Artificial MnS Inclusions in Stainless Steel: Fabrication by Spark Plasma Sintering and Corrosion Evaluation by Microelectrochemical Measurements

Abstract

Spark plasma sintering was used to fabricate type 304L stainless steel specimens containing artificial manganese sulfide (MnS) inclusions, and a microelectrochemical technique was used to characterize the pit initiation behavior at the MnS. A 200 μm square electrode area that included an artificial MnS particle was potentiodynamically polarized in 0.1 M NaCl, and the electrode surface was observed in situ by optical microscopy. The anodic dissolution of the MnS particle was observed in the passive region of the stainless steel. The pit occurred at the boundary between the particle and the steel matrix after the particle dissolved slightly. The dissolution potential and pit initiation behavior at the artificial MnS particles in the sintered stainless steel were confirmed to be similar to those at MnS inclusions in commercial stainless steels.

1. Introduction

To clarify the role of alloying elements in the pitting corrosion resistance of stainless steels, elucidating the electrochemical properties of sulfide inclusions is important because the inclusions readily act as initiation sites of pitting.¹⁾ The relationship between the chemical composition and the electrochemical properties of the inclusions is a topic of interest in corrosion-resistant alloy design.^2,3,4,5,6) A systematic investigation of inclusion modification by alloying necessitates a facile fabrication method of steel specimens containing artificial sulfide inclusions. However, research in this area has been very limited. Williams and Zhu fabricated stainless steel specimens with artificial inclusions by melting and solidifying sulfide powders in a cavity (1 mm diameter) drilled in a stainless steel rod.⁷⁾ However, the artificial inclusions were much larger than the actual inclusions in commercial stainless steels.

In the present study, spark plasma sintering was used to fabricate stainless steel specimens containing artificial MnS inclusions; pit initiation process at the artificial MnS inclusions was compared with that of sulfide inclusions in commercial type 304 stainless steels using a microelectrochemical technique.⁸⁾

2. Experimental

Gas-atomized type 304L stainless steel powder (Kojundo Chemical Lab.) and MnS powder (99.9% purity, Sigma-Aldrich) were used as starting materials. The particle sizes of the stainless steel and MnS powders were less than 105 μm and 44 μm, respectively. The chemical composition of the gas-atomized stainless steel powder is listed in Table 1. The stainless steel powder was mixed with the MnS powder to prepare sintered specimens containing 0.06 mass% S. The mixed powders were loaded into a cylindrical graphite die and subsequently compressed using a hydraulic press. The pressed compact was sintered at 1373 K for 600 s under vacuum and under 30 MPa uniaxial pressure in a spark plasma sintering system (LABOX-110, Sinter Land). The density of the sintered specimen was 7.7 g/cm³. After the sintering process, the surfaces of the sintered specimens were ground with SiC papers and polished with a 1 μm diamond paste. The specimens were finally rinsed with ethanol.

Table 1. Chemical composition of stainless steel powder (mass%).

C	Si	Mn	P	S	Ni	Cr	Mo	Cu	Ti	Al	Nb
0.01	0.22	0.17	0.012	0.0025	11.1	19.0	0.01	0.01	0.001	0.002	0.007

Potentiodynamic anodic polarization was conducted in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The specimen surface was masked to make an electrode area of ca. 200 μm × 200 μm containing a single sulfide particle. All of the potentials reported in this paper refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). The potential scan rate was 3.8 × 10⁻⁴ V/s. During polarization, the electrode area was observed using an in situ real-time optical microscopy system.⁸⁾

An optical microscope and a field-emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system were used to capture images of the specimen surfaces before and after polarization. The secondary electron images and the EDS maps were collected at an accelerating voltage of 20 kV.

3. Results and Discussion

Figure 1 shows an optical micrograph of the as-polished surface of the sintered specimens. The large gray particle at the center of the image is thought to be a sulfide particle produced by the MnS powder. An area of each particle was varied from approximately 100 μm² to 3000 μm². The distribution of the large particles on the specimen surfaces was 3.9 particles/mm². Besides, small gray and black particles (less than 5 μm in diameter) were observed on the entire surface, which appear to be sulfide and/or oxide inclusions. Figure 2 shows a SEM image and the corresponding EDS maps of a large particle. Table 2 shows the results of the EDS analysis at each point in Fig. 2. The particle was mainly composed of Mn and S even though it contained approximately 15 at.% of Cr. Chromium was likely diffused from the stainless steel powders during the sintering process. For simplicity, the large gray particles in the sintered specimens are referred to as MnS in this paper.

