Corrosion Resistance of Stainless Steel Containing Mo-Rich Areas

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), and the transpassive dissolution of pure Mo occurs in the passive region of stainless steels. In this study, a stainless steel that releases Mo-species in chloride solutions and exhibits self-healing properties was fabricated by spark plasma sintering (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 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 specimens. From the observations, the formation of Mo-rich areas was confirmed, and the Mo-rich area was found to be consisted of three phases: inner, middle, and outer phases. Inner phase was rich in Mo, and middle and outer phases contains other elements such as Cr, Ni, and Fe.To assess the corrosion resistance, potentiodynamic polarization curves were measured in 0.1 M NaCl (pH 6.0). The size of electrode area was 300 µm × 300 µm or 5 mm × 5 mm. The stainless steel with Mo-rich areas showed higher pitting corrosion resistance compared with sintered type 316L stainless steel fabricated by SPS. After polarization, inner phase of Mo-rich area dissolved, while middle and outer phase did not dissolve. Pitting corrosion resistance was concluded to be improved by dissolved Mo-species from Mo-rich areas.References; Saito, I. Muto, and Y. Sugawara, Mater. Trans., 61 (2020), 2248-2251. Saito, I. Muto, M. Nishimoto, and Y. Sugawara, Mater. Today Commun., 33 (2022) 104211.