Open AccessArticle

Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys

Wenrui Wang

^1,*,

Jieqian Wang

Honggang Yi

¹,

Wu Qi

¹ and

Qing Peng

School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China

Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA

Author to whom correspondence should be addressed.

Entropy 2018, 20(12), 908; https://doi.org/10.3390/e20120908

Submission received: 6 October 2018 / Revised: 21 November 2018 / Accepted: 26 November 2018 / Published: 28 November 2018

(This article belongs to the Special Issue New Advances in High-Entropy Alloys)

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Abstract

The present work investigates the influence of micro-alloyed Mo on the corrosion behavior of (CoCrFeNi)_100−xMo_x high-entropy alloys. All of the (CoCrFeNi)_100−xMo_x alloys exhibit a single face-centered cubic (FCC) solid solution. However, the (CoCrFeNi)₉₇Mo₃ alloy exhibits an ordered sigma (σ) phase enriched in Cr and Mo. With the increase of x (the Mo content) from 1 to 3, the hardness of the (CoCrFeNi)_100−xMo_x alloys increases from 124.8 to 133.6 Vickers hardness (HV), and the compressive yield strength increases from 113.6 MPa to 141.1 MPa, without fracture under about a 60% compressive strain. The potentiodynamic polarization curve in a 3.5% NaCl solution indicates that the addition of Mo has a beneficial effect on the corrosion resistance to some certain extent, opposed to the σ phase. Furthermore, the alloys tend to form a passivation film in the 0.5 M H₂SO₄ solution in order to inhibit the progress of the corrosion reaction as the Mo content increases.

Keywords:

(CoCrFeNi)_100−xMo_x alloys; high entropy alloy; microstructure; mechanical properties; corrosion behavior

1. Introduction

Traditional alloys only have one major element as a matrix [1]. With the increase of the amount of alloying elements and the concentration of minor elements, the alloy forms a fragile intermetallic phase, which not only increases the difficulty of the microstructure, but also may result in a reduction in the mechanical performance [2,3,4]. To overcome these difficulties, high entropy alloys (HEAs) are invented with extensive research interests; HEAs usually compose of five or more kinds of major elements, with the concentration of each principal element being between 5 and 35 at % [5,6,7]. HEAs tend to generate a face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal closed-packed (HCP) multicomponent solid solution phase [8,9]. Some HEAs have been confirmed to achieve a series of excellent properties, such as high strength, high hardness, and glorious corrosion resistance [10,11,12].

The CoCrFeNi alloy has received extensive attention for its outstanding corrosion resistance, ductility, and structure stability [13,14]. However, because of the poor mechanical strength, the application of the CoCrFeNi alloy in engineering has been limited. It has been shown that an ordered σ strengthening phase can be formed by a certain amount of Mo additives, resulting in precipitation strengthening [15,16,17]. However, the excessive addition of Mo causes a large amount of the coarse σ strengthening phase, which may lead to a rapid increase of the alloy brittleness. Furthermore, because of the low electronic potential of Mo, the excessive content of Mo may reduce the corrosion resistance [18,19,20]. Referring to the chemical composition of austenitic stainless steel, the content of Mo in stainless steel is generally less than 3 wt %. Therefore, we also supply a small amount of Mo in addition to the CoCrFeNi alloy, in order to study the corrosion resistance of the (CoCrFeNi)_100−xMo_x alloy for the development of an HEA system, with a good performance of both strength and corrosion resistance.

In this paper, the as-cast (CoCrFeNi)_100−xMo_x (x =1, 2, and 3 wt %) HEAs have been prepared mainly by vacuum arc melting. The excellent corrosion behavior was investigated by the electrochemical experiments.

2. Materials and Methods

2.1. Samples Fabrication

Elements Co, Cr, Fe, Ni, and Mo with purities of over 99.9 wt % were prepared as raw materials previously. The as-cast (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3, represented by Mo₁, Mo₂, and Mo₃, respectively) were prepared by vacuum arc melting and were fabricated under the WK-∏ vacuum arc melting furnace at least five times in the crucible, so as to ensure the chemical homogeneity. The size of the ingot was approximately Φ 35 × 10 mm, and the ingot was annealed for 3 h under 500 °C and was cooled in the air so as to release the residual stress caused by rapid cooling during casting.

