Influence of sulfur, phosphorus, and antimony segregation on the intergranular hydrogen embrittlement of nickel
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I.
INTRODUCTION
EVIDENCE for a relationship between grain boundary composition and intergranular hydrogen emhrittlement of metals is well documented. In particular, the enrichment of certain metalloid impurity elements at grain boundaries has been shown to induce intergranular failure of nickel, iron, and several of their alloys in the presence of hydrogen. Sulfur segregation was shown to control both the fracture mode and tensile ductility of nickel L2 and iron2'3'4 when tested at cathodic potentials. Threshold sulfur coverages on grain boundary fracture surfaces were identified where the fracture mode changed from transgranular ductile rupture to intergranular. The effects of phosphorus and antimony segregation on hydrogen embrittlement have not been as well characterized. Latanision and Opperhauser 5 demonstrated that the hydrogen embrittlement of nickel 270 was associated with the enrichment of antimony and tin at grain interfaces while phosphorus was suggested to play a part in the hydrogen-induced cracking of a nickel base superalloy by Berkowitz and Kane6 and Fiore and Kargol. 7 Although several elements have been identified to increase a metal's susceptibility to intergranular hydrogen embrittlement, very little is known concerning the relative embrittling potencies of specific elements. The work of Kameda and McMahon (as reported in Figure 10 of Reference 8) on a nickel-chromium steel and that of Jones et a/9'1~ on iron, are the only direct comparisons between intergranular fracture in a hydrogen environment and the grain boundary segregation of different elements. Kameda and McMahon8 found that the relative embrittling potencies of phosphorus, tin, and antimony on fracture toughness in hydrogen gas was consistent with temper embrittlement results. Antimony at the boundaries was determined to be the most effective in lowering the threshold stress intensity,
followed closely by tin, with phosphorus the least effective. The potencies of sulfur and antimony in iron were compared by Jones et al,9 and antimony again was shown to be the more detrimental. As a result of straining electrode tests at cathodic potentials, it was estimated that five times as much sulfur would be necessary at the grain interface to match antimony's effect on ductility and the tendency for intergranular fracture. The primary purpose of this paper is to report the relative hydrogen embrittling potencies (hereafter referred to simply as relative potencies) of sulfur, phosphorus, and antimony in nickel. Straining electrode experiments were performed on zone refined and vacuum melted nickel alloys in an aqueous solution of 1N H2SO4 at cathodic potentials. Grain boundary compositions were analyzed as a function of heat treatment using Auger Electron Spectroscopy. The direct comparison of interfacial compositions and mechanical properties was used to determine relative potencies and gain some further understanding of the cooperative effect of a segregant and hydrogen on the embrittlement of nickel.
II.
The effect of specific impurity el
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