Corrosion Behavior Of X70 Pipeline Steel In Marine Environment

With the rapid increase of energy demand and the exhaustion of land energy resources,the development of marine oil and gas resources has been put on the agenda.The marine environment has diversity,complexity and variability.The static water pressure,oxygen content,temperature,light,p H value,salt content,seawater flow rate and marine biological fouling will affect the integrity of the submarine pipeline.In this paper,the corrosion behavior of X70 pipeline steel base metal and weld in marine environment was studied by electrochemical test,slow strain rate tensile test and SEM.The main conclusions are as follows:(1)When the hydrostatic pressure increases from 0 to 2MPa in 3.5%Na Cl solution,the corrosion rate of base metal and weld of X70 pipeline steel increases,and the corrosion form is pitting corrosion,and the main corrosion product is Fe OOH.When the hydrostatic pressure is increased to 3Mpa,the corrosion forms of base metal and weld metal all develop to comprehensive corrosion state.Because of the formation of protective Fe3O4 on the surface of X70 pipeline steel,the corrosion rate decreases.Under the same hydrostatic pressure,because the components of weld and base metal are different,the corrosion of weld is more serious than that of base metal.(2)When the hydrostatic pressure is 0 ~ 2MPa,the SCC sensitivity of X70 pipeline steel base metal and weld in 3.5% Na Cl solution increases with the increase of hydrostatic pressure,and decreases with the increase of 3Mpa.SCC sensitivity is not related to hydrostatic pressure,but depends on pitting condition of metal surface.The SCC sensitivity of the weld under the same hydrostatic pressure is higher than that of the base metal.With the increase of hydrostatic pressure,the anodic dissolution of the base metal and weld surface of X70 pipeline steel is promoted,and more hydrogen atoms are also promoted into the steel.The cracking mechanism is a mixed mechanism controlled by both anodic dissolution and hydrogen induced cracking.(3)In 3.5% Na Cl solution,the corrosion rate of X70 pipeline steel increases with the increase of flow rate when the flow rate is 0-2m/s,and the corrosion rate decreases when the flow rate continues to increase to 3m/s due to the formation of protective Fe3O4 and Fe OOH on the surface of X70 pipeline steel.Furthermore,the flow rate did not change the corrosion process of the material,but only increased or decreased the corrosion rate.(4)When the flow rate is 1-3 m/s,the corrosion of X70 pipeline steel is overall corrosion,and the corrosion products are mainly Fe OOH and Fe3O4.At 2 m/s,the corrosion products on the surface of X70 pipeline steel sample are washed away,and the corrosion rate is the highest.