CEC-ICMC 2013 Anchorage, AK 17-21 June
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On the Mechanism of Niobium Electropolishing:
Some effects of Gravity and Geometry
Ashwini Chandra
M.D. Sumption, E.W. Collings
G.S. Frankel
This work supported by United States Department of Energy
Grant DE-SC0004217 Nb coupons L Cooley, FNAL
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Motivation
• Surface defects like pits, protrusions, and grain boundaries are detrimental to the performance of SRF cavities
• Electropolishing is so far the best technique to get a high quality surface finish
• The electropolishing mechanism for niobium is not very well understood, specially the association of pits with the EBW region
• Interesting to look at the interface between pure electrochemistry, and its realization in cavity work
• Will look at effects of gravity on film present during EP, and infleunce on EP rate
• Will look at EP on BCP and no BCP Nb coupon in HAZ
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Our Schematic Bath for Nb EP
• Working Electrode: High
purity polycrystalline Nb
(99.999%)
• Counter Electrode: High
purity Al (99.99%)
• Reference Electrode:
mercurous sulfate electrode
(MSE)
• Electrolyte: HF(48%)+
H2SO4(96%) in 1:9 vol ratio Fig: Schematic of the experimental setup.
Aluminum
Counter
electrode
Aluminum
Counter
electrode
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Experimental Setup
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Replicating Acid Ratio/Temperature, and
stirring results in our set-up
Effect of bath
agitation
-10
0
10
20
30
40
-5 0 5 10 15 20 25 30 35
Cu
rren
t D
en
sit
y (
mA
/cm
2)
Anodic Potential vs. MSE (V)
Stirred
Non Stirred
Note: Effect of [F-] supports
acceptor limited model
By know well known effect of
acid ratios, T, and agitation
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Verification of Evolution of
surface roughness with EP time
aRa: 319nm Rq: 415nm
aaRa: 319nm Rq: 415nm b
Ra: 173nm Rq: 241nmbb
Ra: 173nm Rq: 241nmc
Ra: 138nm Rq: 168nmcc
Ra: 138nm Rq: 168nm
EP time: 2 h EP time: 4 h EP time: 6 h
Surface changes from scalloped appearance after 2 h of electropolishing
to a relatively smoother surface after 6 h of polishing.
V= 15V, T= 26C,
HF:H2SO41:9
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Evidence of the presence of surface film while
Niobium electropolishing
• A mass transport limited current plateau is necessary for electropolishing to take place
• There are evidences of some form of film formation on the surface through which there is diffusion limited transport of ions involved in polishing mechanism
• The surface film formed is soluble and dissolves in the electrolyte on switching off the current
Surface Film formed
during electropolishing
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Evidence of the presence of surface film while
niobium electropolishing
• There is an effect of gravity on the
film
• Thinner film at the
top of the sample because of
hydrodynamics
• Higher dissolution
rate at the top of
the sample
TOP
BOTTOM 0.96mm by 15 mm
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Evidence of Possible Gravity-Convective
Effects
A
B
• Natural, gravity driven convection results in thinner diffusion
layer at top of sample, hence higher dissolution rate
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Evidence of Possible Convective Effects
Ridges close to bottom
edge of sample due to
convection caused by
dissolution products (sliding
of the film under gravity)
• Pits at surface of flag electrode that was facing
down.
• Possibly: Gravity + convection of film
prevented the formation of a stable film and
hence pitting instead of electropolishing
• Selective dissolution resulted in pitting
Bottom
edge of
sample
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Surface film, gravity, convective effects and EP rate
for EP “face up” and vertical Flag
• EP rate should depend on film thickness
• Film is weakly held on and may be affected by gravity
• SO –current density vs time was measured for a films only exposed to the EP bath on top and bottom surfaces, respectively
• The face up configuration (blue) had a very low current density and polishing rate (thick film)
• The vertical configuration (red) had a relatively high current density and polishing rate
0
20
40
60
80
100
120
-2000 0 2000 4000 6000 8000
Current density (vertical)Current density (face up)
Cu
rren
t d
en
sit
y (
mA
/cm
2)
Time (s)
I vs t for EP done at 15V for 2
hours at 26oC
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Surface film and polarization curve instability (“noise”)
during niobium electropolishing of VERTICAL flag
• Film thickness increases with the applied potential till it falls under its own weight
• Formation and re-dissolution of the film causes current oscillation – may well be chaotic (in the physics sense)
• Stirring causes the film to remain thin, weight could be supported till relatively higher potential
• No oscillations in case of electropolishing of masked sample due to controlled hydrodynamics at the surface
-50
0
50
100
150
200
250
300
350
-10 0 10 20 30 40
Cu
rren
t d
en
sit
y (
mA
/cm
2)
Cell Voltage (V)
Flag electrode with stirring
Flag electrode without stirring
Masked sample
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Surface (viscous) film, gravity, and EP rate for EP “face up” and “face down”
• Assume weakly adhering surface film
• Gravity assist in “face up” mode leads to thick films
• Gravity “attack” in “face down” mode leads to thin films
• Un-masked vertical films may show preferential attack at bottom edges
• Masked vertical films extra support for film at bottom edge
Critical angle
of the “sand-
pile
“mask”
Erosion at
bottom edge
Face-up Face down
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
How will this play out in a cavity as it is
polished?
