**1. Introduction**

132 Corrosion Resistance

[66] D. Q. Yu, C. M. L. Wu, L. Wang, The Electrochemical Corrosion Behavior of Sn-9Zn and

[67] F. Rosalbino, E. Angelini, G. Zanicchi, R. Carlini, R. Marazza, Electrochemical Corrosion

Congress, Beijing, P.R. China, (2005)19–24.

7231-7237.

Sn-8Zn-3Bi Lead-Free Solder Alloys in. NaCl Solution, 16th International Corrosion

Study of Sn–3Ag–3Cu Solder Alloy in NaCl solution, Electrochim. Acta, 54 (2009)

#### **1.1 Pseudo-unitary lattice with a characteristic parameter as a description of multi-principal alloys – The high-entropy alloys (HEAs)**

In the summer of 1995, J.W. Yeh and the author (SKC) started the study of multi-principalelement alloys which was called, then, alloys with high randomness and now the high-entropy alloys (HEAs). SKC checked the first 10 equal-molar alloys, which was designed by Yeh that contained from 6 to 9 elements in the alloys out of one of Al, Cu, and Mo, together with Ti, V, Fe, Ni, Zr, Co, Cr, Pd, and B, with a home-made vacuum-arc remelter, and the author observed that the alloy series containing Mo can be made most easily, while the ones containing 3 at% B are the ones most difficult in melting, and 6 out of 10 can be formed in the water-cooled copper mold of the remelter, i.e., the existence of the HEAs was demonstrated by experiments. The alloys were aimed at that time to design as another kind of bulk glass alloys, and based on the high configurational entropy of R ln(n), n between 5 and 13, similar to the mixing of different gases [1]. No conclusions were drawn with XRD patterns of these alloys that were found two years later to be composed with peaks from a single simple lattice cell like FCC A1 or BCC A2, although some evidence of existence of amorphous phase was observed from TEM diffraction patterns and high resolution images [2,3]. The simple crystalline phases instead of amorphous ones were continuously found in alloys like in AlCoCrCuFeNi during research of HEAs in these 10 to 20 years, and identified with a so-called extended FCC or BCC unit cell that SKC called it a pseudo-unitary lattice in 2010 [4].

As multiple principal element alloys, high-entropy alloys (HEAs) comprise at least five elements whose concentration for each one ranges between 5 at % and 35 at % [5]. Attributes of forming a simple solid solution and nano-particle precipitation, as well as achieving a high hardness and strength, and excellent high-temperature oxidation resistance make HEAs highly promising for application and research and development of these alloys [6-9]. Properties of AlxCoCrFeNi (0 ≤ x ≤ 1) HEAs vary significantly with x [10]. For instance, the alloy structure changes from FCC to BCC for increased Al content x. Besides, the coefficient of thermal expansion decreases with x. Both properties are closely related to the bond strength of alloys. Moreover, electrical resistivity of AlxCoCrFeNi alloys is large, i.e., approximately up to 200 cm [11].

Electrochemical Passive Properties of AlxCoCrFeNi

Table 1. Composition (wt %) for alloys C-x and SS 304.

**2.1.2 Test solutions and temperatures** 

**impedance spectroscopy (EIS)** 

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 135

sulfuric acid for 1, 3, 5, 8, 11, and 15 days, respectively. All tests, except the weight loss test, were performed at least three times to confirm the data reproducibility. Finally, weight loss

The base solution for all tests was 0.5 M of sulfuric acid. Test temperatures were ambient temperature (~25oC). Test solutions bearing chloride ions were with 0.25, 0.50, and 1.00 M sodium chloride in the base solution. To avoid the dissolved oxygen (aeration) affecting the test solutions, deaeration was simultaneously made by a nitrogen gas flow of 120 ml/min in the test solution. The effect of temperature on polarization was examined under

A three-electrode cell was used for the electrochemical test. The reference electrode was a commercial Ag/AgCl electrode saturated in 3 M KCl electrode (−0.205 VSHE or –0.205 V to standard hydrogen electrode). The auxiliary electrode was made of Pt, and the working electrode was the specimen. Potentiostat was CH Instrument Model-600A. The specimen was cathodically polarized at a potential of −0.4 VSHE for 300 s before the test for the purpose of removing surface oxides. The quasi-steady-state time for an open circuit was 900 s. Scan speed was 1 mV/s for scan potential ranging from −0.6 VSHE to 1.4 VSHE. For EIS, the working potential was that of open circuit at 900 s from the start of immersion with scan

Samples were dipped in sulfuric acid for 15 d to determine the weight-loss rate. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis were performed with samples after a 0.8 VSHE pretreatment plus a 1-h immersion. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was performed on the electrolyte after an 8-d immersion of the samples. The effect of temperature on polarization was examined under

thermostatic control at an interval of 15oC in the temperature range of 20oC–65oC.

**2.4 Scanning electron microscopy (SEM) metallographic examination and energy** 

Samples were fine polished, up to 0.05 μm Al2O3 powder and, then, examined with SEM (JEOL JSM-840A) equipped with an Oxford EDS for topography and elemental

thermostatic control at an interval of 15oC in the temperature range of 20oC - 65oC.

**2.2 Potentiodynamic polarization curve measurements and electrochemical** 

amplitude 10 mV and a frequency ranging from 100 kHz to 10 mHz.

**2.3 Immersion tests and ICP-AES and XPS analyses** 

**dispersed X-ray spectroscopy (EDS) analysis** 

tests were performed twice and the reproducibility was given in an error bar.

Alloys Al Co Cr Fe Ni C-0 0 27.12 23.74 23.99 25.14 C-0.25 3.05 25.14 22.48 24.15 25.18 C-0.50 5.59 25.25 22.13 22.80 24.22 C-1.00 10.02 23.84 21.11 21.99 23.03 SS 304 0 0 19.40 72.68 7.92
