Influence of halide ions and alloy microstructure on the passive and localized corrosion behavior of alloy 22
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INTRODUCTION
THE engineered barriers for the permanent disposal of high-level nuclear waste in Yucca Mountain are designed to maintain isolation of the waste for at least 10,000 years. The engineered barriers include a double wall metallic container and an detached drip shield.[1] Alloy 22 (N06022) was selected for the outer shell of the container.[2,3] The primary purpose of the outer shell is to provide protection against corrosion. If water is present in the repository site, Alloy 22 may undergo different corrosion processes, namely, general or uniform corrosion, stress corrosion cracking, and localized corrosion processes.[4] The environment at Yucca Mountain is basically dry. It is possible (but unlikely) that aqueous solutions may enter in contact with the container. These solutions may form via dripping from the drift wall or by deliquescence of dust that may accumulate on the container. The most likely solutions will contain several anionic species such as chloride (Cl), nitrate (NO3 ), sulfate (SO4 ), carbonate (CO3 ), and silicate (SiO2 aq.). Chloride is one of the most detrimental species since it may promote crevice corrosion in Alloy 22 under specific conditions. The susceptibility of Alloy 22 to crevice corrosion is highly dependent on chloride concentration, temperature, potential, and crevice geometry (tightness). The presence of oxyanions is beneficial (especially NO3 ) since they act as inhibitors against crevice corrosion. Fluoride is found in small amounts in ground water at Yucca Mountain, but it is not expected to concentrate by evaporation since it forms insoluble salts with common cations also present at the site (e.g., Ca2) MARTÍN A. RODRÍGUEZ, Researcher, and RICARDO M. CARRANZA, Principal Investigator, are with the Argentine Atomic Energy Commission, 1650 San Martin, Buenos Aires, Argentina. Contact e-mail: [email protected] RAÚL B. REBAK, Corrosion Scientist, is with Lawrence Livermore National Laboratory, Livermore, CA 94550. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
The maximum allowed temperature by design specifications is 350 °C.[1] Previous studies have shown that the mechanical and corrosion properties of this alloy did not change when it was aged for up to 40,000 hours at 427 °C.[5,6,7] Microstructural changes that occur in the base material have been evaluated at temperatures from 427 °C to 760 °C. Tetrahedral close-packed (TCP) phases precipitate in the Alloy 22 at temperatures of 593 °C and higher.[8,9,10] These phases could have a detrimental effect upon corrosion resistance and cause loss of mechanical ductility. A long-range ordering (LRO) reaction can occur at lower temperatures and produce an ordered Ni2(Cr,Mo) phase.[7,8] This ordering reaction is thou
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