Electropolishing of aluminum: concentration and chemical make up

ASM Metal Handbook
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A. Hi Fab. Read on for one common formulation. And please search the site for 'electropolish aluminum' as we have many threads on the subject with some very detailed & insightful replies to help you get started. In addition to the often-cited Wernick, Pinner & Sheasby, ASM Metals Handbook Vol. 5 has great coverage of aluminum electropolishing. Good luck.

Luck & Regards,

A. I hate to burst your bubble but I know of no electro-polish that gives the same results as "Brite Dipping". It is an age-old problem and if someone out there has an electropolish that can REALLY compete with "Brite Dip" I need to know ASAP.

A. Hi Drew. Coincidentally, a similar question was asked on the same day on topic 51410, which resurfaced Harry Parkes' outstanding response explaining why the move from brite dipping to electropolishing before anodizing is often quite impractical from a production standpoint.

Luck & Regards,

A. Hi Emilio. High purity aluminum is ideal for the electropolishing process. You'll find excellent detailed data about electropolishing aluminum in ⇦[this on eBay, AbeBooks, Amazon affil links] , and you can simply read on here for direct answers to your question. But keep in mind that small pieces can have extremely high surface area so things may proceed a bit differently than you expect if you don't stay mindful of it. Powders may explode in metal finishing processes which are slow and well controlled on large pieces :-(

Luck & Regards,

A. The 7000 alloys are not recommended for electropolishing applications, since is a high alloyed aluminum and may be pitted very easily.

A. Hi Michael. Per the Canning Handbook [on eBay, Amazon, AbeBooks affil links], Brytal is a registered trade mark of the British Aluminum Co. and consists of 150 g/l of sodium carbonate ⇦this on eBay or Amazon] and 115 g/l of trisodium phosphate ⇦this on eBay or Amazon [affil links] . It is operated at 12-15 V, 5-10 A/dm2, 85-90 °C, for 5 to 20 minutes. See the handbook for further details.

You'll see that many readers discourage chemical processing by hobbyists due to safety issues and possible environmental damage.

As you say, some processes work only on some alloys, and Brytal is for 99.8% super purity aluminum -- do you know what alloys you'll be dealing with? Are you restoring previously electropolished aluminum or trying to brighten something that was dull, painted, or plated? The safe way is to mechanically polish and buff the aluminum.

Electropolishing stainless steel is a completely different process, and you won't be able to use your tanks and solutions on the stainless parts. Good luck.

"The Surface
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A. Hello again. Please remember that there are 100 readers for every poster. While your c.v. is impressive, the internet is a giant one-room schoolhouse where people with no chemical training "overhear" stuff and may try it if not warned against it. So we frequently remind readers that hazardous chemicals are for those who have received chemical & haz-mat training, and working in proper facilities with proper personal protection equipment.

Hydrofluoric acid is truly nasty stuff, so it's easy to understand the paste being banned as it can accidentally be left on surfaces which might be touched.

According to an old but detailed paper by A.W. Brace, "European Bright Anodising Practice", published in the 1959 AES Proceedings, the phosphoric-sulfuric bath should be 75-80 percent (vol) phosphoric acid and 20-25 percent (vol) sulfuric acid and, ideally, with 1/2-1 percent nitric acid--but he mentions other formulations.

You may know this, but other readers may not: Wernick, Pinner & Sheasby ⇨
is the exhaustive authority on finishing of aluminum. Good luck.

A. Mike,

EP of aluminum can be a tricky process. The alloys you mentioned are all pretty low in Si, which does make them easier to polish. Most EP tanks are heated (approx. 165 °F), and agitated. You have to make sure that your agitation is good/even, or your parts will be pitted. It's a tricky process for a novice. The other thing you'll want to look at is the amount of current you are putting into your parts, and the fixturing you are using to hold them. In an ideal situation, you'll want to use copper-tipped Ti racks (so no current robbing takes place). Have you considered, perhaps, sending these parts out.

A. Dead spray rinse well
Rinse/Rinse
30-60 sec 10-20 nitric acid
Rinse/Rinse
Hot D.I. dry

If still a problem use a citric acid passivate after the nitric dip and rinses.

A. The EP temperature seems quite high. A phosphating reaction may possibly be occurring after the current is turned off. The EP solutions developed by Charles Faust at Battelle generally have working ranges of 80-175 °F. At the lower end, the solution is highly viscous (excessive drag-out) and you cannot evaporate excess water (as from rinsing). Temperatures of 140-160 °F are common. I suggest 120 °F for all rinses except the final DI, which can be kept at 150 °F for better drying. The sulfuric acid component of the EP rinses more readily than does the phosphoric, and you don't want to bake on any neutralized phosphoric acid (phosphate). Do not allow any liquid residue on the surface to dry while transferring parts between tanks. A longer time in the passivation solution (nitric?) should also help.

