Fabrication of Mo-Dispersed Type 304L Stainless Steel By Spark Plasma Sintering and Its Corrosion Resistance in 0.1 M NaCl
Authors: Saito Haruka , Muto Izumi , Sugawara Yu 
Published: December 2020

Stainless steels are widely used in industries because of their high corrosion resistance. The Cr-rich oxide film on the surface protects the balk metal. For type 316 stainless steel, 2~3 mass% Mo was added to improve corrosion resistance in chloride solutions. The pitting corrosion resistance of type 316 stainless steel is known to be higher than that of type 304 stainless steel1). The role of Mo has been investigated for long time. It was proposed that Mo forms Mo-oxides and inhibits corrosion attack2). In addition, it is suggested that Mo changes the structure of the Cr oxide layers on stainless steels 3).
Spark Plasma Sintering (SPS) has been applied to fabricate various types of composite materials. The advantage of the SPS technique is rapid heating and cooling 4). When two metal powders were mixed and sintered, dual-phase materials are expected to be formed because sintering is conducted below the melting points of the metals.
In this study, we fabricated Mo-dispersed type 304L stainless steel by spark plasma sintering, and the corrosion resistance was assessed in 0.1 M NaCl.
Mo-dispersed type 304L stainless steel was fabricated by SPS. As the starting materials, pure Mo powders and type 304L stainless steel powders were used. Table 1 shows the chemical composition of type 304L stainless powders. Sintering was performed at 1373 K for 20 min. After sintering, the specimen was heat-treated at 1573 K for 5 h in vacuum and quenched in water. After that, the solution treatment was conducted at 1373 K 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 SPS procedure and heart-treatments were applied to prepare those.
The surface of Mo-dispersed type 304L was inspected using an optical microscope. Scanning electron microscope (SEM) was also used. Electron Probe Micro Analyzer (EPMA) analysis of surface was conducted.
Potentiodynamic polarization curves were measured in 0.1 M NaCl solution (pH 6.0) at 298 K. Specimens were covered by epoxy resin, with the exception of the electrode area (5 mm × 5 mm).
On the basis of the surface observations and X-ray diffraction, it was suggested that the steel matrix of Mo-dispersed type 304L is austenite, but many voids were generated. Mo-rich phase were observed on the entire surface. The size of Mo-rich phase was 100 ~ 200 μm. From EPMA analysis, the average concentration of Mo in Mo-rich phase was determined to be 4.3 mass%. In the steel matrix, the average Mo concentration was 2.8 mass%.
In polarization measurements in 0.1 M NaCl, the passive current density of Mo-dispersed type 304L and type 316L was unstable, probably because of the presence of voids. However, the pitting potentials of Mo-dispersed type 304L were found to be higher than those of type 316L. The selective dissolution of Mo-rich phases was thought to prevent the initiation of pitting effectively in chloride environments.
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Figure 1