The effects of hydrogen on oxidation behaviors of Haynes 230 in high temperature steam environments were investigated. Mott-Schottky analysis was used to characterize the oxide layer and understand the oxidation mechanisms. The oxide layer of Haynes 230 exhibited an n-type semiconducting behavior in steam environments. From the slope of the Mott-Schottky plot, the defect density in the oxide layer was quantitatively estimated, which showed that it increased when 20 vol.% H-2 was added to a steam environment. Consequently, the diffusion of the oxidant in the oxide layer was enhanced and the oxidation rate increased in a steam + 20 vol.% H-2 environment. An oxidation mechanism was proposed which could explain the increase in oxidation rate and changes in oxide morphology when hydrogen was added to the high temperature steam environment. (C) 2013 Elsevier B.V. All rights reserved.