Corrosion and Serration Behaviors of TiZr0.5NbCr0.5VxMoy High Entropy Alloys in Aqueous Environments

The corrosion and serration behaviors of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Mo high entropy alloys (HEAs) in NaCl and H2SO4 solutions were studied by potentiodynamic polarizations (PP) and immersion tests. The results show that all the alloys display excellent corrosion resistance no matter in NaCl solution or in H2SO4 solution. The additions of V and Mo increase the pitting corrosion resistance for the three alloys in NaCl solution slightly and greatly improve the corrosion resistance in H2SO4 solution. The corrosion behaviors of TiZr0.5NbCr0.5 and TiZr0.5NbCr0.5Mo alloys are more sensitive to temperature than that of TiZr0.5NbCr0.5V alloy. After immersion, the surface of TiZr0.5NbCr0.5 alloy appears some pitting holes, this may be related to the electrochemical noise and serration behavior on PP curves; localized corrosion initiates mainly on the boundaries of the BCC and Cr2Zr Laves phase for TiZr0.5NbCr0.5V alloy; while for the TiZr0.5NbCr0.5Mo alloy, the dendrites with Mo element rich region exhibit poor corrosion resistance.


Introduction
High-entropy alloys (HEAs), which may be defined as alloys that generally contain more than three principal elements and each of them has an atomic percentage more than 5 at.% [1][2][3][4][5][6][7], have attracted increasing attentions due to their extensive applications prospect.Because of the very high mixing entropy, HEAs usually intend to form face center cubic (FCC) and/or body center cubic (BCC) disordered solid solutions (DSS) rather than intermetallic compounds or other complex ordered phases [8].The particular structure makes multi-component HEAs exhibit excellent properties, such as high strength [7], high ductility [9,10], well magnetic properties [11], excellent resistances to wear, oxidation, irradiation and corrosion [5,12,13], and high thermal stability [14].These properties provide HEAs with many potential applications such as tools, molds, diffusion barriers for integrated circuit (IC) and solar thermal collectors, etc., and a number of HEAs have been developed for both functional and structural applications.
Recently, the HEAs with promising high-temperature mechanical properties are designed by the phase formation rules of HEAs and based on some transition metal elements with high melting point such as Ta, Mo, Nb, W, V, and Ti.Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 alloys have been proven to remain stable, still presenting a single-phase BCC crystal structure after exposure to 1400 °C, and exhibit a strong resistance to high-temperature softening, likely due to slow diffusion effects of atoms in multicomponent alloys [5].NbTiVTaAlx alloys have a single DSS phase with BCC structure and possess high compressive yield strength and high ductility (no fracture under 50% strains) [15].Senkov et al. found that NbCrMo0.5Ta0.5TiZralloy has a good combination of mechanical properties and oxidation resistance after heating at 1273 K for 100 h in flowing air [14].
In this paper, corrosion behaviors of the TiZr0.5NbCr0.5VxMoyalloys in NaCl and H2SO4 solutions were studied by the measurements of PP curves, and electrochemical noise and serration behaviors were observed on the PP curves.

Microstructure Analysis
Figure 1 shows the XRD patterns of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Morefractory alloys.All the alloys exhibit the reflections of a BCC DSS phase and an ordered Cr2Zr phase, indicating that the additions of V or Mo have little effect on the phase constitution of TiZr0.5NbCr0.5 alloy and only make the diffraction peaks shift to the right side.Taking into account that the intensities of the diffraction peaks of BCC DSS phase are frequently stronger than that of ordered phases, the volume fractions of DSS appear to dominate and can be considered as the major phase.As listed in   )among all the constituent elements, we can see that Zr has the most negative mixing enthalpy with Cr, and this may be the reason why intermetallic compound phase Cr2Zr forms.
Figure 2 shows the SEM secondary electron images of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys.Clearly, typical cast dendritic and inter-dendritic morphologies can be observed in all the alloys, as marked by arrows in the figure (marks of "ID" for inter-dendrites, and "DR" for dendrites).The dendrites of TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys are much smaller than that of TiZr0.5NbCr0.5 alloy, which indicates that the additions of V and Mo make the dendrites refined.V and Mo elements can also lead to a significant decrease in the grain size in the Fe based steels, mainly due to the increasing driving force for nucleation of the refractory particles of V(C,N) or other particles [14,15].According to the EDS analysis listed in Table 2, the inter-dendrites of the three alloys are rich in Zr and Cr elements.