2.2. Microstructure of the (CoCrFeNi)_100−xMo_x Alloys

The crystalline phases of the (CoCrFeNi)_100−xMo_x alloys were identified by X-ray diffraction (XRD), using an Ultima IV X-ray diffractometer with Cu K_α radiation. The X-ray diffractometer has an operating voltage of 30 kV and an operating current of 20 mA with the diffraction angle (2θ) from 20 to 90°, at a scanning rate of 4°/min.

Etching the (CoCrFeNi)_100−xMo_x alloys with aqua regia (HNO₃: HCl = 1:3, volume fraction) and using FE-SEM JEOL JEM-7600F scanning-electron microscopy (SEM) (JEOL Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS) allowed for an analysis of the microstructure and composition.

2.3. Mechanical Properties

The microhardness was measured using a Wolpert-401MVD Vickers hardness tester (WOLPERT Co., Norwood, MA, USA) with loads of 500 g and a duration of 10 s. The measurements were performed at 10 different locations on each sample, and the average value of the 10 measurements was calculated. Compressive tests were carried out on the Φ 3 × 6 mm samples, using the universal testing machine (CMT 4305) (MTS Co., Eden Prairie, MN, USA) with a strain rate of 10⁻³ s⁻¹.

2.4. Electrochemical Corrosion Test

The electrochemical experiments were performed on (CoCrFeNi)_100−xMo_x alloys, using the Potentiostat Workstation Versa STAT MC (PARSTAT 4000, AMETEK Co., Princeton, NJ, USA). A three-electrode electrochemical cell using a saturated calomel electrode (SCE) as a reference electrode, a platinum plate as an auxiliary electrode, and a sample as a working electrode were tested. The electrochemical experiments of the (CoCrFeNi)_100−xMo_x alloys were conducted in a 3.5% NaCl and 0.5 M H₂SO₄ solution at room temperature, respectively. The potentiodynamic polarization measurements were taken at a scan rate of 1 mV/s from a potential scanning range of −0.5 V to 1.1 V.

2.5. Corroded Microstructure

The electrochemically tested alloys were cleaned using an ultrasonic cleaner, and then dried in nitrogen. SEM and EDS were used to study the morphology of the corrosion surface of the high-entropy alloys.

3. Results and Discussion

3.1. Microstructure of the (CoCrFeNi)_100−xMo_x Alloys

Figure 1 represents the XRD pattern of the (CoCrFeNi)_100−xMo_x (x = 1, 2, 3) alloys. All of the samples of alloys have a single FCC solid solution structure, which was confirmed by the predecessors [21]. As the Mo content increases, the peak intensity changes, but the FCC phase is kept. As the Mo content becomes 3 wt % (Mo₃ structure), the small peak on the left of the matrix FCC phase in the XRD patterns is identified as Cr and Mo rich σ phase, which agrees with the previous remarks [22].

The SEM images of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys presented in Figure 2 shows that the alloys are composed of typical dendritic structures. Region A is interdendrites and region B is dendrites. Figure 2c presents the SEM image of the Mo₃ alloy, and region D is the grain boundary. Figure 2d is a part of the SEM of the Mo₃ alloy. The EDS results of the (CoCrFeNi)_100−xMo_x alloys are shown in Table 1. According to the EDS, the dendrite is the Co and Fe rich phase, and the interdendrite is the Cr and Mo rich phase. Combined with the XRD and EDS results, the Mo₃ alloy exhibits Cr and Mo rich σ phase in the interdendrite. When the content of Mo is 1 and 2 wt %, there is no formation of a precipitate phase in the alloy because of the high entropy effect. However, HEAs undergo spinodal decomposition inside the crystal grains during cooling, leading to the formation of microstructures with the same structure but different compositions.

