Fabrication of Type 304L Containing Mo-rich Areas and Corrosion Behavior

Abstract

Stainless steels are widely used because of their high corrosion resistance. Mo is main alloying elements that can improve pitting corrosion resistance. Although the role of Mo has been investigated for a long time, there is no unified theory about it.In stainless steels, Mo is added as solid solution. However, in this study, we tried to fabricate a type 304L stainless steel containing Mo-rich areas, and effects of Mo-rich areas on corrosion resistance was investigated1). For fabrication of samples, spark plasma sintering (SPS) was used. The effect of heat-treatments after sintering was also investigated.Type 304L stainless steel containing Mo-rich areas was fabricated by sintering mixed powders of type 304L gas atomized powders and 2.5mass% of Mo powders. Sintering temperature was 1100℃, and it kept for 20 min. After sintering, the sample was heat-treated at 1300℃ for 5 h and water-quenched. After that, a solution-treatment was conducted at 1100℃ for 30 min and water-quenched. The specimen surface was polished down to 1 μm using a diamond paste. Sintered type 304L and type 316L were used as reference materials. The same heat-treatment of the Type 304L stainless steel containing Mo-rich areas was applied to those specimens. After sintering, the same heat-treatment of the heat-treatment of the Type 304L stainless steel containing Mo-rich areas was performed. In addition, type 304L and type 316L sheets were used.Surface observation of the samples were conducted using an optical microscope, a scanning electron microscope (SEM), a transmission electron microscope (TEM). Electron probe micro analyzer (EPMA) analysis of surface was also conducted.From the observation of the sample surface of the Type 304L stainless steel containing Mo-rich areas, it was found that the sintered samples contained many voids. Mo-rich areas were also observed. In addition to Mo, Cr was found to be rich in the Mo-rich areas.To assess the corrosion resistance, potentiodynamic polarization curves were measured in 0.1 M NaCl (pH 6.0). Specimens were covered by epoxy resin except for the electrode area (5 mm × 5 mm).Polarization curves of sintered samples did not show stable value because of the presence of voids. During the polarization measurements of Type 304L (SPS) and type 316L (SPS), current density increased with increasing potential, and no passivation occurred. On the other hand, in the case of type 304L containing Mo-rich areas (SPS), re-passivation occurred at 0.3 V. After that, current density increased at 0.83 V because of pitting growth. Type 304L containing Mo-rich areas (SPS) showed higher corrosion resistance than the other samples.The effect of heat-treatments after sintering was also investigated. Type 304L powders and Mo powders (2.5 mass%) were mixed and sintered. After sintering, four different heat-treated samples were prepared.Polarization measurements of each sample were performed. Passivation occurred in the polarization of the sample heat-treated at high temperature, and it showed the highest corrosion resistance of the four samples.Reference; Saito, I. Muto and Y. Sugawara, Mater.Trans., 61 (2020), 2248-2251