The primary coolant loop of the high temperature gas-cooled reactor contains trace impurities that could cause some corrosion to the materials. When the HTGR operates at a very high temperature, the impurities in the coolant have significant influence on the properties of the alloys used in steam generators. Corrosion tests were conducted on the Inconel 800H, Inconel 617 and Hastelloy X, and the alloys are commonly used in the steam generator of HTGR. They were corroded for up to 48 hours at 950 °C in a given helium atmosphere with trace impurities, and the corrosion results were observed. The SEM with EDX system was utilized for the metallographic examination and surface oxide composition analysis. The AES was used to verify the EDS analysis results. Due to severe internal oxidation and surface carbon transfer, Inconel 800H has the worst corrosion resistance, but Hastelloy X with the dense oxide layer has the best corrosion resistance. For Inconel617, the Cr element depletion occurs near the corrosion layer, which is very consistent with the theory. Since a certain amount of manganese in Hastelloy X may contribute to the formation of a dense oxide layer, thus an appropriate increase of the manganese content may enhance the alloy’s corrosion resistance.

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