Fig. 1.

Optical micrograph of the as-polished specimen surface. (Online version in color.)

Fig. 2.

SEM image and EDS maps of the sulfide particle in the specimen. (Online version in color.)

Table 2. Relative composition (at.%) at points 1–3 in Fig. 2.

Point	Mn	S	Cr	O	Fe	Ni
1	35	49	16	<1	<1	<1
2	36	49	14	<1	<1	<1
3	35	49	15	<1	<1	<1

Potentiodynamic polarization was carried out to characterize the pit initiation behavior at the MnS particles in the sintered specimens. Figure 3 shows the polarization curve of a small electrode area that includes a MnS particle, as measured with the electrode immersed in 0.1 M NaCl (red curve). The area of the MnS particle was 1885 μm². The polarization curve of a small electrode area of a commercial type 304 stainless steel bar with MnS inclusions⁸⁾ is included as a reference (black curve). Polarization was started at −0.2 V and was stopped after the sharp increase in the current was measured. An anodic current density of ca. 1 × 10⁻² A/m², corresponding to the passive current density of stainless steels, was measured above ca. 0 V. The current increase above ca. 0.35 V was likely due to the anodic dissolution of the MnS particle. MnS inclusions in type 304 stainless steels dissolve above ca. 0.2 V and pits occur in the dissolution potential range of the inclusions.^{8,9,10,11,12)} The amount of dissolved MnS was calculated using Faraday’s law:   

$$d=\frac{q}{nF}\cdot \frac{M}{DS}$$

where d is the average depth of dissolved MnS (μm), q is the charge passed by MnS dissolution (C), n is the number of electrons involved in the MnS dissolution reaction, F is the Faraday constant (C/mol), M is the molar mass of MnS (g/mol), D is the density of MnS (g/cm³), and S is the surface area of MnS (μm²). The charge passed owing to MnS dissolution was calculated after subtracting the current on the steel matrix from the total current. The current on the steel matrix was estimated by the dashed line in Fig. 3. Four electrons are involved in the MnS dissolution reaction:¹³⁾   

$$\text{MnS}+\frac{3}{2}{\text{H}}_2\text{O}\to {\text{Mn}}^{2+}+\frac{1}{2}{\text{S}}_2{\text{O}}_3^{2-}+3{\text{H}}^{+}+4{\text{e}}^{-}$$

The molar mass and the density of MnS are 87.0 g mol⁻¹ and 4.0 g cm⁻³, respectively.¹⁴⁾ The charge passed by MnS dissolution was 7.3 × 10⁻⁶ C. The average depth of the dissolved MnS during polarization was calculated to be 0.22 μm.

Fig. 3.

Polarization curve of a small electrode area that includes a MnS particle in the sintered specimen, as measured in 0.1 M NaCl. The polarization curve of the specimen prepared from a stainless steel bar was quoted from our previous research:⁸⁾ reproduced with permission from J. Electrochem. Soc., 159 (2012), C341. Copyright 2012, The Electrochemical Society. (Online version in color.)

At 0.56 V, a sharp increase in the current density was recorded. This current increase is indicative of pit initiation. No metastable pitting events occurred in the sintered specimen, whereas a metastable pit was initiated at 0.41 V in the commercial steel specimen. Polarization was repeated three times on different specimens. In all cases, no metastable pit was observed even though a stable pit was initiated. It is likely that stable pitting tends to occur on the sintered specimens. The current density of the sintered specimen immediately before pit initiation (0.7 A/m²) was larger than that of the commercial stainless steel (0.14 A/m²). The difference in the current density was probably attributed to the area of MnS. In Fig. 3, the areas of the artificial MnS particle in the sintered specimen and the MnS inclusion in the commercial steel were 1885 and 122 μm², respectively. Figure 4 exhibits a SEM image of the MnS particle after polarization. The surface of the particle dissolved slightly, and a pit occurred at the boundary between the MnS particle and the steel matrix. The polarization curve and the pit initiation site of the sintered specimen were quite similar to those of commercial stainless steels.^{8,9,10,11,12)}

Fig. 4.

