Corrosion-resistant sintered stainless steels with non-equilibrium Mo-rich phases

Abstract

As corrosion-resistant alloys with non-equilibrium metallurgical structures, stainless steels containing Mo-rich areas were fabricated by sintering Type 304 L and Mo powders. The effects of the heat-treatment conditions and Mo concentration on the pitting corrosion resistance were assessed, and an appropriate microstructure for achieving higher pitting corrosion resistance and the corrosion-resistant mechanism were determined. Heat treatment at high temperatures resulted in the formation of Mo-rich areas consisting of BCC (Mo- and Cr-enriched) and FCC (Ni-enriched) phases. As the heat-treatment temperature increased, the fraction of Mo-rich areas increased. The increase in the pitting potentials in 0.1 M NaCl with an increasing heat-treatment temperature indicated that the increase in the fraction of Mo-rich areas resulted in higher pitting corrosion resistance. Galvanostatic polarization indicated that the pitting initiation sites were located in the steel matrix. Polarization measurements of the model alloys of the BCC and FCC phases revealed that the BCC phase in the Mo-rich area functioned as a barrier against pit growth. The increase in the amount of added Mo resulted in an increase in the fraction of Mo-rich areas.