Electropolishing Procedures

Electropolishing Procedures
C. Hammond, M. Bowles, R. Bunker, R. Schnee, B. Wang, and J. White
Department of Physics, Syracuse University, Syracuse, NY 13244, USA
(Dated: October 4, 2012)
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I.
INTRODUCTION
This document outlines safe electropolishing procedures for the removal of radon daughters from the surfaces of stainless-steel components of a low-radioactivity, multi-wire proportional chamber called the “BetaCage.” Additionally, a number of stainless-steel plates
will be electropolished by varying degrees to study radon contamination as a function of surface smoothness. A successfully electropolished surface will be characterized by the uniform
(to within 3%) removal of at least 100 nm of material, easily conforming to the BetaCage’s
required mechanical tolerances [1].
II.
REFERENCES
The primary references used to construct this procedure include: the CDMS Wiki on
the Queen’s University website [2] (to find information about the procedures, devices, and
chemicals required for the process); a paper from some mechanical engineers in Taiwan [3];
Bob Nelson’s BetaCage gain calculation [1]; an electropolishing article in the January 2000
issue of Metal Finishing [4] by K. Hensel; and a particularly informative article by Zuzel
and Wójcik [5]. The latter includes results for electropolishing steel and germanium.
Additional information was obtained from industry websites. In particular, Electro Polish
Systems, Inc.’s website [6] contains some useful facts that compliment [4] (K. Hensel is the
president of EP-Systems), and the Stainless Steel Information Center’s website [7] is a useful
resource for better understanding the properties of stainless steel.
III.
PRIMARY CHALLENGE
The main challenge is to design a procedure capable of removing a specific amount of material consistently. The 25 µm-diameter stainless-steel wires constituting the anode planes of
the BetaCage amplify drifted electrons via the electric fields directly surrounding the wires.
This amplification depends sensitively on the diameter of the wires. Consequently, if electropolishing results in nonuniform wire diameters, the BetaCage might exhibit significant,
position-dependent gain variations (and correspondingly poor energy resolution). Such gain
variations could also obscure the reconstruction of ionizing tracks caused by electron- and
alpha-particle interactions.
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IV.
CONSIDERATIONS
In industry, the composition of an electropolishing solution typically depends on the type
of metal being polished (see, e.g., EP-Systems’ list of chemicals [6]). The procedure and
electropolishing-solution recipe described in this document may not be equally effective (or
even appropriate) for metals other than stainless steel. Furthermore, stainless steel comes
in a variety of grades. According to [7], the difference between 304 and 316 stainless steel
is that 304 contains 18% chromium and 8% nickel, while 316 contains 16% chromium, 10%
nickel and 2% molybdenum. The “moly” is added to help resist corrosion to chlorides (like
sea water and de-icing salts). Keep in mind that the electropolishing rates for different
grades of stainless steel may be different. The electropolishing solution described in this
document is particularly effective for 300 series stainless steel.
V.
EQUIPMENT
• DC Power Supply with 3 Wires Attached to Alligator Clips - to create an
electric field such that material is migrated from the positively biased surfaces of a
sample to the negatively biased surfaces of the copper electrodes. We will use the BK
Precision 1670A dc regulated power supply we have on hand. It is a current limited
(up to 3 A) supply with an adjustable dc output voltage (up to 30 V). Two black
wires are connected to the negative terminal, while one red wire is connected to the
positive terminal. The ends of all three are terminated in alligator clips to facilitate
electrical connections to the electropolishing electrodes (black wires) and stainlesssteel samples (red wire). The output voltage should be adjusted to 2.4 V and then
turned off before attaching the alligator clips. Additionally, the current limit should
be set to its maximum value of 3 A by turning the current-limit knob clockwise as far
as it will go. Note that the electropolishing rate can be increased by increasing the
voltage. However, for large stainless-steel samples (e.g., the 2-inch squares), a voltage
greater than ∼3 V could be unsafe and is not advised.
• 316L Stainless Steel Vessel - container in which the electropolishing will occur. It
must withstand temperatures in excess of 190◦ F. See Figure 1 for further details.
3
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FIG. 1. Illustration of the stainless-steel container and electrode-sample geometry that will be used
during electropolishing.
• Ultrasonic Cleaner - to clean the samples and electrodes before electropolishing. We
will use the Branson model 1510 we have on hand, with isopropyl (inside a glass beaker)
as the cleaning agent. The ultrasonic cleaner should never be filled with just isopropyl.
