The Effect of Impurity Segregation on the Hydrogen Embrittlement of a High Strength Nickel Base Alloy in H2S Environments

Several nickel and cobalt base alloys are being investigated for application as tubulars in deep sour gas wells. The high strength and excellent weight loss corrosion resistance of these materials appear to meet the demands of deep sour gas well environments which contain large concentrations of H2S, CO2, and salt water. In the present study, C-ring specimens cut from cold reduced Hastelloy alloy C-276 tubulars, were coupled to steel and stressed in an aqueous NACE solution (5% NaCl, 0.5% acetic acid, saturated with H2S) and were found to exhibit adequate embrittlement resistance. However, moderate aging treatments in the range of 200 to 500 C resulted in increased susceptibility to hydrogen induced intergranular failure when tested under the same conditions. Based upon the increasing evidence of the effect of tramp impurities on environmentally induced embrittlement, it was hypothesized that grain boundary impurity segregation may be playing a role. Thus, samples of Hastelloy alloy C-276 were charged cathodically with hydrogen and then fractured in vacuum. Scanning Auger spectroscopy was used to determine the grain boundary composition of the unaged and aged specimens. The Auger results revealed considerable phosphorus segregation to the grain boundaries which correlated with the aging treatments. Experimental low phosphorus heats were tested in the same environment and showed improved resistance to embrittlement when bulk phosphorus levels were lowered significantly below commercial levels. The results of this investigation suggest a possible relation between the hydrogen embrittlement of this alloy and phosphorus segregated at the grain boundaries.