| Stainless steel was widely used because of its excellent corrosion resistance.Non-metallic inclusions in steels have a significant detrimental effect on Corrosion resistance.As a kind of local corrosion of stainless steel,pitting corrosion has the characteristics of high destruction and concealment.Therefore,how to reduce or eliminate the pitting damage caused by inclusions has always been a research hotspot.Inclusions were generally used as the starting point of pitting initiation,which will further expand later,resulting in larger and larger pitting pits,fracture under the action of external force,and finally material failure.Therefore,the research on the influence of inclusions on the pitting corrosion resistance of stainless steel was of far-reaching significance.By studying the modification effect of Rare Earth La on inclusions in stainless steel,this paper expounds the mechanism of rare earth elements improving the pitting corrosion performance of stainless steel,which has high theoretical and industrial engineering significance.Thirteen distinct 304 stainless steels with varying S and La contents in steel were prepared using laboratory experiments.The effect of La on the composition,shape,size,quantity,and area fraction of inclusions in the steel was evaluated using a scanning electron microscope（SEM）and an energy dispersive spectrum（EDS）.Through corrosion weight loss experiments and electrochemical experiments,the effect of rare earth La on the pitting corrosion resistance of 304 stainless steels was investigated.The optimal addition amount of La was discovered,and the relationship between the optimal addition amount and S content was investigated.Immersion corrosion tests and scanning electron microscopy were used to investigate the corrosion process of MnS inclusions,oxide inclusions,and rare earth inclusions,as well as the corrosion resistance and mechanism of various inclusions.Inclusions in stainless steels with varying La contents were analyzed,and it was found that:1)Inclusions of MnS and Mn-Al-Si-O were the most common inclusions in 304stainless steels.2)When La was added,the number density of MnS inclusions decreased.For the stainless steel with the sulfur content of 80 ppm,the number density of MnS inclusions decreased from 49 to 18#/mm2with the content of La increasing from 0 to 230ppm.3)The transformation process of oxide inclusions was Mn-Al-Si-O→La-Al-Si-O→La-Al-O→La-O-S.Inclusions of La-O-S was converted to La-S inclusions in the stainless steel with the sulfur content of 80 ppm and the lanthanum content of 230 ppm.4)The number density and area fraction of inclusions increased with the increase of rare earth content,while the average size firstly decreased and then increased.Through electrochemical experiments,it was found that:1)The pitting corrosion resistance of stainless steels was able to be considerably improved with an optimal quantity of rare earth La addition.When the content of sulfur in the steel was about 80 ppm,the pitting potential of the sample with La content of 84 ppm was the maximum value of0.208VSCE.2)The optimal content of rare earth La was proportional to the content of S in the stainless steel.The pitting corrosion resistance of stainless steels reached the best value when the value of La/S was approximately 1.3)The constant voltage polarization curve showed that when La was added to stainless steels,the peak value increased,and the number of peak values increased with the increase of La content,while the metastable pitting resistance of the stainless steel decreased.4)Electrochemical impedance spectroscopy showed that the polarization resistance of stainless steel reached the maximum value when the value of La/S was approximately 1.Immersion corrosion experiments revealed the following conclusions:1)Pitting corrosion preferentially occurred at the interface between inclusions and the matrix.2)MnS inclusions were easily dissolved,thus forming gaps at the interface,which further promoted the occurrence of pitting corrosion.Composite inclusions of Ti-Mn-O and Al-Si-O were hardly dissolved.3)Rare earth oxide inclusions were difficult to dissolve,thus the breakdown of the matrix promoted pitting corrosion,whereas rare earth sulfides and rare earth oxysulfides were dissolved together with the matrix at early stages of pitting corrosion.4)The pitting corrosion resistance of various types of inclusions was La-Al-O>La-Al-Si-O>（Ti-Mn-O and Al-Si-O）>La-Si-O>（La-O-S≈La-S）>MnS through analyzing the dissolving process of various types of inclusions. |
