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topic 4408

Dissolved Aluminum in Anodizing bath -- How Much to Have, How to Analyze/Titrate, How to Correct/Adjust It


Q. Dear Sir /Madam,

It has been said that higher Al content (German belief: 12 g/L , American thought: 20 g/L ) in sulfuric anodizing bath has detrimental effects i.e. Higher required Voltage (at the same current density ), softer anodic oxide and nonuniform coloring. We performed anodizing on 1100 & 6064 in 24 g/L sol. with subsequent inorganic coloring in the lab, but saw no above-mentioned bad effects on colored specimens (No microscopic examination was done). Are there really any bad effect on oxide characteristic and uniformity of colour (excluding aluminum sulfate deposition on specimens)? What is the Al content upper limit and why?


Amir Koorosh Zarrabi
- Isfahan, IRAN

May 3, 2011

A. Hi
I think 12 g/l Aluminium sulphate is the maximum. Above this the solution loses conductivity and more voltage is needed to compensate for this. Also for colouring purpose we have to keep the aluminium sulphate under control.

Vivekananda Pai Bachodi
- Auckland, New Zealand

January 2014

A. Hi Amir. While there may well be Americans who think 20 g/L is good, I don't think that's the prevailing thinking. Just going by what I hear on these pages, 8 is about ideal, and more than 12-15 is thought to be problematical. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

How to titrate for dissolved aluminum


Q. Could someone please advise their method for titrating the anodizing bath? The method that we have used for years seems very subjective to me, especially the aluminum content portion of the test.

Thanks in advance for any help you can give.


Rick Watson
- Greenville, South Carolina


thumbs up signHi, Rick.

Please tell the readers what procedure you are doing that seems so subjective to you, and they'll be more likely to suggest an alternative. They might fear looking stupid, suggesting as non-subjective a test which you've already dismissed as very subjective.

Thanks. Regards,

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. You are right, the aluminum results derived by calculation from free versus used acid titrations do not give repeatable and accurate dissolved aluminum results. Depending upon how you are controlling the bath and the frequency of additions the range of free acid versus used over time, etc. it may not matter, particularly if your processes depend upon a range of say 8-12 g/l of Al. One shop I worked at where we had tight process controls, we had AA performed weekly. Interestingly enough, they never matched the dissolved aluminum results reached from titration. Wet bench chemistry has an error of +/- 5% right off the bat, depending upon the technique and the technician.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


A. We recommend the same procedure as everyone else. SG measurement with a titration of the H2SO4. Plotting this on a graph yields the aluminum content. Another way is to use the back titration with CuSO4 and EDTA but that may be overkill. Really all you need is Al between 2-12 g/l for anodizing and the simpler analysis should give you that.

Lee Branch
Richmond, Virginia

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Anodizing bath with no dissolved aluminum


Q. What happens if anodizing bath has nil dissolved aluminum ?

Dado Macapagal
- Toronto


A. If nil Al(+3) is present in your anodizing tank you find you will consume more electrical energy to create anodic oxide film on aluminum substrate. It is advisable to have 5-15 g/l, preferably 10 g/l Al(+3) in the sulphuric acid anodizing solution. If more Al(+3), then your electricity consumption will be higher resulting in difficulty for cooling the solution. It is advisable to use ion-exchange equipment to have constant Al(+3) in the solution.

Timur Ulucak
aluminum extrusions & finishing - Istanbul, Turkey


A. Aluminum oxide will always dissolve in sulfuric acid anodizing solution, thus, it's impossible to have NO ALUMINUM once the first part is immersed in the bath and power is turned on!

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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A. You should keep the aluminum around 12-16 g/l. if the aluminum gets too high you can decant half the tank then fill it with water, mix it good with air, run test, and then add sulfuric to g/l you require to run. Make sure you let the tank cool to the temperature you want to run at. Running with no or too low aluminum will give you too shiny a finish and you will not get the coating you need.

Kris Heim
- Walkerton, Indiana

How do I obtain 5-15 g/l dissolved aluminum in my electrolyte?


Q. Hello all you wonderful people at Long time, no chat! Things are progressing slowly after a small personal setback but things are good; it looks as though I am back in the aluminum anodizing race. All of the equipment is in place and I am working on mixing my initial bath batches. Exciting stuff! It has been a long journey and is sure to only get longer; however, I have prepared myself on many levels and invite the challenges and obstacles (I have so much more to learn). It should be an amazing learning experience and hopefully with proper judgement I can build a lasting small business.

