Effect of Sulfur and Antimony on the Intergranular Fracture of Iron at Cathodic Potentials
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INTRODUCTION
STRAINING electrode tests conducted at cathodic potentials on iron and iron-antimony alloys heat treated to produce a range of grain boundary sulfur and antimony :oncentrations have revealed that antimony is a potent em3rittler of iron tested in the presence of hydrogen. In a previous study, ~,2 the fracture mode of iron tested in IN H2504 at a potential of - 0 . 6 V and - 0 . 7 5 V (SCE) ~r shown to be uniquely related to the grain boundary ~ulfur concentration. The fracture mode changed from a'ansgranular to intergranular at a grain boundary sulfur :oncentration of 0.13 monolayers. This paper reports the ~,ffect of antimony additions on the fracture mode and duc:ility of iron when tested in IN H2804at cathodic potentials. This work was undertaken because temper embrittled steels have been shown by several authors 3,4,5 to be sus:eptible also to hydrogen embrittlement. Elements such as , S, Sb, As, and Sn, which segregate to grain boundaries and cause temper embrittlement, also enhance hydrogen induced intergranular fracture of steels. The role of antimony in temper embrittlement of steels was demonstrated by Low et al,6 where the presence of nickel and chromium was necessary for the antimony induced embrittlement, It has since been shown by several authors 7,8,9that the antimony concentration in the grain boundary is the controlling variable in this steel and that nickel enhances antimony segregation. Antimony in the grain boundaries of nickel has been suggested by Latanision and Opperhauser 10 to be a contributor to intergranular fracture in a hydrogen environment. Similar observations have not been reported for iron; how~,ver, Viswanathan and Hudak 4 found that the K~scc of steel doped with phosphorus, antimony, and tin was altered by the presence of phosphorus at the grain boundaries. Antimony and tin were not present in the grain boundaries at :letectable levels. Therefore, either antimony had a neglif i n e effect on the hydrogen embrittlement of iron and steels R.H. JONES, Staff Scientist, S.M, BRUEMMER, Senior Research ,cientist, M.T. THOMAS, Staff Scientist, and D.R. BAER, Senior ".esearch Scientist, are all with Metallurgy Research Section, Pacific 4orthwestLaboratory,P.O. Box 999, Richland, WA 99352. Manuscript submittedFebruary 17, 1981. r
TRANSACTIONSA
or its effect was manifested at grain boundary concentrations below the detectable limit of one pct of a monolayer. Latanision and Opperhauser 1~ proposed that some grain boundary impurities enhance hydrogen permeation and therefore enhance intergranular fracture. Their proposed mechanism is supported by hydrogen permeation studies in iron and steel which show that hydrogen permeation is strongly enhanced by compounds of elements belonging to the V-A and VI-A periodic groups. For instance, the addition of antimony trioxide, 5b203, to sulfuric acid enhances the permeation rate of hydrogen through iron by a factor of 1000X.I~ In aqueous solutions, the electrochemical stability of the grain boundary impurity may prove to be a significan
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