂ and O₂). The results show that pH of the thin layer electrolyte under the disbondment varies from near neutral to the value as high as 12, depending on the amount of CP current and CO₂. CP current on pipeline surface initiates a high pH environment on the steel surface. Due to the CP shielding effect and a high CO₂ content, a near-neutral pH environment may exist under disbonded coating. The CP effectiveness and the shielding effect of the disbondment can be estimated by measuring pH of the thin layer electrolyte under disbonded coatings.

Key words: pipeline steel soil corrosion 3PE coating cathodic protection thin electrolyte layer stress corrosion cracking (SCC)

ZTFLH:

TG172.9

Fund: Supported by National Natural Science Foundation of China (No.51131001) and Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education

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	Maocheng YAN
	Jianqiu WANG
	En-hou HAN
	Cheng SUN
	Wei KE

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https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00156 OR https://www.ams.org.cn/EN/Y2014/V50/I9/1137

Fig.1 Schematics of electrochemical cell simulating the thin electrolyte layer environment in a crevice on pipeline steel under disbonded coating (RE—reference electrode, CE—counter electrode)

Fig.2 Distributions of local potential (E) of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice without cathodic protection (Bulk solution at the holiday bubbled with 5%CO₂ + 95%N₂)

Fig.3 Distributions of local potential of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice during cathodic protection potential E_CP = -1000 mV applied and interrupted at the holiday (Bulk solution at the holiday bubbled with 5%CO₂ + 95%N₂)

Fig.4 Distributions of local potential of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice at various CP levels applied at the holiday (Bulk solution at holiday bubbled with 5%CO₂+95%N₂)

Fig.5 Distributions of local potential of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice during E_CP= -1000 mV applied and interrupted at the holiday (Bulk solution bubbled with N₂)

Fig.6 Distributions of local potential of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice during E_CP= -1000 mV applied and interrupted at the holiday (Bulk soil solution is open to air)

Fig.7 Variation of local potential of X70 steel (a) and pH (b) at 180 mm in the crevice (Bulk solution at the holiday bubbled with 5%CO₂+ 95%N₂, pure N₂ and air, respectively)

Fig.8 Distributions of local potential of X70 steel (a) and pH of thin layer electrolyte (b) in the crevice before CP interruption and after 4 h of CP interruption (E_CP= -1000 mV, bulk solutions at the holiday bubbled with 5%CO₂+95%N₂, pure N₂ and air, respectively)

Fig.9 Polarization curves for X70 steel in the soil solutions at pH 6.8, at pH 11 and in the high pH SCC solution (pH 9.5, containing Na₂CO₃+NaHCO₃, i—current density)

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