on the mechanism of niobium electropolishing: …cec-icmc 2013 anchorage, ak 17-21 june 1 on the...

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


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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


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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


4


Experimental Setup


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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?


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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.


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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


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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,


18


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.


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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.


20


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.







and f) 240 min.


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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


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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.







and f) 240 min.


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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.

on the mechanism of niobium electropolishing: …cec-icmc 2013 anchorage, ak 17-21 june 1 on the...

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