Abstract: A leakage problem occurred during the operation of the high-pressure cylinder side exhaust temperature measurement sleeve in a nuclear power plant. The causes of casing leakage were analyzed using macroscopic observation, chemical composition analysis, hardness testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the casing profile underwent thinning, with white shiny oxides and black matrix metals mainly presented at the thinning area. Under the erosion and corrosion of high-temperature steam, the casing was continuously thinned during the oxidation and detachment cycle, ultimately led to leakage of the casing.