**3.5 Metallographic examination and EDS analysis**

Microstructures for not H2SO4-immersed alloys C-0, C-0.25, C-0.50, and C-1.00 are with single FCC, single FCC, duplex FCC-BCC, and BCC-ordered BCC phases, respectively [6]. Table 6 lists the EDS composition for each phase in different alloys.


Table 6. EDS analyses (at %) for alloys C-0, C-0.25, C-0.50, and C-1.00.

Electrochemical Passive Properties of AlxCoCrFeNi

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 147

microstructure. The composition of BCC phase after alloy immersion is close to the overall alloy composition before immersion, indicating that the BCC phase is a corrosion-resistant phase. Moreover, the change in overall composition after immersion is attributed to the

selective dissolution of Al and Ni in the ordered BCC phase of this alloy (Table 6).

Fig. 12. Metallograph of alloy C-0.50. (a), before; (b) & (c), after immersion; and

containing this bonding readily react with (OH)-

procedure.

passive film.

(d), EDS line-scan results of the location indicated in (c). Retained holes were from cast

This selective corrosion in Al and Ni-rich phase in C-0.50 and C-1.00, which results in the corrosion attack on Al and Ni, is due to the large bonding in Al and Ni [37]. Alloys

and dissolve in a sulfuric solution. Accordingly, after immersion, the remaining compound in the less corrosive-resistant Al and Ni-rich phase is an oxide, rich in Cr, in the residual

and (SO4)2- to form Al and Ni complexes

Figs. 11(a)-(b) show the microstructure of C-0 before and after 3-d immersion in 0.5 M H2SO4, respectively. Figs. 11(c)-(d) show the microstructure of C-0.25 before and after 3-d immersion in 0.5 M H2SO4, respectively. General corrosion occurs for both C-0 and C-0.25, as revealed by EDS analyses (Table 6).

Fig. 11. Metallograph of alloys C-0 ((a) & (b)) and C-0.25 ((c) & (d)). (a) & (c), before; and (b) & (d), after immersion. Retained holes were from cast procedure, and Al2O3 residuals were from polishing procedure.

Figs. 12(a)-(b) show the microstructure of C-0.50 before and after 3-d immersion in 0.5 M H2SO4, respectively. According to these figures, after immersion the FCC phase remains smooth while the BCC phase shows a rough morphology. Fig. 12(c) shows a line-scanned area across the FCC and BCC phases for an immersed sample. Fig. 12(d) summarizes the line-scanned results, indicating that the BCC phase of C-0.50 before immersion is rich in Al and Ni. However, after immersion, it is poor in Al and Ni and rich in Cr.

Fig. 13 shows the microstructure and line-scan analysis of C-1.00 before and after immersion. Before immersion, BCC and ordered BCC phases cannot be resolved from the

Figs. 11(a)-(b) show the microstructure of C-0 before and after 3-d immersion in 0.5 M H2SO4, respectively. Figs. 11(c)-(d) show the microstructure of C-0.25 before and after 3-d immersion in 0.5 M H2SO4, respectively. General corrosion occurs for both C-0 and C-0.25,

Fig. 11. Metallograph of alloys C-0 ((a) & (b)) and C-0.25 ((c) & (d)). (a) & (c), before; and (b) & (d), after immersion. Retained holes were from cast procedure, and Al2O3 residuals were

Figs. 12(a)-(b) show the microstructure of C-0.50 before and after 3-d immersion in 0.5 M H2SO4, respectively. According to these figures, after immersion the FCC phase remains smooth while the BCC phase shows a rough morphology. Fig. 12(c) shows a line-scanned area across the FCC and BCC phases for an immersed sample. Fig. 12(d) summarizes the line-scanned results, indicating that the BCC phase of C-0.50 before immersion is rich in Al

Fig. 13 shows the microstructure and line-scan analysis of C-1.00 before and after immersion. Before immersion, BCC and ordered BCC phases cannot be resolved from the

and Ni. However, after immersion, it is poor in Al and Ni and rich in Cr.

as revealed by EDS analyses (Table 6).

from polishing procedure.

microstructure. The composition of BCC phase after alloy immersion is close to the overall alloy composition before immersion, indicating that the BCC phase is a corrosion-resistant phase. Moreover, the change in overall composition after immersion is attributed to the selective dissolution of Al and Ni in the ordered BCC phase of this alloy (Table 6).

Fig. 12. Metallograph of alloy C-0.50. (a), before; (b) & (c), after immersion; and (d), EDS line-scan results of the location indicated in (c). Retained holes were from cast procedure.

