Degradation mode surveys of high performance candidate container materials Page: 4 of 18
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The Ti alloys considered are commercially pure and low alloy content materials. Ti Grade
2 (CP grade) is the most widely used titanium material. Ti Grade 7 is a Ti-Pd(0.2) alloy that has
significantly better localized corrosion resistance. Ti Grade 12 is a Ti-Ni(0.8)-Mo(0.3) alloy that
has similar corrosion properties to Grade 7. The titanium alloys are less dense than the nickel
alloys, which means less material by weight may be required.
For geologic disposal the most aggressive media that are expected to contact the metallic
barrier are aqueous solutions. Because of the unknown composition of the vadose water that
might come into contact with the metallic barrier and possible concentrating effects, a wide range of
solution compositions are being considered. Solutions range from dilute to highly concentrated
The nuclear waste will significantly perturb the surrounding environment. Gamma
radiation levels may be as high as 1 x 105 rads/h, and the decay heat of the waste will cause the
temperature of the surrounding environment to rise. The temperatures of the metallic barriers are
expected to be in the range 50 to 25C0C during the containment period. The pressure will remain at
slightly less than one atmosphere, with the boiling point of water at 96*C. Corrosion concerns are
considered over this entire temperature range, and the effects of gamma radiation on corrosion
processes need to be considered also.
EFFECT OF PROMOTERS
Promoters are necessary to cause pitting and crevice corrosion in aqueous solutions. The
most widely studied promoters are the halide ions, although other species may increase the
corrosion susceptibility of the titanium and the Ni-Cr-Mo alloys. Halide ions are generally
thought to participate in crevice corrosion by acting as electrical conductors, aggressive species,
and solvating ions.
Both titanium and the nickel alloys are susceptible to pitting corrosion in halide ion
solutions during anodic polarization. This susceptibility is indicated by the pitting potential. The
lower the critical potential for pitting, the greater is the susceptibility to pitting. For titanium,
pitting potentials vary significantly among halide ion solutions at room temperature [4-7). Bromide
and iodide solutions are the most aggressive, followed by chloride solutions. Fluoride solutions
are the least aggressive halide solutions. In bromide and iodide ion solutions the pitting potentials
are the lowest (1-3 V, SCE). In chloride ion solution the pitting potential is significantly higher
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Gdowski, G.E. & McCright, R.D. Degradation mode surveys of high performance candidate container materials, article, December 1, 1990; California. (digital.library.unt.edu/ark:/67531/metadc626669/m1/4/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.