Display omitted •Three different roles of H were identified in structural modification of oxide film.•H modified the double-layer oxide film into a defective single-layer oxide film.•A redox chain reaction of Ni2+(NiO) → Ni0 → Ni2+(aq) selectively dissolved NiO.•Thickness of the oxide film was reduced by the in-situ charged H.•Growth of the oxide film under H charging followed a layer-by-layer mechanism. The roles of H in the modification process of the oxide film of alloy 600 was investigated through a dual-exposed oxidation experiment with an in-situ H-charging method in a high temperature high pressure water environment. The selective dissolution of the NiO modified the oxide film into a defective and porous, single-layered Cr2O3 skeleton after oxidation with H charging. The modification process depended on three different chemical states of H with a varied distribution: i) H2 molecule in water, ii) neutral H in the grain boundary of the oxide film, and, iii) proton (H+) in the lattice of the oxide film.