Nickel base alloys 617 and 230 are promising candidates for the Intermediate Heat eXchanger (IHX) of GenIV Very High Temperature gas cooled Reactors. The capability to maintain an oxide layer as an efficient barrier against corrosion under mechanical loading is investigated through SEM in situ tensile test. The mechanical properties of external oxide layers are so compared between the two alloys. Cracks and spallation are observed. Few differences could be observed between these two alloys when pre oxidized in impure helium.