•Sn precipitated in Cr2O3 layer after steam oxidation of Cr-coated Zry-4 at 1000 ℃.•The outward diffusion and selective oxidation of Zr in Cr layer were studied.•Larger outward mass flow caused in Kirkendall cavities close to Zr/ZrCr2 interface.•Voids formed at oxide/metal interface of coating due to vacancy condensation.•Amorphization of Cr2O3 was observed which due to the decomposition of Cr2O3 grains. In this study, the steam oxidation behavior of Cr-coated Zry-4 alloy at 1000 °C and the various interesting phenomena observed during the oxidation process were investigated. During oxidation, Sn in the Zry-4 substrate diffused into the coating and precipitated in the Cr2O3 scale. The Kirkendall effect of these Sn-containing precipitates then generated microcavities. Furthermore, the outward diffusion behavior of Zr and the selective oxidation mechanism of the Zr particles were studied. Voids detected at the Cr2O3/Cr interface result mainly from vacancy condensation. The amorphization of Cr2O3 was also observed adjacent to the voids, and the amorphization mechanism was discussed.