This selective corrosion in Al and Ni-rich phase in C-0.50 and C-1.00, which results in the corrosion attack on Al and Ni, is due to the large bonding in Al and Ni [37]. Alloys containing this bonding readily react with (OH) and (SO4)2- to form Al and Ni complexes and dissolve in a sulfuric solution. Accordingly, after immersion, the remaining compound in the less corrosive-resistant Al and Ni-rich phase is an oxide, rich in Cr, in the residual passive film.

Electrochemical Passive Properties of AlxCoCrFeNi

including Icri, Ipass, Wloss, and Rf, for AlxCoCrFeNi in H2SO4.

Fig. 14. (a) Icri, (b) Ipass, (c) Wloss, (d) Qf, (e) Rf, and (i) Icorr values vs. Al content x plots.

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 149

Ipass locate at the passive region of polarization curve. The same tendency for Icri, Ipass, and Wloss here indicates that the spontaneous passivation occurs for AlxCoCrFeNi, i.e., the open circuit potential (OCP) is readily in the passive region of polarization curve. The above phenomenon can be attributed to the spontaneous passivation of pure Al in H2SO4 [24]. EIS equivalent circuits reveal that the passive layers of AlxCoCrFeNi consist of an oxide layer and an adsorption layer mentioned in Section 3.3. Here, only parameters associated with the oxide layer, i.e., Qf and Rf, are discussed. The oxide layer thickness is evaluated by using the Helmholtz model mentioned above and denoted by d, as d =0S/Qf, where denotes the permittivity of free space (8.85 x 10-14 F/cm), denotes the dielectric constant of the medium, and S denotes the surface area of the electrode. Assuming that and S for oxide layers of alloys are the same allows us to compare relative values of d for all samples by 1/Qf. Figs. 14(d)-(e), whose data were listed in Table 5, show the Qf and Rf vs. x plot, respectively. Both Qf and Rf decreases with x. This represents that d increases with Al content x, and a thicker oxide layer implies a smaller value of impedance. Therefore, one can explain this phenomenon by considering both the thickness and the density of oxide layer. Related study reported Al oxide easily forms a porous structure [25]. Hence, it is easily understood that in addition to causing a thicker oxide layer, Al promotes the dispersive and porous oxide layer. In summary, Al has a negative effect to the passive parameters,

Fig. 13. Metallograph of alloy C-1.00. (a), before; (b) & (c), after immersion; and (d), EDS line-scan results of the location indicated in (c).

#### **3.6 Comparison among potentiodynamic polarization, electrochemical impedance spectroscopy, and weight-loss immersion tests**

As discussed above, the corrosion current density (Icorr), the critical current density (Icri), and the passive current density (Ipass) were obtained from potentiodynamic polarization. The capacitance (Qf) and the resistance (Rf) of oxide layer were obtained from electrochemical impedance spectroscopy (EIS) equivalent circuits. And the weight-loss rate (Wloss) was obtained from weight-loss immersion test. All these data were taken from experiments at ambient temperature (25oC) in 0.5 M H2SO4.

Figs. 14(a)-(b), whose data were listed in Table 2, show Icri and Ipass vs. Al content x plots, respectively. One can easily see that both Icri and Ipass increase with x. This implies that the passive corrosion property of AlxCoCrFeNi decreases with Al content x. Fig. 14(c) shows Wloss vs. x plot. Like Icri and Ipass, Wloss also increases with x. Notice that, unlike potentiodynamic polarization, immersion weight-loss test is a natural electrochemical reaction, i.e., without applying any voltage on the test sample. On the other hand, Icri and

Fig. 13. Metallograph of alloy C-1.00. (a), before; (b) & (c), after immersion; and (d), EDS

**3.6 Comparison among potentiodynamic polarization, electrochemical impedance** 

As discussed above, the corrosion current density (Icorr), the critical current density (Icri), and the passive current density (Ipass) were obtained from potentiodynamic polarization. The capacitance (Qf) and the resistance (Rf) of oxide layer were obtained from electrochemical impedance spectroscopy (EIS) equivalent circuits. And the weight-loss rate (Wloss) was obtained from weight-loss immersion test. All these data were taken from experiments at

Figs. 14(a)-(b), whose data were listed in Table 2, show Icri and Ipass vs. Al content x plots, respectively. One can easily see that both Icri and Ipass increase with x. This implies that the passive corrosion property of AlxCoCrFeNi decreases with Al content x. Fig. 14(c) shows Wloss vs. x plot. Like Icri and Ipass, Wloss also increases with x. Notice that, unlike potentiodynamic polarization, immersion weight-loss test is a natural electrochemical reaction, i.e., without applying any voltage on the test sample. On the other hand, Icri and

line-scan results of the location indicated in (c).

ambient temperature (25oC) in 0.5 M H2SO4.

