| GUO Yong,WANG Jue,DING Ji-feng,YANG Hai-yang,ZHANG Bo,YANG Zhao-hui,LI Xiang-yang.Cause Analysis for Cathodic Protection Potential Coupons Corrosion Failure Behavior in Soil Environment[J],51(10):235-242 |
| Cause Analysis for Cathodic Protection Potential Coupons Corrosion Failure Behavior in Soil Environment |
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| DOI：10.16490/j.cnki.issn.1001-3660.2022.10.024 |
| KeyWord:cathodic protection buried pipelines steel coupons AC corrosion instant-off potential |
| Author | Institution |
| GUO Yong |
Qingdao NCS Testing and Protection Technology Co., Ltd., Shandong Qingdao , China;Central Iron and Steel Research Institute, Beijing , China |
| WANG Jue |
Guangdong Dapeng LNG Co., Ltd., Guangdong Shenzhen , China |
| DING Ji-feng |
Qingdao NCS Testing and Protection Technology Co., Ltd., Shandong Qingdao , China;Central Iron and Steel Research Institute, Beijing , China |
| YANG Hai-yang |
Qingdao NCS Testing and Protection Technology Co., Ltd., Shandong Qingdao , China |
| ZHANG Bo |
Qingdao NCS Testing and Protection Technology Co., Ltd., Shandong Qingdao , China |
| YANG Zhao-hui |
Qingdao NCS Testing and Protection Technology Co., Ltd., Shandong Qingdao , China |
| LI Xiang-yang |
Beijing Advanced Innovation Center for Materials Genome Engineering, China Iron & Steel Research Institute Group, Beijing , China |
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| Abstract: |
| The work aims to analyze the cause of cathodic protection potential coupon of buried pipelines, so as to clarify the failure of the cathodic protection potential coupon of pipelines and corrosion mechanism, and put forward guidance for accurate detection and analysis of cathodic protection coupon instant-disconnect potential of buried pipelines. The cathodic protection potential coupons are more and more widely used under the interference of stray current, the coupons which connected to a buried pipeline in the field was used as the research object. A multifunction data recorder was used to record the cathodic protection parameters of the coupon. The reasons for the failure of the cathodic protection potential coupon are comprehensively analyzed by collecting the ON-potential, instant-OFF potential, DC current density, pipeline AC influence voltage, AC current density and detecting the components of the corrosion product on the surface of the coupon. The results of the macroscopic analysis showed that the corrosion products on the surface of the coupon had been corroded, and the corrosion products were mixed with the soil together, so that formed a hard and hard block. After the surface corrosion products were removed, the surface of the coupon had a number of uneven size corrosion pits, and the surface of the coupon was corroded and the maximum pit depth was 1mm.The element surface analysis results of the product show that the main elements in the rust layer are mainly Fe and O elements, and the corrosion products are mainly iron oxides, mixed with soil impurities and calcium and magnesium incompatible salts. The historical data of the cathodic protection potential coupon showed that the cathodic protection potential meet the specification requirements, but the AC current density was relatively high, the maximum AC current density was 208.3 A/m2, and the maximum AC interference voltage was 7.52 V. Combined with the surface morphology and the analysis of AC corrosion phenomena and conclusions made in the laboratory，the cathodic protection coupon had eventuated AC corrosion. AC corrosion caused serious local corrosion of the coupon. Under the action of AC and DC current, the corrosion products and insoluble deposits were doped together, which increased the diffusion resistance of the interface, changed the properties of the interface, and hindered the cathodic current of the interface, so that The cathodic protection parameters of the coupon are invalid. The failure of the cathodic protection potential coupon was related to AC corrosion. At higher AC current density, although the instant-off potential of the cathodic protection potential coupon meets the potential specified by cathodic protection, the AC interference causes local corrosion of the cathodic protection potential coupon, especially under the cathodic protection current, which accelerates the AC corrosion of the test piece. Under the action of AC stray current and cathodic protection current, the surface corrosion products of the coupon accumulated, and the tolerate salt and iron oxide had formed a thick isolation layer between the coupon and soil environment, so that it was difficult for the cathodic protection current to flow to the surface of coupon through soil environment, and it was also impossible to accurately measure the cathodic protection potential. |
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