Corrosion Enhanced Enrichment of Sulfur and Implications for Alloy 22 Page: 4 of 20
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INTRODUCTION
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 20 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. Therefore, adsorption from the environment and enrichment during
corrosion are the two most likely processes to cause breakdown of the passive film over repository
lifetimes.
Sulfur and phosphorus both enrich at interfaces and surfaces of metals and alter the corrosion behavior
of nickel base alloys. But, there are many differences in the behavior of these two elements. Some of the
reported differences include: 1) sulfur is more surface active than phosphorus so for a given bulk
concentration it will be more highly enriched and once adsorbed to the surface it is more stable, 2) sulfur
has been shown to enrich on the surface during corrosion of some nickel alloys while phosphorus has
not, and 3) sulfur degrades passive film stability over a range of pH values while phosphorus can be
detrimental at low phosphorus concentrations and low pH's but beneficial at high phosphorus
concentrations over a wide pH range. Therefore, sulfur is considered the more potentially detrimental
element for the long-term stability of the passive film on Alloy 22, although an experiment to determine
whether phosphorus can enrich on the surface of Alloy 22 during corrosion is warranted.
There have been a number of studies that demonstrate the effect of surface adsorption of sulfur on
passive film breakdown of metals. This adsorption has occurred from the environment, from biological
processes and from enrichment due to corrosion. The studies showing surface enrichment during
corrosion required concentrations of sulfur in the bulk material higher than the impurity concentration in
order to accelerate the enrichment process. It was shown in studies where sulfur was enriched on the
surface from the bulk and from the environment that the chemical state of sulfur is critical to its affect
on passive film stability. Reduced forms of sulfur are more deleterious and oxidized forms less, or not,Paper 06621 for Corrosion/2006, San Diego, 12-16 March 2006
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Jones, R H; Baer, D R; Windisch Jr., C F & Rebak, R B. Corrosion Enhanced Enrichment of Sulfur and Implications for Alloy 22, article, November 15, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc876113/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.