3.2. Mechanical Properties

Figure 3 shows the Vickers hardness (HV) of the (CoCrFeNi)_100−xMo_x alloys as a function of the Mo content. The alloy hardness increases from 124.83 to 133.60 HV, with the Mo content increasing from 1 to 3 wt %. When the content of the Mo element is 3 wt %, the presence of the σ phase results in a remarkable increase in the hardness.

Besides the hardness, we have examined the stress–strain relationships. The compressive stress–strain curves and the inner longitudinal-section SEM images of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys are shown in Figure 4. After yielding, the strength of the alloys increases continuously. All of the three samples do not break under about a 60% compressive strain, indicating that the alloys possess a good ductility, flexibility, and fracture strain. As shown in Figure 4b–d, the deformation of the Mo₃ subgrain boundaries is more prominent in the angle of 45°, probably due to the resolved shear stress.

Table 2 lists the mechanical properties of the (CoCrFeNi)_100−xMo_x alloys. The Mo₁ and Mo₂ alloys exhibit a similar behavior. The yield stress of the Mo₃ alloy increases significantly because of the second-phase hardening by the σ phase, as reported in the literature [23,24].

3.3. Environmental Effect on Corrosion Behavior

3.3.1. Corrosion Behavior in Chloride-Containing Solutions

Figure 5 lists the polarization curve of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in a 3.5% NaCl solution. The corrosion potential of the alloys gradually shifts to more positive potentials with a decreasing Mo content.

Table 3 presents the electrochemical parameters of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in a 3.5% NaCl solution. The corrosion current densities (i_corr) of the Mo₁, Mo₂, and Mo₃ alloys were 0.4, 0.24, and 6.6 μA/cm², and the corrosion potentials (E_corr) were −199, −277, and −493 mV, respectively. The breakdown potential (E_b) gradually shifts to more positive potentials. E_pit is a primary passivation potential. ∆E is the passive region width, defined as the difference between the E_b and E_pit. The i_corr value of the Mo₃ alloy is an order of magnitude higher than the other two. The corrosion resistance of Mo₃ was dropped. Therefore, the presence of the Cr and Mo rich σ phase in the Mo₃ alloy leads to the diminution of the corrosion resistance.

Figure 6 presents the microstructure of the (CoCrFeNi)_100−xMo_x alloys after potentiodynamic polarization in a 3.5% NaCl solution. DR is dendrite and IR is interdendrite. According to the SEM images, the majority of types of corrosion were mainly the pitting corrosion. The Mo₃ alloy is more susceptible to pitting corrosion, which is consistent with the polarization curve results. Region A is a Cr-rich phase and region B is a Mo-rich phase. There is an element of segregation that causes the corrosion to occur. The interdendritic phase of Mo₃ is the Cr- and Mo-rich phase, and the dendrite is a Cr- and Mo-depleted phase. Therefore, galvanic corrosion occurred at the junctions of dendrites. The XRD and SEM result show that the Mo and Cr rich σ phase appeared inside the interdendrite, the corrosion occurred at the interfaces around σ phase, as shown in Figure 6c,d. The results of the EDS of the alloy after corrosion are summarized in Table 4.

3.3.2. Corrosion Behavior in Acid Solutions

Figure 7 shows the polarization curve of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in 0.5 M H₂SO₄. Table 5 presents the electrochemical parameters of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in 0.5 M H₂SO₄. The i_pp is the lunt current density. It can be seen from the electrochemical parameters that the i_corr was 34.1, 28.0, and 15.4 μA/cm², respectively. The E_corr shifted to more positive potentials, and the i_corr value dropped as the Mo content increased. This suggests that the alloys tend to form a passivation film to inhibit the progress of the corrosion reaction.

Figure 8 shows the SEM microstructure of the (CoCrFeNi)_100−xMo_x alloys after potentiodynamic polarization in a 0.5 M H₂SO₄ solution, combined with EDS, because the low potential of Mo is enriched and the Cr₂O₃ is insufficient in region A. Consequently, region A is more susceptible to corrosion in the H₂SO₄ solution. When the content of Mo is 3 wt %, as Figure 8c indicates, the results of the EDS analysis show that the concentration of element Cr in region A is reduced, and the content of Mo in region B is significantly higher than that in region A, the effect of Mo was to form Mo (VI) oxyhydroxide or molybdate (MoO₄²⁻), decreasing the rate of dissolution in active zones. Hence, the corrosion of the Mo₃ alloy is concentrated in region A.

