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In this work, the Al content-dependent microstructure evolution and oxidation behavior of co-sputtered CrAl alloyed coatings on Zr-4 alloy were systematically investigated in 1000 ℃ steam. Experimental results demonstrated that the microstructure of generated oxide scale transformed from a Cr 2 O 3 oxide layer with embedding discrete α-Al 2 O 3 particles at low Al contents to a continuous outer (α, θ)-Al 2 O 3 and inner Cr 2 O 3 bi-oxide layer at high Al contents. However, there appeared a unique eutectoid-like Cr 2 O 3 /α-Al 2 O 3 multilayered oxide scale in Cr-18.3Al coating, which conferred the sample with optimal oxidation resistance. The underlying oxidation mechanisms were rationalized by revealing the Al content dominated selective oxidation behavior. • Appropriate Al addition can improve the steam oxidation resistance of Cr coating. • Al content dependent microstructure transformation of oxide scale is revealed. • Unique Cr 2 O 3 /α-Al 2 O 3 multilayered oxide scale in Cr-18.3Al coating contributes to high oxidation resistance. • Oxide-scale structure dominated failure mechanisms of CrAl coatings are discussed. [ABSTRACT FROM AUTHOR]