An Investigation Of Cathodic Protection Potential And Potential Measuring Of Pipe In Soils

Generally, buried pipelines are protected against corrorion by the combined use of cathodic protection and organic coating. Although installed with cathodic protection systems, most of the buried pipes are not properly protected. Because of the incorrect measurement and misunderstanding of potential criteria, the corrosion of buried pipes are also serious. Evaluating the cathodic protection of natural gas distribution systems present significant challenges to the proper application of CP criteria, the measurement must be taken properly, the appropriate criteria applied, and every measurement appropriately assessed. As a result, how to measure the true potential of buried pipes is important to assess the level of cathodic protection systems. According to NACE RP0169-96, all potential criteria for cp require consideration and elimination of voltage drops in soil during potential measurement. In recent years, more and more interests are given to the investigation concerning on potential measuring techniques and potential criteria, especially in the use of coupons for monitoring the level of cathodic protection on gas and oil buried pipelines.In this paper, the different types of soils along natural gas pipes belonging to South-Western Oil and Gas Company are chosen for laboratory tests. Electrochemical measurement techniques are used to study the soil corrosivity, as well as the cathodic polarization behavior of 16Mn steel in soils. At the same time, EIS technique and weight loss experiment are used to investigate the different potential criteria, ie.-100mV polarization shift and -850mV criteria, with the sample being analyzed by SEM and EDS. On the case of potential measurement,coupons are used to assess the level of cathodic protection.According to the EIS analysis of 16Mn steel in four types of soil with different moisture contents, the corresponding equivalent circuits were proposed respectively. By analyzing RPand Rt, it is found that moisture content is the prominent factor which influence the corrosion behaviors. Based on EIS results, the Rt-Ee,c curves are obtained, from which the optimum protection potential of steel in soil can be determined.On the discussion of -100mV polarization shift and -850mV criteria, it is drawn that the protection effect under the two potential criteria, on some cases, the protection effect under -100mV polarization shift criterion than that under -850mV criteria, which is verified properly and in agreement with results drawn from weight loss method. Therefore the -lOOmV criterion is particularly useful for structures that are bare or poorly coated, because attempting to reach of the -850mV criterion on some structure may need far more current than is actually required for protection. On the basis of experimental results, it is proposed that the maximum protection potential for coated pipes is -1.2V at low moisture content, and -1.0V at medium and high moisture content.The work performed to verify the theory that the polarization behavior of the coupons represent that of a holiday of similar size on the pipe. There is a trend for the small coupons to be polarized more than the large coupons, only when the exposed area of the coupons is similar to that of the holiday on the pipe, the potential of coupons can represent that of the pipe. At the same time, coupons respond to changes in the CP level in a similar manner as the pipe, this close relationship between coupons and pipe is critical in verifying the usefulness of coupons in establishing the effectiveness of a CP system.Finally buried coupons in site were used to monitoring the CP level of pipes, and proved to be a practical method to determine the polarization behavior of buried pipes, as well as an excellent method to determinethe IR drop of buried pipes.This study firstly in china performed to investigate the cathodic polarization behavior in details by EIS, and put forward the equivalent circuit in different conditions. According the Rt-Ee,c curves, the optimum protection potential can be obtained. At the same time, the use...