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topic 21345

Complexing agent for aluminum? Avoid rock hard scale formation?

Current question and answers:

January 7, 2021

Q. Looking for ways to Reduce / Eliminate 'rapid scale buildup' in Drain Lines of Rinse Tanks following Black Oxide Treatment of Iron and following Caustic Etch of Aluminum.

Both Treatment Processes involve strong Sodium Hydroxide materials and these 'salts' attach aggressively to the walls of the drain lines, regardless if constructed of Steel or PVC.

This requires frequent A) Replacement of Drain Lines {Where Accessible} and/or B) Aggressive Mechanical Clean-out.

Can anyone share Means to address this problem???


Lyndon Tschetter
- Mishawaka, Indiana

January 2021

A. Hi Lyndon. We appended your inquiry to a generally similar thread where some ideas have been offered...
Regarding black oxide, EPI (Electrochemical Products Inc.) [a finishing.com supporting advertiser] has an article at
which suggests that some black oxide formulations can hold more iron, thereby limiting the problem. This thread offer some ideas for the aluminum etch sludge, including addition of sodium gluconate.

It may be impractical for rinse tank drain lines, but if possible, keep recirculating the sludge and it will not attach and harden. Anything that keeps the sludge moving is key.

My own experience suggests that such sludges are not actually a scale; rather, that when they first form they are rather light and fluffy. But that when they are allowed to sit undisturbed, the heavier-than water sludge components quickly drive out all the interstitial water, leaving a rock. I know that that 'rock' can be gradually washed away, and have suggested a way that such sludge could be handled with a paper "gravity" filter, but no one has actually tried it to my knowledge :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

Previous closely related Q&A's starting in:


Q. I am looking for a complexing agent for aluminum to use in one of our etching baths. The pH of the bath is about 2 and right now we are getting AlF3 scale (rock hard) formed in the bottom of our test tank. This bath will be used for cleaning and etching aluminum alloys. EDTA is an obvious complexing choice, but I have doubts on how well it would hold aluminum at pH 2.

Terry Tomt
- Auburn, Washington, USA


A. Dear Terry,

Look for aluminum fluoride is high 19.37. So you can use 5SSA 5 sulphosalycilic acid with log k=29.

Hadi Khosravi
- Tehran, Iran


A. Try sodium gluconate.

Ken Vlach
- Goleta, California

contributor of the year

Finishing.com honored Ken for his countless carefully
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which the finishing world continues to benefit from.


thumbs up sign Thank you for the responses. I am concerned that sodium gluconate is made to work in alkaline pH region and not in acid as my case is. I will look into the salicylic acid for sure. Also I will research the gluconate.

Terry Tomt
- Auburn, Washington, USA


A. It may be of only academic interest to you, or no interest, but this 'rock hard' sludge might only become rock hard when it's allowed to settle and squeeze out the interstitial water. If you can keep it moving, it may never harden. Even after it has hardened, though, I think you can remove it by getting it back into solution via patient washing, and then filter it out. I have seen a number of aluminum sludges/slurries that do this.

I haven't actually tried it, but I'll bet you can clean it out by using an automatic paper filter and recirculating water through the tank, directing the output at the scale

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

Avoid AlF3 rock hard scale formation in acid etching

February 10, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am looking for advise how to avoid AlF3/AlPO4 scale in our hydrofluoric/phosphoric acid etching baths. The baths are used to etch aluminium alloys. We currently have to shut down the production every 3 months to carry a "dig out" with drilling pneumatic hammers and remove all the AlF3/AlPO4 scale (rock hard)from the walls and bottom of the etching tanks...
What Al complexing agents do you recommend other than EDTA? EDTA is the most obvious choice but don't know if it would work at pH 2 and represents a serious problem to the environment. What could we do to avoid the Al sludge to become rock hard? Continuous stirring? Rising the temperature?Any practical solutions?

Any help would be greatly appreciated. Thank you :-)

Nuno Loureiro
- Peterborough/UK

February 2018

A. Hi Nuno. Any chance of hooking up an automatic paper filter, either for continuous use or for periodic cleanup per the sketch on this thread we appended your inquiry to?


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

February 11, 2018

Q. Hi Ted, thank you for your prompt answer. This thread has been the most informative...
Do you know if actually Ted's idea had been successfully put into place? Wouldn't the paper filter be quickly deteriorated for continuous use due to the very low pH of the etch bath?
One last question: do you know if using chelating agents such the ones mentioned would work and would it be economically feasible? Are there any new ones you're aware of?

Thank you once again and congratulations on this knowledge exchange platform. Wish there were much more...:-)

Kind regards,

Nuno Loureiro, chemist [returning]
- Peterborough, Cambridgeshire, United Kingdom

February 2018

A. Hi Nuno. If you collect a plastic bottle full of your slurry, I think you'll discover that it settles out in a few hours to a rock hard sludge and a decantate ... but that if you turn the bottle over and back a hundred times or so the 'rock' gradually washes back into a slurry. If your aluminum scale acts as I describe, I think it is possible to continuously filter the solution or, lacking that, to recirculate the decantate to wash the 'rock' back into a slurry for periodic filtration.

Automatic paper filter systems use disposable newsprint which unrolls onto a continuous belt of mesh. So the fact that newspaper isn't particularly acid resistant shouldn't matter -- although the rest of the filter parts would have to hold up to the solution. I haven't tried it.

I am not a chemist and cannot comment on the complexing suggestions offered by Hadi, Ken, and Terry.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

February 12, 2018

thumbs up sign Thanks Ted, your help was precious! I'm going to study and explore all these ideas. I'll keep you posted :-)
Hopefully with good news...

Kind Regards,

Nuno Loureiro [returning]
- Peterborough, Cambridgeshire, United Kingdom

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