Untersuchungen zum Oxydationsverhalten von Modellegierungen auf Ni-Cr-Basis hinsichtlich der Eignung oxidischer Deckschichten als Kohlenstoff-Barrieren im Primärkreis-Helium fortgeschrittener HTR

To study the corrosion mechanisms in the complex gas-metal systems of high temperature reactor (HTR) primary coolant circuits, 26 nickel-chromium experimental alloys have been exposed in different gas compositions. The alloys contained systematically varied Cr, Al, Ti and Si levels. For the first exposure of the specimens (oxidizing conditions), the composition of the test gas was selected to give the same oxidation potential as the primary coolant of an HTR by adding to the helium controlled amounts of hydrogen and water. For the second exposure (carburizing conditions) methane was added to the test gas to investigate the effectiveness of the oxide in preventing carburization. The concentrations of the impurities in the test gases were a factor of 100 higher than in the PNP Standardhelium, in order to accelerate the reactions, and so shorten the test durations. The results of these experiments were compared with those obtained for the same alloys exposed in PNP Standardhelium. The corrosion products which formed on the specimens surfaces were investigated using optical metallography, scanning electron microscopy, electron probe microanalysis, and secondary ion mass spectroscopy. The carbon content of the alloys after exposure was also measured. The influence of the alloy and test gas composition on the corrosion processes was determined. In the post exposure examination of specimens, emphasis was given to the definition of the conditions necessary for the formation of oxide films which remained stable in methanecontaining test gases and therefore prevented carbon diffusion into the alloy