Effect of Plasma Carburizing Treatment on Pitting Corrosion Resistance of Type 304 Stainless Steel

Abstract

Stainless steels suffer from pitting in corrosive environment. Pitting corrosion resistance of stainless steels is known to be improved by carburizing at low temperatures.1,2)Plasma carburizing is suitable for stainless steel, since it is possible to keep low temperatures during the treatments to prevent sensitization. In this study, we analyzed the effect of plasma carburizing treatments on the pitting corrosion resistance of type 304 stainless steel. Specimen used was Type 304 stainless steel. Chemical composition in mass% was C: 0.036, Si:0.61, Mn:0.82, Cr:17.92 and Ni:7.93. The specimen was heat-treated at 1373 K and quenched in water. Before plasma carburizing, the surface of specimen was polished using SiC grinding paper and 1μm diamond paste. Plasma carburizing was carried out at 573 K or 743 K under the atmosphere of CH4:H2:Ar = 1.5:13.5:15 (sccm). The total pressure was kept at 550 Pa (at 573 K) or 650 Pa (at 743 K). The analysis of lattice parameters and phases was conducted using X-ray diffraction (XRD). The surface composition was analyzed by grow discharge optical emission spectroscopy (GD-OES). A scanning transmission electron microscope (STEM) equipped with an X-ray energy dispersive spectroscopy (EDS) system was used to identify the microstructure of the carburized specimen. In the electrochemical measurement, the surface of specimen was coated using epoxy resin and paraffin without the electrode area (ca. 0.05 mm2 or 100 mm2). The counter electrode was a Pt plate. The reference electrode was an Ag/AgCl electrode (3.33 M KCl). Anodic polarization measurements were conducted in 0.1 M NaCl (298 K).The potential scan rate was 23 mV min-1. After polarization, the specimen surface was observed using confocal laser scanning microscope and a field emission scanning electron microscope (FE-SEM). Figure 1a shows the XRD patterns of the carburized and untreated specimens. The lattice parameter of the austenitic phase was changed from 0.3599 nm to 0.3688 nm. And also, after the carburization, the austenitic phase was partly transformed into martensite. Figure 1b shows the GD-OES depth profile of the relative concentration of C, Ni, Cr and O in the specimen treated at 743 K. The thickness of the carburized layer was determined to be around 16 μm. The average of relative concentration of C was 2.5 mass%. Figure 1c shows the anodic polarization curves of specimens. In the case of untreated specimen, pit was initiated around 0.55 V, but the large current increases were observed at lower potentials for the carburized specimens. Figure 1d shows the surface appearance of the electrode area after the polarization. The intergranular corrosion occurred on the surface of the carburized stainless steel at 743 K. From STEM observation, Cr23C6 was precipitated after the plasma carburizing treatments. References 1) F.J.Martin, E.J.Lemieux, T.M.Newbauer, R.A.Bayles, P.M.Natishan, H.Kahn, G.M.Michal, F.Ernst, and A.H.Heuer, Electrochem.Solid-State Lett,10,C37(2007) 2) A.Chiba, S.Shibukawa, I.Muto, T.Doi, K.Kawano, Y.Sugawara, and N.Hara, J. Electrochem. Soc, 162,C270-C278(2015) Figure 1