An Auger Electron Spectroscopic Study of Phosphorus Segregation in the Grain Boundaries of Nickel Base Alloy 600

The grain boundary composition of nickel base alloy 600 has been studied by means of Auger electron spectroscopy. After being cathodically charged with hydrogen in a water saturated salt bath at 200 C, all the specimens could be intergranularly fractured in the Auger chamber. Phosphorus was the only element found to segregate at the grain boundaries of the two materials studied in all conditions of heat treatment considered; sulfur appeared essentially as a contaminant, which built up on the surfaces after fracture. The segregation of P was shown to be of the equilibrium (McLean) type, whereas Si did not segregate appreciably to the grain boundaries. The results, discussed in connection with published corrosion and stress corrosion data, explain the influence of P and the virtual absence of influence of Si on the sensitivity of alloy 600 to intergranular corrosion in HNO₃ + Cr⁶⁺ solutions. They also indicate that the segregation of P is not the cause of intergranular stress corrosion cracking of this alloy in pure water and caustic environment, and that hydrogen embrittlement is very unlikely to be the mechanism of this phenomenon.