Corrosion And Reliability Of Bimetal Compound Pipeline For Oil And Gas Transportation

With the highly corrosive oil and gas fields of mining, oil and gas pipelinescorrosion has become the focus of the study. Bimetal compound pipe not only has highmechanical properties and corrosion resistance, but also reduce the cost of corrosionresistant alloy pipes, which is widely used in highly corrosive oil and gas fields. Due tolack of reliability evaluation methods of the corrosion and the domestic composite pipetechnical specifications, the corrosion resistance of the weld evaluation method has notyet formed the corresponding standard, that to measure the corrosion resistance of thecomposite pipe weld reliability by using standard methods, and domestic need toconduct in-depth research on corrosion resistance of the composite pipe welds.Therefore, to make composite pipe the base pipe selection, evaluation liner, compositepipe connection, bond strength evaluation helps to supplement the lack of research, toprovide the basis for the bimetal compound pipe applications. Thesis topics combinedthe practical application of energy companies needs, while related to materials science,chemistry and chemical engineering, energy science and other areas and disciplines,which has practical value and basic theoretical significance.In this thesis, to select a certain representative316L-20G the mechanicalcomposite pipe as the object of study, which in the oil and gas transportationenvironment with S2-and Cl-, to seek316L-20G composite pipe corrosion technology asresearch purposes, to explain316L-20G composite pipe corrosion mechanism as thebasis, to research by modern electrochemical research methods as the means,systematically studied the corrosion and anti-corrosion technology of the316L-20Gcomposite pipe liner corrosion resistant alloys in the corrosive media with S2-and Cl-,and analyzed corrosion mechanism in the interaction of various corrosive factors, andstudied the galvanic corrosion behavior in leakage conditions of the composite tubejunction, and discussed protection potential and mechanism of grassroots metals andalloys liner in the different conditions of cathodic polarization, and verified themigration and diffusion of the alloying elements in the composite pipe junction, andanalyzed the dispersal mechanism and electrochemical corrosion behavior of the weldconnection element. Based on the findings, drawn as follows conclusions:①Composite pipe lined with metal316L and304steel in different Na2S corrosivemedia, have shown with increasing of Na2S concentration the corrosion potential negative shift, corrosion current increases, and increased corrosion; while in the lowtemperature range, as the temperature rise, corrosion potential negative shift, corrosioncurrent increases, and accelerated corrosion of the pipeline. Analyzed the impact ofNa2S concentration on the corrosion rate by the basic principles of chemical equilibriumshift; Explained the temperature on the properties of corrosion by mass transfer theory.②Composite pipe lined with metal316L and304steel in the coexistence of S2-and Cl-corrosive media have shown that, because of the metal surface passivation filmhas some integrity, lined with alloy corrosion potential slightly positive shift, and thecorrosion current lower. With etched time extended, as Cl-pitting corrosion on the metalsurface, the metal surface passivation film is destroyed, and the corrosion currentincreases rapidly. But when the Cl-concentration is increased to a certain extent, reachedor exceed the S2-concentration, a certain amount of Cl-in priority combined with metalions, hindered other anionic (S2-) in the electrode reaction, and also reduced adsorbedoxygen in the solution, and corrosion current gradually decreased, but was inhibited.③In galvanic corrosion reaction of316L-20G composite pipe lined grassrootsmetals and alloys, the cathodic process is the reduction of oxygen molecules, which issubject to the oxygen molecule diffusion control; anodic process is oxidation of themetal atom, which is subject to the electron transfer process control;Potential difference exists between the two materials,20G in an anode, which activedissolution state, and316L in cathode,which incurs protection status; when thecomposite tube junction leakage occurs, for corrosive media20G and316L cathodicprotection, both of which have potential difference, and316L and20G in the corrosivemedium to form galvanic corrosion cell, the overall increased corrosion. In the choice ofcathodic electrochemical protection,20G and316L protection potential preferably inoxygen concentration diffusion region.④By the Fick and EDS element scan316L-20G double metal composite pipeweld analysis, researched that alloying element of transition weld is diluted with carbonsteel, and the carbon atom diffused to316L stainless steel layer and the weld area, whilethe alloy elements Cr, Ni of316L and the wire migrated to carbon steel. At the sametime, weld transition played a good isolation effect in migration of the alloy elements,maintained the corrosion resistance of the base material316L, but also reduced thecarbon activity, hindered the diffusion of carbon, and ensured that the weld has goodmechanical performance.⑤Test electrochemical corrosion of316L-20G bimetal compound pipe, and compared to Nyquist plot and Bode diagram of AC impedance spectroscopy, can bedrawn that,316L-20G bimetal compound pipe welds after reaching the maximum ACimpedance, the impedance decreases slightly until relatively stable, and its magnitude isstill reached105, which means that the weld has good corrosion resistance, and canpromote the use of in the oil and gas sewage pipeline.