To examine the compatibility with liquid sodium possessed by structural materials for liquid-metal fast breeder reactor (LMFBR) steam generators, tests were conducted on selected steels in a sodium loop facility. Mass-transfer behavior of specimens exposed to sodium flow were examined for three conditions:

  1. 1000 h in sodium containing 8 ppm oxygen
  2. 1000 h in sodium containing 50 ppm oxygen
  3. 3000 h in sodium containing 9 ppm oxygen.
  The materials tested were ferritic 2¼ Cr—1 Mo, 2¼ Cr—1 Mo—1 Nb, high Cr—1 Mo, and austenitic AISI Type 316 stainless steels. The specimens were set during all three runs in identical positions and with identical temperature distribution for each of the three measurements. A method is proposed for estimating the rate of decarburization of the specimen by integrating the distribution curves of the carbon content determined from emission spectroscopic analysis of layers underlying the surface, which are successively uncovered by precision grinding. The decarburization behavior was found to reflect differences in the heat treatment history of the specimen, carbon loss being found to be more rapid on annealed than on normalized and tempered steels. On the other hand, differences in oxygen content of the sodium did not prove to affect decarburization behavior to a significant extent. The observed decarburization rate constants arranged in Arrhenius plots against reciprocal of absolute temperature reveal that for the annealed 2¼ Cr—l Mo steel, the plots fall unexpectedly in the region above the recommended design curve given by Armijo et al., though all the other specimens yield plots duly underlying Armijo’s line. Another finding worth noting is that when the chromium content of the specimen was altered, the carbon concentration immediately beneath the specimen surface was found to increase consistently with the chromium content, and in all three runs the decarburizing behavior shown by the lowchromium specimens changed to carburization in the range of chromium content between 3 and 5 wt% chromium. This also indicates that the circulating sodium did not modify its carbon activity to any significant extent during the three runs totaling 5000 h.