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"Aluminum etch additives?"

An ongoing discussion beginning back in 2000 ...


Q. Happy Holidays Fellow Finishers! My company does both rack and barrel sulfuric acid anodizing. Many parts need to be etched prior to finishing. We currently use sodium hydroxide with no additives and have to deal with the difficult aluminum hydroxide sludge that develops over time. We would like to start using an additive (sodium gluconate?) that will keep more aluminum in solution (and possibly make tank clean-up easier). Does anyone have any info on the concentration of sodium gluconate needed or if there is a better choice of additive?

Victor Waldman
- Naugatuck, Connecticut


A. You might be just as well off buying a pre-mixed caustic etch. That's what I use. Liquid based, it has all the agents pre-mixed into the make-up. Takes the guesswork out of the whole situation really. There are tons of suppliers of these types of products, listing them would be somewhat pointless as anyone that deals with anodizing and anodizing chemical additives will have access to such a product.

Matthew Stiltner
J & J Plating Company - Toledo, Ohio


A. Hello fellow anodizer! just browsing and saw this. We use an additive called ax-1888 from Houghton chemical. It is a good chelator; it keeps the solids in suspension therefore gets drug out into the rinse tanks. We have no-dump etch tanks. The aluminum stays between 140 g/l to 160 g/l. I will say that it gives the material a matte finish, not shiny. If that's a problem there are other additives. Well I've probably gabbed enough. It's just a thought.

Brian Vandivier
- franklin, Indiana

How much sodium gluconate should be in an aluminum etch


Q. I have a question regarding the amount of sodium gluconate I need to add to my caustic bath that I use in my aluminium anodizing plant.The literature I have suggests a "few" grams/liter. Could any one tell me what "few" is? Thanks, Sanjay

Sanjay A. Bulchandani
- Bombay, India


A. I was looking for information about sodium gluconate in solutions of sodium aluminate. I have found something that may helps you: I think you should add 1% to 5% of gluconate to your bath. I found that in Vulcan Chemicals-Technical and environmental services: "Caustic Etching of Aluminum"

Prouvez Charlotte
- France


A. My friend I am also involved in the anodizing process. You can use 10 grams/liter of Sodium gluconate.

Syed Iqbal Hussain
- Pakistan

Non-proprietary aluminum etch alternatives?

December 5, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello all, I own a small metal finishing business and have to save money where I can when I can at this point. Previously I worked for a larger metal finishing company that acquired all of their treatment chemicals from Macdermid. I am currently working on getting an aluminum chromate conversion line (immersion) set up and will be getting most of the chemicals for this process from Macdermid. However I have been doing reading and have seen that caustic NaOH bath can be used for etching in this process. Currently the proprietary etch chemical from Macdermid, is about $xxxx [Ed. note: Sorry, we can't post prices here] per barrel and isn't sold in smaller quantities than a 350 lb drum whereas I can buy a 50 lb bag of soda beads for $yyy ....

Can someone elaborate on whether this is a viable option or not and how using NaOH as a caustic etch instead of a proprietary might effect salt spray testing?

As a side question as well, is it possible to use the soap as an etch instead since the Data sheet from Macdermid indicates that it can etch aluminum fairly easily at the right concentration.

Also, since someone will probably ask, here is my current process setup:

Soap (Isoprep 740)
Deox (Isoprep 184)
Iridite 14-2

Paul Hurley
owner, metal finishing business - Oregon, USA

December 2014

A. Hi Paul. Yes, it's possible to use a single tank both as a soak clean and an etch, and some small labs certainly do so. But it's not good practice for a production shop to combine the two processes because it opens the door for unintended consequences -- as soon as you have a cleaning problem, you'll find yourself over-etching, which means you'll have to over de-smut; plus you may have more carry over of oils this way; finally, you'll probably be dumping more than you should because the tank will fill with both soils & aluminum, and you'll have to dump as soon as either becomes a problem.

As for proprietaries vs. home brew, I think you should try to find out from experience how much the proprietary really costs you in practice vs. how much you really spend in practice if you try to do without. You might find that the additives prolong the solution life more than you anticipated, such that a pound to pound comparison isn't really useful.

