To better understand the differences between R30003 alloy corrosion in tritiated water and in H2O, a detailed study was made of the oxide layers produced in the former medium. The R30003 alloy was selected because of its nuclear corrosion resistance and its hardness, ensuring leaktightness when assembled with soft alloys. The characteristics and morphology of the formed oxide were investigated in corrosion potential, passive, and passive-transpassive regions where breakdown occurs. With tritiated water, the repassive potential is slightly lower than that obtained with H2O. Consequently, localized corrosion, which leads to corrosion in oxide sublayers, is greater and is produced by the effects of excited radiolytic products formed by time-dependent O3H− diffusion into the oxide. If enough tritium decay energy is absorbed by the oxide layer, excited and ionized states of the oxide are formed. Thus, reactive radiolytic species and voids accumulate in a small volume below the oxide surface. Spreading of these voids causes oxide cracking, leading to peeling and wall formation. Mechanisms for both processes and the electrochemical properties are described. The majority of the ionic carriers are in the peels, contributing to oxide delamination and thus steel degradation.