This past week, the batch mixing has progressed without a hitch, up until this morning. Regarding the electrolyte, I fully understand that by volume the batch is to be made up of 9.3% H2SO4 and 90.7% H20, however, I am unclear on the dissolved aluminum content within the tank. I realize these are elementary questions for most of you so please bare with me. In the past I would always mix my H2SO4 into my H20 and disregard the dissolved aluminum piece of the puzzle. Though I never ran into any complications (that I recognized anyway) I feel that this is something I must understand before I move forward.

- How does one get the dissolved aluminum into the solution?
- How does one analyze the solution down the road to determine the dissolved aluminum content?

Lastly, I would like to thank Drew Nosti at Anodize USA. I obtained my cathodes from Anodize USA and would like to mention the outstanding product that Mr.Nosti stands behind. The engineering behind Anodize USA's cathodes provided me with a simple to use, high quality product. Thank you for your support and advice Drew, and thank you for making such a product available, particularly to a small fry such as myself. Cheers!

daniel degueldre
Daniel DeGueldre
   anodizing shop entrepreneur
Ste. Anne, Manitoba, Canada


A. To get 15% aluminium content into your bath, just dissolve the required weight in a beaker of sulphuric acid and add it to the tank. You may need to gently heat it to ensure it is all dissolved. Analysis can be done by titration, so look in a good analytical chemistry book.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


A. With regards to the aluminum, it is my understanding that (on a theoretical level) the bath won't work without some dissolved aluminum. On a practical level, the minute you put parts in the acid you start getting some dissolved aluminum present and it starts no longer being an issue.

With regard to maximum concentrations: before beginning work for my company, it had been established that letting the dissolved aluminum get up to about 3 oz per gallon (22 g/l) resulted in really poor anodizing with the aluminum settling out of the bath and onto the parts. We then, for a while, just started a brand new bath with no dissolved aluminum whenever we went over 2 oz/gallon. Now we use a Anopur unit to filter out the dissolved aluminum and try to keep it at about 9 g/l (1.2 oz/gallon).

For testing purposes, see letter 38674.
You may also want to check out letters 30612, 22036, 12274, 5032. According to my notes you might find them interesting.

Good luck!

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
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A. I have recently started up a line here in Westminster and after running for almost 3 months I have finally reached 4.5 g/l AL in the sulfuric tank.

I had little to no startup issues, but then again Bob Probert and myself plated 10-15 runs of scrap 6061 and 2011 alloy aluminum before plating production parts the next day.

Purification systems are extremely expensive and I am shying away from that style right now although it is by far the best way to keep the Aluminum level at 9 g/l. I will decant my bath in half when I reach 11 g/l and then add fresh sulfuric and remake the bath.

This is only an option. Please make sure you have proper disposal methods before decanting the tank this way.

Ryan Cook
Ryan Cook
Toccoa, Georgia

January 30, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Friends,
Can you tell me how to eliminate the Aluminium content from the sulfuric acid anodizing bath?

Aijazullah Tajir
- Abu Dhabi, UAE

probert book
Aluminum How-To

by Robert Probert
$89 New
The Chromating - Anodizing - Hardcoating Handbook

January 2014

A. Hi, Aijazullah. As you see, we appended your inquiry to a thread where it has been answered.

Some additional info on how to reduce dissolved aluminum: in Letter 52410 it is suggested that, short of inexpensive decanting, expensive ion exchange is the only alternative, and Eco-Tec is suggested as another possible source of the equipment. Robert Probert's "Aluminum How-To" includes a worksheet for adjusting dissolved aluminum. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Anodizing Solution Concentration and Monitoring Questions

February 3, 2016

Q. Hello Sir,

I write to this community regarding my doubt about the basic set up to start an anodizing plant. I'm aware of the equipment but I lack the knowledge of the composition of the chemicals. I'm interested in Sulfuric acid anodizing. PLEASE GUIDE ME THROUGH AND KINDLY PROVIDE ME WITH THE BEST EXPLANATION FOR UNDERSTANDING. My doubts are as follows:
1) What is the ratio of DM water: Sulfuric acid (98% grade) in liters to be added? How to determine it?
2) The amount of Aluminium to be present in the electrolyte: should we add Alum powder manually initially or just maintain the Al2SO4 concentration as the process goes on?.
3) Can you please tell me where can I find detailed procedure for titration analysis for determination of Al?
4) What are the essential quality maintenance practices to be ensured and how often to check the liquid?
5) What is the best ratio of composition for obtaining the most efficient and best results of anodizing?
6) Does the voltage of play a major role in the output? How do determine the necessity voltage for the process?.


