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

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 dependence of corrosion behaviour in various solutions.