The flash point of isopropyl is ∼50 ◦ F. Consequently, any ultrasonic cleaner filled with
isopropyl represents an explosion hazard. Instead, the parts to be cleaned should be
placed in a loosely covered glass beaker filled with just enough isopropyl for them to
be fully submerged. The beaker is then placed in the ultrasonic cleaner’s basket with
the ultrasonic cleaner filled with deionized water. The radon-daughter-implanted
stainless-steel squares from the University of South Dakota should not be
ultrasonic cleaned.
• Electronic Scale - to gauge the amount of material removed from samples (such as
the stainless-steel plates) following electropolishing. We will use the Mettler AE 200
we have on hand. It has a precision of ∼1 mg (with its glass doors closed).
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• 3 1-liter Griffin Beakers - to mix/measure the electropolishing acid bath, to
hold/collect rinsing water (as it runs off of samples while cleaning with ultra-pure water), and to hold/collect residual rinsing water while drying samples with dry nitrogen
(or equivalent).
• 2 Copper Electrodes - thin copper plates that will be negatively biased and will
act as collectors for metal ions that are removed from the surfaces of samples during
electropolishing.
• Electropolishing Samples - stainless-steel 2-inch squares and 25 µm wire will be
used to gauge the electropolishing rate.
• Time Keeper - to time electropolishing sessions, helping to establish a consistent
method and thus ensure the accuracy of our methods.
• 2 Plastic Rods - to suspend electrodes and sample in electropolishing acid bath.
• Metal Hook - to suspend samples (from plastic rods) into acid solution.
• Plastic Bags and Labels - for sanitary and ordered storage of samples following
ultrasonic cleaning and electropolishing.
• Plastic Funnel - to facilitate the safe transfer of discarded acid bath and rinse water
into hazardous-waste containers.
• Hazardous-Waste Containers - to collect our used solution and rinse water. Double
containment of all hazardous waste (with appropriate hazardous-waste labels) will be
stored in a clearly marked hazardous-waste storage area (located under the fume hood)
at all times.
• Drip Tray - used as a stage for the entire procedure to collect spurious drips or
splashes of the acid bath or rinse water, thereby helping to ensure containment of
hazardous materials.
• LDPE Bottles - Used to hold/store and transport ultra-pure water, without contaminating it.
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• Scanning Electron Microscope (SEM) with Material Holder - to measure the
diameter and quality of electropolished wires.
• Profilometer - to measure the surface roughness of large samples (for relatively course
surface features down to ∼1 µm).
• Atomic Force Microscope (AFM) - to measure surface roughness (for relatively
smooth surfaces with sub-micron features).
• Pouring Rod - used to transfer acids and ultra-pure water into mixing beaker (where
their volumes are measured) and then into the stainless-steel electropolishing vessel,
thus reducing splashing of hazardous materials.
• Digital Thermometer and Thermocouple - used to monitor the temperature of
the electropolishing solution.
• Voltmeter - used to check the quality of electrical connections.
• Squeeze Bottles - used to hold/squeeze ultra-pure water for rinsing.
• Small Plastic Tray - Used to hold samples (and thus maintain their cleanliness)
while weighing on the scale. We’re using a circular (red) flange cap for this.
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VI.
SAFETY
A.
Safety Procedures
It is important to ensure that no part of the solution is allowed to be flushed
down the drain or splash on any part of the researcher.
All safety equip-
ment/apparel should remain on until the solution has been disposed of (or
stored) properly (see Section VIII below for further details). Small spills may be left on
the spill tray until the completion of the procedure, at which time they should be transferred
to a hazardous-waste container. Larger spills should be soaked up (or at least contained)
with the Hazmat spill kit (located next to the fume hood in a cardboard box; see, e.g., Figure 3), and emergency services should be contacted (see green-colored spill procedure posted
on lab door). Any area of the researcher splashed with solution must be rinsed thoroughly
(via use of the shower and/or eyewash if appropriate). While pouring and mixing acids,
always pour down the glass pouring rod to avoid splashes (as demonstrated in Figure 2).
