Anomalous Creep and Corrosion Effects Caused by Annealing Specimens in Evacuated Silica Capsules

Early tests on Alloy 800 done by the Metals Programme of the OECD Dragon Project showed that the creep and corrosion resistance of one particular cast was much poorer in HTR helium than in air. The work reported here showed that this effect was not an intrinsic property of the alloy, but was caused by an anomalous annealing treatment in evacuated silica capsules. This treatment led to severe carburisation of the alloy which was believed to cause the reduction in creep strength. Electron probe microanalysis has shown that the annealing treatment produced a thin film of titanium/aluminium oxide on the surface of the specimen. This oxide film appeared to prevent the subsequent formation of the protective chromium/manganese oxide layer normally found on Alloy 800 in HTR helium. The chromium free oxide can only form at solution annealing temperatures in the presence of a very restricted supply of oxygen (less than about —20 10 atm. in a thermodynamically stable system). Such conditions are unlikely to be met in a normal, commercial heat treatment. Therefore the possibility that this anomalous behaviour of Alloy 800 could occur in practical circumstances is believed to be extremely remote. (author)