Effect of grain boundary chemistry on the intergranular fracture of iron at cathodic potentials
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A relationship between temper embrittlement and hydrogen embrittlement in quenched and tempered steels has been suggested by Yoshino and McMahon, ~ Viswanathan and Hudak, z and Briant, Feng, and McMahon. 3 These authors concluded that impurities such as P, Sn, Sb, As, and so forth which segregate to grain boundaries and induce intergranular fracture in temper embrittled steels also promote hydrogen embrittlement. Yoshino and McMahon I suggested that impurities and hydrogen each lowered the cohesive force required to separate two adjacent grains and that their effects were additive. Briant, et aP also assumed an additive impurity and hydrogen effect when relating the critical maximum principle stress at the tip of a crack and the cohesive strength of the weakest crystallographic plane. Briant, et al 3 proposed that hydrogen induced fracture occurs when the reduction in cohesion due to hydrogen equals the critical maximum principal stress. A mechanism where grain boundary impurities enhance hydrogen absorption was put forth by Latanision and Opperhauser. 4,5 In this mechanism, metalloid impurities such as S, P, Sn, and Sb which are hydrogen recombination poisons, segregate to grain boundaries and enhance the grain boundary permeation rate of hydrogen. They supported their hypothesis with results obtained from Ni-270 which contained Sn and Sb in the grain boundary and was found to fracture in an intergranular mode when tensile tested at cathodic potentials in 1N HESO4. Bernstein6 found that the fracture mode in hydrogen charged iron could be changed from transgranular to intergranular by heat treating temperature, cooling rate, and interstitial content. Solute partitioning between the grain boundaries and the grain was suggested as a controlling factor in the hydrogen induced cracking. R. H. JONES, TechnicalLeader,MetalPhysics,S. M. BRUEMMER, SeniorResearchScientist,M. T. THOMAS,TechnicalLeader, SurfaceScience,and D. R. BAER,SeniorResearchScientist,are with PacificNorthwestLaboratory,(operatedby BattelleMemorialInstitute), Richland,WA 99352. Manuscript submittedMay 29, 1980.
There is little doubt that grain boundary impurities enhance hydrogen induced intergranular fracture of steels, iron, and nickel; however, there is little evidence to support a particular mechanism. A better understanding of the impurity-hydrogen mechanisms will clarify whether they act independently or dependently and the role of various impurities such as S, P, As, Sn, Sb, C, 0 , N, and so forth, which can be present in varying amounts in the grain boundaries of many commercial or high-purity materials. It is important to recognize that both the chemical and electrochemical properties of these elements must be considered since their effect on stress corrosion cracking or hydrogen embrittlement of materials in aqueous solutions may depend on both. In order to elucidate the complex relationships between grain boundary chemistry and hydrogen embrittlement, straining electrode tests have been conducted on zone refined and vacuum melted iron which w
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