At the end of the day, before leaving the lab, any waste materials that have been collected
over the course of the procedure should be transferred (from the waste-containing beaker) to
an appropriate hazardous-waste container via the plastic funnel. Particular care should be
taken to avoid overflowing the funnel, accidental spillage, and/or splashing. Additionally, all
containers should be clearly labeled (indicating contents), and any containers holding hazardous materials (even if only temporarily) must be labeled with proper hazardous-waste
labels.
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FIG. 2. Demonstration of proper technique for pouring acids with a glass pouring rod.
B.
Safety Equipment
• Fume Hood
• Shower and Eyewash (in very close proximity to work area and fume hood)
• Safety Glasses
• Rubber and Nitrile Gloves
• Splash Shields
• Hazmat Spill Kit
• Lab Coats
• Glass Pouring Rods (mentioned above)
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VII.
CHEMICALS
The following list includes the ingredients for the electropolishing solution, as well as
liquids for rinsing and (ultrasonic) cleaning of samples.
• H3 PO4 (orthophosphoric acid, 85% aqueous solution) - to compose 40% (by weight)
of electropolishing solution. An 85% solution of phosphoric acid has a molarity of 14.7
and a density of ∼1.7 g/ml.
• H2 SO4 (sulfuric acid, 96% aqueous solution) - to compose 40% (by weight) of electropolishing solution. A 96% solution of sulfuric acid has a molarity of 18.0 and a
density of ∼1.84 g/ml.
• CrO3 (chromium trioxide, solid) - to compose 3% (by weight) of electropolishing
solution. The density of CrO3 is ∼2.7 g/cm3 . No longer used.
• Ultra-pure Water - to compose ∼20% (by weight) of electropolishing solution and
for rinsing samples following electropolishing. Normal deionized water should be used
in the ultrasonic cleaner.
• C3 H8 0 (isopropyl alcohol, 70% aqueous solution) - for use in ultrasonic cleaner.
• N2 (compressed molecular nitrogen) - for drying samples and other electropolishing
parts following ultrasonic cleaning or rinsing with ultra-pure water. Either safety
glasses or a splash shield should be worn while using nitrogen gas.
A total of ∼650 ml of electropolishing solution is just enough to fill the stainless-steel
vessel to a (safe) level that allows samples to be completely submerged in the solution while
suspended between the copper electrodes (as shown in Figures 1 and 5). Consequently, the
test-run procedure outlined below is based on a total electropolishing volume of ∼650 ml.
If a different stainless-steel vessel is used, a dry run should be performed in which the
electrodes and sample are suspended (via the plastic rods) into the empty vessel. Using
ultra-pure water and a clean beaker, determine the total volume of electropolishing solution
required by filling the vessel with measured amounts of water until the sample is submerged
to the desired level. The amount of each electropolishing-solution ingredient should then be
recalculated based on this new total and the percentages listed above.
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VIII.
TEST-RUN PROCEDURE
Ultimately we would like to remove a consistent amount of material from a sample for
a given electropolishing time. To establish the electropolishing rate for our solution and
electrode configuration, we will conduct several test runs. Between test runs, material removal will be diagnosed with a combination of the scale, an SEM (for wires), a profilometer,
and (possibly) an AFM. Such diagnostic measurements will be iterated with electropolishing test runs as many times as required to derive a consistent electropolishing rate. The
electropolishing test-run procedure is outlined below.
1. Obtain ultra-pure water from Martin Forstner’s lab in sub-basement room 229 (Physics
Building). About 2 liters should be sufficient for the procedure. Store this in the LDPE
bottles in the lab.
2. Before entering the electropolishing area (shown in Figure 3), always put on a lab coat.
There are three lab coats of varying sizes hanging from hooks on the wall directly
behind the shower. If pouring acids (i.e., the electropolishing solution or any of its
components) or handling anything that has been exposed to acids, always wear rubber
or nitrile gloves and a splash shield. In general, when handling samples (particularly
following ultrasonic cleaning), nitrile gloves should be worn. Additionally, all acids
should be poured/mixed inside the fume hood with its sliding glass door approximately
half closed (but not so far that it becomes an impediment). While the acid solution
is exposed to air (e.g., during an electropolishing test run) and access to the interior
of the fume hood is unnecessary, keep it closed if possible.
3. Use the ultrasonic cleaner to clean the copper electrodes, stainless-steel samples, metal
hook, and any other parts that might come into contact with the acid solution (except
for the USD radon-daughter-implanted SS squares, which should not be ultrasonic
cleaned). Before inserting parts to be cleaned, degas the cleaning solution for 5 minutes. Parts should be cleaned in isopropyl inside a glass beaker for at least 30 minutes
at ≥60 ◦ C (see manual located next to cleaner for detailed instructions).
