Effect of sulfuric acid concentration on corrosion behavior of Al0.1CoCrFeNi high-entropy alloy
Introduction
Currently, high-entropy alloys (HEAs), a new type of alloys that typically contain five or more alloying elements in approximately equal molar concentrations, are under intensive research and investigation [[1], [2], [3]]. Although HEAs were only recently proposed in 2004, emergence of HEAs has ignited the imagination of the scientific community since they open up so many possibilities for researchers to explore new types of engineering of viable high-performance alloys. It has been reported that HEAs exhibit several outstanding mechanical and physical properties: high strength [4], enhanced thermal stability [5], high hardness [6], extensive or compressive properties [7], unique electrical and magnetic properties [8], outstanding oxidation resistance [9], irradiation resistance [10] and excellent anticorrosive properties [[11], [12], [13]].
The proposal of HEAs provided a broad prospect and research ideas for alloys with outstanding corrosion resistance [14]. Studies on HEAs often indicate that single-phase solid solution alloys present superior corrosion resistance [15], had noble corrosion potential [16], a wider passivation range and superior passivation ability [17]. The corrosion behavior of HEAs has been extensively studied in typical environments that the alloys may encounter in different applications, e.g. in sulfuric acid (H2SO4) solution. Xiao et al. [18] studied the effect of adding Cr and Ti on the corrosion behavior of AlCoCrFeNi HEAs in H2SO4 solutions. The effects of Al on the corrosion behavior of CrFe1.5MnNi0.5 HEAs were also investigated in 0.5 M H2SO4 solution [19]. The HEAs CrFe1.5MnNi0.5, Al0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5MnNi0.5 showed a wide passivation region in the potentiodynamic polarization curves in 0.5 M H2SO4 solution, and the general corrosion resistance decreased with increasing Al content. Qiao et al. [20,21] studied the effect of rare-earth elements on corrosion behavior and passivation film compositions of CrMnFeNi HEAs. Lanthanum doping can effectively improve the corrosion resistance of HEAs in 0.5 M H2SO4 and positively affect the composition of the passivation films.
The relatively extensive research on the corrosion behavior of HEAs has been based on an equimolar CoCrFeNi system [[22], [23], [24]]. Wu et al. [24] found that the corrosion resistance of as-cast CoCrFeNiCux (x = 0, 0.5, 1.0) HEAs decreased with increasing Cu content. Addition of Mo to CoCrFeNi HEAs has also been studied [25]. Results of X-ray photoelectron spectroscopy (XPS) show that Mo can improve the composition of the passivation film and enhance the corrosion resistance of CoCrFeNi HEAs. In addition, effects of Al addition on the microstructure and mechanical properties of CoCrFeNi HEAs have been studied [8,26].
The corrosion behavior of AlxCoCrFeNi HEAs in different aqueous solutions has also been reported [27,28]. Research on corrosion behavior of AlxCoCrFeNi HEAs showed that when the Al content exceeded 0.3, the body centered cubic phase was formed and hence caused severe selective pitting in solutions containing Cl− [29]. In addition, greater Al contents increased critical current density for passivation film in H2SO4 solutions [19]. As a single-phase solid solution with face-centered cubic (FCC) structure and almost no component segregation [30], Al0.1CoCrFeNi HEA shows surprising pitting resistance in halogen-containing solutions [31], and all elements can be passivated in H2SO4 solutions [32]. Notably, it was shown that AlxCoCrFeNi (x = 0 to 2) HEAs showed a different anodic passivation behavior in different concentrations of acid solutions [27,32,33]. This may be due to the influence of the acid solution concentration on the corrosion mechanism of AlxCoCrFeNi HEAs. Because of the multi-elemental nature of HEAs, it is possible that variation in weak oxidative acids such as H2SO4 might bring out the distinctive anodic behavior of its several passive elemental constituents. Therefore, this work is devoted to investigating the corrosion behavior of as-cast Al0.1CoCrFeNi in H2SO4 medium of different concentrations: 0.1, 0.2, 0.3, 0.5 and 1.0 M. The general corrosion resistance and passivation ability of the material is assessed and the relationship between the chemical composition, microstructure, and corrosion behavior is discussed.
Section snippets
Experimental procedure
The HEAs with a nominal composition of Al0.1CoCrFeNi were made by a vacuum arc melting furnace using five corresponding elemental metals with purity of 99.95%. Every as-cast rod sample (diameter 3 mm) was cut into smaller cylindrical pieces of length 3 mm using wire electrical discharge machining. Subsequent test surfaces all refer to the cross-section faces of approximate area of 0.07 cm2. Before electrochemical tests, the specimen surfaces were ground to 2000. Afterwards alumina polishing
Potentiodynamic polarization curves
Fig. 1 shows the potentiodynamic polarization curves of the Al0.1CoCrFeNi HEA in 0.1, 0.2, 0.3, 0.5 and 1.0 M H2SO4. In general, Al0.1CoCrFeNi HEA exhibits good passivation capability and corrosion resistance in H2SO4 solution (Fig. 1a). Interestingly, as the H2SO4 concentration changes, the Al0.1CoCrFeNi HEA exhibits different anodic passivation behavior. In-depth analysis shows that for 0.5 and 1.0 M concentrations, two passivation stages are prominent in the corresponding polarization
Conclusion
The effect of concentration of H2SO4 solution on corrosion behavior of the as-cast Al0.1CoCrFeNi HEA was studied. The potentiodynamic polarization curves, EIS, SEM and XPS provided evidence concerning the anodic polarization behavior, corrosion morphology and chemical composition of the passive film of Al0.1CoCrFeNi HEA in H2SO4 solution of various concentrations. The major conclusions follow:
- (1)
The anodic polarization behaviors of Al0.1CoCrFeNi HEA in H2SO4 solutions of various concentrations
Author statement
Thank you very much for providing us with the opportunity to modify the manuscript! The manuscript and highlights have been carefully reviewed and revised based on the revised content of the language editor.
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.
Acknowledgment
The authors acknowledge the financial support of the Natural Science Foundation of Shanxi Province, China (Nos. 201901D111105 and 201901D111114), Transformation of Scientific, Technological Achievements Programs of Higher Education Institutions in Shanxi (2019) and State Key Lab of Advanced Metals and Materials of China (Grant No. 2020-Z09).
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