Corrosion characteristics of Ni-base superalloys in high temperature steam with and without hydrogen

摘要

The hot steam corrosion behavior of Alloy 617 and Haynes 230 were evaluated in corrosion tests performed at 900 °C in steam and steam + 20 vol.% H _2 environments. Corrosion rates of Alloy 617 was faster than that of Haynes 230 at 900 °C in steam and steam + 20 vol.% H_2 environments. When hydrogen was added to steam, the corrosion rate was accelerated because added hydrogen increased the concentration of Cr interstitial defects in the oxide layer. Isolated nodular MnTiO_3 oxides were formed on the MnCr_2O_4/ Cr_2O _3 oxide layer and sub-layer Cr_2O_3 was formed in steam and steam + 20 vol.% H_2 for Alloy 617. On the other hand, a MnCr_2O_4 layer was formed on top of the Cr _2O_3 oxide layer for Haynes 230. The extensive sub-layer Cr_2O_3 formation resulted from the oxygen or hydroxide inward diffusion in such environments. When hydrogen was added, the initial surface oxide morphology was changed from a convex shape to platelets because of the accelerated diffusion of cations under the oxide layer.