10
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FIG. 3. Digital photograph of the electropolishing setup inside and near the fume hood in room
304 of the Physics Building.
4. Set up 3 beakers: one for measuring ingredients for the electropolishing solution (“mixing beaker”), another to collect rinsing waste (“waste beaker”), and a third to collect
any waste expelled while drying samples (“drying beaker”). These beakers, as well
as the stainless-steel container (for the electropolishing solution), should be placed
on the drip tray within the fume hood and should stay there for the duration of the
procedure. Any water rinsed from anything that was in contact with the solution
should be directed into the waste beaker, while subsequent drying with dry nitrogen
(or equivalent) should be directed into the drying beaker.
5. Measure out 33 grams of chromium trioxide onto the small plastic tray using the digital
scale and carefully transfer to the (previously empty and clean) stainless-steel vessel.
(Chromium trioxide no longer used) If necessary, clean the stainless-steel vessel by
rinsing with ultra-pure water (into waste beaker), and wiping with isopropyl-dampened
lint-free wipes.
6. Measure 100 ml of ultra-pure water into the mixing beaker, and pour carefully into
the stainless-steel vessel using the glass pouring rod (as demonstrated in Figure 2).
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7. Measure 300 ml of orthophosphoric acid into the mixing beaker, and pour carefully
into the stainless-steel vessel using the glass pouring rod.
8. Measure 250 ml of sulfuric acid into the mixing beaker, and pour carefully into the
stainless-steel vessel using the glass pouring rod. Be sure to store the bottles of acid
in the (yellow and clearly labeled) acid cabinet (in the gray tubs), only removing them
(one at a time) temporarily in order to dispense in the manner described.
9. Stir the solution slowly and evenly with the pouring rod. Caution! Mixing the
electropolishing solution causes an exothermic reaction; the solution and containment
vessel will become extremely hot. The vessel will not melt the drip tray. However,
avoid unnecessary contact with your hands or other items inside the fume hood.
10. While mixing and throughout the remainder of this procedure, monitor the temperature of the solution with the digital thermometer (by attaching its thermocouple to the
outside of the stainless-steel vessel). Allow the acid bath to cool to room temperature
(≤30 ◦ C) before using it to electropolish for the first time.
11. Rinse the mixing beaker, transfer resulting waste to the waste beaker, and rinse the
pouring rod (into the waste beaker). If (at any point during the process) the waste
beaker becomes too full, transfer waste to an appropriate hazardous-waste container
(stored in the labeled cabinet below the fume hood) via the plastic funnel. Finish
waste transfers by rinsing the funnel into the hazardous-waste container.
12. Using the 2 plastic rods, suspend the 2 copper electrodes at a distance of 2 inches
apart from the top of the stainless-steel vessel (as shown in Figure 4). Make sure the
electrodes are not touching the bottom or sides of the stainless-steel vessel.
13. Make sure the dc power supply is turned off. Use the 2 black wires with alligator clips
to attach the electrodes to the negative pole of the dc power supply, and (carefully)
use the voltmeter to check the resistance between the power supply and the electrodes.
A good electrical connection is typically characterized by less than half an ohm. Do
not dip the voltmeter tips into the solution.
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FIG. 4. Digital photograph of the electrode geometry for the electropolishing setup.
14. Measure the starting weight of the stainless steel square (this step can be skipped if
electro-polishing wires) and record the result in the log book.
15. To electropolish a wire, loop desired amount of wire through a washer (at one end)
and the metal hook (at the other end). Then suspend the metal hook from the
central plastic rod (between the copper electrodes) such that the washer hangs into
the electropolishing solution, thereby submerging a portion of the wire. Make sure the
washer does not rest on the bottom of the stainless-steel vessel, and that the hook,
washer or wire are not in (direct) contact with either electrode. To electropolish a
stainless-steel plate, insert the metal hook through the hole in the plate, and (similarly)
suspend the hook from the central plastic rod. Again, ensure the sample and hook are
not in direct contact with the vessel or either electrode.
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FIG. 5. Digital photograph of a properly configured sample-electrode geometry suspended into an
electropolishing solution and wired to a dc power supply.
