Study On Local Corrosion Law Of Damaged Inner Coating Of Natural Gas Pipeline

The application of internal coatings in large natural gas pipelines is available to improve the gas throughout,as well as to play a certain anti-corrosion role.However,it is not suitable to use Internal Corrosion Direct Assessment?ICDA?to identify the internal corrosion in pipelines with inner coatings,and there are few studies on the internal corrosion in such pipelines both at home and abroad.Therefore,it is helpful to understand the local corrosion of the defected inner coating,which can provide some suggestions for the modification of the lining and the amendment of ICDA.According to the comprehensive statistical analysis of the internal detection data of an X80 natural gas pipeline,the common internal coating defect size was selected out.Based on the thermodynamics and kinetic theory of electrochemical corrosion of metal,same factors which have significant effects on the metal corrosion were filtered in this study,such as temperature,the total pressure and the concentration of chloride ions.With X80 pipeline steel/epoxy coating as the samples,the size of defects,temperature,pressure and Cl-concentration were considered to study the corrosion regularities on the defects by using VersaScan system?Tafel polarization curves/EIS/SKP?,high temperature and high pressure autoclave,and corrosion products test system?SEM/EDS?,and many macroscopic and microscopic data were caught.And the graphene oxide?GO?modified coating was made in order to reduce the possibility of damage of coating and was evaluated on its anti-corrosion effect.Through the experimental studies,the following conclusions are drawn:?1?The wide crack defects?3mm×1mm?was easy and continuous to form corrosion in the open corrosive circumstance.?2?The ionic activities were lower at 10? and were more active at 30?;and the corrosion rates in cracked coating area was higher than those without coatings.And EIS results showed that corrosion process was controlled by diffusion at 20? and 30?.SEM figures revealed that the corrosion product structure was looser at 10? while the amount of corrosion products increased and the structure was more compact at 30?.However,there existed a few chloride salts at the highest temperature,resulting in reduced charge transfer resistance,thereby promoting metal corrosion.?3?The pipeline pressure from 1MPa to 8MPa promoted the transportation ability of H₂O and corrosive media,which made internal coatings fail quickly,and increased the corrosion rate of the metal at the breakage of the coating,almost 2 to 2.5 times faster than metal without coatings.SEM plots showed that some holes was in the surface of products at high pressure,which might bring about the pitting corrosion or hole corrosion.?4?At the concentration of 0.05mol/L chloride ions,the corrosion current density reached the top on defect of X80 pipeline steel/internal lining aera.EIS results indicated that the higher the Cl-concentration,the better the conductivity of the solution,resulting in the ions exchanging with easy,and the earlier corrosion process controlled by the diffusion.The results of SKP mapping showed that the large potential difference push the local corrosion at the interface to have a trend into inner coatings.?5?It was proposed to add graphene oxide?GO?into the epoxy coating to effectively inhibite the metal corrosion at the breakage.Because of the existence of GO,the modified coating had a better physical properties and had the effective infiltration of H₂O and Cl-into the coating.