Study Of The Effect Of High Voltage Transmission Lines On Buried Metal Pipeline Corrosion

Rapidly growing energy demands promote construction of a large quantity of high-voltage transmission lines and buried oil and gas pipelines, the land for energy construction become increasingly crowded. Thus, the steel pipelines and overhead electric power transmission lines have to share proximal rights-of-way. In such cases, the buried pipeline is subject to an alternative current (AC) electromagnetic interference caused by the neighbouring power lines, which will result in AC corrosion problems on the pipelines and affect the effectiveness of cathodic protection (CP) system seriously. However, some aspects of AC corrosion are still unknown in China, the further related research is needed.Interference of AC on the corrosion and CP efficiency of X65 steel coupons was investigated in typical loam soil taken from Zhong-Wu pipeline field with weight loss tests in laboratory. The AC interference characteristics of high-voltage transmission lines and its effect on the CP system of a buried pipeline were investigated by means of online-monitoring of AC, DC current / potential and weight loss of X65 steel coupons simulating the coating defects of the buried pipeline on field.Laboratory experiment results show that AC interference greatly enhances the corrosion of the steel in the soil in the absence of CP, with the increase of AC current density, the corrosion rate of the steel in soil increases very quickly, then slowly, and rapidly again at last. There is a tendency that the increase of AC current density results in a change of the morphology of the corroded surface from uniform corrosion to local corrosion. However, the unfavorable effect of the interfering AC is effectively inhibited by the CP system with reducing AC corrosion rate more than 90%. The corrosion rate at each CP current density increases exponentially with the increase of AC current density, but it is possible to reduce the corrosion rate smaller than the critical corrosion rate of 0.01 mm/a recommended by ISO 15589-1 by adjusting the direct current density of the CP system.For a given DC current density used for CP, there is a threshold AC current density. It is further found there was a clear correlation between the applied CP current density and the tolerable interfering AC current density. On the basis of this correlation, a new AC/DC current density criterion is proposed for assuring the effectiveness of CP of buried steel pipelines being interfered with AC, which suggests there is not any AC corrosion risk on buried pipelines only if i_ac < 90 A/m² and 0.8 A/m²≥idc > (i_ac-10)/100≥0.01 A/m² within the range of experimental condition.Field test results show that there are similarities between the current variation and potential variation when the soil conditions keep roughly unchanged; AC corrosion can be mitigated effectively by the CP system. Using either AC voltage alone or AC current density alone as the only key parameter is inappropriate for evaluating the AC corrosion risk, and thus the joint-type criterion considering both AC and DC current density is recommended.A good agreement between results estimated by AC/DC current density criterion proposed in laboratory and the actual on-field measured corrosion degree indicates the AC/DC current density criterion could be applied in field works.