The uniform corrosion rate of Alloy 22 will define the lifetime of a component such as a waste container if all other degradation modes are not operative. This represents the best-case scenario because the experimentally determined uniform corrosion rates from multi-year tests is 0.01 µm/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; however, this stability is unknown. One potential breakdown mechanism is corrosion-enhanced enrichment of sulfur to the surface leading to 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 exists 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 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.
The ultimate barrier protecting the nuclear waste from the environment in the Yucca Mountain repository will be a nanometer thick passive film. Even though the waste will be contained in a 25 mm thick vessel made from a highly corrosion resistant nickel alloy, this corrosion resistance is the result of a nanometer thick oxide film that forms in aqueous solutions. The lifetime of the container may then be defined by the uniform corrosion rate and the stability of the nanometer thick passive film. Therefore, mechanisms that could cause failure of the passive film during the long lifetime of the container are very important. There are mechanisms that are not measurable during standard laboratory test times, even long-term testing that may take years, that could cause film breakdown. One such mechanism is the enrichment of a species on the surface of the metal during extended periods and the breakdown of the passive behavior due to this enrichment. In this paper we first review chemical species that can cause a breakdown of the passive film over extended periods that would not be detectable during short laboratory test times then report on preliminary experimental results on corrosion enhancement of sulfur enrichment on the surface of Alloy 22 and the affects of sulfur on the corrosion behavior of Alloy 22.
Sulfur and phosphorus have been identified as two species that have the propensity to enrich at surfaces of metals and alloys and have been demonstrated to alter the stability of passive films on nickel and nickel base alloys. This enrichment could occur by the following processes: 1) thermal processing of the YM container, 2) long-term aging at repository temperatures resulting in surface segregation, 3) diffusion from the grain boundary or inclusions intersecting the surface, 4) adsorption from the environment and 5) enrichment during corrosion. Enrichment during thermal processing of the Alloy 22 container is detectable by standard laboratory tests since similar enrichment would occur during annealing of laboratory test samples. Segregation modeling has demonstrated that at repository relevant temperatures thermally activated enrichment does not reach significant concentrations within a 10,000 year time period so enrichment following emplacement is not likely. Diffusion from the grain boundaries and inclusions is also not likely since it depends on thermally activated processes and will be similarly limited by bulk diffusion. There
