Study On The Corrosion Behavior Induced By Typical Inclusions In 304 Stainless Steel

High quality steel is required to have good strength and toughness.It also requires excellent corrosion resistance.As we all know,inclusion in steel is the important factor to affect the corrosion resistance of steel.And the type,quantity,size and distribution of inclusions have serious impact on corrosion resistance of steel.Rare earth is known as a "flavor" of the iron and steel industry.Adding rare earth into the steel could modify the morphology and size of inclusions so that it can improve the properties of steel.So,it can be seen that the modification of inclusions is very important for steel to improve its corrosion resistance.This paper adopted electrochemical experiment,immersion corrosion experiment to study the change of morphology and size of MnS inclusions and Al2O3 inclusions during the immersion corrosion.And also,it explored the mechanism of corrosion dissolution of steel matrix.In experiment,the type and distribution of inclusions were analyzed by SEM-EDS.And through immersion corrosion,it analyzed the dissolution of inclusions and steel matrix before and after corrosion in 304 stainless steel and concluded the mechanism of different types of inclusion inducing pitting.Additionally,it also studied the change rules of inclusions treated by earth rare through adding the Ce and Y into the steel.And using electrochemical and immersion experiment,the effect of different content of rare earth on the change of inclusion and on the corrosion behaviors was analyzed in 304 stainless steel.The results of MnS and Al2O3 inclusions inducing pitting corrosion drew four conclusions.1)the sensitivity of corrosion of steel was changed with the average size of inclusions and also the size had a critical value.If the average size of MnS inclusions greater or less than the critical value,the sensitivity of corrosion would be increased.2)The effect of MnS inclusions on the grain boundary is greater than that of MnS inclusions in the grain.The micro cracks around the clustered MnS inclusions were connected each other and it affected the corrosion.The single distribution of MnS inclusions also had effect on pitting corrosion.And the clustered distribution of MnS inclusions had a greater effect on corrosion than the MnS inclusions distributed alone.The MnS inclusion which was buried deeply induced corrosion into steel matrix.The influence caused by MnS inclusions which was buried deeply was greater.3)the clustered distribution of Al2O3 inclusions had a greater effect on corrosion than the Al2O3 inclusions distributedalone.The Al2O3 inclusions which were buried shallowly appeared with the dissolution of steel matrix.However,the micro cracks caused by Al2O3 were smaller than MnS inclusions.And the effect of MnS inclusions on corrosion was greater than that of Al2O3 inclusions.The results of the behaviors of inclusions inducing corrosion indicated that:1)the inclusions in 304 stainless steel included MnS inclusions,hybrid oxides inclusions and oxide-sulfide hybrid inclusions.2)the type of corrosion in 304 stainless steel included pitting corrosion and intergranular corrosion.And pitting corrosion was mainly induced by inclusions.And MnS inclusions had the most obvious effect on pitting corrosion.And intergranular corrosion was mainly caused by intergranular embrittlement and the intergranular embrittlement was caused by inclusions on the grain boundary.The intergranular corrosion was more serious than pitting corrosion.For the experiment of rare earth modifying the inclusions and further influencing the corrosion,the results indicated that:1)the main inclusions in 304 stainless steel was MnS inclusions and hybrid oxides inclusions.And the addition of Ce was 0.012%,all of the inclusions was modified into globular inclusion containing Ce.And the size of modified inclusions was the smallest.If the addition of Ce was further increased,the inclusions were modified into irregular shape inclusions containing Ce.And the size was a little increased.2)Rare earth Ce could improve the corrosion resistance in stainless steel.And the addition of Ce was 0.012%,the corrosion resistance was best.3)with the content of Y increasing,hybrid oxides inclusions and MnS inclusions were modified into hybrid oxides inclusions containing Y.And the size of inclusions was decreased.When the addition of Y was 0.013%,the inclusions were modified into Y2O3.And the average size was smallest.If further adding the content of Y,it formed the YN inclusions which the chemical property was unstable.And the average size of inclusions was increased.4)When the addition of Y was 0.007%and 0.049%,the corrosion potential was lowest.And the effect of content of Y on inclusion inducing corrosion was greater and the corrosion resistance was bad.When the addition of Y was 0.013%,the corrosion potential was highest.And the corrosion resistance of not adding Ce and addition 0.019%Ce was neither too good nor too bad.The corrosion resistance of adding 0.019%Y was better than that of none Y addition.