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00226 DIFFUSION KINETIC ISSUES DURING INTERGRANULAR CORROSION OF Ni-BASE ALLOYS

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Product Number: 51300-00226-SG
ISBN: 00226 2000 CP
Author: E. P. Simonen, L. E. Thomas and S. M. Bruemmer
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Exposure of Ni-base alloys to hot water results in corrosion reactions at advancing stress corrosion cracks and intergranular degradation at crack walls. The morphology of the corrosion is characterized by very narrow widths of oxides for long distances along grain boundaries. Of particular interest is that this response of alloys occurs during exposure to non- oxidizing water at 330°C. The mechanisms by which oxidation reactions occur in these tight restricted pathways have critical impact on stress corrosion cracking susceptibility. In the present paper, diffusion kinetic analyses are related to observations of corrosion behavior ahead of crack tips and at grain boundaries intersecting cracks behind crack tips. It is concluded that solid-state diffusion of oxygen within alloy grain boundaries is not significant at the low temperatures of interest by conventional diffusion mechanisms. It is not possible for grain boundary internal oxidation to occur by direct analogy to high temperature matrix internal oxidation. However, solid-state diffusion is predicted to be highly promoted by assumed vacancy injection during corrosion and may contribute to reactions within the grain boundary. Nonequilibrium vacancy injection enhances diffusion of substitutional solute (Ni, Cr, Fe) and promotes ingress of interstitial oxygen by vacancy-oxygen binding interactions. Liquid-state diffusion and gas-state diffusion in the restricted open crack volumes of interest are very rapid and provide adequate kinetics to support observed penetration and cracking rates. Although ionic transport is rapid in aqueous solutions, the expected ionic concentrations are too low to rationalize oxide precipitation from aqueous solution during cooling from the exposure temperature. Finally, gas phase kinetics are shown to be very rapid and capable of supporting observed penetration and crack advance rates. However to maintain rapid transport over significant distances, the interconnected porosity required for transport must not be blocked by any solid path length. Kevwords: Grain boundary diffusion, corrosion, oxidation, Ni-base alloys.
Exposure of Ni-base alloys to hot water results in corrosion reactions at advancing stress corrosion cracks and intergranular degradation at crack walls. The morphology of the corrosion is characterized by very narrow widths of oxides for long distances along grain boundaries. Of particular interest is that this response of alloys occurs during exposure to non- oxidizing water at 330°C. The mechanisms by which oxidation reactions occur in these tight restricted pathways have critical impact on stress corrosion cracking susceptibility. In the present paper, diffusion kinetic analyses are related to observations of corrosion behavior ahead of crack tips and at grain boundaries intersecting cracks behind crack tips. It is concluded that solid-state diffusion of oxygen within alloy grain boundaries is not significant at the low temperatures of interest by conventional diffusion mechanisms. It is not possible for grain boundary internal oxidation to occur by direct analogy to high temperature matrix internal oxidation. However, solid-state diffusion is predicted to be highly promoted by assumed vacancy injection during corrosion and may contribute to reactions within the grain boundary. Nonequilibrium vacancy injection enhances diffusion of substitutional solute (Ni, Cr, Fe) and promotes ingress of interstitial oxygen by vacancy-oxygen binding interactions. Liquid-state diffusion and gas-state diffusion in the restricted open crack volumes of interest are very rapid and provide adequate kinetics to support observed penetration and cracking rates. Although ionic transport is rapid in aqueous solutions, the expected ionic concentrations are too low to rationalize oxide precipitation from aqueous solution during cooling from the exposure temperature. Finally, gas phase kinetics are shown to be very rapid and capable of supporting observed penetration and crack advance rates. However to maintain rapid transport over significant distances, the interconnected porosity required for transport must not be blocked by any solid path length. Kevwords: Grain boundary diffusion, corrosion, oxidation, Ni-base alloys.
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