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available high purity metal sheet. For some metals (e.g. Ca,
Mg), foil or sheet forms were not available and therefore
other metallic forms were used, as indicated in Table 1. No
polishing or any other control of surface finish was em-
ployed, so the results obtained must be regarded as only
qualitative.
The individual preweighed metal coupons were placed
in PyrexTM glass tubes containing a small TeflonTM-coated
stirring bar, ∼3 ml of 25% SbF5/HSO3F were added, the
tube was sealed, and the mixture was agitated gently for
1 h. The coupons were then removed from the superacid
solution, washed with water, air dried, and reweighed.
Afterwards, the coupons were returned to the superacid
solutions and heated at 94 ◦C for an additional hour by
immersion of the tube in a boiling water bath. The coupons
were then again removed, washed, dried, and weighed.
Runs at ∼155 ◦C (the boiling point of 25% SbF5/HSO3F)
were carried out similarly, using an oil bath as a heat
source.
In contrast, the actinide metals tested (Th, U, Np, Pu) are
relatively inert except when exposed as high surface-area
turnings. As observed with the powerful superacid HF/SbF35,
thorium metal is essentially unaffected at room temperature
by 25% SbF5/HSO3F.
Aluminum, molybdenum, Zircalloy II, and Hastelloy
C-22, are essentially unaffected by Magic Acid even
at elevated temperatures. However, most other common
metals of construction are attacked at moderate to rapid
rates.
Other metals in Table 1 are attacked at highly variable
rates that do not correlate in an obvious way with the usual
chemical reactivity of the metals. For example, lead, silver,
and copper are attacked aggressively while beryllium and
titanium are unaffected at room temperature. Molybdenum
is relatively inert, even in refluxing superacid, which may
explain the resistance of Hastelloy C-22 (21% Cr, 13% Mo,
66% Ni) and the much greater corrosion rate of 304 stainless
steel versus 316 stainless steel. The stainless compositions
are quite similar except for the presence of 2–3% Mo in the
316 variety.
2.3. Dissolution of plutonium oxide (PuO2)
Formation of insoluble surface complexes undoubtedly
plays a role in limiting the extent of superacid attack on the
metal surfaces. This was visually evident—especially in the
case of Hastelloy C-22—as a clear lacquer-like film on the
metal coupon when removed from the superacid. This film
deliquesced on exposure to moist air for a few days.
Common glass (quartz and PyrexTM) and fluoropolymers
(TeflonTM, FEP, PFA, and Kel-F) are not attacked at a sig-
nificant rate even at elevated temperatures, which allowed
these materials to be used as vessels and apparatus in this
series of experiments
For some active metals (e.g. Mg, Np, and Pu), initial gas
evolution was observed which diminished quickly. In only
one case was massive gas evolution observed. This result
suggests that SbF5 is the net oxidant—rather than protons
or H2F+—and that strong hydrogen evolution does not gen-
erally occur. Of course, this does not exclude the possibility
that the metal surface is attacked by protons and the gener-
ated hydrogen is immediately oxidized by SbF5. With highly
active metals (e.g. alkaline earth and lanthanide metals),
intensely colored blue or purple solutions were sometimes
observed when rapid corrosion occurred. In the exposure of
calcium at room temperature, rapid reaction and gas evo-
lution was observed. The color of the solution in this case
(indigo blue) was determined to be due to the formation of
S28+—via reduction of HSO3F—was verified by the close
match of the UV–Vis spectrum of the blue solution with
that reported [8]. The gas evolved by the reaction of Ca
with Magic Acid was most likely a mixture of hydrogen
and HF judging by the effect on the drybox atmosphere.
The extent of reaction in this case was clearly limited by the
quantity of acid available to react with the metal. Neither
the blue solution nor the vigorous reaction was observed,
even at 155 ◦C, when calcium possessing an intact surface
oxide layer was exposed to the superacid.
In a glovebox designed and equipped for handling
transuranics, 200 mg (1.355 mmol) of low-fired PuO2 and
9.23 g (4.4 ml) of 25 mol.% Magic Acid were added to a
tared 1/2 in. O.D. PyrexTM glass tube fitted with a glass
seal (Fischer-Porter Solv-sealTM). The tube was heated
in an oven to ∼125 ◦C for 2 h. At the solid–liquid inter-
face, the solid turned from dark brown to pink, similar
in appearance to PuF4. In a few minutes, the solution
began to turn brown. After about 8 h of heating, all of
the solid had dissolved, yielding a deep brown solution
with a UV–Vis-NIR absorption spectrum indicative of
Pu(IV).
3. Results and discussion
Tables 1 and 2 summarize the qualitative metal corrosion
experiments using 25% SbF5/HSO3F at 25, 94, and 155 ◦C.
In considering the results, it should be kept in mind that
for those cases where substantial reaction occurred, an ap-
preciable fraction of the superacid was consumed and con-
dition, in some cases the final product was a rather vis-
cous suspension. These effects no doubt limited the rate
and extent of attack on the more reactive metal substrates,
therefore the weight losses given in Table 1 for the more
reactive metals would undoubtedly be higher for higher
initial superacid/metal ratios than might be inferred from
Table 1.
Not surprisingly, most of the platinum-group metals are
essentially inert to 25% SbF5/HSO3F under the conditions of
our experiments. The exception is iridium, which is attacked
slowly at elevated temperature. The lanthanide (Pr, Gd, and
Lu) metals are attacked aggressively at room temperature.
Eller, P. Gary
