**4. Conclusions**

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 long-term dipping.

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.

Electrochemical Passive Properties of AlxCoCrFeNi

Alloys & Comp. 488 (2009) 57-64.

stainless steel, Corros. Sci. 47 (2005) 2257-2279.

stainless steel, Corros. Sci. 47 (2005) 2679-2699.

environments, Corros. Sci. 50 (2008) 2053-2060.

acid, Thin Solid Films 517 (2008) 1301-1305.

Electrochem. Soc., 154 (2007) C424-C430.

Electrochem. Soc. 149 (2002) B187-B197.

duplex stainless steel, Corros. Sci. 49 (2007) 1847-1861.

B 163 (2009) 184-189.

1026-1034.

10 (1970) 709-718.

61 (2005) 3-11.

866 (1985) 197-206.

York, 2002.

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 155

[8] J. Tong, S.K. Chen, J.W. Yeh, T.T. Shun, C.H. Tsau, S.J. Lin, S.Y. Chang, Mechanical

[9] J.W. Yeh, S.K. Chen, J.Y. Gan, S.J. Lin, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang,

[11] H.P. Chou, Y.S. Chang, S.K. Chen, J.W. Yeh, Microstructure, thermophysical and

[12] Y.Y. Chen, T. Duval, U.D. Hung, J.W. Yeh, H.C. Shih, Microstructure and

[13] Y.Y. Chen, U.T. Hong, H.C. Shih, J.W. Yeh, T. Duval, Electrochemical kinetics of the

[14] C.P. Lee, C.C. Chang, Y.Y. Chen, J.W. Yeh, H.C. Shih, Effect of the aluminium content of

[15] C.P. Lee, Y.Y. Chen, C.Y. Hsu, J.W. Yeh, H.C. Shih, Enhancing pitting corrosion

[16] C. P. Lee, Y. Y. Chen, C. Y. Hsu, J. W. Yeh, and H. C. Shih, The Effect of Boron on the

[17] Y.F. Kao, T.D. Lee, S.K. Chen, Y.S. Chang, Electrochemical passive properties of

[18] V. Ashworth, P.J. Boden, Potential-pH diagrams at elevated temperatures, Corros. Sci.

[19] Y.Y. Chen, L.B. Chou, L.H. Wang, J.C. Oung, H.C. Shih, Electrochemical polarization

[20] M. Femenia, J. Pan, C. Laygraf, In situ local dissolution of duplex stainless steels in 1 M

[21] I.H. Lo, W.T. Tsai, Effect of selective dissolution on fatigue crack initiation in 2205

[22] F. D. Bogar, M. H. Peterson, A comparison of actual and estimated long-term corrosion

[23] P. Marcus, Corrosion Mechanisms in Theory and Practice, 2nd ed., Marcel Dekker, New

elements, Metall. Mater. Trans. A 36A (2005) 1263-1271.

performance of the AlxCoCrCuFeNi high-entropy alloy system with multiprincipal

Formation of simple crystal structures in Cu-Co-Ni-Cr-Al-Fe-Ti-V alloys with multiprincipal metallic elements, Metall. Mater. Trans. A 35A (2004) 2533-2536. [10] Y.F. Kao, T.J. Chen, S.K. Chen, J.W. Yeh, Microstructure and mechanical property of as-

cast, -homogenized, -deformed AlxCoCrFeNi (0 ≤ x ≤ 2) high-entropy alloys, J.

electrical properties in AlxCoCrFeNi (0 ≤ x ≤ 2) high-entropy alloys, Mater. Sci. Eng.

electrochemical properties of high entropy alloys—a comparison with type-304

high entropy alloys in aqueous environments—a comparison with type 304

AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous

resistance of AlxCrFe1.5MnNi0.5 high-entropy alloys by anodic treatment in sulfuric

Corrosion Resistance of the High-Entropy Alloys Al0.5CoCrCuFeNiBx, J.

AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00) alloys in sulfuric acids, Corros. Sci. 52 (2010)

and stress corrosion cracking of alloy 690 in 5-M chloride solutions at 25°C, Corros.

H2SO4 + 1 M NaCl by electrochemical scanning tunneling microscopy, J.

rate of mild steel in seawater, Laboratory Corrosion Test and Standards, ASTM STP

In C-0.50 and C-1.00, the secondary passivation phenomenon in polarization curve results from selective dissolution of the Al and Ni-rich phase.

Moreover, Icorr increases with x at higher temperatures (> 27 C), while Icorr decreases with x at lower ones (< 23 C). That more closely examining Arrhenius plots of Icorr reveals that both pre-exponential factor A and activation energy Ea increase with Al content. However, A affects Icorr more significantly than it does so for Ea at higher temperatures (> 27 C) and, conversely, at lower temperatures (< 23 C).

Al is an inferior factor to the passive corrosive resistance but helpful for the general corrosive resistance for AlxCoCrFeNi in H2SO4. The thickness and the density of oxide layers promoted by the addition of Al compete with each other at various temperatures. At ambient temperature, the thick oxide layer dominates Icorr value; at temperatures higher than 27 C, the loss oxide layer does. Intuitionally, one may improve the corrosion performance for AlxCoCrFeNi by adjusting Al content.
