Corrosion of Advanced Steam Generator Alloy Weldments in 1100 and 1200 F (595 and 650 C) Steam

In gas-cooled steam generators for advanced nuclear systems, it is expected that economic considerations will dictate that certain materials will be used in high-temperature regions; others will be specified for intermediate regions; and still others will be justified at lower temperatures. Thus, dissimilar and similar metal welds joining various base metals will be subjected to the steam coolant. This paper discusses the results of initial studies at the lower temperatures of interest, that is 1100 and 1200 F. Additional studies at 1300 and 1400 F (705 and 760 C) are planned. The investigation concerned general and preferential steam corrosion and test conditions consisted of isothermal exposures to superheated steam at 1100 and 1200 F (595 and650 C) for periods to 8000 hr. The steam was of good commercial quality, flowing at a 5-ft/sec velocity and 900 psi (63.5 kg/sq cm) pressure. While similar studies have been made on several steam generator materials in wrought form,^1-3  this program marks the first comprehensive investigation of weldments in these alloys with their inherent compositional variations. Of particular concern was the prospect of aggravated attack at fusion-line areas, especially in dissimilar metal welds.