The effect of Al addition on the corrosion behavior of high entropy alloys Al x CoCrFeNi (x = 0, 0.1, and 1.0) in supercritical water was investigated. The weight gain and oxide film thickness reduced with the increase of Al content. A duplex-structure oxide film, consisting of FeCr 2 O 4 outer and Cr 2 O 3 inner layer, was developed on CoCrFeNi alloy. A triple-layered oxide film consisting of FeCr 2 O 4 outer, Cr 2 O 3 middle, and Al 2 O 3 inner layer was developed on Al 0.1 CoCrFeNi and AlCoCrFeNi alloys. The improvement of corrosion resistance by adding Al was attributed to the high Al concentration that accelerated the formation of Al 2 O 3. • The Al addition on the corrosion behavior of Al x CoCrFeNi HEAs in SCW was investigated. • The weight gain and oxide film thickness reduced with the increase of Al content. • Duplex oxide layer was formed on CoCrFeNi alloy, while triplex oxide layer was developed on Al 0.1 CoCrFeNi and AlCoCrFeNi alloys. • The increase of Al content improved the corrosion resistance of Al x CoCrFeNi HEAs in SCW. [ABSTRACT FROM AUTHOR]