Effects of Nb and C Addition on Corrosion Resistance of Metal-Injection-Molded 440C Stainless Steel

The effects of Nb and C addition on the corrosion performance of metal-injection-molded 440C martensitic stainless steel were investigated. In addition to 440C and Nb440C samples, samples with different carbon contents were prepared from the Nb440C pre-alloyed powder mixed with graphite powders. The results show that Nb is uniformly distributed in the matrix in the form of carbides. This can reduce the activity of C in the matrix and inhibit grain growth. The corrosion resistance of all the samples was characterized by anodic polarization, electrochemical impedance spectroscopy, and double-loop electrochemical potentiokinetic reactivation analyses. The added Nb consumes C atoms in the matrix, allowing more Cr atoms to diffuse to the surface to form a passive film. As a result, resistance to pitting corrosion and intergranular corrosion is improved. However, increased carbon content negatively affects corrosion resistance.


Introduction
As a martensitic stainless steel, 440C stainless steel has high strength and hardness is widely used for transportation, machining, and in medical instruments [1][2][3]. However, stainless steel parts with small sizes and complex shapes are difficult to manufacture by traditional methods such as casting and machining. Hence, as a near-net-shape method, metal injection molding (MIM) is employed for the production of stainless steel parts [4]. of carbon and prevent decarburization. In addition, the formation of precipitate can refine the grain and help to increase the hardness and thermal stability of the steel [9][10][11]. At the same time, some researchers found that the addition of Nb in austenitic and ferritic stainless steels is beneficial as it stabilizes the passive film and enhances the corrosion resistance of the steel [12,13]. Dalmau, et al. [14] also found the same behavior in martensite. Jin, et al. [15] found that the interaction of Nb with C promotes the formation of a continuous film, which improves the corrosion resistance of low-alloy steel. These findings indicate that the addition of Nb has a positive effect on the corrosion resistance of stainless steel. While the addition of Nb and C into MIM of 4xx stainless steel is a popular topic [16][17][18][19], attention is generally focused on the densification and mechanical properties; the effects of Nb addition on the corrosion resistance of MIM440C stainless steel have rarely been reported. This paper reports the effects of Nb and C additions on the corrosion resistance of MIM440C stainless steel, with different carbon contents, and a preliminary analysis of the resistance mechanism.

Experimental Procedure
In this work, 440C and Nb440C pre-alloyed powders, supplied by Hunan Hengji Powder Technology Co. Ltd., were used. The chemical composition and particle size distributions of the powders are shown in Table 1. Graphite powders were added to the Nb440C stainless steel powders to obtain samples with different carbon contents: 0.15, 0.35, and 0.45 wt.%. The powders were mixed with the same wax-based binder, with a loading of 57 vt.%. The samples were prepared by injection molding after mixing and pelletizing. Solvent de-binding was performed by immersing the compacts in methylene chloride at 36 °C for 6 h. Then, thermal de-binding and pre-sintering were carried at 900 °C for 1 h in argon. Finally, the samples were sintered at 1340 °C for 2 h in a vacuum atmosphere of 1 × 10 -1 Pa. The metallographies of the sintered samples were observed using a Leica DM2700M metallographic microscope. Phase analyses of the sintered samples were performed by X-ray diffraction (XRD) using a RINT2000 vertical goniometer at a scanning rate of 10°/min. The phase compositions were determined by comparing with the standard card. The elemental distributions of the samples were determined using an electron field emission JXA-8530F electron probe microanalyzer. The particle size of the raw material powder was detected by a Micro plus laser diffraction particle size analyzer. The powder carbon content was measured using a CS600 carbon-sulfur analyzer, and the oxygen content was measured using a TCH600 oxygen-nitrogen-hydrogen analyzer. Tensile strength was measured using an Inston3369 mechanical testing machine at a tensile rate of 2.0 mm/min. Rectangular specimens with dimensions of 10 × 10 × 8 mm were cut from the sintered samples. After ultrasonic cleaning in alcohol, the 10 × 10 mm sides of the samples were exposed to the outside, while the other sides were connected to a copper wire. They were then embedded in epoxy resin, leaving an exposed working area of 10 × 10 mm. A CHI660E electrochemical workstation was used to characterize the corrosion properties of the materials. A conventional three-electrode cell was used, equipped with a saturated calomel electrode (SCE) as the reference electrode and a platinum plate as the counter electrode. Electrochemical impedance spectroscopy (EIS) tests were conducted in a 3.5 vol% NaCl solution. These were performed at the open-circuit potential, in the potentiostatic mode, with a voltage perturbation amplitude of 5 mV in the frequency range of 0.01-10 6 Hz. After this, the Tafel test was performed. The polarization curve scan started from -1 V to the anode, at a scan rate of 0.5 V/s, until the current density reached 500 mA/cm 2 . Meanwhile, double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted in a solution of 0.5 mol/L H 2 SO 4 + 0.01 mol/L KSCN. The samples were kept immersed in the test solution for 0.5 h at the open-circuit potential. After obtaining a stable E corr , the potential was raised by 0.1 mV/s. A reactivation scan followed the

Microstructure and mechanical properties
The carbon content, density, and average grain size of the sintered samples are shown in Table 2. With increasing initial carbon content, the carbon content of the sintered samples also increased. attainment of the predetermined potential (300 mV), returning to E corr . The intersection of a straight line with the curve, generated by the application of a current of 10 -4 A to each anodic polarization, was used as the pitting potential (E b ) of the samples. All the potentials were referred from the SCE. All the tests were performed at 20 °C.   increasing carbon content in Nb440C, the tensile strength decreased remarkably. The higher the carbon content, the more NbC second phases precipitated in the matrix. The NbC precipitates are the origins of cracks during deformation and so decrease the tensile strength of the material. Moreover, a smaller grain size of Nb440C can promote strength.

