Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces Page: 1 of 9
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ALTERNATE ELECTROLYTE COMPOSITION FOR
ELECTROPOLISHING OF NIOBIUM SURFACES*
Jean R. Delayen, John Mammosser, Larry Phillips, and Andy T. Wu
Thomas Jefferson National Accelerator Facility,
Electropolishing has shown promising results for the
treatment of Nb cavities to be used in particle
accelerators, particularly in the attainment of high surface
electric fields. In support of the CEBAF Upgrade project
and as part of a longer-term R&D program we have
investigated the properties of several electrolyte recipes.
A particularly promising one consists of a mixture of
lactic, sulfuric, and hydrofluoric acids. Initial tests reveal
that smooth Nb surfaces can be achieved with no
observable grain boundaries under optical microscope.
We report on the results of the investigations of the
parameters that control the polishing process using this
particular acid mixture.
Electropolishing has been a standard technique widely
used in industry to obtain smooth and lustrous surface
finish on various metals. Smoothening is achieved due to
the dissolution of surface metal away from the bulk
according to Faraday's law, which takes place
preferentially at convex locations of a metal surface. A
bright metal surface can be obtained if all the extremely
fine surface asperities that causes dullness by scattering
visible light is removed. Although electropolishing
always was part of the surface preparation techniques
available to the superconductivity radio frequency (SRF)
community, there was a sharp increase of interest
following the investigations that took place at KEK .
Since then, tremendous progress has been made in this
area, as one can see from the proceedings of SRF
workshops. However, to the best of our knowledge, all
the reported electropolishing on Nb SRF cavities has been
done using exactly the same electrolyte recipe firstly used
by Diepers et al.  and subsequently adopted by Saito et
To explore other alternatives, we have done
electropolishing on Nb using various recipes including the
one used by KEK. One particular recipe, consisting of
appropriate mixture of lactic, sulfuric, and hydrofluoric
acids, has shown very encouraging results in terms of
surface smoothness. This type of acid mixture for
electropolishing on Nb was first briefly described  by
Monti in a French journal Mitaux Corrosion Industries in
1958, although the ratio of the mixture described there is
*Work supported by the U.s. Department of Energy, contract
12000 Jefferson Avenue, Newport News, VA
not the same as the one adopted here- In this paper, we
report the results of the investigations on the parameters
that control the polishing process using this particular
recipe. The relationship between current density vs.
applied voltage during polishing, the dependence on the
distance between anode and cathode, the effect of cathode
material on the finish of the polished Nb surfaces, and the
effect of polishing time are studied. Optimized process is
established through inspection of the polished surfaces of
Nb samples using appropriate microscopic tools. We also
report here the results of our initial attempts to
electropolish half and single cell Nb cavities using this
particular acid mixture.
Our motivation for designing an experimental set-up for
electropolishing on Nb is to try to avoid using Nb cavities
as samples, since it is too costly. We believe that optimal
polishing conditions can be obtained by working on small
Before trying to electropolish on Nb cavities, we need
to know the following three parameters: a) limiting
current density, b) polishing rate, and c) cathode material.
A determination of the limiting current density is
important, if current density is selected to be the
controlling factor for electropolishing. The other
alternative is voltage control. We decided to try current
control first, because it has been concluded by KEK 
that current density is a more relevant parameter. It is
well established  now that a clean Nb surface can be
obtained only after the removal of a surface layer of
thickness of about 100 m for a newly fabricated cavity.
Generally speaking, this process will enable us to remove
any effect from previous treatments. To minimize
unnecessary electroploshing time, we need to know the
polishing rate at optimal polishing conditions- One of the
requirements for electropolishing is that the cathode
should not react with the electrolyte. In Monti's paper 
platinum was selected as the cathode material due to the
inertness of platinum. However, platinum is very
expensive. We noticed that pure aluminum is not reactive
[11 with the mixture of hydrofluoric and sulfuric acids,
although reactions do take place when pure aluminum is
in contact with each one of the acids. This fact motivates
us to find alternative materials for the cathode. We
intended to try pure aluminum, stainless steel, gold plated
aluminum, and platinum plated aluminum-
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Delayen, Jean R.; Mammosser, John; Phillips, Larry & Wu, Andy T. Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces, article, September 1, 2001; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc739035/m1/1/: accessed December 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.