Influence of dissolved hydrogen on nickel alloy SCC in high temperature water

Stress corrosion crack growth rate (SCCGR) tests of nickel alloys were conducted at 338 C and 360 C as a function of the hydrogen concentration in high purity water. Test results identified up to a 7 x effect of hydrogen levels in the water on crack growth rate, where the lowest growth rates were associated with the highest hydrogen levels. At 338 C, the crack growth rate decreased as the hydrogen levels were increased. However, different results were observed for the test conducted at 360 C. As the hydrogen level was increased in the 360 C tests, the crack growth rate initially increased, a maximum was exhibited at a hydrogen level of {approximately} 20 scc/kg, and thereafter the crack growth rate decreased. Based on this testing and a review of the commercial literature, the thermodynamic stability of nickel oxide, not the dissolved hydrogen concentration, was identified as a fundamental parameter influencing the susceptibility of nickel alloys to SCC. These test results are discussed in relation to the accuracy of extrapolating high temperature SCC results to lower temperatures.