Characteristics Of Inclusions And Their Effects On The Initiation And Propagation Of Pitting Corrosion In Rare Earth Treated Steels

Along with the rapid development of marine economy,low alloy steel with high strength and toughness was more and more widely used in marine engineering structures,the corrosion resistance of low-alloy steel is urgently need to be further improved.In this research,rare earth treatment was used to modify the inclusions in steel and microalloying to reduce the pitting sensitivity of inclusions and improve the corrosion resistance of steel substrates.In the high-frequency induction furnace,four groups of experimental steels with different refining process?Al deoxidation,three groups of Al deoxidation+Re treatment?were smelted,and the precipitation behavior of rare earth inclusions and the characteristics of inclusions in the rolled steel were preliminary studied.The pitting corrosion resistance and corrosion behavior of experimental steel in 3.5%NaCl solution were discussed.The mechanism of pitting corrosion induced by different types of inclusions?sulfide inclusions and oxide inclusions?was analyzed from the microscopic level.The composition,morphology,homogeneity and compactness of the surface rust layer of the experimental steel with different content of rare earth in the simulated seawater was explored from the macroscopic level.The main conclusions obtained are as follows:?1?The amount of rare earth is different,and the composition of inclusions and sulfide precipitation in steel are different.The inclusions were mainly Al₂O₃+Mn S in Al-deoxidized steels;After addition of rare earth,the inclusions were mainly CeAlO₃+Ce₂O₂S+Mn S in 2^#steel?0.011%Ce?;The inclusions in 3^#steel?0.015%Ce?are similar to the 2^#steel,but no separate MnS inclusions are found.When the content of rare earth was 0.02%,Al was gradually replaced by Ce in the oxide,and the inclusions in4^#steel were dominated by Ce₂O₃+Ce AlO₃+Ce₂O₂S.With the addition of rare earth,the single elongated Mn S inclusions gradually transformed into nearly spherical rare earth sulfides,which can improve the pitting corrosion resistance of steel to a certain extent.?2?For the number,size,and deformability of inclusions,the area content of inclusions in 3^#steel is the smallest?0.067%?,the number is the least?215 pieces/mm²?,and the average particle size is the smallest?1.91?m?when the amount of rare earth added is 0.015%.Nearly 87%of the inclusions are smaller than 2.5?m,deformation of inclusions less than 3?m is about 92%,and the average deformation of the inclusions is 1.83?m.The area,number,size,and deformability of inclusions are better than those of 1^#,2^#,and 4^#steels to a certain extent.Inclusions are less likely to induce pitting,and the pitting corrosion resistance of steel is stronger.?3?The thermodynamic calculation and inclusion observations shows that during the solidification process,the main inclusions formed in rare earth-treated steel are CeAlO₃ and Ce₂O₂S.CeAl O₃ is homogenize nucleation precipitated at the solidification front,Ce₂O₂S precipitates easily on the surface of CeAlO₃ during solidification,and all of them are precipitated at the solidification front,it is beneficial to the dispersion distribution of inclusions.?4?Polarization curves result show that the number of pitting corrosion on the surface of 3^#steel is the least,the distribution is more dispersed,the source of pitting corrosion is less,the pitting potential is the highest,and the pitting corrosion resistance is the strongest in the early stages of corrosion.AC impedance spectroscopy result show that the polarization resistance and the corrosion resistance of steel gradually increases with the increase of rare earth additionin the late stage of corrosion,and the corrosion resistance of 4^#steel is the best,so adding more rare earth can improve the corrosion resistance of steel.?5?In the Cl^-medium,both MnS and Ce₂O₂S would dissolve in test steel.After the inclusion of Mn S was dissolved,the sulfur would be easily gathered on the side where the radius of curvature of inclusions was small,so that the induced pitting was easy to expand in the direction of rolling and depth.However,after the dissolution of Ce₂O₂S,the sulfur diffused to the surface of the matrix.the pitting was easily extended laterally,and the harm of pitting was suppressed.?6?For oxide inclusions,during the corrosion process,the chemical properties of Al₂O₃?Ce AlO₃ and Ce₂O₃ were relatively stable.The electrode potential was higher than that of the substrate,and the oxide inclusions would not dissolve.The adhesion of inclusions and matrix was different,and pitting usually occured at the gap between the oxide inclusion and the matrix interface.When the inclusions fell off,the erosive anions were less in the pitting pit,and the pitting would easy to expand laterally.The sulfur content in the center of the rare earth oxide inclusion was high,which inhibits sulfide precipitation to a certain extent,it reduced the pitting corrosion source and improved the pitting corrosion resistance of the steel.?7?The results of full immersion corrosion test showed that the surface of the steel could form a more continuous,uniform and better adhesion rust layer after the treatment of rare earth.After the rust removal,the corrosion of the 4^#test steel was more uniform and its corrosion resistance was stronger.The observation of rust layer section and XRD diagram showed that the density and stability of rust layer increased with the increase of rare earth content.In 4^#steel,the percentage of?-Fe OOH in the rust layer was the highest,which was most conducive to the formation of a dense and stable rust layer.The weightlessness test results showed that with the increase of rare earth addition,the average mass loss and the corrosion rate of the test steel decreased,and the corrosion resistance of the steel increased.The 4^#steel which was deoxidized by 0.02%Ce shows the best corrosion resistance during the long immersion corrosion process.