Oxide Film Aging on Alloy 22 in Halide Containing Solutions
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0985-NN08-09
Oxide Film Aging on Alloy 22 in Halide Containing Solutions Martín A. Rodríguez1, Raúl B. Rebak2, and Ricardo M. Carranza1 1 Dept. Materiales, Comisión Nacional de Energía Atómica, Av. Gral. Paz 1499, Villa Maipú, 1650, Argentina 2 Lawrence Livermore National Laboratory, 7000 East Ave, L-631, Livermore, CA, 94550-9698 ABSTRACT Passive and corrosion behaviors of Alloy 22 in chloride and fluoride containing solutions, changing the heat treatment of the alloy, the halide concentration and the pH of the solutions at 90°C, was investigated. The study was implemented using electrochemical techniques, which included open circuit potential monitoring over time, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements carried out at open circuit and at passivity potentials. Corrosion rates obtained by EIS measurements after 24 h immersion in naturally aerated solutions were below 0.5 µm/year. The corrosion rates were practically independent of solution pH, alloy heat treatment and halide ion nature and concentration. EIS low frequency resistance values increased with applied potential in the passive domain and with polarization time in pH 6 - 1M NaCl at 90ºC. This effect was attributed to an increase in the oxide film thickness and oxide film aging. High frequency capacitance measurements indicated that passive oxide on Alloy 22 presented a double n-type/p-type semiconductor behavior in the passive potential range. 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 thousand of years. These barriers include a double wall metallic container and a detached drip shield[1]. Alloy 22 (N06022) was selected for the outer shell of the container[1]. 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[2]. Chloride is one of the most detrimental species since it may promote crevice corrosion in Alloy 22 under specific conditions. Different concentrations of fluorides and chlorides can be naturally found in ground waters. The effect of chlorides and fluoride ions on the passive state and localized corrosion have been extensively studied[3-7]. The maximum allowed temperature by design specifications is 350ºC[1]. Tetrahedral close packed (TCP) phases precipitate in the Alloy 22 at temperatures of 593ºC and higher[8-10]. These phases could have a detrimental effect upon corrosion resistance and cause loss of mechanical ductility. A long range ordering reaction (LRO) can occur at lower temperatures and produce an ordered Ni2(Cr,Mo) phase[8]. This ordering reaction is thought to cause little or no effect on corrosion and causes only a slight loss in ductility.
The aim of this study was to investigate the effects of pH, and thermal aging on passivity and susceptibility to un
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