The X70 and X80 steel electrochemical specimens were soaked shortly in NS4 solution which was simulated as a typical neutral soil environment. After that, ulcerlike, loose and easilyfalling rust spots were found on them. Shallow corrosion pits occurred in larger rust spots. The electrochemical behaviors of specimens were investigated with polarization curve method and electrochemical impedance scan method. The results show that the rust spot increased corrosion trend of metals and accelerated the corrosion rate. This was due to the imperfection and separation of the early rust spots reduced the metalenvironmental polarization resistance. The roughness of the surface increased the working area of the specimens and the corrosion current.

By introducing PCM in the multifrequency principles and using DM produced by Radiodetection company, the article showed respectively the detection results of the anticorrosion layers from Shuwulian to first station, from Shuyilian to Shuwulian and from Shusilian to first station gathering pipelines in Shuguang oil production plant in Liaohe oilfield. The inferior and poor level of the three pipelines was accounted for 50.2%,51.8% and 42.9% respectively on all the anticorrosion layers. The detect results and the verification of suspected leaks were consistent. There were certain limitations when PCM was used only to detect underground pipes .If it was combinated with other detection means, the results would be more accurate.