Figure 9a shows the comparison of the corrosion behavior between the HEAs and the conventional corrosion resistant alloys in a 3.5% NaCl solution, compared with those of the conventional corrosion resistant alloys [25,26]. The HEAs are located in the upper part of Figure 9a, the E_corr of the HEAs are more positive than those of the Mn alloys, Ni alloys, and some of the Ti alloys. On the other hand, the i_corr of the HEAs are much lower than some of Mn alloys and are comparable with the Ti alloys, which indicates that the corrosion resistance of the HEAs is comparable or even better than those of the conventional alloys. However, the partial i_corr of the HEAs is higher than that of the total, because the presence of the σ phase is catastrophic for HEAs. Figure 9b presents the comparison of the corrosion behavior between the HEAs and the conventional corrosion resistant alloys in the 0.5 M H₂SO₄ solution. Compared with the conventional alloys [27,28], the E_corr of the HEAs are much more positive than those of the Ti alloys and Ni alloys. The i_cor_r of the HEAs are much lower than the Ti alloys, Ni alloys, and some of the Cu alloys. As a general trend, the corrosion resistance of the HEAs in the 0.5 M H₂SO₄ solution is better than those of the conventional alloys.

4. Conclusions

The (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys have been synthesized. Their microstructures, mechanical properties, and corrosion behaviors have been experimentally investigated. The microstructures of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys belong to a single FCC structure. The increase in Mo promotes the formation of the Cr- and Mo-rich σ phase. The hardness and compressive yield strength increase obviously with an increase of the Mo content from 1 to 3 wt %. Regarding the potentiodynamic polarization curves of the (CoCrFeNi)_100−xMo_x alloys in a 3.5% NaCl solution, the curves of the Mo₁ and Mo₂ alloys indicated that the increase of the Mo content increases the corrosion resistance of the chloride environment to some extent. However, the Cr and Mo rich σ phase is present at the grain boundary of the Mo₃ alloy, resulting in a decreasing in corrosion resistance in the 3.5% NaCl solution. Furthermore, the potentiodynamic polarization curves of the (CoCrFeNi)_100−xMo_x alloys in the 0.5 M H₂SO₄ solution yielded an extensive passive region, and as the content of the Mo increased, the corrosion current density gradually decreased. Therefore, in an acidic solution, the addition of Mo has a positive effect on the corrosion resistance of the (CoCrFeNi)_100−xMo_x alloys.

Author Contributions

Conceptualization, W.W.; formal analysis, J.W.; methodology, J.W. and W.Q.; resources, W.W.; writing (original draft), J.W.; writing (review and editing), J.W., H.Y. and Q.P.

Funding

The authors are grateful for the financial support provided by the Fundamental Research Funds for the Central Universities (FRF-TP-16-044A1 and FRF-GF-17-B18) and the National Natural Science Foundation of China (21703007).

Acknowledgments

The authors would like to thank the project team assistance with funding. Many thanks to the tutor for his thoughtful and thorough guidance.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD (X-ray diffraction) patterns of the (CoCrFeNi)_100−xMo_x (x = 1, 2, 3) high entropy alloys.

Figure 2. SEM (scanning-electron microscopy) images of the (CoCrFeNi)_100−xMo_x high-entropy alloys: (a) x = 1; (b) x = 2; (c) x = 3; (d) 10 × magnification.

Figure 3. Vickers hardness of the (CoCrFeNi)_100−xMo_x high-entropy alloys as a function of the Mo content.

Figure 4. (a) Compressive stress–strain curves and the inner longitudinal-section SEM images of the (CoCrFeNi)_100−xMo_x alloys after compression deformation: (b) x = 1; (c) x = 2; (d) x = 3.

Figure 5. Polarization curves of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in a 3.5% NaCl solution.

Figure 6. SEM images of (CoCrFeNi)_100−xMo_x after potentiodynamic polarization in a 3.5% NaCl solution: (a) x = 1; (b) x = 2; (c) x = 3; (d) the interdendrite morphology of the Mo₃ alloy.