**spectroscopy, and weight-loss immersion tests** 

Ipass locate at the passive region of polarization curve. The same tendency for Icri, Ipass, and Wloss here indicates that the spontaneous passivation occurs for AlxCoCrFeNi, i.e., the open circuit potential (OCP) is readily in the passive region of polarization curve. The above phenomenon can be attributed to the spontaneous passivation of pure Al in H2SO4 [24]. EIS equivalent circuits reveal that the passive layers of AlxCoCrFeNi consist of an oxide layer and an adsorption layer mentioned in Section 3.3. Here, only parameters associated with the oxide layer, i.e., Qf and Rf, are discussed. The oxide layer thickness is evaluated by using the Helmholtz model mentioned above and denoted by d, as d =0S/Qf, where denotes the permittivity of free space (8.85 x 10-14 F/cm), denotes the dielectric constant of the medium, and S denotes the surface area of the electrode. Assuming that and S for oxide layers of alloys are the same allows us to compare relative values of d for all samples by 1/Qf. Figs. 14(d)-(e), whose data were listed in Table 5, show the Qf and Rf vs. x plot, respectively. Both Qf and Rf decreases with x. This represents that d increases with Al content x, and a thicker oxide layer implies a smaller value of impedance. Therefore, one can explain this phenomenon by considering both the thickness and the density of oxide layer. Related study reported Al oxide easily forms a porous structure [25]. Hence, it is easily understood that in addition to causing a thicker oxide layer, Al promotes the dispersive and porous oxide layer. In summary, Al has a negative effect to the passive parameters, including Icri, Ipass, Wloss, and Rf, for AlxCoCrFeNi in H2SO4.

Fig. 14. (a) Icri, (b) Ipass, (c) Wloss, (d) Qf, (e) Rf, and (i) Icorr values vs. Al content x plots.

Electrochemical Passive Properties of AlxCoCrFeNi

a very small amount of Ni(OH)2 appears in our case.

\*General corrosion, \*\*Selective dissolution in Al and Ni

C-1.00.

with the results of EIS.

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 151

A value for the identified sputter rate for SiO2 in this AES device is 7.5 nm per min. Because the immersion for the samples is in the same 1-h period of time, the oxide layers of C-0, C-0.25, C-0.50, and C-1.00 can be distinguished from the terminals of the negative slope shown in each of the profiles. Look at the vertical red-dashed line, i.e., the end-terminal of each profile. It represents the interface between the oxide layer and the intrinsic metal. One can see that the thickness of the oxide layer increases with Al content x that is in accordance

XPS analyses attempt to investigate the binding energy profile of 2psub<3/2>sub for Al, Co, Cr, Fe, and Ni. Compared to Co, Cr, Fe, and Ni, Al reveals relatively low atomic sensitivity factor [38]. The signals of Al for alloys C-0 to C-0.50 are too small to identify. Hence, only C-1.00 was used for XPS analysis. Figs. 16(a)-(b) show the Al(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. The raw profile revealing two main peaks represents the exhibition of the selective dissolution. The oxides consists of Al2O3, Al(OH)3, and Al25Ni75Ox. Al tends to form oxides in H2SO4 [24] can explain the formation of Al2O3 and Al(OH)3. The existence of Al25Ni75Ox results from the relatively negative enthalpy of Al and Ni. Corresponding to the ICP-AES analysis in the next section, Al-Ni selective dissolution undoubtedly exists for C-1.00. Figs. 16(c)-(d) show the Co(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. The binding types of Co2+ and Co3+ can be seen. Compare Fig. 16(c) with Fig. 16(d), one can see that the peak intensity of Co2O3 is very small in the deep region of the oxide layer. Figs. 16(e)-(f) show the Cr(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. Three kinds of oxide, including Cr2O3, Cr(OH)3, and CrO3, exist for Cr [39]. However, CrO3 merely forms at high temperatures. Hence, only Cr2O3 and Cr(OH)3 are revealed in the profile. One can see that the deep region of oxide layer remains in relatively small amount Cr2O3. Figs. 16(g)-(h) show the Fe(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. Similar to references [23,40] Fe3O4 and Fe2O3 oxides can be found. Figs. 16(i)-(j) show the Ni(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. In resemblance with reference [41], NiO and Ni(OH)2 can be observed. However,

Table 7 lists the results of ICP-AES of immersion solutions for C-x. To trace the ions resulting from the intrinsic metal, one can study the selective dissolution of the alloy elements. Compared with C-0 and C-0.25, C-0.50 and C-1.00 reveal relatively greater Al-Ni

Alloys Al Co Cr Fe Ni Remarks

selective dissolution. This event is consistent with the results of the XPS analysis.