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A. Arek,

I think aging is unnecessary and wasteful in terms of EP bath life and sludge generation. Not recommended for any proprietary product that I am aware of, and it isn't practiced by the large EP companies that produce tubing and fittings for the semiconductor industry. However, I have met EP'ers who believed it necessary to add some old solution when making up a new one. Anyone else with opinions or experiences on this subject?

Jon,

I can't find a reference re. my rinsing statement. Made a move several years ago, and may have lost or not unpacked some relevant papers. However, it makes sense both scientifically (sulfuric mixes very exothermically with water, phosphoric has a higher viscosity, phosphoric can form insoluble phosphates on ferrous metals) and empirically (Mr. Dyrdol found phosphorous residue). Also, have observed that a 60-20 sulfuric-phosphoric solution rinses better than a 65-15 (wt. percentages).

A. Hi folks:

AES is an unusual way to get surface contamination from parts coming out of wet process... IPC-TM-640 calls out a standard test method for 7 most common anions found clinging to wet process parts.

I primarily suspect your rinse water, the cleaning process, the atmosphere quality above your tools, and your test method, rather than the EP process. Additionally, you never told us what is your offending surface concentration of Phos? 316SS comes with intrinsic phosphorous, maybe 0.05%W/W so it will never be "all gone." It may be trapped in the EPOx matrix so that the IPC test will detect no free, dissolvable, surface phosphorous, but AES will still pick it up because you blast the matrix away with the Auger sputter. The closer you get to the base material, the higher the concentration the AES will detect until it reads the intrinsic level.

The air above the process has some Phos in it. Just wave the part around in humid air and AES will pick that up. Every worker breaths out aerosols loaded with Phos.

The DI water may be bad, and can get contaminated just by exposing it to the atmosphere. Remember, 18.3 MOhm is true only inside the closed teflon pipes leading up to your process. You indicated the problem worsens in ultrasonic DI, which usually indicates bad DI. The recording head industry(a famous example was in Goleta!) are probably the best surface cleaners in the business. They use expensive DI, multiple staged rinse processes, staged ultrasonic DI with a variety of additives, alcohol or some other inert water-displacer at the end of process, and they blanket the whole line in inert gas. That's how you get truly clean parts. Your requirements may not be so strict.

Also, there is no "best" temperature. Sometimes I run my baths below the freezing point for special reasons.

Requires a hog power supply.

Regarding 316 SS & Sulfuric/Phosphoric baths, general rules are:
1. Lowering temperature helps
2. Lowering EP bath water concentration helps(the bath will pull humidity right out of the air)
3. Raising Current density helps.

Hope this helps. Maybe send out some parts to a test lab for the IPC anion test, and compare to your AES test.

A. Hello,

I'm not sure whether or not it would work for a electropolishing bath, but for our anodizing bath we use a specially designed filter (I'm not sure whether or not I can mention the manufacturer, as they are not an advertiser of this site) that removes the vast majority of the dissolved aluminum. There may be similar technology available for electropolish solutions as well.

Good luck!

A. Hi,
As this is a college project I expect that you know during the electropolishing of aluminium an oxide film continuously forms and dissolves. When you stop the current flow, the remaining oxide film dissolves into the electrolyte. The speed at which it dissolves depends on bath temperature, water content and when used, sulfuric acid concentration.

If you rinse quickly then some oxide may remain undissolved. It is necessary to remove this oxide in a suitable stripper. Nitric acid is not effective. I suggest that you try 5% w/v sodium hydroxide at room temperature few seconds. You can tell when the oxide has dissolved because the aluminium begins to gas. If the immersion is too long the sodium hydroxide will etch the surface.

Conversely, if the time between stopping polishing and rinsing is too long, the electropolishing solution will start to etch the aluminium. This etching can also result a white bloom. However, the defect is not a surface coating but the aluminium itself and cannot be removed by chemical treatment.

A. Hi Erik.

There is no fixed answer to that question, but the basic proposition would be that open top rectangular tanks are ideal for processing, but poorly suited to long term storage. So, if the solution will not be used for a very long time, transfer it to drums or closed round tanks. But if you will be alternating frequently, just cover the tank when not in use and use another tank for processing.

Regards,