阴极保护试片在土壤环境中的腐蚀失效分析

Cause Analysis for Cathodic Protection Potential Coupons Corrosion Failure Behavior in Soil Environment

目的针对埋地管道阴极保护测试用试片的失效问题进行原因分析,阐明了管道阴极保护测试用试片的失效现象及原因,为准确检测试片的阴极保护电位提供指导。方法以野外埋地管道相连接的试片为研究对象,采用数据记录仪采集试片的阴极保护参数,利用采集失效试片的通断电电位、交直流电流密度,干扰电压及试片表面腐蚀产物等来综合分析试片失效原因。结果试片表面宏观分析发现,试片表面腐蚀产物聚集,腐蚀产物和土壤夹杂在一起,形成坚硬的硬块,除去表面腐蚀产物后,试片表面有大小不均的腐蚀坑,最大坑深1 mm。试片表面腐蚀产物的元素面分析结果表明,锈层中的主要元素以Fe和O以元素为主,腐蚀产物以铁的氧化物为主,并夹杂有土壤杂质及钙镁难容盐等。试片历史数据显示,阴极保护电位满足规范要求,但交流电流密度较大,最大交流电流密度208.3 A/m^(2),最大交流干扰电压7.52 V。结合试片表面形貌,试片发生了交流腐蚀。结论阴极保护测试用试片的失效与交流腐蚀有关,试片在交流杂散电流和阴极保护电流的作用下,表面腐蚀产物聚集,难容盐与铁的氧化物形成厚厚的隔离层,隔离了试片与土壤介质,造成阴极保护电流流入困难,无法准确测量阴极保护电位。

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/m^(2),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.