Effect of Aging on the Anodic Behavior of Ni-Cr-Mo Alloys

Abstract

The precipitation response of corrosion resistant alloys based on the Ni-Cr-Mo phase diagram is complex with both intermetallic and carbide phases being formed during aging in the 650 to 1090 C (1202 to 1994 F) range. Although these phases markedly affect the corrosion resistance, their composition and structure have not been fully defined. Data are presented to show that the precipitating intermetallic phase in Hastelloy alloy C-276 is a (Ni, Fe, Co)3 (W, Mo Cr)2 Mu phase and the carbide is an M6C. Both phases have been shown to significantly increase the anodic dissolution rate of the passive region in boiling 10% sulfuric acid. Intergranular attack on aged alloy C-276 samples occurred due to attack of depleted regions in reducing acids and direct dissolution of precipitates under oxidizing conditions.

Prediction of the Lifetime Integrity of a Nuclear Waste Container Material Based on Thermal Stability Studies

“…At higher temperatures (above about 600X), several inter-metallic phases have been observed to form in Ni-Cr-Mo alloys (Leonard 1969, Hodge 1973, Tawancy et al 1983, Raghavan et al 1982, Tawancy 1996and 1980, and Summers et al 1998. Two of the intermetallics that form, p. phase and P phase, are similar both chemically and crystallographically (Leonard 1969, Raghavan et al 1982, Cieslak et al 1986.…”

“…This temperature is low from a metallurgical point of view, but the container lifetime, defined as the time for which there is reasonable assurance of no leakage, must be greater than 10,000 years. Because of the extremely long times involved and because the phases that form in C-22 and other similar Ni-base alloys are known to have a detrimental effect on their mechanical (Matthews 1976, Tawancy 1980 and corrosion properties (Leonard 1969, Hodge 1973, Hodge and Kirchner 1976, phase stability of the heavily alloyed C-22 alloy is of concern.…”

“…Also at the higher temperatures, carbides have been observed (Hodge 1973 andTawancy et al 1983) and are generally believed to be MO-rich MC or M& type carbides. These phases tend to be rich in either MO or Cr, which are the elements that give C-22 alloy its excellent corrosion resistance.…”

“…These phases tend to be rich in either MO or Cr, which are the elements that give C-22 alloy its excellent corrosion resistance. Precipitation of these phases leads to local Cr and MO depletion, which can cause an increased susceptibility to localized corrosion (Leonard 1969, Hodge 1973, Tawancy et al 1983, Heubner et al 1989, Tawancy 1996and 1980. At lower temperatures (below approximately 6OO"C), long-range ordering has been observed to occur in Ni-Cr-Mo alloys (Tawancy et al 1983, Tawancy 1996and 1980, Rebak and Koon 1998, and Summers et al 1998.…”

“…Although the phases forming and their effect on the properties have been studied extensively, very little work has been done on the kinetics of precipitation. Hodge (1973) measured volume fraction of precipitation in C-276, a Ni-Cr-Mo alloy similar to C-22 alloy with nominally 16% Cr, 16% MO, 4% W, 5% Fe, and found the activation energy to be 260 kJ/mol (62 kcal/mol). Heubner et al (1989) looked at aged C-22 alloy samples optically and produced a Time-Temperature-Transformation (TTT) diagram, but it was not within the scope of that paper to extrapolate the kinetics of precipitation to lower temperatures.…”