Hi David, I have worked in the nuclear industry previously and I do not know right offhand why the publication states that SCC is not an issue in Alloy 600 for caustic concentrations below 70 weight % for the range of temperatures you cite. Early in the commercial nuclear industry phosphate-based chemistries were used on the secondary side of steam generators. This led to the generation of caustic conditions in the heat transfer crevices and cracking in mill annealed Alloy 600 steam generator tubing. As a result, tests were performed to understand cracking in Alloy 600 tubing under caustic conditions. The tests were generally performed in 10 weight % NaOH solutions using U-bend specimens manufactured from Alloy 600 tubing and SCC readily occurred in the U-bend specimens. Thermally treated Alloy 690, however, provides much better performance in caustic environments and is one reason the industry is moving towards Alloy 690. Attached are three seminal papers from Roger Staehle summarizing the nuclear industry experience with Alloy 600 and Alloy 690, as well as the testing done to compare the two alloys in a variety of environments, including 10 weight % NaOH with and without contaminants (e.g., lead, cooper) added to the test solution.
Hello all,Thank you for your responses. In particular, the three-part paper by Staehle and Gorman is comprehensive and was very illuminating. It seems that there a few main differences between the nuclear applications and general caustic service: among them are electrochemical potential, deaerated vs. aerated environments, and the fact that the caustic environment in steam generators is formed in crevices where heat transfer occurs.
That being said, it is also clear that in laboratory environments in deaerated caustic--even without crevices--SCC readily occurs at high stress levels with certain applied potentials.
I think in our case, it is a matter of reviewing these factors more closely, and I believe that our application sits in a good position with regard to applied stress and no mechanism for further caustic concentration increase. Our environment also has a high dissolved oxygen level. We will have to give these details some more thought, but the discussion has certainly moved us in the right direction, so thank you again.