**2.1.2 Test solutions and temperatures**

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

#### **2.2 Potentiodynamic polarization curve measurements and electrochemical impedance spectroscopy (EIS)**

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

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

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 dispersed X-ray spectroscopy (EDS) analysis**

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

Electrochemical Passive Properties of AlxCoCrFeNi

Ipass increases with x.

[25].

(x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids 137

Table 2 reveals that the corrosion potential (Ecorr) and the corrosion current density (Icorr) for all of the alloys differ only slightly, and no obvious trends occur for Ecorr and Icorr vs. x variation. The above phenomenon can be attributed to the spontaneous passivation of pure Al in H2SO4 [22]. Al metal spontaneously passivates in H2SO4, explaining why its corrosion potential is ready in the passive region, i.e., this passivation explains why the polarization curve of Al does not display an apparent active-passive transition region. However, elements such as Cr and Fe exhibit a large critical current density (Icri) for passivation, explaining why Cr and Fe dissolve more than Al before the alloy reaches its passive state. Thus, the variation of Al affects the active region of the polarization curves slightly. Furthermore, in H2SO4, all Al, Co, Cr, Fe, and Ni metals show passivity. Among them, Al has a relatively high passive current density (Ipass) [22,23] because only Al oxide can easily form a porous film on the metal surface [24]. Therefore, protection by oxide layer on the alloys with higher Al content is inferior to that with lower Al content. Fig. 1 thus reveals that

The results of potentiodynamic polarization were compared via performing 15-day-dipping and weight loss experiments. In the 15-day-dipping and weight loss experiments, the corrosion rates for C-0.50 and C-1.00 were markedly higher than those of C-0 and C-0.25 (Fig. 2). This observation differs substantially from the values of Icorr obtained from polarization experiment (Fig. 1), in which the two groups only differ slightly, despite the fact that the trend is the same. A previous study found a similar deviation in corrosion current densities obtained from weight loss test and potentiodynamic polarization method

Fig. 2. Diagram showing change in corrosion rate (g m-2 h-1) in the 15-day-dipping and

Fig. 1 shows potentiodynamic polarization diagrams for the AlxCoCrFeNi alloys and SS 304. The alloys have better overall general corrosion behaviour, with a larger Ecorr and smaller

weight loss measurement for alloys C-x.

Icorr, Icri, and Ipass than SS 304.

compositions. Finally, samples were examined before and after 3 days immersion of 0.5 M H2SO4.
