High Temperature Oxidation Behavior of Alloy 617 and Haynes 230 in Impurity-Controlled Helium Environments

Abstract

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 A degrees C and 1000 A degrees C in impure helium environments containing H-2, H2O, CO, CO2, and CH4. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr2O3 layer, TiO2 ridges on the grain boundaries, and isolated MnCr2O4 grains on top of the Cr2O3 layer. On the other hand, those of Haynes 230 consisted of a Cr2O3 inner layer and a protective MnCr2O4 outer layer, which increased the oxidation resistance. The effect of small amounts of CH4 and H-2 on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr2O3 oxides for alloy 617 and MnCr2O4 platelets for Haynes 230, was formed in the impure helium environment at 900 A degrees C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr2O4 oxide crystals.