Inhibition of hydrogen environment embrittlement by SO₂

Results of tensile tests conducted on SAE 4340 steel and Inconel 718 exposed to H₂/SO₂ mixtures are presented. The tests were conducted to determine if small (less than or equal to1.5 percent) amounts of SO₂ could effectively inhibit hydrogen environment embrittlement (HEE) over a wide range (20 to 7000 psi) of hydrogen pressures. The tensile tests were conducted at a strain rate of 0.05 in./in.-min on notched round specimens having a K/sub t/ of 8.4. The test conditions and results are summarized in tables for SAE 4340 steel and Inconel 718. Considering the Inconel 718 test results, 0.5 percent SO₂ is a less effective inhibitor of HEE than an order-of-magnitude smaller concentration of O₂, which gives complete inhibition. A π.5 percent concentration of SO₂ is a better inhibitor of low (20 psi) than at elevated (7000 psi) hydrogen pressure. At approx.1 atm total pressure, π.5 percent SO₂ was able to arrest crack growth in a compact tension specimen, whereas tensile specimens exposed to the same concentration of SO₂ underwent a 17 percent loss in tensile strength. This difference in inhibition can be attributed to differences in crack growth rates between the two types of tests. Results of calculations show that crack arrest due to inhibition occurred at a stress intensity of approximately 32 ksi in.² The stress intensity calculated for the maximum load and the initial notch geometry is approximately 63 ksi in.² with a crack formed, the stress intensity would be higher yet. Thus, during much of its growth period the crack in the tensile specimen would be growing at a faster rate than in a precracked specimen with a reduced opportunity for inhibition by the SO₂. (authors)