When moist residual solids resulting from evaporation of simulated concentrated unsaturated zone (UZ) pore water were heated, corrosive gases produced by the hydrolysis of crystallized magnesium salts were released. Gases detected included Cl2, NO and NO2, along with the acid gases HNO3 and HCl. As acid gases were produced, the pH of condensed water droplets in these systems decreased from pH 6-7 to pH 1 or less. Samples of the high nickel material Alloy 22 (UNS NO6022) were placed in the system of wet residual solids such that the surface of the sample was exposed to both the heated solids and vapor space above the solids. When solids were heated to various temperatures over the range of 110°C to 145°C, corrosion rates on the order of 5-100 μm/year was observed on the foil within 10 days. In all cases, corrosion in the form of green spots was observed primarily on the areas of the metal that were exposed to the vapor space. The green corrosion product was identified by UV-visible spectroscopy as solubilized Ni(II) and Cr(III).

In addition, samples of Alloy 22 were found to corrode rapidly, on the order of 600-2000 μm/year, when the metal was exposed to reflux solutions formed when most of the simulated UZ pore water solution had been evaporated. In a final experiment, U-bends exposed to repeated dryout and rewetting of simulated UZ pore water at 110°C exhibited similar rates of corrosion in the crevice that was formed between the U-bend legs and the PFTE spacer.

Subject
Acids, Foil, Water, Evaporation, Corrosion rate, Nickel based alloys, Metal coupons, Solids, Vapors, Acidity, Pores, U bends, Alloys
Keywords:
Alloy 22, Yucca Mountain, corrosion, evaporation, unsaturated zone
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2004
Association for Materials Protection and Performance (AMPP)
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