[en] The effect of corroded surface layer on the tensile properties at room and high temperatures was studied for Hastelloy X and Incoloy 800 by comparing the properties of corroded specimens with those of aged specimens. Corroded specimens were those subjected to 3,000-h and 50,000-h exposures at 1,000⁰C to air and VHTR helium. A small effect of exposure was observed both to helium and air in the case of Hastelloy X. In the case of Incoloy 800, however, exposure to air was found to reduce tensile properties markedly whereas exposure to helium scarcely reduced. Metallographic observation indicated that reduction of tensile properties was caused by intergranular oxidation. The ratio of the tensile strength (corroded specimen/aged specimen) being plotted against the amount of intergranular oxidation, data of the two alloys were superimposed on the same curve. Ductility minimum point observed on the solution treated materials at intermediate temperature was not detected on the aged and the corroded Hastelloy X, but was detected on the aged and the corroded Incoloy 800. (author)