1. 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-SC
Nb coupons L Cooley, FNAL

2. 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

3. 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.

4. Experimental Setup

5. Replicating Acid Ratio/Temperature, and stirring results in our set-up
By know well known effect of acid ratios, T, and agitation
Effect of bath agitation
Note: Effect of [F-] supports acceptor limited model

6. Verification of Evolution of surface roughness with EP time
V= 15V, T= 26C, HF:H2SO41:9
Verification of Evolution of surface roughness with EP time
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.
economic consideration for optimizing time. 6

7. 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

8. There is an effect of gravity on the film
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
0.96mm by 15 mm
BOTTOM

9. Evidence of Possible Gravity-Convective Effects
Jerry suggested that there is not much difference in film thickness, it is the diffusion layer which is thinner at the top and thicker at the bottom because of natural convection.
Natural, gravity driven convection results in thinner diffusion layer at top of sample, hence higher dissolution rate 9

10. Evidence of Possible Convective Effects
Bottom edge of sample
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
Ridges close to bottom edge of sample due to convection caused by dissolution products (sliding of the film under gravity) 10

11. I vs t for EP done at 15V for 2 hours at 26oC
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
I vs t for EP done at 15V for 2 hours at 26oC

12. 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

13. 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
Face-up Face down
Erosion at bottom edge
Critical angle of the “sand-pile
“mask”

14. 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?

15. 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.

16. 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

17. 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.

19. 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.
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.

21. 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

22. 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.

23. 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 ( ). 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.