Corrosion Behavior And Mechanism Of Carbon Steel Under The Multi-media Interaction

Pipeline corrosion has always been one of the major problems in the development of oil and gas industry.Leakage cracking of pipelines caused by corrosion will lead to major safety accidents and environmental pollution,which has caused huge economic losses to the country.The subject comes from a case of pitting failure of pipelines.After the pipeline worked for about 2 years,its wall underwent a serious pitting perforation.When the existing media is applied to the pipeline material alone,the corrosion effect is far less than the engineering failure behavior,and it is not only a simple superposition of these corrosion behaviors.There may be one or several interactions between these complex media,which may accelerate the corrosion failure process of materials."Multi-media Interaction"is often the most important form of corrosion in the engineering environment.The internal environment of the pipeline is complex and severe.The main corrosive media includes CO₂(a large amount of injection for improving the production during the oil and gas exploration),O₂(a large amount of air injection is used to assist oil recovery),formation water,Cl^-(contained in the formation water),and HCO₃^-(injection into the pipeline flow as a p H stabilizer).Internal pipeline is faced with a huge risk of corrosion and even fracture due to the interaction of multi-media,and the corrosion behavior and reaction mechanism of pipelines become complex and changeable;however,research in this regard is scarce.Thus,the investigation of the interaction mechanism between/among various media is of central significance to solve the engineering failure problems encountered by oil and gas pipelines in the service environment,along with the monitoring and protection of pipeline corrosion.In this paper,the interaction of muti-media on the corrosion process of carbon steel pipelines is studied.Based on this,the engineering failure case is analyzed.The main results are as follows:(1)The corrosion behavior of carbon steel in solutions containing CO₂,Cl^-and CO₂-Cl^-was systematically studied,and the interaction mechanism between them was clarified.The results show that the enhanced relaxation process of water-iron species adsorbed on the iron surface has appeared for all CO₂ saturated solutions.In addition to the local damage to the corrosion product film,Cl^-can restrict the precipitation and growth of Fe CO₃ corrosion products by adjusting the solubility of CO₂,reduce the coverage percentage of Fe CO₃ corrosion product,and thus accelerate the localized corrosion of carbon steel;(2)For the injection of p H stabilizer into the pipeline flow,the corrosion behavior of carbon steel pipeline in saturated CO₂ solutions containing HCO₃^-,Cl^-and HCO₃^--Cl^-was systematically studied.The results indicate that there is a competitive adsorption relationship between HCO₃^-and Cl^-on the surface of carbon steel.Increasing HCO₃^-concentration can gradually replace the adsorption of Cl^-on the surface of carbon steel.The mathematical equations based on the corresponding reactions are established to verify the consistency between the experimental results and theoretical calculation.The interaction mechanism is as below.HCO₃^-accelerates the local precipitation of Fe CO₃ corrosion products by increasing solution p H and surface adsorption;whereas Cl^-increases the contact between carbon steel and corrosive solution containing CO₂ and accelerates the internal expansion of pitting corrosion.When HCO₃^-concentration reaches 0.5mol·L^-1,the effect of Cl^-in the corrosion process is weakened obviously;(3)In view of the local mild alkaline environment of the carbon steel pipeline flow along with considerable amounts of air injection in the first stage of oil and gas exploitation,the corrosion behavior of carbon steel pipeline in solutions containing HCO₃^-,Cl^-and HCO₃^--Cl^-was systematically studied under the natural oxygen environment.The results show that there is a synergistic relationship between HCO₃^--Cl^-media during the pitting process of carbon steel.The micro alkaline environment containing HCO₃^-has accelerated the formation of corrosion product film,which is a prerequisite for the de-passivation behavior of Cl^-on the surface of carbon steel.The diffusion of Cl^-in the closed corrosion pit promotes the expansion of corrosion pits.Dissolved natural oxygen can induce the transformation of corrosion products to Fe₂O₃,and the density of corrosion products decreases obviously.When the corrosion product film is locally destroyed,a large amount of Fe²⁺will react with HCO₃^-to promote the formation of new corrosion products.(4)The subject of"multi-media interaction"is put forward in dealing with a failure case of oil pipelines.For the complex and severe environment of internal pipeline,its corrosion process is the most typical embodiment of multi-media interaction.Therefore,the interaction mechanism between the main media in the pipeline is studied and clarified.This section is to analyze the corrosion failure case and restore the failure process of the oil pipeline based on the multi-media interaction mechanism.The results show that,under the wet/dry cycles,the interaction of Cl^-,HCO₃^-,CO₂ and O₂ results in the formation and propagation of corrosion pits on the surface of carbon steel pipeline.The main role of HCO₃^-is to enhance the passivation of carbon steel surface,and provides a prerequisite for the subsequent de-passivation by Cl^-.The presence of natural oxygen can reduce the compactness of corrosion products on the carbon steel surface by the oxidation effect,thereby increasing the contact between the steel matrix and corrosive media containing CO₂,and accelerating the formation,propagation and connection of corrosion pits.When the upper part of the pit is in a dry state,the bottom of the pit is still in the corrosive medium with the decline of water level of solutions in the corrosion pit,which promotes the continuous increase of depth of corrosion pits.In the meanwhile,the fatigue pattern at the edge of each pit is due to the difference of corrosion degree of pits at different depths.After the continuous cycles mentioned above,the oil pipeline is perforated.