SEM image of the MnS particle after polarization.

To analyze the relationship between the polarization curve and the surface appearance on and around the particle, the electrode surface during polarization was observed in situ. Figure 5 displays optical micrographs of the electrode surface with the MnS particle during the polarization shown in Fig. 3. The surface of the particle was gray when polarization was started at −0.2 V. As evident in Figs. 5(a)–5(d), the particle surface did not substantially change until 0.3 V. The particle color changed as the current density gradually increased above 0.35 V (Figs. 5(e)–5(i)): the color of the particle changed to brown-gray at 0.4 V, dark-blue-gray at 0.45 V, and dark-gray above 0.5 V. Finally, the pit was formed at the MnS/steel boundary, coinciding with the sharp current increase observed in the polarization curve. The gradual increase in the current density above 0.35 V was attributed to the dissolution reaction of the MnS particle, and the sharp current increase was attributed to pit initiation.

Fig. 5.

Optical micrographs of the electrode surface with the MnS particle during the polarization shown in Fig. 3. (Online version in color.)

Figure 6 shows enlarged optical micrographs of the area surrounded by the dashed lines in Fig. 5(j). The electrode surface at 0.56 V (immediately before pit initiation) is shown in Fig. 6(a); this instant is defined as 0 s in the following analysis. The electrode potential changed by as little as 0.8 mV during the period from 0 to 2 s. As indicated by the arrow in Fig. 6(b), a small pit was generated at the MnS/steel boundary at 0.1 s. The pit expanded over time from 0.1 to 0.8 s. At 1.0 and 1.5 s, small holes suddenly appeared (marked by the arrows in Figs. 6(g) and 6(h)). This observation implies that the pitting grew inside the steel matrix. From 1.5 to 2.0 s, little or no change was observed in the pit mouth, whereas a large current density (ca. 80 A/m²) was measured continuously. No notable change was observed in any other area than the enlarged area. This result suggests that the pitting proceeded in the depth direction. The pit depth was estimated using Faraday’s law. The mean valance was assumed to be 2.19 for stoichiometric dissolution of type 304 stainless steel.¹⁵⁾ The mean atomic weight and the density were 55.8 g and 7.9 g/cm³, respectively.¹⁵⁾ The area of the pit mouth was 45 μm². The charge passed by steel dissolution was 42.4 × 10^–6 C. The average pit depth was calculated to be 31 μm. The pit initiation process at the artificial MnS particle is well correlated with that at MnS inclusions in commercial stainless steels.¹³⁾ Moreover, the dissolution potential and polarization behavior of the artificial MnS particles in the sintered specimens were very similar to those at the MnS inclusions in commercial stainless steels.^{8,9,10,11,12)} The combination of spark plasma sintering and microelectrochemical measurements is expected to provide an effortless and useful method to understand the electrochemical properties of not just the sulfide inclusions in stainless steel but also other non-metallic inclusions or second-phase particles in metals and alloys.

Fig. 6.

Enlarged optical micrographs of the area surrounded by the dashed lines in Fig. 5(j). (Online version in color.)

4. Conclusions

Stainless steel specimens with artificial MnS particles were fabricated by spark plasma sintering, and the pit initiation behavior was evaluated by microscale polarization measurements. The MnS particles dissolved slightly in the passive state of the stainless steel, and pitting occurred at the MnS/steel boundary. The dissolution potential and pit initiation behavior at the artificial MnS particles in the sintered specimens were confirmed to be similar to those at the MnS inclusions in commercial stainless steels.

Acknowledgments

This work was supported by JSPS KAKENHI (grant Nos. JP17H01331 and JP17J03625). This work was also supported by the Program for Leading Graduate Schools, “Interdepartmental Doctoral Degree Program for Multi-dimensional Materials Science Leaders, Tohoku University,” MEXT, Japan.

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