Study On Mechanism And Inhibition Strategy Of Under-deposit Corrosion Of Carbon Steel Pipelines In Oil And Gas Fields

Oil and natural gas are important resources for national development,which cannot be replaced by other kinds of energy sources.The safe and efficient development of oil and gas resources can provide guarantee for the good development of the national economy.However,the exploitation,transportation and storage of oil and gas are facing increasingly serious corrosion problems,which can not only cause huge economic losses,but also can give rise to serious accidents.Under-deposit corrosion(the accelerated corrosion of metals beneath deposits due to the coverage of deposits)is ordinary in the water-injection pipelines,oil wells,transportation pipelines and other facilities,which has become a persistent problem in oil and gas production.The influencing factors of under-deposit corrosion are numerous,resulting in the complicated corrosion mechanism of under-deposit corrosion which needs to be further clarified.As for the prevention and control methods for the under-deposit corrosion in the oil and gas field,the commonly used methods are not efficient,and there is a lack of effective prevention and control methods for the under-deposit corrosion.Therefore,it is of great theoretical and practical significance to study the mechanism and prevention and control strategies of the under-deposit corrosion.In this thesis,the formation of deposits,mechanism of the under-deposit corrosion and the prevention and control methods of the under-deposit corrosion were systematically studied.For the formation of deposits,it was found that the corrosion process can promote the CaCO3 scaling,and the critical corrosion rate phenomenon of the promotion effect was discovered.For the mechanism of the under-deposit corrosion,it was clarified that the in-situ damage of the porous corrosion products can promote the localized corrosion beneath deposits,and the influence mechanism of the atmospheres on the under-deposit corrosion was figured out.For the prevention and control methods of the under-deposit corrosion,the interaction mechanism between the corrosion inhibitors and deposits was elucidated,and the design strategy of the high-efficiency corrosion inhibitor for under-deposit corrosion was proposed and verified.The main results are summarized as follows:The corrosion of carbon steel can promote the CaCO3 scaling,which is mainly due to the increased local pH at the metal/solution interface caused by the cathodic reaction in the corrosion process.The corrosion can promote CaCO3 scaling when the corrosion rate exceeds the critical value,and the critical phenomenon was related to the diffusion of corrosion products.Combined with the method for the in-situ removal of deposits and the wire beam electrode technique,it was found that the in-situ local damage of the porous corrosion products would promote the corrosion of the surrounding area,resulting in the localized corrosion beneath deposits.It can be explained as follows:at the initial stage of the removal of the corrosion products,the increased corrosion rate of the damaged area can result in the formation of more protective corrosion products film.And the surrounding area acted as an anode with an accelerated corrosion rate when coupled with the damaged area.The effect of the dissolved oxygen on the corrosion rate of carbon steel fully covered by the CaCO3 deposit depended on the cathodic reaction under open circuit potential.When the cathodic reaction was the oxygen reduction reaction,the dissolved oxygen can accelerate the cathodic reaction and increase the corrosion rate of carbon steel beneath deposits;when the cathodic reaction was the carbonic acid reduction reaction,the dissolved oxygen can inhibit the cathodic reaction and decrease the corrosion rate of carbon steel beneath deposits.Besides,the addition of O2 can promote the formation of oxygen concentration cells under the CaCO3 deposit and result in the localized corrosion under deposits.2-hydroxy-benzimidazole(2-OH-BI),2-amino-benzimidazole(2-NH2-BI)and 2-sulfhydryl-benzimidazole(2-SH-BI)showed good inhibition performance on the under-deposit corrosion beneath the CaCO3 deposit,but only 2-SH-BI had a higher inhibition efficiency on the under-deposit corrosion beneath the SiO2 deposit.The above phenomena were reasonably explained from three aspects:the corrosion inhibition performance of corrosion inhibitors on the bare metal surface,the adsorption of corrosion inhibitors on the surface of deposits and the diffusion ability of corrosion inhibitors through deposits.Based on the above,the design strategy of high-efficiency corrosion inhibitor for under-deposit corrosion was proposed:good corrosion inhibition performance for bare metals,low adsorption tendency on deposits,and easy penetration to the metal surface through deposits.