February 2016

A. Hi Lalith. Please start with our Anodizing FAQ for a quick intro.

1). Is answered in topic 4.3 of Robert Probert's "Aluminum How To", with warnings about using only Electronic Grade sulphuric acid. It is also mentioned in the FAQ.
2). Just maintain the aluminum content; do not add alum.
3). The titration procedure you seek is p.74 of "Aluminum How To". But as you will see in the discussion above, many people have limited confidence in such titrations.
4). Please tell us whether you are talking about just the anodizing tank per se, or are including other processes like etching and the very critical sealing step. What anodizing books and/or standards do you have available to you, that we can refer you to?
5). I'm not sure how this questions differs from Q1 -- perhaps I'm misunderstanding it -- but about 15% be weight (10% by volume) is the approximate correct ratio.
6). Yes, the voltage is critical and depends on the alloy, and is about 15 V for most 1xxx, 5xxx, 6xxx alloys, 16 V for 7075, and 20-21 V for 2xxx alloys. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

March 20, 2016

A. The best method for anodising aluminium is to make bath consisting of sulphuric acid 10% by volume and 90% water; the temperature of the bath must be 0-4 °C, that is, cold bath in order to have hard enough layer of oxide. Above this temperature the oxide layer tends to dissolve in the bath but at this low temp the oxide layer acccumulates with high thickness and you can use higher amperage without a problem. No need to add aluminium powder to the bath as it ages the bath. The layer looks dark grey and is very resistant.


ahmad abo awf
- cairo,egypt

April 29, 2016

A. Here's the classic titration for a sulfuric anodize bath, it is very easy to do, and does not require much equipment.

You will need:
- a 50 mL capacity buret (looks like a graduated cylinder with a rotating stopcock at the base, and it is numbered upside down)
- A 5 mL pipet which you can buy off Amazon for about $30. The 'sure-pette' from accu bio tech is a lot better than I expected for the price.
- A liter of Sodium Hydroxide 1.0N which you can get at any lab supply house
- A small bottle of phenolpthalein indicator, also a common lab item
- A bottle of Potassium Fluoride 50%w/v. You can also buy it as solid and make it yourself with deionized water.
- 2 glass beakers
If you don't want to sit there swirling your beaker during the titration a magnetic stir plate and stir bar are very useful!

Start by pH stabilising your KF. Just put a couple drops of PTH indicator in the bottle. Shake it. It will turn pink. Add one drop of very diluted sulfuric acid- Like put a couple drops of the fresh 98% in 10 mL deionized water to use for this- NOT out of the tank. Shake the KF bottle again. Repeat until it goes clear. You only have to do this once, it is now ready for use.

Next label your beakers "A" and "B". One will give you JUST your Sulfuric value, and the other will give you the value of the Sulfuric and Aluminum combined. There's a calculation later for that.

In beaker A, add 5mL of your plating bath, 10mL of your KF solution, some DI water, and a couple drops of PTH indicator. You will see the solution get cloudy as the KF reacts with the free Aluminum and makes it fall out of solution. This is how you get ONLY your sulfuric number.

In beaker B, add everything EXCEPT the KF. Here, you WANT to titrate the Aluminum out as hydroxide.

Start with beaker A. Fill the buret to the 0 mark all the way at the top with NaOH 1.0N. While you are swirling the beaker, open the stopcock and let it slowly drip into the solution. For a while, nothing will happen. Then you will see pink trails forming with each drop. You are nearing the end point of the reaction so slow down. When the solution does not turn clear again with the addition of one drop of NaOH, read the buret and write down that number as value "A" in your log book.

Repeat with beaker B.

You now can calculate your concentrations!
9.807A= g/L H2SO4 in solution
(B-A) x 1.797 = g/L Aluminum in solution.

One thing to note is that if your Aluminum is so high that it is out of limits, you may need to add more KF at first to get a true Sulfuric value, but if your aluminum is high, you need to decant and re-test the tank first anyway before taking any other action.

To test your titration skills, make up a mock tank sample of just sulfuric of known concentration, and see if you can hit it within +/-5% accuracy.

Hope this helps, and good luck!

Rachel Mackintosh
Plating Line Solutions Control / Industrial Metals Waste Treatment - Brattleboro, Vermont, USA

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