Environment-Assisted Intergranular Cracking
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ns when leaking natural gas ignites. C o m m o n r e q u i r e m e n t s for IGSCC include an aqueous or nonaqueous envi ronment containing a corrosive, depassivating species (acids, halides, sulfur Compounds, etc.), along with depletion of a protective alloying dement along grain boundaries; or the segregation of atoms that weaken grain-boundary strength (atomic hydrogen, sulfur, phosphorus, liquid mercury, etc.). Another requirement is sustained tensile stress. The precise role of stress varies considerably,
d e p e n d i n g on whether intergranular Separation is controlled by locally favorable electrochemical dissolution along a chemically weakened interface or by lo cally favorable fracture along a mechanically weakened interface. In the former case, stress can alter the thermodynamics of the corrosion reaction through the influence of lattice strain energy on the free energy of the atom being corroded. However, strain energy only alters the free energy (and hence the equilibrium oxidation potential of a stressed metal) by a negligible amount, compared with an unstressed metal. More likely, stress acts to separate and lift away corroded boundaries, to fracture remaining uncorroded ligaments that hold boundaries together, and to rupture any oxide films that protect boundaries from corrosive attack. In the latter case, the local stress level exceeds the environmentally degraded interfacial cohesive strength, and local boundary failure occurs. Grain boundaries are often susceptible crack paths because they have low inter facial energies as well as different nanostructure and nanochemistry, compared with grain interiors. These differences can establish a preferred intergranular crack p a t h along h o m o p h a s e (grain b o u n d a r i e s in a s i n g l e - p h a s e alloy)
Figure 1. Intergranular stress-corrosion cracking (IGSCC) of sensitized AISI 304 (100 h at 600°C) stainless steel during slow-hsing tensile testing in 0.5 M H2S04 + 0.01 M KSCN Solution under conditions where >23% of grain boundaries were activated by Cr depletion.65
MRS BULLETIN/JULY 1999
Environment-Assisted Intergranular Cracking
as well as heterophase interfaces. Differences in s t r u c t u r e and energy result from the creation of a solid-state interface, even in high-purity metals. Homophase-interface energy was first described by nearest-neighbor brokenbond modeis,' later by dislocation and disclination-based modeis, 2 and most recently by geometric modeis in combination with atomic Simulation.3 Given the five macroscopic and three microscopic degrees of freedom of homophase bound aries, energies and structures can vary greatly. 1 Their chemical character often differs as a result of segregation of detrimental impurities to boundaries or depletion of beneficial elements d u r i n g precipitation and growth of a second phase. Segregation often further lowers the energy of the interface.1 Detrimental foreign-atom impurities (S, P in Fe, Pb, Sn in Fe, Bi in Cu, S in Ni, etc.) are often dissolved in dilute solid Solution within
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