Potentiodynamic Polarization (PP) Measurements
PP curves of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys in 3.5 wt.% NaCl solution at room temperature are given in Figure 3a and the electrochemical parameters are listed in Table 3.All the alloys exhibit good resistance to general corrosion, and show low corrosion current density (icorr) which are about 4.41 × 10 −9 , 9.74 × 10 9 , 9.40 × 10 −8 A/cm 2 , respectively.They all show excellent ability to passivation and exhibit a wide passive region (ΔE) extending >1400 mV in the sodium chloride solution.Generally, many alloys cannot develop passivation or have a narrow passive region in NaCl solution [3,[16][17][18][19], this is due to chloridion (Cl − ) may penetrate and damage the passive film.The elements, Ti, Zr, Nb, Cr, Mo, and V, have strong passivation abilities which facilitate to form oxide films, and the films are with multicomponent high entropy alloys, with dense packing structure, and chloridion cannot easily penetrate through.This film resists the penetration of chloridion and improves their pitting potential which is associated with an increasing resistance to pitting.3.In sulfuric acid solution, three alloys all show excellent resistance to the general corrosion, and their corrosion current density can reach to 4.52 × 10 −7 , 2.04 × 10 −8 , and 5.26 × 10 −8 A/cm 2 , respectively.Compared with TiZr0.5NbCr0.5 alloy, V and Mo elements make the corrosion potential (Ecorr) shift positively and the corrosion current density decrease, which show that TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys have better corrosion resistance than that of TiZr0.5NbCr0.5 alloy in the H2SO4 solution.The corrosion behaviors of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys at 25 °C , 40 °C and 55 °C in NaCl solution are showed in Figure 4. From Figure 4a-c, a rising temperature increases the corrosion current density, the corrosion potential, and the current density of passivation, but decreases the pitting potential for TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys.For the three alloys, with temperature increasing, the corrosion rate which is directly related to icorr increasing and the pitting corrosion is easier to happen.As the solution temperature increasing, the corrosion potential shifts positively, the critical passive current density increases.Thus, the resistance to corrosion decreases as the temperature increases.TiZr0.5NbCr0.5Valloy is the least sensitive to temperature among the three alloys, indicating that the passive film generated on the surface of TiZr0.5NbCr0.5Valloy is the most stable and insensitive to external environment.However, for the TiZr0.5NbCr0.5 and TiZr0.5NbCr0.5Moalloys, their electrochemical parameters change a lot with the increasing of temperature.More serrations can be seen on the PP curves at 40 °C and 55 °C than at room temperature for the three alloys, but the reason still remain unknown.

Immersion Tests
The surface morphology of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys after immersed in 3.5 wt.% NaCl solution for 15 days are showed in Figure 5 and the EDS analyses are listed in Table 3.The major corrosion model of TiZr0.5NbCr0.5 alloy belongs to pitting, because some pitting holes are found, as shown by arrows.This indicates that TiZr0.5NbCr0.5 alloy has a poor resistance to pitting corrosion.For TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys, some regions on the surface initiate mild localized corrosion.The regions in which the corrosion originates for TiZr0.5NbCr0.5Valloy concentrate mainly on the boundaries of the BCC phase and Cr2Zr phase, while the locations of corrosion for TiZr0.5NbCr0.5Moalloy are in the dendrites, as marked by the arrows.From the EDS analyses, there are no obvious difference between the samples before and after immersion for TiZr0.5NbCr0.5 and TiZr0.5NbCr0.5Valloys.However, for the TiZr0.5NbCr0.5Moalloy, the content of Mo in the dendrites is decreased, indicating that Mo element in the dendrites is prone to corrosion.This result coincides with the surface appearances of TiZr0.5NbCr0.5Moalloy which easily produces the selective corrosion in the dendrites.

