Effects of W addition on the Corrosion Behaviors of FeCoNiCrMn High Entropy Alloy Composites in the 3.5 wt.% NaCl solution

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Abstract

The effects of W addition on the corrosion behaviors of FeCoNiCrMn high entropy alloy composites in 3.5 wt.%NaCl solution are investigated in this study. The X-ray diffraction and scanning electron microscopy suggest that the quantity of added W particles results in the increasing quantity of particles in the composites and large amount of cohesionless phases covering the matrix occur in the alloy with 1.0 molar ratio of W element. The trend of corrosion potential current density means that all W-added high entropy alloy composites have a better corrosion resistance referring to the base high entropy alloy composites and 0.8 molar ratio of W element into base generates a best corrosion resistance in the current work. The Electrochemical impedance spectra and compositional analysis of corroded surface of as-cast high entropy alloys demonstrate that W particles act as anode in the composites and Fe-Mn-Cr is as the cathode.

Introduction

As a kind of promising metallic materials, high entropy alloys (HEAs) has been proved to be as future structural materials in the engineering practices, due to their excellent mechanical performances and distinguished corrosion resistance. In the design conception of a HEA, it is consisted of at least 5 principal elements without principal element and with a compositional range from 5 to 35 at.% of each element [1]. HEAs break the limitation of conventional alloys and form simple crystal structures including face-centered cube (FCC), body-centered cube (BCC), hexagonal close-packed (HCP) as well as their mixture [2].

In the previous works, the classic FeCoNiCrMn HEA, with single FCC phase microstructure, has been paid much attention and demonstrated to be of better corrosion resistance than S304 and S316 stainless steels [3,4] and the corrosion resistance of FeCoNiCrMn HEA can also be enhanced through mechanical process and the improvement of manufacture methods [5,6]. On the other hand, the relatively low yield strength of traditional cast FeCoNiCrMn HEA has also been noticed and particle reinforcement method can remarkably enhance the strength [2,7]. In the previous works, the particles that has been utilized in the HEA matrix usually are ceramics (oxides or carbides) [8,9]. Theoretically, these particles have large differences with matrix in the electric potential causing local corrosion. To suppress the corrosion, the distributionand morphology of introduced particles has been considered [10].

In the past decade, many works have devoted to understanding the corrosion resistance of HEAs and promoted this performance to the higher level[4,11,12]. Some of the published literatures reveal that additional element can strongly affect the mechanical properties and corrosion resistance of HEAs [12], [13], [14]. For example, the as-annealed CoCrFeNiMo0.5 HEA possesses excellent corrosion resistance because appropriate Mo content makes passive film denser, and brings down the overall dissolution rate [15]. The Ti content in Co1.5CrFeNi1.5Ti0.5Mo0.1 high entropy alloys which are manufactured by a combination of using selective electron beam melting (SEBM) and solution treatment (ST) also has a positive impact on tensile strength and corrosion resistance [16]. However, for AlxCoCrFeNi high entropy alloys, the addition of Al enhances the tensile strength evidently, nevertheless, it also lead to the volume fraction of the Cr-depleted phase increasing, and degrade the localized corrosion resistance in 3.5 wt.% NaCl solution [17] and in H2SO4 solution, results also proved that Al deteriorates the corrosion resistance due to the porous and inferior nature of the protection oxide film of Al [18]. This result is similar to copper content in FeCoNiCrCuxalloys, the increasing copper content in FeCoNiCrCux alloys also facilitates the tendency of localized corrosion in these alloys [19]. Similarly, Fe50Mn30Co10Cr10 dual-phase HEAs show superior mechanical properties but poor corrosion resistance both in acid solution [20] and 3.5 wt% NaCl solution [21]. And for CoFeNiMnCr high-entropy alloys in a 0.1M H2SO4 solution,surface analysis indicated that Mn addition had little influence on the composition and thickness of the passive films and CoFeNiMnCr high-entropy alloy remained almost unchanged with the addition of Mn [22]. Another method which can enhance the HEAs mechanical properties as well as corrosion resistance at the same time is particle reinforcement. It has been proved that WC particles are dispersed homogeneously in substrate and WC–HEA has higher hardness than traditional WC/Co sintering body [23,24], in addition, WC–HEA perform high corrosion resistanceand the pitting resistance of WC-HEA composites can be improved through reducing the content of Cr-rich carbide [10]. Similarly, the addition of 5wt.% of SiC nanoparticles improves the mechanical properties (e.g. hardness, yield stress) simultaneously boost the corrosion resistance of HEAs [25]. Xu et al designed and prepared graphene encapsulated equiatomic CoCrFeMnNi high-entropy alloy by in-situ mechanical exfoliation graphite crystal. The corrosion resistance of the as-prepared HEAs was significantly improved in 0.1M H2SO4 solution.

Till now, Tungsten (W) element played an important role in the enhancement of corrosion resistance of stainless steels due to the prevention of element dissolution [26]. Alternatively, the melting point of W element (~3410°C) is much higher than FeCoNiCrMn HEA (~1334°C) [27]. Therefore, under the case of melting of HEAs, the W can be retained in the matrix. Nevertheless, due to the alloying reaction, few W can be melted into the matrix which will form the transition region between W and HEA matrix. In the current work, the pure W particles were melting with FeCoNiCrMn HEA and the composite structure with W particles and HEA matrix was constructed. The aim of this work is focused on the corrosion behavior of FeCoNiCrMnW HEA composites in 3.5wt.% NaCl solution. The results indicate that the best corrosion resistance of HEA composites can be achieved at the 0.8 molar ratio of W that added into FeCoNiCrMn HEA. The finding of this work benefits the development of HEA application in the submarine and other marine engineering.

Section snippets

preparation of alloys

The Wx-FeCoNiCrMn HEAs (x =0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively) were prepared by the melting of mixture of high pure Fe, Co, Ni, Cr and Mn (≥99.9%), and W powders as raw materials by vacuum arc melting. The HEAs ingots were flipped and remelted at least six times to ensure the solution of elements in the melt. Subsequently, the φ 6 mm × 70 mm Wx-HEA rods were obtained through suction casting.

Characterization

The crystal structures of phases which present in Wx-HEAs were identified by X-Ray

Results and discussion

Fig. 1 shows the XRD patterns of as-cast Wx-HEAs (x=0.0, 0.2, 0.4, 0.6, 0.8, 1, hereinafter, they were marked as W0.0, W0.2, W0.4, W0.6, W0.8 and W1.0, respectively.) HEAs. Only FCC phase is observed in the W0.0-HEAs. Nevertheless, with increasing W content, the diffraction peaks which can be assigned to the W element appear in each W-added specimen and the intensity was enhanced remarkably with the quantity of W particles. In addition, two peaks, at 35.1 ° and 37.8 °, that are not part of base

Conclusion

In the current work, the effect of W on the electrochemical corrosion behaviors of HEA composites in 3.5 wt.% NaCl solution was investigated. The main conclusions can be summarized as follows.

  • (1)

    The quantity of added W particles results in the quantity of particles in the Wx-HEA composites. The XRD and SEM demonstrate that the particles are consisted of W phases. The microstructures of W1.0 indicate large amount of cohesionless phases covering the matrix.

  • (2)

    The trend of Ecorr means that all W-added

Data Availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Thanks for financial support from the National Natural Science Foundation of China (grant numbers 51601050), Jiangsu Key Laboratory for Advanced Metallic Materials (grant number BM2007204) and the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology (No. ASMA201702).

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