| As a result of no nickel in ferritic stainless steel(FSS),it's price is much lower and more stable than that of traditional austenitic stainless steel(ASS).More and more FSS is considered to replace 300 series ASS in some ASS applications.With the development of industry,recycling of scrap steel becomes more and more important,while getting rid of the common residual elements in steel,such as tin and copper ususlly increases the cost of recycling.But under some circumstance the addition of tin and copper could improve the performance of FSS.Through analyzing current researches,and using of the laboratory environment,this thesis tend to study Nb-Ti stabilization on ferritic stainless steel bearing tin and copper and its corrosion resistance.FSS with 0.25%Sn and 0.2%Cu was used as matrix.Throngh changing the C,N contents and the ratio of Nb/Ti,the inclusion,grain size,room temperature impact,pitting corrosion,intergranular corrosion and the passivation film of the FSS were studied.Inclusion analysis results showed that the addition of tin and copper decreased the size of inclusions.And after stabilization,large size inclusions in FSS with high content of carbon and nitrogen would increase.While in low carbon and nitrogen FSS,the size and number of inclusions increases with the decreasing of Nb/Ti.And the content of Nb and Ti in composite inclusions was positively related to the variation of this ratio.The microstructure showed that tin and copper bearing FSS had smaller grain size.And Nb-Ti stabilization could lead to further grain refinement.With the increasing proportion of Nb/Ti added in steel,the grain refinement effect would be more significant,and the size distribution became more uniform.However,in the range of this experiment the impact toughness of the steel would be reduced by adding tin,copper,niobium and titanium.The polarization curves showed that the addition of tin and copper in low C,N FSS significantly increased the pitting potential,while after Nb-Ti stabilization the pitting potential increased at first and then decreased.The maximum pitting potential was 285.4 mV when the value of Nb/Ti was 3.However,in high C,N FSS,adding tin and copper barely had no influence on pitting potential while Nb-Ti stabilization would reduce the pitting potential.Electrochemical impedance spectrum(EIS)tests showed that after adding tin and copper,passivation film impedance of ferritic stainless steel would increase,and based on this,the impedance of passivation film reach the peek at ratio of Nb/Ti=3 stabilization.The pitting corrosion resistance of Nb-Ti stabilized FSS bearing tin and copper decreased with the increase of temperature and concentration of chloride,and increased together with the increase of pH.One of proper solution to test the intergranular corrosion sensitivity of experiment steel is 0.25 mol·L-1 H2SO4+0.01 mol L-1 KSCN.The results showed that tin and copper could improve the intergranular corrosion of FSS to a certain extent when the content of C,N was high in steel.Titanium and niobium addition could significantly reduce steel intergranular corrosion sensitivity.Within the scope of this study,single niobium stabilization in low carbon and nitrogen ferritic stainless steel bearing tin and copper showed the lowest intergranular corrosion sensitivity,which means it had the highest intergranular corrosion resistance.XPS results showed that niobium oxide and tin oxide were found in the passivation film,which contributes to the stability of passivation film. |
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