PNNL

Abstract

The uniform corrosion rate of Alloy 22 will define the lifetime of the Yucca Mountain container if all other degradation modes are not operative. This represents the best case scenario since the experimentally determined uniform corrosion rates from 5 year tests is on the order of 0.01 um/yr or 0.1 mm in 10,000 years. This lifetime depends on the stability of the passive film over the lifetime of the container which is possible but not known. One potential breakdown mechanism is corrosion enhanced enrichment of sulfur to the surface and sulfur induced breakdown of the passive film. There are numerous studies that confirm that sulfur causes passive film breakdown in nickel based alloys and evidence for corrosion enhanced enrichment of sulfur in nickel and nickel alloys. However, neither sulfur enrichment nor sulfur induced breakdown of the passive film on Alloy 22 has been demonstrated. The results of preliminary studies conducted at the Pacific Northwest National Laboratory do suggest sulfur enrichment does occur at the alloy surface and that sulfur shifts the corrosion potential to more active potentials. No clear affect of sulfur was noted on the corrosion rate but the sulfur concentrations were about only 2-3 % of a monolayer, well below the concentrations that are possible.