16. Use the red wire with alligator clip to attach the metal hook (from which the sample
is suspended) to the positive pole of the dc power supply. A properly configured
sample-electrode geometry is shown in Figure 5.
17. Begin electropolishing by turning the power supply on. Again, make sure beforehand
that the power supply has been set to output 2.4 V, and that the current limit is set
to its maximum allowed value of 3 A. Also, remember to time your electropolishing
sessions and record the values in the log book. The electropolishing time is defined as
the period of time during which the power supply is on. It is also useful to monitor the
current draw displayed by the power supply as a function of time. The electropolishing
rate (i.e., the rate at which mass is removed from the sample) is correlated to the
current draw. The current draw may barely register at the 10–20 mA level when
electropolishing wires (due to their small surface areas), while it can vary between a
few hundred milliamps and the full 3 A for the stainless-steel plates. Additionally, it
may be useful to monitor (and record in the log book) the temperature of the acid
bath as a function of time.
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18. After the desired electropolishing time, turn off the power supply. Remove the red
alligator clip from the metal hook and (carefully and slowly) lift the hook off of the
plastic rod, allowing the acid solution to run off of the sample and fall back into the
stainless-steel vessel. It is vital that the sample be removed from the solution
promptly after the voltage has been turned off, otherwise some of the removed
material may plate back onto the sample. Rinse the sample off with ultra-pure water,
directing the runoff into the waste beaker. After a thorough rinse, hang the sample over
the drying beaker and use dry nitrogen (or equivalent) to blow the sample dry. The
idea is to push off any residual liquid (into the drying beaker) rather than evaporate
it. Remember to stop the flow of nitrogen (i.e., close the flow valve) once the sample
is completely dry.
19. Measure the weight of the sample (without the metal hook) and record the value in
the log book. If polishing a wire, transfer it to a clean plastic bag and label it with a
sample number, the electropolishing time, the date, and your initials.
20. If additional polishing is desired, gently stir the acid solution with the glass pouring rod
and repeat steps 15–19, making sure to record the electropolishing time and sample
weight each time. Stirring the solution between uses appears to help maintain the
viability of the solution, and thus the consistency of the procedure. Be sure to clean
the glass rod each time, properly directing rinse water into the waste beaker. Once the
desired (total) electropolishing time has been achieved, place the (rinsed and dried)
sample in a clean bag and label it with a sample number, the (total) electropolishing
time, the date, and your initials.
21. Disconnect the black wires from the electrodes and slowly lift them out of the solution,
allowing excess solution to fall back into the stainless-steel vessel. Place the electrode
apparatus over the waste beaker and rinse thoroughly with ultra-pure water. Move
the apparatus to the drying beaker and blow both electrodes dry with nitrogen.
22. Use the funnel to pour the rinse waste into a labeled hazardous-waste container.
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23. The electropolishing solution should remain viable in the the stainless-steel vessel for
several days, and possibly several weeks. It is therefore necessary to dispose of the
solution only when a fresh solution is desired. A mixed solution can be stored in
the stainless-steel container inside the fume hood when not in use provided the fume
hood’s glass door is kept fully closed. Additionally, it’s recommended that a sign be
posted to warn others of the presence of an acid solution. When it is deemed necessary
to dispose of the acid solution, it should be transferred to a hazardous-waste container
via the plastic funnel while wearing full safety apparel (gloves, splash shield and lab
coat), and particular care should be taken to avoid overflowing the funnel, accidental
spillage and splashing.
[1] R.
Nelson,
“Calculations
of
the
Mechanical
Tolerances
for
the
BetaCage,”
http://wiki.phy.syr.edu/doku.php?id=physics:schnee:mechanical_design (2010).
[2] W. Rau, “Group Website,” http://cdms.phy.queensu.ca/cdms_restricted.
[3] S-J.
Lee
et
al.,
“The
Study
of
Electrolyte
Agitation
on
Electropolishing,”
http://www.premalab.re.kr/seminar/seminar_data/Lee-Lai-Wang-Lee-Tsui-Taiwan.pdf.
[4] K.B. Hensel, Metal Finishing 98, 440 (2000).
[5] G. Zuzel and M. Wójcik, Nuclear Instruments and Methods in Physics Research A 676, 140
(2012).
[6] EP-Systems, “Company Website,” http://www.ep-systems.com.
[7] Stainless Steel Information Center, http://www.ssina.com.
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