Study On Cathodic Protection Technology Of Underground Pipeline

Coating and applied cathodic protections are two important methods to protect underground pipeline. Cathodic protection technique has been widely used in long-distance pipeline domestically but reports of studies on parallel pipeline have been rarely seen. In this paper, coating and cathodic protection of parallel underground pipeline were studied. The simulated parallel pipelines system was designed, and the effect of equalizer was primarily analyzed. A mathematical method was proposed to calculate the potential and current distribution in cathodic protection systems of parallel pipelines in layered soil. The result indicates that the current in the pipeline with good coating is larger than that for polarization. And at the same time the equalizer can minish the potential differences between the two pipes. When difference of the coating insulating resistance is great, it is more harmful for protection of pipeline with good coating. And it is good for cathodic protection of parallel pipelines when the coating property is similar. The industrial pure zinc buried in soil was anodized by galvanostatic polarization. The EIS and potential of electrode buried in soil for deferent time were measured and analyzed. The results were compared with those obtained on traditional zinc electrode. It shows that the potential of the polarized zinc electrode has kept quite stable for a long time. Electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) methods were used to study the performance of four high impedance pipeline coatings and ZM99 coating in soil or NaCl solution. The capacitance of FBE coating is the largest, three layer PE , Coal tar, Oil bitumen were similar, but the capacitance of three layer PE is the smallest both in soil and NaCl solution. The noise resistance and the wavelet noise resistance of the four coatings ranks as follow: three layer PE coating> FBE coating>Coal tar coating>Oil bitumen coating. EIS of ZM99 coating has two time constants. The impedance modulus at the same frequency decreased and the characteristic of relaxation became more and more notable with the elapse of time, which indicated that corrosion reaction began in the interface when the electrolyte moved to the coating/metal interface. The close interval potential survey (CIPS) and the galvanostatic transient response method were used to detect the coating state of buried pipelines. The coating damage point and the coating state were diagnosed with the analysis system on the spot. The detection results of real pipeline in Zhongyuan oil field between Liaocheng and Puyang are satisfactory.