CORROSION ANALYSIS: LOW-CAPACITY PUMP. Large Sodium Pump Study, Phase II. Page: 9 of 30
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g. The UKAEA work indicated that above about 10 to 15 fps, corrosion
rates became independent of velocity for all oxygen levels investiga-
ted (Figure 1). G.E. work, however, indicated an almost linear
variation of corrosion with velocity (actually VO.884) up to the test
limit of 25 ft/sec.
Observations on Corrosion Processes
Thorley and Tyzak~l] reported an initial weight increase with time that was
eventually superseded by a linear weight loss. The weight gain was due to
the formation of a sodium chromite film CmaCrO ) resulting in a denudation
of chromium in the substrate material. Once this film was removed by the
flowing sodium, nickel was selectively removed. A layer of ferrite was thus
formed and remained in contact with the sodium for the remainder of the test.
This selective leaching of nickel and chromium was also observed in the G.E.
work at low-oxygen levels. They did not observe a weight increase or the
presence of an oxide. As all low-nickel materials eventually end up with a
ferrite surface layer, it is not surprising that their corrosion rates are
similar. At higher oxygen levels (50 ppm) G.E. reported a selective re-
moval of iron.
Observations on Corrosion Rates
Effect of Oxygen Content
Between 5 ppm and 50 ppm oxygen content in the sodium, the corrosion rate
was almost directly proportional to the oxygen content. This marked
variation is shown in Figure 2. Thorley and Tyzak postulated that the
iron reacted with the sodium oxide in the sodium to form a double oxide
that then went into the solution.
Na20 + Fe = FeO + 2 Na (Liquid)
FeO + 2 Na20 = FeO (Na20)2
giving 3 Na20 (in Na) + Fe (Solid) = FeO(Na2O) (in Na) + 2 Na(Liquid).
Mottley[31 proposed that the following reaction occurred without the
formation of a double oxide:
Fe (Solid) + Na20 = FeO (Solid) + 2 Na
FeO (Solid) = FeO (in Na)
Although the exact mechanism is not yet clear, the effect of oxygen on
corrosion rate is extremely important, and is probably the most critical
factor in sodium corrosion.
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Thomas, K.C. & Shiels, S. CORROSION ANALYSIS: LOW-CAPACITY PUMP. Large Sodium Pump Study, Phase II., report, October 31, 1970; Madison, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc1034653/m1/9/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.