Discussion
If the surface examination shows the major types of corrosion are localized corrosion and/or pitting, the corrosion rate is usually determined using electrochemical-polarization measurements [20].The corrosion rate r is expressed as: 3 (mm / year) 3.27 10 ρ where ρ is the density of the alloy in g/cm 3 , EW is the alloy equivalent weight, which is given by: where ni is the valence of the ith element of the alloy, fi is the atom fraction of the ith element of the alloy, and Wi is the atomic weight of the ith element of the alloy.
In this work, as listed in Table 3, TiZr0.5NbCr0.5 alloy has the minimum corrosion current density and the most active corrosion potential, and TiZr0.5NbCr0.5Moalloy has the maximum icorr and medium Ecorr in the NaCl solution at 25 °C among the three alloys.According to Equations ( 1) and ( 2), the corrosion rates of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys are 3.46 × 10 −5 , 6.85 × 10 −5 and 6.60 × 10 −4 mm/y, respectively.The result indicates that the resistance to general corrosion was slightly decreased after adding vanadium to the refractory TiZr0.5NbCr0.5 alloy, and molybdenum makes the anti-corrosion properties declined more significantly.The reason why the TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys has larger corrosion rate than TiZr0.5NbCr0.5 alloy may be that V and Mo elements prompt the precipitation of Cr2Zr phase and increase its dispersion which can make potential differences in small areas and induce the general corrosion.Furthermore, there is little difference in the passive current density (ipass) for the TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys, which indicate the dissolution rates of the oxide films formed are very similar.The pitting potentials are 1.180, 1.448, and 1.400 VSCE for TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys, respectively, and pitting holes are only found on the surface of TiZr0.5NbCr0.5 alloy.Molybdenum can improve the ability of passivation in the non-oxidization medium and the resistance to pitting.General view is that the molybdenum atoms are adsorbed to the surface of the active metal in the form of MoO4 2− , which inhibit the dissolution of active metal atoms and improve the pitting resistance.Like the Mo element, Vanadium also can form a dense passive film and make the ability of the resistance to pitting corrosion improved.V and Mo can significantly improve the pitting-corrosion resistance of the alloys.
According to Equations ( 1) and ( 2), the corrosion rates of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys are 3.55 × 10 −3 mm/y, 1.44 × 10 −4 mm/y and 3.69 × 10 −4 mm/y, respectively, in in 0.5 M H2SO4 solution at 25 °C.It has been reported that CoCrFeNi alloy has the best resistance to corrosion in 0.5 M H2SO4 solution than other HEAs, and its corrosion rate is 0.12 mm/year [21].However, compared with our alloys, its corrosion rate is much larger, indicating that the corrosion resistance of the three alloys in H2SO4 solution is better than CoCrFeNi alloy.Generally, the austenitic stainless steels are ternary iron-chromium-nickel alloys, and their structures are FCC type.The austenitic stainless steels normally have greater corrosion resistance than the ferrite stainless steels (BCC structure) [20][21][22][23][24]. Generally, there are always potential differences between different phases, and which facilitate to form micro-batteries. So, the resistance to corrosion of single DSS phase is better than that of multiphase alloys.Furthermore, single DSS FCC phase has a good resistance to corrosion than BCC phase or other multiphase structure, possibly due to its higher atomic packing density.Compared with traditional HEAs, especially CoCrFeNi alloy which has a FCC structure, this system of alloys that are composed of BCC DSS phase and Cr2Zr phase has excellent corrosion resistance in the H2SO4 solution, possibly due to the passive elements and the high entropy phase structure in the alloys.TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys have lower corrosion rate than TiZr0.5NbCr0.5 alloy in H2SO4 solution, related with the strong ability to passive for Mo and V which can form protective passivation coatings on the surface of alloys in the acid solutions.
For TiZr0.5NbCr0.5Valloy, corrosion initiates mainly on the boundaries of the BCC phase and Cr2Zr Laves phase in NaCl solution at room temperature.The boundaries between different phases are the areas where are packed with crystal defects, impurities, alloying elements and where atomic arrangement is loose and disorder, which lead to be active in the boundaries and initiate corrosion.So, in the TiZr0.5NbCr0.5Valloy, the boundaries of the two phases are the weak zones and preferential to initiate corrosion.

