Corrosion Behavior Of Different Grade Pipeline Steels Welded Joint Under Simulated Deep-sea Environment

The oil and gas industry is an important foundation for the rapid development of the national economy.With the increasing living standards and quality,the demand for energy is also growing.For the past few years,energy of land and offshore has dried up,and the exploration of deep-sea oil and gas resources has mentioned in the schedule.The study found that more than 80 percent of marine oil resources reserved in sea water depths less than 500 m,Deep-sea environment is a high hydrostatic pressure,low temperature,low dissolved oxygen content and strong electrolyte environment,so that pipeline corrosion in the deep sea environment has a great difference from shallow marine corrosion.Up to now,there were many researches on the shallow sea corrosion of pipeline welded joint and few researches about the deep-sea corrosion.We choosed the welded joints of API X70 and X80 pipeline steel for the work in this paper.Contrastively studied the corrosion behavior of the based metal and weld metal of their straight and spiral seam welded joints in the deep sea environment,and observed the galvanic corrosion characteristics by electrochemical test methods.The electrochemical tests showed that increasing the temperature or reducing the dissolved oxygen content could make the corrosion potential of the sample dropped.Decreasing the temperature or reducing the dissolved oxygen content could make the self-corrosion rate decreased.These samples have no passivation process in the simulated deep-sea environment.The anodic process was active dissolution reaction,and the cathodic process consisted oxygen diffusion process and water depolarization process,that was was controlled by mixing process.By the galvanic corrosion tests,we found that,when the base metal specimen and weld metal specimen coupled in a simulated deep-sea environment,straight and spiral weld metal of the API X70 pipeline steel welded joints specimens were the localized anode of the galvanic couple participated in the reaction.For API X80 pipeline steel specimen the localized anode of the galvanic couple was the base material.The galvanic current of seam welded joints was smaller than straight seam welded joints.For the specimen we have selected,the X70 pipeline steel spiral seam and straight seam welded joint have security risks in the actual production and transportation process.