Pitting resistance
The anodic polarization curves of the different samples in the 3.5 vol% NaCl solution are shown in Figure 5. The curves have no obvious passivation zone, and the corrosion current density increases rapidly after the dynamic potential sweep through the corrosion potential. Table 3 presents the parameters of the anodic polarization curves of the different samples; Nb440C had a lower I corr and higher E corr and E b than the other samples. When E b is higher, pitting resistance improves. Thus, the results show that Nb440C has good corrosion resistance. With increasing carbon content, the I corr of the samples increased, while the E corr and E b decreased and corrosion resistance was reduced. Table 2 shows that the carbon content of 440C and Nb440C are similar; hence, the effects of the carbon content on corrosion resistance may be ignored. It is evident, therefore, that the addition of Nb improves the pitting resistance of 440C stainless steel.
Moreover, the samples with Nb exhibited a lower decarburization rate during sintering. Because Nb can form carbides with carbon and retard the diffusion of carbon, the carbon atoms become more difficult to shift to the surface of the sample for reaction with the atmosphere [8]. The densities of the samples reached much the same level, and densification was almost completed (> 97%), as shown in Table 2; therefore, the effects of density on corrosion resistance may be ignored.
The microstructures of the different samples are shown in Figure 1. The addition of Nb resulted in a smaller grain size and more carbide precipitation in the grain. The XRD results of Nb440C and 440C are shown in Figure 2. The addition of Nb did not change the main phase composition of the materials, yet NbC peaks were observed. The elemental distributions of Nb440C are shown in Figure 3. Most of the NbC precipitates were distributed on the grain boundaries. During sintering, these particles would inhibit grain growth. Consequently, Nb440C had a smaller grain size, as shown in Table  2.
The mechanical properties are shown in Figure  4. The hardness was similar for all samples. However, the tensile strength was significantly improved with the addition of Nb. On the other hand, with  primarily in the form of carbonitrides, in the matrix [21]. However, the nitrogen content of 440C steel is extremely low, and it mainly exists in the form of carbides. The presence of Nb-containing carbides is evident in Figure 2 and Figure 3. The formation of these carbides consumes the C atoms in the stainless steel and thus reduces the Cr content held in carbides. In this way, more Cr can be used to form the passive film, which in turn improves the pitting resistance. Additional carbon content may offset this tendency. Figure 7 shows the DL-EPR curves of different samples in the 0.5 mol/L H 2 SO 4 + 0.01 mol/L KSCN solution. The reactivation current density (I r ) and Figure 6 presents the impedance spectra in the form of Nyquist curves of the samples in the 3.5 vol% NaCl solution. Table 4 shows the EIS data of the samples. It can be seen that the Nb440C sample has a larger diameter arc than the 440C sample. This indicates that Nb440C has a larger charge transfer resistance, but this gradually decreases with increasing carbon content. Thus, the addition of Nb increases corrosion resistance. However, addition of carbon has a negative effect on resistance.

Resistance to intergranular corrosion
The pitting resistance of stainless steel is mainly influenced by a passive film, formed by Cr 2 O 3 , with a thickness of ten to hundreds of nanometers [20]. Some research has shown that Nb is present,   specimens. The R a value was found to increase with increasing carbon content. This indicates that increasing the carbon content reduces resistance to intergranular corrosion in Nb440C stainless steel.
activation current density (I a ) were measured and their ratio R a (%) = I r /I a was calculated, as shown in Table 5. The value of R a represents the degree of sensitization to intergranular corrosion of the   Susceptibility to intergranular corrosion of stainless steels is related to the depletion of chromium along the grain boundaries as a result of precipitation of chromium carbides or other chromium-rich phases [22]. Nb is a strong carbide stabilizing element, the formation tendency of NbC is larger than that of Cr 23 C 6 . Thus, the carbon atoms preferentially form NbC with Nb. As a result, the amount of chromium consumed by the formation of Cr 23 C 6 near the grain boundary is also reduced. At the same time, Nb440C has a smaller grain size than the other samples, as shown in Figure 1. The smaller grain boundary size means that the grain boundary area is greatly increased. To form an equal amount of Cr 23 C 6 , chromium may be drawn from a larger area, resulting in less chromium atoms being consumed near the grain boundary [23]. Therefore, the addition of Nb to MIM440C stainless steel can improve resistance to intergranular corrosion. However, increased carbon content has the opposite effect.

1.
In MIM Nb440C stainless steel, NbC precipitates are formed. Most NbC precipitates are distributed on the grain boundaries, which inhibits grain growth.
2. The formation of NbC precipitates in stainless steel consumes carbon atoms, and reduces the tendency to form carbides with chromium. Thus, the pitting resistance of the 440C steel is improved. However, carbon content has the opposite effect.
3. NbC reduces the degree of chromium depletion near the grain boundary and refines the grains. It also reduces R a and improves resistance to intergranular corrosion of 440C stainless steel. On the other hand, increase in carbon content will increase R a and reduce resistance to intergranular corrosion.