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Figure
1 contains 5 mpy (mil per year corrosion rate) iso-corrosion
curves for Nb, Zr, Ta, and the Ti-Pd alloy (tailored
for oxidizing conditions).
As
you can see, the resistance of niobium in pure HCl is
good to above the boiling point up to about 13% concentration.
At higher concentrations, the 5-mpy line drops gradually
to 50°C. The Ti-Pd alloy is useful in pure acid
to boiling condition at concentrations less than 5%.
Its 5-mpy curve drops to near room temperature at just
over 25% concentration. These curves for Zr and Ta,
on the other hand, remain above the boiling point curve
at all concentrations.
The
current testing program was aimed at determining the
gain in corrosion resistance of Nb by adding an oxidizer
(ferric ion) to the HCl acid.
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