• Will topology of
the surface be
an issue in any
gravity and
convection
moderated
effects?
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Opticals of EP of HAZ I (Nb with no BCP)
• Top two micros-- the
HAZ near the weld
before EP
• Bottom two micros --
pits formed on the
surface of Nb
electropolished for
• 15 min.
• Possible the pits
initiated at surface
defects such as
impurities, structural
defects, etc. that
formed during the
welding process.
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ II (Nb with no BCP)
• HAZ after 30 min
eliminates the pits
on the surface
Conditions: applied voltage of 15 V, a
working temperature of 25 °C and a
HF:H2SO4 acid volume ratio of 1:9
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ III (Nb with no BCP)
• Top two Micros --
Further EP (60 min)
further smoothens the
surface of the Nb.
• Lower two micros --
After EP for 120 min, a
slightly enhanced
dissolution at grain
boundaries provides a
contrast that allows
grains over the entire
surface to be imaged,
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ IV (Nb with no BCP)
• EP for 240 min re-
roughens the
surface -- and a
fine pit structure is
seen.
• In addition,
preferential
dissolution of
certain grains is
found in some
places, which
leaves large holes
on the surface of
the electropolished
Nb
Conditions: applied voltage of
15 V, a working temperature of
25 °C and a HF:H2SO4 acid
volume ratio of 1:9 for different
electropolishing durations: (a
and b) 60, (c and d) 120 and (e
and f) 240 min.
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ I (Nb with BCP)
• Top two figs – BCP Nb no
EP -- grain morphology is
readily evident by optical
microscopy. Some grains
darker from surface
morphology.
• Bottom two micros – EP
for 15 min. More
dissolution of the dark
grains than that of the
white grains with the
formation of pits on both
grains.
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ II (Nb with BCP)
• EP for 30 min. More
dissolution of the dark
grains than that of
the white grains with
the formation of pits
on both grains.
Conditions: applied voltage of
15 V, a working temperature of
25 °C and a HF:H2SO4 acid
volume ratio of 1:9 for different
electropolishing durations: (a
and b) 60, (c and d) 120 and (e
and f) 240 min.
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ III (Nb with BCP)
• Top two micros – EP
for 60 min. The
smaller grains almost
disappear, the pits
are more apparent on
the dark grains.
• Bottom two micros –
EP for 120 min.
Formation of pits on
the dark grains while
the light grains do not
have the significant
pitting -- indicates
that the light grains
have much higher
resistance to pitting
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Optical of EP in HAZ IV (Nb with BCP)
• EP for 240 min. Severe
pitting of the treated
surfaces, on both dark
and light grains.
• It is clear that the
electropolishing for
240 min cannot
smoothen the surface
of the treated Nb by
levelling the
protruded grains
above the surface.
Conditions: applied voltage of
15 V, a working temperature of
25 °C and a HF:H2SO4 acid
volume ratio of 1:9 for different
electropolishing durations: (a
and b) 60, (c and d) 120 and (e
and f) 240 min.
CEC-ICMC 2013 Anchorage, AK 17-21 June
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Department of Materials Science & Engineering
Conclusions
• Film forms during EP, thickness influenced by topilogy and gravity,
in turns affects etch rate
• BCP and no BCP Nb were electropolished for different durations,
the HAZ was examined
• EP of the no BCP Nb after led to pits after 15 min, but further EP
(30-60-120). EP up to 120 min continued to smoothen the surface,
but EP at 240 minutes cause severe pitting.
• BCP significantly changed the Nb surface morphology which was
covered by grains of different size, appearing light or dark in the
optical microscope.
• BCP Nb was susceptible to pitting during the EP sequence
• Dark grains (more texture) had more susceptibility to pitting than
the light grains.
• EP for 240 min again resulted in severe pit formation.