Pinhole defects were found in a main combustion chamber made from NARloy-Z after an unexpectedly short time in service. Analysis indicated that the throat section of the liner had been exposed to very severe environmental conditions of high temperature and high oxygen content, which caused ductility loss and grain-boundary separation. The excessive oxygen content in the liner was attributed to diffusion from an oxygen-rich environment that had resulted from nonuniform mixing of propellants. The internal oxygen embrittled the alloy and reduced its thermal conductivity, which resulted in a higher hot-gas wall temperature and associated degradation of mechanical properties.
A Stirling engine heat pipe failed after only 2h of operation in a test situation. Cracking at the leading edge of an evaporator fin allowed air to enter the system and react with the sodium coolant. The fin was fabricated from 0.8 mm (0.03 in.) thick Inconel 600 sheet. The wick material was type 316 stainless steel. Macro- and microexaminations of specimens from the failed heat pipe were conducted. The fin cracking was caused by overheating that produced intergranular corrosion in both the fin and the wick. Recommendations for alleviating the corrosion problem included reducing the heat flux, redesigning the wick, and reducing the oxygen content of the sodium.