(Digital Presentation) Effects of Secondary Phases in Stainless Steel on Corrosion Resistance

Abstract

Stainless steels are widely used because of their high corrosion resistance. It is well known that Mo improves the pitting corrosion resistance of stainless steels1, 2).Mo dissolves at the passive region of stainless steels because of transpassive dissolution. In this study, we tried to provide dissolved species of Mo into the solution by utilizing transpassive dissolution of Mo. The type 304L stainless steel containing Mo phases was fabricated and corrosion resistance was measured. For fabrication of samples, Spark Plasma Sintering (SPS) was used because non-equilibrium microstructure can be obtained by short sintering time and low temperature.Type 304L stainless steel containing Mo phases was fabricated by SPS of type 304L gas atomized powder and 2.5 mass% of Mo powder. Sintering temperature was 1100℃ and it kept for 20 min. The specimen surface was polished down to 1 μm using a diamond paste. The same procedure was applied for fabricating sintered type 304L as comparison material.The as-polished and etched surfaces of the samples were observed by an optical microscope. A field emission scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy system was also used to observe the surface of the samples.From the observation of sample surface, it was found that the samples contained many voids. Mo phases were also observed in entire surface. In Mo phases, the concentration of Mo was more than 90 mass%, and other elements were hardly detected.To assess the corrosion resistance, potentiodynamic polarization curves were measured in 0.1 M NaCl (pH 6.0). Electrode area was set from 100 µm × 100 µm to 5 mm × 5 mm depend on the measurement. Specimens were covered by epoxy resin except electrode area.Polarization curve of the samples were unstable because of the presence of voids. In measurement of Type 304L (SPS), current density increased with increasing potential and no passivation was occurred. On the other hand, in the case of type 304L containing Mo phases (SPS), increase of dissolution currents was inhibited with dissolution of Mo phases. After polarization, the surfaces of the samples were observed. Mo phases were preferentially dissolved, but the possibility of improvement of pitting corrosion resistance by dissolved Mo species from Mo phases.If the effect of dissolved species of Mo can be obtained by Mo oxide was investigated. The sample was fabricated by sintering of mixed powder of 304L and MoO3 powder. After the polarization measurement, no pitting was observed. This indicate that corrosion resistance was improved.References; Saito, I. Muto and Y. Sugawara, Mater.Trans., 61 (2020), 2248-2251 Nishimoto, I. Muto, Y. Sugawara, N. Hara, J.Electrochem.Soc., 166 (11), C3081-C3089