C-0 alloy 0 25.93 25.73 24.21 24.13 solution 0 24.91 24.89 25.17 25.01 \*

C-0.25 alloy 6.16 23.27 23.58 23.59 23.40

C-0.50 alloy 11.01 22.77 22.61 21.70 21.92

C-1.00 alloy 18.88 20.55 20.63 20.01 19.93

solution 7.90 23.01 23.07 23.20 22.80 \*

solution 14.92 21.90 16.51 19.96 26.70 \*\*

solution 31.52 20.98 4.86 14.92 27.71 \*\*

Table 7. ICP-AES composition (at%) of immersion solution for alloys C-0, C-0.25, C-0.50, and

Interestingly, Al makes a different effect on general corrosion. Fig. 14(f) shows Icorr vs. x plot. One can see that Icorr decreases with x. This implies that Al promotes the general corrosion resistance, but degrades the passive one.

#### **3.7 AES, XPS, and ICP-AES analyses of oxide layers**

Figs. 15(a)-(d) show the AES results for C-0, C-0.25, C-0.50, and C-1.00, respectively. Owing to the slight difference of atomic number, the signals of Fe, Co, and Ni overlap in AES analysis. Hence, one can see the signals of Co are higher than that of Fe or Ni even for the equal-mole nominal chemical composition of Fe, Co, and Ni. What mentioned above, only the longitudinal composition profiles of O are discussed. A negative and a near-zero slopes are revealed in the relative concentration vs. sputter time profiles in Figs. 15(a)-(d).

Fig. 15. AES analyses for (a) C-0, (b) C-0.25, (c) C-0.50, and (d) C-1.00.

Interestingly, Al makes a different effect on general corrosion. Fig. 14(f) shows Icorr vs. x plot. One can see that Icorr decreases with x. This implies that Al promotes the general corrosion

Figs. 15(a)-(d) show the AES results for C-0, C-0.25, C-0.50, and C-1.00, respectively. Owing to the slight difference of atomic number, the signals of Fe, Co, and Ni overlap in AES analysis. Hence, one can see the signals of Co are higher than that of Fe or Ni even for the equal-mole nominal chemical composition of Fe, Co, and Ni. What mentioned above, only the longitudinal composition profiles of O are discussed. A negative and a near-zero slopes are revealed in the relative concentration vs. sputter time profiles in Figs. 15(a)-(d).

resistance, but degrades the passive one.

**3.7 AES, XPS, and ICP-AES analyses of oxide layers** 

Fig. 15. AES analyses for (a) C-0, (b) C-0.25, (c) C-0.50, and (d) C-1.00.

A value for the identified sputter rate for SiO2 in this AES device is 7.5 nm per min. Because the immersion for the samples is in the same 1-h period of time, the oxide layers of C-0, C-0.25, C-0.50, and C-1.00 can be distinguished from the terminals of the negative slope shown in each of the profiles. Look at the vertical red-dashed line, i.e., the end-terminal of each profile. It represents the interface between the oxide layer and the intrinsic metal. One can see that the thickness of the oxide layer increases with Al content x that is in accordance with the results of EIS.

XPS analyses attempt to investigate the binding energy profile of 2psub<3/2>sub for Al, Co, Cr, Fe, and Ni. Compared to Co, Cr, Fe, and Ni, Al reveals relatively low atomic sensitivity factor [38]. The signals of Al for alloys C-0 to C-0.50 are too small to identify. Hence, only C-1.00 was used for XPS analysis. Figs. 16(a)-(b) show the Al(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. The raw profile revealing two main peaks represents the exhibition of the selective dissolution. The oxides consists of Al2O3, Al(OH)3, and Al25Ni75Ox. Al tends to form oxides in H2SO4 [24] can explain the formation of Al2O3 and Al(OH)3. The existence of Al25Ni75Ox results from the relatively negative enthalpy of Al and Ni. Corresponding to the ICP-AES analysis in the next section, Al-Ni selective dissolution undoubtedly exists for C-1.00. Figs. 16(c)-(d) show the Co(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. The binding types of Co2+ and Co3+ can be seen. Compare Fig. 16(c) with Fig. 16(d), one can see that the peak intensity of Co2O3 is very small in the deep region of the oxide layer. Figs. 16(e)-(f) show the Cr(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. Three kinds of oxide, including Cr2O3, Cr(OH)3, and CrO3, exist for Cr [39]. However, CrO3 merely forms at high temperatures. Hence, only Cr2O3 and Cr(OH)3 are revealed in the profile. One can see that the deep region of oxide layer remains in relatively small amount Cr2O3. Figs. 16(g)-(h) show the Fe(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. Similar to references [23,40] Fe3O4 and Fe2O3 oxides can be found. Figs. 16(i)-(j) show the Ni(2psub<3/2>sub) spectra of C-1.00 after the sputter times of 20 and 35 s, respectively. In resemblance with reference [41], NiO and Ni(OH)2 can be observed. However, a very small amount of Ni(OH)2 appears in our case.

