SCC Initiation Testing of Nickel-Based Alloys Using In-Situ Monitored Uniaxial Tensile Specimens

Stress corrosion cracking (SCC) initiation tests were conducted on Alloy 600 and Alloy 600 weld metal (EN82H) at elevated temperatures (315°C to 360°C). Tests were conducted with insitu monitored smooth tensile specimens under a constant load in both hydrogen dearated and aerated environments. The use of uniaxial tensile specimens is an improvement over conventional U-bend specimens in which the stress state is difficult to characterize due to stress relaxation. Three different loading methods were assessed: ring-loading, pressure-loading, and active-loading (i.e., electric actuator or dead weights). Three insitu monitors were investigated: DC-electric potential drop, linear variable differential transformers, and electrochemical noise. Electric potential drop was the only in-situ crack detection method that consistently detected the onset of SCC. While under equivalent initial conditions each loading method initiated SCC, reproducible results were only obtained with pressure and active loading. In ring loading, the extent of SCC initiation was much less relative to active or pressure loading which is likely due to differences in plastic strain. A load history dependency was observed in oxygenated-sulfate solutions. The thermal activation energy was measured as 140 kJ/mol in hydrogenated water. SCC initiation appears to have a dissolved hydrogen dependency similar to SCC crack growth. Preliminary results suggest that the mechanical parameter which controls SCC initiation is plastic strain and not stress.