**2.1 Test materials and conditions for electrochemical and weight loss tests**

## **2.1.1 Test specimens for electrochemical tests and weight loss measurements**

As-cast AlxCoCrFeNi alloys were prepared according to molar ratios of x = 0, 0.25, 0.50, and 1.00 (called C-0, C-0.25, C-0.50, and C-1.00, respectively) in a vacuum arc remelter. Table 1 lists the composition of the alloys. Test specimens were cut in 0.8 cm x 0.8 cm x 0.3 cm and cold-mounted in epoxy with the outside surface from a surface of 0.8 cm x 0.8 cm of specimens. The specimens were subsequently ground and polished with grit #1000 silicon carbide paper, rinsed and dried in preparation for electrochemical tests and weight loss measurements. During determination of the weight loss, six sets of samples were dipped in

Corrosion properties of AlCoCrCu0.5FeNiSi [12,13], AlxCrFe1.5MnNi0.5 [14,15], and Al0.5CoCrCuFeNiBx [16] HEAs have been extensively studied in recent years. Among these HEAs, AlCoCrCu0.5FeNiSi alloy (HEA 1) displays, at room temperature, a better general corrosion resistance than SS 304 in 1 N H2SO4; however, it exhibits a worse pitting corrosion resistance than SS 304 in 1 N H2SO4 and in 1 M NaCl, respectively. The general corrosion resistance of each of HEA 1 and SS 304 decreases when exceeding room temperature. The effect of temperature on corrosion resistance of HEA1 is less severe in 1 M NaCl than in 1 N H2SO4 [13]. AlxCrFe1.5MnNi0.5 alloys (HEA 2) reveal that in each of the 0.5 M H2SO4 and 1 M NaCl solutions, corrosion resistance increases with a decreasing x; in addition, the susceptibility to general and pitting corrosion of HEA 2 increases with an increasing x [14]. AlxCrFe1.5MnNi0.5 alloys (called hereinafter as HEA 2a and 2b for x = 0 and 0.3, respectively) in 0.1 M HCl exhibit different corrosion behaviours for different x values. Although HEA 2a is susceptible to localized corrosion, HEA 2b has a stable passive film on the surface. In 0.1 M HCl, anodized treatment of HEA 2a and 2b alloys in 15 % H2SO4 gives higher corrosion resistance than the untreated [15]. In deaerated 1 N H2SO4, Al0.5CoCrCuFeNiBx alloys are more resistant to general corrosion than SS 304, and are not susceptible to localized corrosion. Additionally, the corrosion resistance of Al0.5CoCrCuFeNiB0.6 alloy is inferior to Al0.5CoCrCuFeNi alloy [16]. Above HEAs show an extremely close compositional

Although many interesting topics have been explored for AlxCoCrFeNi alloys [10,11], investigation on their corrosion property is still lacking. Therefore, this study elucidates how Al affects their corrosion behaviour. The electrochemical properties of the alloys in sulfuric acids are investigated using the potentiodynamic polarization curve and a weight loss measurement method. Additionally, these alloys are compared with SS 304, especially with respect to the effect of temperature. Moreover, based on use of electrochemical impedance spectroscopy (EIS), the effect of Al on corrosion behaviour is analyzed. Furthermore, the relationship of stability of oxide film with Al content is examined by varying the chloride concentration in a sulfuric solution. Additionally, this study, which extends [17], also attempts to investigate the mechanism of the passive layers influenced by Al content x at

**2.1 Test materials and conditions for electrochemical and weight loss tests 2.1.1 Test specimens for electrochemical tests and weight loss measurements** 

As-cast AlxCoCrFeNi alloys were prepared according to molar ratios of x = 0, 0.25, 0.50, and 1.00 (called C-0, C-0.25, C-0.50, and C-1.00, respectively) in a vacuum arc remelter. Table 1 lists the composition of the alloys. Test specimens were cut in 0.8 cm x 0.8 cm x 0.3 cm and cold-mounted in epoxy with the outside surface from a surface of 0.8 cm x 0.8 cm of specimens. The specimens were subsequently ground and polished with grit #1000 silicon carbide paper, rinsed and dried in preparation for electrochemical tests and weight loss measurements. During determination of the weight loss, six sets of samples were dipped in

**1.2 Corrosion resistance for HEAs and conventional alloys** 

dependence of corrosion behaviour in various solutions.

**1.3 Aim of this study** 

various temperatures in detail.

**2. Experimental details** 


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

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