Table 7 lists the results of ICP-AES of immersion solutions for C-x. To trace the ions resulting from the intrinsic metal, one can study the selective dissolution of the alloy elements. Compared with C-0 and C-0.25, C-0.50 and C-1.00 reveal relatively greater Al-Ni selective dissolution. This event is consistent with the results of the XPS analysis.


\*General corrosion, \*\*Selective dissolution in Al and Ni

Table 7. ICP-AES composition (at%) of immersion solution for alloys C-0, C-0.25, C-0.50, and C-1.00.

Electrochemical Passive Properties of AlxCoCrFeNi

**3.8 Corrosion current density (Icorr) at various temperatures** 

dispersive oxide dominates. Therefore, this special phenomenon occurs.

Fig. 17. Icorr values for alloys C-x (Alx) at various temperatures.

**4. Conclusions** 

long-term dipping.

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 153

As mentioned in Section 3.1, Icorr decreases with Al content at 25oC. However, this differs from temperatures to temperatures. Fig. 17 shows the Icorr values of C-x at various temperatures. One can see that Icorr decreases with Al content x at low temperatures (< 27oC), and, conversely, at high temperatures (> 27oC). The EIS results (Section 3.1) indicate that more Al content x makes the oxide layers thicker and more dispersive. At low temperatures, the thicker oxide is the dominator for Icorr; whereas, at high temperatures, the

Owing to the spontaneous passivation of Al element in H2SO4, the variation of Al reveals a more apparent effect in a passive region rather than in an active one. Therefore, in contrast with Ipass, which increases with x, no obvious trends occur for Ecorr and Icorr vs. x variation. In particular, the weight loss experiment indicates that Ipass is a proper index to evaluate the weight loss of samples since AlxCoCrFeNi alloys are found to have passive behaviour in

EIS results indicate that the passive films of AlxCoCrFeNi alloys become increasingly thicker and more dispersive with an increasing x. Therefore, Ipass increases with x. As x value increases to 1.00, the inductance effect appears in the equivalent circuit for severe dissolution of Al and Ni-rich phase. As for the effect of chloride on the anti-corrosion property, chloride eases the passive layer to form metastable ion complexes and further dissolve into H2SO4. With an increasing chloride concentration and Al content, the metastable ion complexes easily form, allowing Epit to shift to a more active region. Additionally, the microstructure of both C-0 and C-0.25 is single FCC phase, while those of C-0.50 and C-1.00 are duplex FCC-BCC and complex BCC-ordered BCC phase, respectively.

Fig. 16. XPS analyses after pre-sputtering for (a) Al-20 s, (b) Al-35 s, (c) Co-20 s, (d) Co-35 s, (e) Cr-20 s, (f) Cr-35 s, (g) Fe-20 s, (h) Fe-35 s, (i) Ni-20 s, and (j) Ni-35 s.

Fig. 16. XPS analyses after pre-sputtering for (a) Al-20 s, (b) Al-35 s, (c) Co-20 s, (d) Co-35 s,

(e) Cr-20 s, (f) Cr-35 s, (g) Fe-20 s, (h) Fe-35 s, (i) Ni-20 s, and (j) Ni-35 s.

### **3.8 Corrosion current density (Icorr) at various temperatures**

As mentioned in Section 3.1, Icorr decreases with Al content at 25oC. However, this differs from temperatures to temperatures. Fig. 17 shows the Icorr values of C-x at various temperatures. One can see that Icorr decreases with Al content x at low temperatures (< 27oC), and, conversely, at high temperatures (> 27oC). The EIS results (Section 3.1) indicate that more Al content x makes the oxide layers thicker and more dispersive. At low temperatures, the thicker oxide is the dominator for Icorr; whereas, at high temperatures, the dispersive oxide dominates. Therefore, this special phenomenon occurs.

Fig. 17. Icorr values for alloys C-x (Alx) at various temperatures.
