Study On Localized Electrochemical Behavior Of Crevice Corrosion For Oil & Gas Buried Pipelines With Disbonded Coating Defects

As the oil gas pipelines are built underneath the earth surface,the corrosion protecting coating may have different kinds of defects with the natural factors 、 human factors 、mechanic factors and cathodic disbonding in the soil. After break or disbond happen to the coating,it will form crevice between the coating and the metallic substrate,the corrosive dielectric enter crevice through the opening of it and make crevice corrosion happen to the metallic substrate,what is worse,it lowers the protection effect of the coating and shorten the life of pipelines,and it can even lead to poisoning effect、fire and exploding after the pipelines have perforation leakage.Nowadays,it is a key problem to research the corrosion failure and service safety of pipelines underground,there are plenty of findings about the pipelines corrosion in different kinds of environment. However, with the deep going research of the oil gas pipelines corrosion,it is hard for traditional macro electrochemical methods to satisfy the need of researches. The advanced micro electrochemical measuring methods which are represented by the Scanning Kelvin Probe(SKP) and Local Electrochemical Impedance Spectroscopy(LEIS)are being given more attention,and they are used into the researches of pipeline corrosion. Micro-electrochemical measuring methods can obtain the local corrosion information from micro perspective,reveal the whole process and mechanism of pipeline corrosion and tell the micro electrochemical corrosion parameters and information of the pipelines to offer basic data for the underground pipelines protection.This paper adopted the simulation experiment and theory analyze together, which included the soil environment simulating of Chongqing area,and promoted crevice corrosion micro electrochemical behavior of the pipelines with peeling defect coating, the main research content and results are listed as following:(1)According to the analyzing results of soil of Chongqing area,we made up three different kinds of soil simulating solution and adopted the three-electrode electrochemical measuring system to measure and analyses the change laws of Free Corrosion Potential、Polarization Curves and Electrochemical Impedance Spectroscopy of 20#steel in different solution,and the basic corrosion behaviors and corrosion parameters of 20# steel in different solution are obtained.(2)Two different size of coating defect(200μmand800μm) were made to simulate the pipelines with peeling coating defect,and measured the Kelvin potential and Local Impedance modulus at defect with the SKP and LEIS,together with the EDS(Energy Dispersive Spectrometer) and XRD(X-Ray Diode)to measure and analyses the corrosion products,the micro electrochemical characters and its effect laws at defect under different defect sizes、different immersing period of time、different solution potassium system and different cathodic protection potential are obtained.(3)The Kelvin potential at defect have negative shift,the distribution of Kelvin potential is not even. And the Local Impedance Modulus at the defect is the lowest,Local Impedance Modulus increases when it is far from the defect,and Local Impedance Modulus at 200μm is higher than that of 800μm.(4)With the increase of kinds of ions in the simulating soil solution,the Kelvin potential at defect increased and the high potential areas appeared the trend of uneven;the Local Impedance Modulus at defect is the lowest and reduced with the increase quantity of ions in solution.(5)With the time of immersing extend,the Kelvin Potential is the highest at the defect and the high potential areas appeared the trend of uneven;The Local Impedance Modulus at defect is the lowest and become higher when the time of immersing is longer.(6)When the under protective potential(-775mV)、protective potential(-925mV)and the over protective potential(-1025mV) are loaded to the experimental species,the Local Impedance Modulus at defect have different changing trends, and the Local Impedance Modulus under the protective potential is the highest,which means give the best protective effect to the species.