Figure 7. Polarization curves of the (CoCrFeNi)_100−xMo_x (x = 1, 2, and 3) alloys in a 0.5 M H₂SO₄ solution

Figure 8. SEM images of (CoCrFeNi)_100−xMo_x after potentiodynamic polarization in a 0.5 M H₂SO₄ solution: (a) x = 1; (b) x = 2; (c) x = 3; (d) partial view of the Mo₃ alloy.

Figure 9. Comparison of the i_corr and E_corr between high entropy alloys (HEAs) and conventional alloys: (a) in a 0.5 M NaCl solution; (b) in a 0.5 M H₂SO₄ solution.

Table 1. Element concentration determined using the energy dispersive spectrometer (EDS) of the three samples of the (CoCrFeNi)_100−xMo_x alloys (at %).

Element		Cr	Fe	Co	Ni	Mo
Mo₁	Dendrite	25.25	23.54	26.07	24.53	0.61
Mo₁	interdendrite	27.69	22.31	24.03	24.94	1.04
Mo₂	Dendrite	26.02	25.17	24.55	23.56	0.69
Mo₂	interdendrite	27.20	23.20	23.33	23.87	2.40
Mo₃	Dendrite	24.99	23.47	25.28	24.96	1.31
Mo₃	interdendrite	26.82	22.54	24.81	24.35	1.50

Table 2. Mechanical properties (yield stress, compressive strength, and fracture strain) of the (CoCrFeNi)_100−xMo_x alloys for x = 1, 2, and 3 as Mo₁, Mo₂, and Mo₃, respectively.

Alloy	Yield Stress σ_y (MPa)	Compressive Strength σ_max (MPa)	Fracture Strain ε_p (%)
Mo₁	113.6	Not fractured	>60
Mo₂	119.7	Not fractured	>60
Mo₃	141.1	Not fractured	>60

Table 3. Electrochemical parameters of the (CoCrFeNi)_100−xMo_x alloys in a 3.5% NaCl solution.

Alloy	E_corr (mV)	I_corr (μA/cm²)	E_b (mV)	E_pit (mV)	∆E (mV)
Mo₁	−199	0.402	992	−5	997
Mo₂	−277	0.235	968	−108	1076
Mo₃	−493	6.610	1053	−358	1411

Table 4. EDS results for the Mo₃ alloy after the potentiodynamic polarization in a 3.5% NaCl solution.

Element	Region	Cr (%)	Fe (%)	Co (%)	Ni (%)	Mo (%)
Mo₁	A	26.17	24.56	23.32	25.01	0.93
Mo₁	B	30.79	23.79	22.87	22.51	0.04
Mo₂	A	26.44	23.72	24.32	23.83	1.70
Mo₂	B	35.81	23.79	19.78	20.11	0.52
Mo₃	DR	24.96	25.21	24.72	24.88	0.23
Mo₃	IR	29.06	23.21	22.43	22.76	2.54

Table 5. Electrochemical parameters of the (CoCrFeNi)_100−xMo_x alloys in a 0.5 M H₂SO₄ solution.

Alloy	E_corr (mV)	I_corr (μA/cm²)	E_pp (mV)	i_pp (μA/cm²)	∆E (mV)
Mo₁	−294	34.1	−225	117	655
Mo₁	−290	28.0	−225	68	681
Mo₁	−279	15.4	−239	25	751

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Wang, W.; Wang, J.; Yi, H.; Qi, W.; Peng, Q. Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys. Entropy 2018, 20, 908. https://doi.org/10.3390/e20120908

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Wang W, Wang J, Yi H, Qi W, Peng Q. Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys. Entropy. 2018; 20(12):908. https://doi.org/10.3390/e20120908

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Wang, Wenrui, Jieqian Wang, Honggang Yi, Wu Qi, and Qing Peng. 2018. "Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys" Entropy 20, no. 12: 908. https://doi.org/10.3390/e20120908

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Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)_100−xMo_x High-Entropy Alloys

Abstract

1. Introduction