Study On Corrosion Mechanism Of Rare Earth Ce On X100 Pipeline Steel

As a kind of steel for transporting oil and natural gas,pipeline steel not only undertakes the transportation task in the marine environment,but also puts forward new requirements for its corrosion performance.The water pressure,low temperature,flow rate,oxygen content,bacterial microorganisms and a variety of salts in the marine environment will affect the corrosion resistance of pipeline steel.At present,the understanding of the corrosion of pipeline steel in the marine environment is not comprehensive,Therefore,it is necessary to improve the corrosion resistance of pipeline steel in marine environment.Taking X100 Pipeline Steel as the research object,this paper explores the effect of rare earth Ce on the corrosion properties of pipeline steel by adding different contents of rare earth Ce to pipeline steel（0#test steel without rare earth Ce,1#test steel 0.0017wt.%Ce,2#test steel0.0017wt.%Ce,3#test steel 0.0028wt.%Ce and 4#test steel 0.0049wt.%Ce）.The immersion experiment was carried out in 3.5%Na Cl solution to simulate the marine environment.The systematic research was carried out by SEM,XRD,Raman,EBSD and electrochemistry.The results show that the rare earth inclusions formed after modification are mainly cealo3and ce2o2s,and the size and quantity of rare earth inclusions decrease and increase with the increase of Ce content.The size and quantity of rare earth inclusions decrease and increase with the increase of Ce content.The addition of rare earth Ce can modify Mn S,Mn O and Si O₂.At the same time,the microstructure of four groups of test steels was observed.It was found that the addition of rare earth Ce played a pinning role on the grain boundary and refined the microstructure of the test steel.When the addition amount of Ce was 49ppm,there were obvious small grains at the boundary of grain boundary.The results of EBSD-KAM diagram show that0#the lattice distortion around the typical inclusions in the test steel is serious and there is obvious stress concentration,while there is no obvious stress concentration around the rare earth inclusions modified by Ce.The results of in-situ corrosion experiments on the above inclusions show that the pitting corrosion sensitivity of inclusions in 0#test steel is significantly higher than that of rare earth inclusions in the same soaking time.The effect of rare earth inclusions on the surrounding steel matrix is smaller.The electrochemical test results show that when the amount of rare earth Ce is 49ppm,the corrosion current density of the test steel decreases and the corrosion resistance is stronger.After removing the rust layer and observing the surface of the test steel,it is found that the corrosion pits on the surface of 0#test steel are mostly narrow and deep,while the corrosion pits on the surface of 1#,2#and 3#test steel added with Ce are mostly wide and shallow.Through the analysis of corrosion rate,it is found that the corrosion rate decreases with the increase of Ce content,that is,the addition of Ce improves the corrosion resistance of X100Pipeline Steel.The outer rust layer scraped by machine and the inner rust layer by Raman were detected by XRD.It was found that the main components of the rust layer were Fe₃O₄ in the same soaking environmentα-Fe OOH andγ-Fe OOH The outer rust layer scraped by machine and the inner rust layer by Raman were detected by XRD.It was found that the main components of the rust layer were Fe₃O₄ in the same soaking environmentα-Fe OOH andγ-Fe OOH.In the same soaking time,with the increase of rare earth Ce content,α-The peak strength of Fe OOH is improved,a stable and dense rust layer is formed,and the protective effect on the steel matrix is strengthened.Combined with the electrochemical results of circumferential immersion corrosion,the corrosion current density is lower and the corrosion resistance is stronger.