Passivation behavior of CoCrNiZrx medium-entropy alloy in the sulfuric acid solutions
Introduction
High entropy alloys (HEAs), which are considered to be composed of five or more principal elements with the atomic percentage of each principal element is between 5 at.% and 35 at.% and exhibit high configuration entropy (>1.5R, R is the gas constant), attract great potentials for engineering applications in many fields [1], [2], [3]. Along with the development of HEAs, researchers find that medium-entropy alloys (MEAs) with fewer components (two to four elements) and moderate configuration entropy (1.0R ≤ ΔS ≤ 1.5R) could have even better performance than HEAs [4], [5]. CrCoNi medium-entropy alloy, the most classical alloy among medium-entropy alloys, has been extensively studied due to its superior properties of impressive ductility and damage tolerance, compared with other ternary alloys [6] and CrMnFeCoNi HEA [7], [8]. Different from the traditional alloy design which consists of one main solvent element and one or more solute elements with low relative atomic ratio, the excellent performance of MEAs does not come from a single dominant component, but is stabilized by the synergistic effect of equimolar multi-elements, which have the potential of unique superior performance combination [9], [10].
Corrosion behavior of several types of HEAs and MEAs in different aqueous solutions has been investigated [11], [12], [13], [14], [15]. Muangtong et al. [11] studied the corrosion properties of four equiatomic HEAs of CoCrFeNi system alloyed with Al, Cu and Sn in NaCl solution. The results showed that CoCrFeNiSn exhibited the most positive breakdown potential and stable passive film, attributed to the extensive formation of stable Cr2O3 and SnO2. Lu et al. [12], [13] investigated the corrosion behavior of Fe50Mn30Co10Cr10 HEA in H2SO4 and NaCl solution and suggested that corrosion resistance of this dual-phase alloy was worse than that of the equiatomic CoCrFeMnNi HEA, attributed to the lower ratio of Cr2O3/Cr(OH)3 and the higher Mn content in the surface film. Wang et al. [16] revealed that the passive film formed on CoCrNi was enriched in Co and Ni. The high Cr content and thicker passive film made CoCrNi present high corrosion resistance in 1.0 mol/L H2SO4 solution. Quiambao et al. [14] investigated the passivation behavior of a novel HEA of 38Ni-21Cr-20Fe-13Ru-6Mo-2W and found that HEA exhibited excellent corrosion resistance in sulfate solution with pH 1 and pH 12, and maintained passivation in a wide range of potential. Corrosion resistance and passive behavior of CoCrNi in H2SO4 and NaCl solutions were studied by Feng et al. [15]. The results displayed that low metastable pitting sensitivity and dense passive film improved the corrosion resistance of CoCrNi.
Zirconium (Zr) alloying is always used in the HEAs and MEAs in the literature [17], [18]. Tong et al. [17] found that the formation of partial Zr-Zr metallic bonds effectively improved the strength and ductility of NbMoTaW HEA. Zhi et al. [18] investigated the effect of Zr content on the mechanical properties of Al2NbTi3V2Zrx alloy and reported that the Al2NbTi3V2Zr0.4 alloy exhibited the superior comprehensive mechanical property because of its simplest microstructure. From the perspective of corrosion, Zr is reported to exhibit good corrosion resistance in human fluids and form a highly-protective surface film on the alloys [19]. It is always alloyed with Ti, Nb, Hf, Ta, and Mo to develop corrosion resistant HEAs [20], [21]. To our best knowledge, the performance of Zr alloying in CoCrNi MEA is rarely reported. How does the Zr affect the corrosion behavior of this alloy should be clarified.
In the present work, corrosion behavior of CoCrNiZrx alloy in 0.1 M H2SO4 solution was investigated. Electrochemical impedance spectroscopy (EIS), Mott-Schottky curves (M-S), and X-ray photoelectron spectroscopy (XPS) were used to study the passivation behavior and surface chemistry CoCrNiZrx alloy. The microstructure of the samples was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effects of Zr alloying on the electrochemical behavior and passive film properties (composition and semiconductor characteristics) were evaluated and discussed.
Section snippets
Material and solution
Co, Cr, Ni and Zr elements with purities over 99.9 wt% were used as raw materials to prepare the CoCrNiZrx (x = 0, 0.1, 0.2, 0.3) MEAs. Solidified ingots were prepared by arc melting and pouring under protective atmosphere. During the manufacturing process, remelting and re-solidification were carried out eight times to ensure the uniformity of each sample. The chemical compositions (wt.%) of the material is listed in Table 1. The alloys were denoted as Zr0, Zr1, Zr2, and Zr3 alloys in the
Microstructure characterizations
The XRD results shown in Fig. 1 indicate that the CoCrNiZr0 MEA consists of a single solid solution face-centered cubic (FCC) phase. The addition of the fourth alloying element may lead to the evolution of the second phase, forming a two-phase structure, or a new single phase [11]. No obvious Zr peak is detected on the Zr0 alloy because of the low Zr content. As the Zr content reaches 0.2 and 0.3, additional peaks at 38.4°, 68.9°, and 82.2°, which correspond to the Zr-rich hexagonal
Effect of Zr addition on the microstructure of CoCrNiZrx MEAs
Fig. 1, Fig. 2, Fig. 3 show that the microstructure of CoCrNiZrx MEAs is significantly affected by the addition of element Zr. A new HCP phase is generated with the addition of Zr. The hardness of the phases was measured by the microhardness tester to study the characteristics of the two different phases and the results are shown in Fig. 14. It can be seen that the overall hardness of the MEAs increases with the addition of Zr. The microhardness of CoCrNi MEAs without Zr is around 175 HV0.1.
Conclusion
The effect of Zr alloying on the microstructure, electrochemical passivity, and film composition of CoCrNiZrx MEAs in 0.1 M H2SO4 solution has been investigated in this work. The following conclusions can be drawn:
- (1)
CoCrNiZrx MEAs are composed of dendritic FCC matrix phase and Zr-rich HCP phase. By increasing the Zr content, the proportion of Zr-rich phase increases and becomes the dominant phase. The hardness of the alloy increases with the increase of Zr content.
- (2)
All of the MEAs reveal a broad
Data availability statement
The raw/processed data required to reproduce these findings cannot be shared at this time as the data is related to an ongoing study.
CRediT authorship contribution statement
Yong Wang: Investigation, Writing - original draft, Funding acquisition. Longhua Zhang: Investigation, Methodology, Writing - original draft. Bo Zhang: Investigation, Methodology, Visualization. Huiyun Tian: Investigation, Writing - review & editing. Xin Wei: Writing - review & editing, Funding acquisition. Zhongyu Cui: Conceptualization, Supervision, Writing - review & editing, Funding acquisition.
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.
Acknowledgements
The authors wish to acknowledgement the financial support of the Ministry of Industry and Information Technology Project (No. MJ-2017-J-99), the Shandong Provincial Key R & D plan (Nos. 2019GHY112050 and 2019GGX104072), and The Open Project of Key Laboratory of Structure and Functional Regulation of Hybrid Materials (Anhui University), Ministry of Education in 2020: DFT study of the storage and transport of alkali metal (Li/Na/Mg) ions on layered 2D InSe surface (No. 8).
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