You obviously know more about your business than I do, so please consider this just the spouting of an aphorism, rather than me presuming to "lecture" anyone ... but there is a saying in the industry: If you can't afford the cost of the customary chemistry, you might be doing the wrong work for the wrong customer at the wrong price :-)

Luck and Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Milky grey aluminum etch bath - can it be fixed?

April 4, 2018

Q. Hi,
I have a problem with our etching bath. In my company, we use etch with sodium hydroxide concentration 50 g/l, aluminium content 120 g/l and long-life additive (Al5000, Metachem).
About a week ago, there was a failure of the heating system which caused that the bath for a week was 20 °C. After reheating, the solution is milky gray, it does not help to add Al5000 or sodium hydroxide.
Is there any way to fix this bath or do I need new etching solution?
Thank you for any help.

Agnieszka Mackiewicz
- London, United Kingdom

April 7, 2018

A. How's the etching results? Basically etching is generally dependent on caustic concentration and aluminum content and temperature of bath. I have also faced same problem many times but the results were same though the colour of bath changed. If etching is okay don't worry about the colour.


Sumit Lodha
- Ahmednagar Maharashtra India

April 18, 2018

A. Your etch bath had been out of control:)

Firstly, you should analyse the bath. Caustic soda had fallen down in the bath as aluminium hydroxide. This situation is not sustainable for production. Also you should check bath's bottom for any precipitate. If you realize any precipitate you should [transfer] etching bath to another available bath for cleaning the bottom of bath. If you don't realize any (or acceptable) precipitate, than you should increase caustic soda to 85-95 gr/lt (if you find result 50 gr/lt). You should catch a balance between caustic soda and dissolved aluminium. Also temperature should be 60-70 °C. during production. Balance of bath will improve day by day, but you should focus on these parameters during every day.
Good luck,

alaattin tuna
- sakarya,TURKEY

December 10, 2019

Q. How to make the etching solution for aluminium anodizing. Tank size is 19000 Liters, final volume is 15000 Liters

Manauwar Azam
- Gaya Bihar India

December 11, 2019

? Hi Manauwar,

Please explain your situation, so that we can have an idea what you are trying to do. Are you wanting to etch as a preparation for anodizing, or are you looking at getting a dull, matte finish?

Brian Terry
Aerospace - Yeovil, Somerset, UK

December 11, 2019

Q. How to make etching solution for anodizing finishing matte for light champaign bronze color?

Manauwar Azam
- Gaya bihar india

December 2019

"Surface Treatment & Finishing of Aluminium and Its Alloys"
Wernick, Pinner & Sheasby
from Abe Books

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A. Hi Manauwar. You haven't introduced yourself yet, so we don't know what you know and what you don't know ... but people usually buy these kinds of process solutions rather than trying to mix them from commodity chemicals. Wernick, Pinner & Sheasby =>
has a 13-page chapter on the subject. As explained there, caustic soda at 4 to 10%, and operated at 40-90 °C can be used, with or without a wetting agent.

They go on to say that dealing with the widely varying dissolved aluminum content is a significant problem which can be addressed with bleed & feed, precipitation with alkali metal silicates, use of an etch recovery system, or (perhaps preferably) by chelation of the aluminum with sodium gluconate. Etches can also be accelerated with nitrites or nitrates. The same book also lists the effects of 47 other different additives on the etching process while claiming that there is no cohesive theory about the subject, just empirical results.

Probert's "Aluminum How-To" offers a more condensed and "problem solving" approach to the subject, and includes some practical hints about formulation like using about 4 oz./gal of chelater, but doesn't actually offer an exact mix. I think I'd urge you to buy a proprietary etching process unless there is a compelling reason not to. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

December 12, 2019

Q. When aluminium profile comes out after caustic soda, it looks very rough.

Manauwar Azam [returning]
- Gaya bihar india

December 2019

A. Hello again Manauwar. If it's not a matter of formulation, you may be etching too long, or too hot ... or perhaps an acid etch would be better for your needs.

Apologies, but I have the feeling that English might not be your first language, and you might ask an interpreter to help. Because you've posted on this site many times, and each time you do I refer you to our page: "Why won't they answer me?!", and ask you to please post full details of your situation. You never do, you never acknowledge our answers, you never reply with requested data, you just move on and post another abstract sentence fragment as your next question, so I don't know if you've received any help or we're just wasting each other's time :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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