Corrosion Behavior Of High Nitrogen Austenitic Stainless Steels

A great attention has been paid to high nitrogen austenitic stainless steels, due to their excellent mechanical properties and good corrosion resistance.The research of corrosion is a key to the development of high-nitrogen austenitic stainless steels.This study is a part of major project, "Mechanism of nitrogen in high nitrogen steels and its effect on properties" financed by National Natural Science Foundation of China and Shanghai BaoSteel Corporation. The high nitrogen austenitic stainless steels have been manufactured by the vacumm induction melting combined with the electrosalg remelting under nitrogen atmosphere. In this study, the uniform corrosion, pitting corrsoion, crevice corrosion and intergranular corrosion of the steels have been investigated by the methods of immersion, anodic polarization and metallographic microscope and SEM.The uniform corrosion of A1, A2 and A3 had been studied by the method of immersion in 40% H2SO4 at 60℃, compared with 316L stainless steel. The results show that, the 316L stainless steel have more excellent uniform corrosion resistance than high nitrogen austenitic stainless steels; The uniform corrosion resistance of A2 steel is better than that of A1 and A3; The high nitrogen stainless steels are not suitable to be used in the solution of 40% H2SO4 at 60℃.The semiconducting properties of passive films of A1, A2 and A3 high nitrogen stainless steels in 3.5wt% NaCl solution were investigated by electrovchemical impendace spectroscopy (EIS). Based on the capacitance results, the semiconducting parameters can be obtained from the Mott-Schottky plots. The flat-band potentials of A1, A2 and A3 are-0.4573V,-0.4686V and-0.4804V respectively. When the potential is lower than this potential, the plots indicate p-type semiconducting behavior. While above this potential, the plots indicate n-type semiconducting behavior. The donor and acceptor density (ND and NA) as well as the thickness of the space-charge layer (W) decrease with the increasing of nitrogen contents in steels.The structure and chemical composition of passive films of A1, A2 and A3 high-nitrogen stainless steels were investigated using X-ray photoelectron spectroscopy (XPS). The outer oxidization layer of the passive flims is mainly composed ofα-Fe2O3 and Fe3O4. While the inner oxidization layer is composed ofα-Fe2O3, Fe3O4, Cr2O3, MnO. The nitrogen is enriched on the out layer in the form of CrN, NH3, N(atom), NH4+and N2(free). The synergy effect of molybdenum and nitrogen modifies the passive flim, which impoves the pitting resistance of the high nitrogen stainless steels.The pitting corrosion resistance of high nitrogen austenitic stainless steels was investigated by the anodic polarization method in NaCl solution. The results show that the high nitrogen austenitic stainless steels have better pitting corrosion resistance compared with 316L stainless steel. With the increasing of nitrogen contents in steels, the breakdown potential and critical pitting corrosion temperature increase wich indicates that nitrogen greatly improves the pitting corrosion resistance. With the increasing of NaCl concentration and the decreasing of the pH in 3.5% NaCl solution, the breakdown potential Eb10 of A2 decreases slightly. The results indicate that the A2 steel has excellent pitting corrosion resistance in strong acid and high concentration chloride ion solution.The crecive corrosion of high nitrogen austenitic was studied used by immersion test in 6% FeCl3+0.05 mol/L HCl solution at 35℃. The results shows that the A1, A2, and A3 high nitrogen austenitic stainless steels exhibite excellent crevice corrosion resistance than 316L stainless steel. With increasing nitrogen contents in steel, the crevice corrosion resistance of the steel is improved.The 2mol/LH2SO4+1mol/LNaCl+0.01mol/LKSCN of EPR solution can be used to check the intergranular corrosion sensibility of Fe-Cr-Mn-Mo-N series steels, but is not suitable for 316L stainless steel. With the decreasing of the scan rates, the Ir/Ia and Qr/Qa increase. The suitable scan rate is 1.6667mV/s. When the A2 heat treated during the temperature range from 650℃to 950℃for 2h follows by air cooling, the amount of precipitation is the maximum at 850℃, and the intergranular corrosion sensibility is also the maximum. With the temperature changing from 650℃to 850℃, the amount of precipitation increases as well as the intergranluar corrosion sensibility. While the temperature higher than 850℃, the amount of precipitation decreases with the increasing temperature and which is consistant with the intergranluar corrosion sensibility. The amount of precipitation of A3 steel heated at 700℃for 2h followed by air cooling is the most, and its intergranular corrosion sensibility is the maximum. While the intergranular corrosion sensibility of A2 steel is the minimum.