Test Specimens for Electrochemical and Immersion Tests
Three refractory alloys were prepared by vacuum arc melting of the mixtures of high-purity metals with the purity better than 99 wt.% under a Ti-gettered high-purity argon atmosphere on a water-cooled Cu hearth.The alloys were remelted 4-5 times in order to improve homogeneity, the experimental details can consult Ref. [5].For electrochemical test, the specimens were cut in 0.7 × 0.7 × 0.3 cm 3 , electrically connected to an isolated copper wire, and cold-mounted in epoxy with the outside surface of 0.7 × 0.7 cm 2 exposed.For immersion test, the samples cut in 0.7 × 0.7 × 0.3 cm 3 were carefully mechanically polished with silicon carbide paper and dipped in 3.5% NaCl solution at 25 °C for 15 days.

Electrochemical Test
PP measurements were carried out in a typical three-electrode cell setup with the specimen as working electrode (WE), a saturated calomel electrode as reference electrode (RE), and a platinum plate as auxiliary electrode (AE), as shown in the Figure 6

XRD and SEM Analysis
The crystal structure was identified by X-ray diffraction (XRD) using a PHILIPS APD-10 diffractometer with Cu Kα radiation and 2θ range of 20°-90°.The microstructure was analyzed with the use of a FEI Quanta250 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) equipped with energy dispersive spectrometry (EDS, FEI, Hillsboro, OR, USA).

Conclusions
A new series of TiZr0.5NbCr0.5VxMoyrefractory HEAs have been successfully prepared and their corrosion behaviors have been studied.All of TiZr0.5NbCr0.5, TiZr0.5NbCr0.5V and TiZr0.5NbCr0.5Moalloys consist of BCC DSS phase and Cr2Zr phase and exhibit obvious dendrite structures.BCC phase mainly concentrates on the dendrites and the inter-dendrites are ordered Cr2Zr phase.
All alloys show excellent corrosion resistance in 3.5 wt.% NaCl and 0.5 M H2SO4 solutions.At room temperature, all alloys possess high corrosion potential, low corrosion current density, very wide passive region (>1400 mV) and positive pitting potential, which indicates they have good resistance to general corrosion and pitting.
The addition of V and Mo decreases the resistance to general corrosion and increases the pitting corrosion resistance for the series of alloys in NaCl solution slightly, while they greatly improve the corrosion resistance in H2SO4 solution.With temperature increasing, the corrosion rate of the alloys increases and the TiZr0.5NbCr0.5Valloy is the least sensitive to temperature.
After immersion, no obvious corrosion regions are found on the surface of TiZr0.5NbCr0.5 alloy; localized corrosion initiates mainly on the boundaries between the BCC phase and Cr2Zr phase.
In addition, obvious serration and electrochemical noise are observed on the PP curves, which may be related to the pitting process.
. The quasi-steady-state time for an open circuit voltage was 600 s.The specimen was scanned potentiodynamically at a rate of 1 mV/s from the initial potential of −1.0 V versus open circuit potential to the final potential of 1.5 V.The base solutions for the test were 3.5 wt.% NaCl and 0.5M H2SO4.The effect of temperature on polarization was examined at an interval of 15 °C in the temperature range of 25-55 °C in sodium chloride solution.