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Whats the best way to etch alumnium ?

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redd504

Sun Jul 13 2008, 11:05PM

Registered Member #1588 Joined: Sun Jul 13 2008, 05:56PM
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I'm trying to etch small parts for my model car hobby out of alumnium . I was wondering if anyone had any good ideas as to how I could acheive this at home.What would be a good etching solution , and any other materials I would need.I tried peroxide and muratic acid , but it didn't work to well.Any tips would be greatly appreciated. THANKS ALOT ( I know nothing about chemistery!!!! )

Chris

Sun Jul 13 2008, 11:50PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
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I would recommend strong sodium hydroxide or 33% sulfuric acid (battery acid). Hydrochloric acid works the fastest but it fumes and spatters a lot during the reaction and the vapors can rust tools easily, not to mention irritating your respiratory tract. Sodium hydroxide might be the most convenient as any spattering from the bath will dry to a solid. The products of the reaction are sodium aluminate and hydrogen gas. All these chemicals will leave a fairly rough etched surface that may need polished before anything else is done.

redd504

Mon Jul 14 2008, 02:24PM

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Thanks for the info. chris , can I still use a black marker for a resistor ,or will the acid eat this to.Where can I purchase this stuff

aonomus

Mon Jul 14 2008, 03:57PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
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Do it outdoors and away from any ignition source, the reaction does produce hydrogen gas.

Chris

Mon Jul 14 2008, 06:18PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
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I don't know if any of those chemicals will eat through permanent marker. You will have to test it out. I suspect that some sort of enamel or polyurethane paint would hold up fine as a resist however.

I forgot to add last time, if you live in the US, saturated NaOH/KOH solution can be found at many hardware and grocery stores (including Wal-Mart and Home Depot) as a heavy duty drain opener. It is extremely dense (about 1.5x as much as water) and usually comes in black or yellow half gallon jugs. It will need diluted a little bit to be effective as an etchant since the saturated solution is not as active on metal. 33% H2SO4 can be found at auto parts stores like NAPA for topping up car batteries, which is an ideal concentration as is. It comes in either small plastic bottles or in 6 quart boxes with a polyethylene bladder inside much like box wine.

Which ever one you choose, keep safety in mind when working with extremely corrosive chemicals like these, and realize how reactive they are to aluminum and how much the bath can heat up if you are doing a large piece.

redd504

Mon Jul 14 2008, 09:25PM

Registered Member #1588 Joined: Sun Jul 13 2008, 05:56PM
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Thanks again,I think I'm gonna look for a few other ways to do it.I have kids and want the least dangerous method.Thanks again for the info. it was very helpful.

Chris

Mon Jul 14 2008, 11:36PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
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In my opinion if you are doing very deep etching it is likely a waste of time and money to use anything other than the strong chemicals I mentioned before, just keep them where the kids can't get them. However if you are just taking off a thin layer you might look into ferric chloride or other oxidizing salts.

A whole range of weaker acids and oxidizing salts that are less dangerous will etch it, but it will be much slower and may require heating. Some examples are: acetic acid, phosphoric acid, ferric chloride, cupric chloride, ammonium nitrate, ammonium perchlorate, and many others. Most of these are corrosive too, but not nearly to the extent of the strong acids or bases (most metal salts are considerably toxic though). Pretty much any salt of a metal or cation that can be replaced by aluminum or is in a high oxidation state will work, but ferric chloride is commonly used as a copper etchant, indeed by many people on this forum. It is the most easily obtained and the anhydrous salt can be bought for about $10/lb. Radio Shack sells it in solution for PCB etching but it is somewhat exorbitantly priced for the amount you get. Ferric chloride is advantageous in that there is no gas given off by the reaction, so no fumes and no splatter, but it is rather "inefficient" on a weight basis in that 3 moles of FeCl3 (a rather heavy molecule at 162.2/mole of anhydrous salt or 486.6g for 3 moles) are required to oxidize just one mole of aluminum (26.98g), not to mention its slow etching rate. The reaction is 3FeCl3 + Al --> 3FeCl2 + AlCl3.

In comparison one mole of NaOH (40g or about 55mL of saturated solution) will dissolve a mole of aluminum according to the overall reaction 2NaOH + 2Al + 2H2O --> 2NaAlO2 + 3H2, and 3 moles of sulfuric acid (about 650mL of battery acid) will dissolve 2 moles of aluminum according to the reaction 3H2SO4 + 2Al --> Al2(SO4)3 + 3H2, with both of those being relatively cheap per amount as well. Looking at it that way sodium hydroxide is the cheapest to use by an enormous margin. A half gallon jug of the saturated solution would be enough to dissolve 34 moles or about 2lbs of aluminum before it was depleted. Last time I bought such a bottle it cost about $7. A 1lb jar of pure lye (usually about $4 if you can find it) is about 25 moles. Of course with anything, as the concentration of active etchant drops, the rate at which it works will also drop, but you will probably want to use a fairly dilute solution anyway.

Steve Conner

Tue Jul 15 2008, 08:47AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've used NaOH solution to etch aluminium before, and can recommend it. Even quite dilute, it will etch quick, fizzing and bubbling like crazy. I spray-painted the whole workpiece with clear lacquer, and then scraped off the paint at the places I wanted etched.

It does leave a rough, greyish surface like Chris said it would. I think it would also undercut viciously, so don't expect to be able to etch parts out of 1/8" sheet or whatever.

As for safety, just don't get it on your skin or in your eyes, and since it's drain cleaner, you can just pour it down the drain when you're done, with your eco-conscience intact.

redd504

Wed Jul 16 2008, 04:43AM

Registered Member #1588 Joined: Sun Jul 13 2008, 05:56PM
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Thanks for the info. guys, I've been experimenting w/ the muratic acid and peroxide method and it seemed to work very well. ( YOU MUST KEEP IT AWAY FROM THE KIDS ) It etched a part I needed 1/16 x 1/8 with 2 holes in it, perfect. I can also do smaller pieces if need (some times I do ) w/ the same results , so I think I've found my " method to my madness ". The one downside to this method is you have to keep adding acid or peroxide to the mixture , but w/ a little expermenting it shouldn't be to much of a problem.It's funny how building model cars has lead me to this.LOL Thanks again . Knowledge Is Power !!!

aonomus

Wed Jul 16 2008, 05:15AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Oh, another big warning: when diluting ANY strong base or acid, always add ACID or BASE TO WATER, never water to acid/base. The reaction generates heat relative to the amount of acid + water, so adding a drop of water to 90% sulfuric is much more exothermic than a drop conc. sulfuric to water, and can easily go out of control (boiling, spattering, etc) without being careful. Gloves, goggles, ventillation, and a floor that you don't mind having to repair are important.

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