Corrosion Resistance Of 439M Ferritic Non - Induced Steel And Its Influencing Factors

439M ferritic stainless steel is a new type of ferritic stainless steel which was studied in this paper. The 439M should decrease the gap elements of C and N content by refined and added a certain amount of Ti and Nb carbonitride formation elements on the basis of traditional ferritic stainless steel. Also the 439M ferritic stainless steel was according to the condition of oils in our country and then developed. This product is mainly used in automobile exhaust muffler system and the pitting corrosion damage is the main failure form. Many studies have shown that the sulfide inclusion is the main sources of pitting corrosion. However, the sulfur grade has dropped to a very low level in stainless steels recently, with the improvement of desulfurization technology and there is few MnS in 439M. Therefore, the influences of inclusions, such as oxides and nitrides, on the corrosion resistance of 439M stainless steel have become more and more important.The pitting corrosion resistance and crevice corrosion resistance of 439M ferritic stainless steel using Ti as single stabilizing elements and Ti, Nb as double stabilizing elements was studied by electrochemical testing system. Also the influence of inclusion, alloy elements and tensile deformation on the corrosion resistance was studied. In addition, the induction and propagation of pitting corrosion was studied by micro-area electrochemical.It is known that the inclusion which may exist in steel and the growth trend of the inclusion through thermodynamic calculation on the formation of inclusion in 439M ferritic stainless steel. Inclusions in steel were analysised by optical microscope (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The non-metallic inclusion (NMI) in the Ti single stabilization steel was mainly TiN. However, the dominant NMI in the Ti and Nb double stabilization steel was TiN or NbN. The other NMIs were Ti(Nb)N、Al2O3、MgO·Al2O3、TiOx、CaO and complex inclusions in these three steels. Complex inclusions based on (Mg, Al, Ti)-oxides as the nucleation core, periphery wrapped in CaO or (Ti, Nb)-nitrides. The grain elongated increases accordingly with the increase of tensile deformation degree and plastic inclusions will occur in the corresponding deformation, and brittle inclusions will occur rupture. The crack or hole may occur in the interface between inclusion and matrix after deformation.The research results of pitting corrosion resistance in 439M ferritic stainless steel show that Cr is the most effective element to improve the pitting corrosion resistance. The pitting corrosion resistance of Ti, Nb double stabilization stainless steel is better than Ti single stabilization of stainless steel. The addition of Nb can improve the pitting potential of 439M ferritic stainless steel. The pitting corrosion resistance of 439M ferritic stainless steel is gradually decreased when the tensile deformation increased from 5% to 35%. The pitting corrosion was found to be initiated at the composite oxide inclusion. The research results of crevice corrosion resistance in 439M ferritic stainless steel show that Cr have greatly influence on crevice corrosion resistance and Nb have few influence on crevice corrosion resistance. When the deformation reach to 35%, crevice corrosion resistance of 439M ferritic stainless steel decreased sharply.The results of micro-area electrochemical showed that the CaO part in composite oxide inclusions is preferential to dissolve. As pitting continue, hydrolysis acidification enhancement makes pH decrease and composite oxide inclusions containing (Al, Mg)-oxides and matrix will be dissolved. Finally, the model of the initiation process and pitting propagation of the composite oxide inclusion-induced pitting corrosion in 439M ferritic stainless steel exposed to chloride ions environments was established.