Abstract

Because of hydrogen generation from thermal decomposition of cesium formate, there were concerns that the use of cesium formate in certain applications may induce hydrogen embrittlement to CRA equipment after extensive exposure to elevated temperatures followed by cooling to low temperature under stress. This study focused on evaluating the severity of hydrogen embrittlement of seven nickel-based alloys at room temperature following exposure at elevated temperature to cesium formate of H2S/CO2 acid gases and consequently reduced pH. Unstressed slow strain rate (SSR) test specimens were previously exposed in an autoclave of cesium formate saturated with H2S/CO2 at an elevated temperature of 275 °F for 90 days. After exposure, significant hydrogen uptakes were observed under the tested conditions by measurement of total hydrogen concentration. The hydrogen-charged SSR specimens were then tested in air with 1 × 10−6 in./in./s strain rate at room temperature and compared with performance of the pristine specimens. In addition, three charged CRA alloys were also heated in furnace to release dissolved hydrogen and then tested in air. Two of the seven CRAs were also strained in situ in cesium formate at 275 °F. The SSR results (yield strength, ultimate tensile strength, reduction of area and elongation) at the different test scenarios are compared to identify the hydrogen embrittlement severity. Variation in hydrogen embrittlement susceptibility among the seven CRA alloys were observed. The HE severity did not seem to correlate with the measured total hydrogen concentration but relate to the yield strength of the material to a certain extent. The low yield strength (66 ksi) of alloy 625 is the only outlier which did not suffer from hydrogen embrittlement while the other high strength alloys with YS ranging from 118 ksi to 160 ksi suffered hydrogen embrittlement. The highest yield strength of alloy 945X (159 ksi) exhibited the lowest resistance to hydrogen embrittlement. The hydrogen embrittlement at room temperature also appears to be reversible after thermal removal of hydrogen.

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