Effect Of Bimodal Structure And Nanocrystalline On Pitting Corrosion Of 304 Austenitic Stainless Steel

Austenitic stainless steel(SS) has been used widely rafor its excellent general corrosion resistance and formability, but it is easily corroded in the solution containing aggressive anions(such as Cl-). The issue of pitting corrosion resistance of SS is more important especially for SS with the nanocrystalline and bimodal structure. In this paper, the behaviors and evolution rule of the pitting corrosion of SS with the bimodal and nanocrystalline structure was systematically studied and compared with that of theas-received SS with the coarse grain.Firstly, the mono-nanocrystalline SS with about 100 nm grain size was obtained by ECAP of SS with the coarse grain(50μm) for 8 passes. Then, the as-ECAPed SS with nanocrystalline microstructure was annealed at 750℃ for 30 min, which resulted in the bimodal microstructure including the ultrafine grains with a volume fraction of 62 % and an verange grain size of 350 nm and the coarser grains with a volume fraction of 38% and an average size of 1.4μm.Secondly, the pitting corrosion behavior of three SS with different grain size(the coarse grained structure the bimodal structure; the nanocrystalline structure) was systematically investigated by static immersion, potentiodynamic polarization curves, potentiostatic polarization curves, electrochemical impedance spectroscopy(EIS) and electrochemical noise(EN). Additionally, the influence of the nanocrystalline and bimodal structure on the pitting corrosion of SS was also analyzed emphatically. The results showed that the pitting corrosion resistance of the bimodal structure and nanocrystalline SS was better than that of the coarse grained SS, and the nanocrystalline SS was superior in the pitting corrosion resistance to the bimodal structure SS. The reason is that passive films formed on the bimodal structure and nanocrystalline SS are more compact than that formed on the coarse grained SS. In addition, the growth rate of pitting corrosion for the bimodal structure and nanocrystalline was much slower than that of the coarse grained SS,contribuating to the better pitting corrosion resistance of the formers.Meanwhile, the pitting morphologies were also studied. The diameter of pitting hole in the coarse grained SS was the biggest in the same immersion time and same corrosive medium, the pitting hole both longitudinally and horizontally in three-dimensional direction and ultimately the “gully” like morphology is formed on the SS. The pitting holes in the bimodal structured and nanocrystalline SS were smaller, and finally the “ridging” “layering” morphology was formed on both SS, respectively.EIS test showed that the impedance values of SS increased in the decreasing order of the grain size in the coarse, bimodal structured and nanocrystalline SS. The coarse grained SS came into pitting after immersion for 76 h in 3.5% Na Cl, however, this time is 120 h and 160 h for the bimodal structured and nanocrystalline SS, respectively. The results suggested that the nanocrystalline SS had more stable electrochemical properties, the bimodal structured SS is in the second position and the coarse grained SS was in the worst position.Finally, EN of three types of SSs were analysized including potentio EN and current density EN. The results showed that the nanocrystalline SS had the longest duration of passive period(120h) and stable pitting period(70h), and the corresponding value in terms of two preiods mentioned above respectively was 48 h and 12 h, which is smaller than that of NC SS. The corresponding values for the bimodal stracttured SS are in betweent that of the nanocrystalline SS and that of the coarse grained SS. Passive film formed on the surface of nanocrystalline SS obtained higher protection ability. When the passive film of SS was compromised, the growth rate of pitting corrosion of the bimodal structured and nanocrystalline SS was much slower than that of coarse grain after the passive film was destroyed.In a word, the mono-nanocrystalline SS exhibts better pitting corrosion resistance, the bimodal structured SS is in the second position, the coarse grained SS is in the last position. The causes why these phenomena occurs results from a fact that the compactness of the passive film on three SSs decreases in the increasing order of the grain size in three SSs.