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topic 32137

Burning problem in Hard Anodizing of Aluminum

A discussion started in 2004 but continuing through 2020


Q. We have hard anodize process in a chemical shop.

Anodize bath contains sulfuric acid (190~210 g/l) and oxalic acid (max.10% of sulfuric acid).

We have been processing mainly "AL 7075-T7451/7175-T7351". But sometimes we get burning on the anodized part (even "melting away").

About 3 Amp-hour/ is applied at 3 - 5 °C. It is not much current density, I suppose.

My questions are the following:
(1) I was told that oxalic acid helps to make current flow stable in anodize bath. Is that right? What is proper range for oxalic acid?
(2) What should I do to avoid burning defects in hard anodize process?

Thanks in advance.

Fr. Sleepless in Korea.

Dong-cheol Park
Aerospace - Busan, Kyungsang-namdo, South Korea


A. Oxalic is old fashioned. Glycolic/Glycerine is the state of the art and protects from burning better than oxalic. Be sure to ramp up very slowly with 7075, be sure to rack so as to GET THE CURRENT ON THE PART.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina


A. Hard coat temps in the 32 °F range, and very very good solution agitation are extremely important. We run at 30 ASF for hard coat.

When the agitation was not sufficient, the aluminum racks as well as some parts would burn.

Get a good rolling solution. I would bet that's where your problem is.

Todd Huehn
- Blaine, Minnesota, USA


thumbs up signThank you.

Agitation is wonderful thing.

I was able to get great hard coating by strong agitation.

Dong-cheol Park [returning]
Aerospace - Busan, Kyungsang-namdo, South Korea

January 8, 2014

Q. For hard anodize bath, is it recommended to work with solution mixing or scrambling conditioning?

- Tefen, Israel

January 8, 2014

affil. link
"The Surface Treatment & Finishing of Aluminum"
from Abe Books


A. Hi Iris. Sorry, but I do not recognize the term "scrambling conditioning" ... maybe you are alluding to turbulent vs. laminar flow?

Hard anodizing requires very good air agitation, and it is not sufficient to merely agitate the tank contents as a whole. You must provide agitation right below the individual parts because the hard anodizing process generates a lot of heat and the process will not function except at the intended operating temperature. Good luck.


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

January 12, 2014

Q. I would like to know how to calculate the power of cooling unit for hard-anodizing.
Data are bathtub depth 1000 millimeters x width 1100 millimeters x length of 2000 millimeters.
The plan is 1500A and 80V
What is the minimum distance to leave between the cathode parts?

IRIS VAX [returning]
- Tefen Israel

January 13, 2014

A. Hi Iris. The cooling unit must remove from the bath all of the heat that the rectifier adds. If the rectifier were running at 1500A and 80V all of the time, that would be 120,000 Watt-hours per hour or 120 KW-hours per hour or 410,000 BTU/hour, or 34 tons of refrigeration. You can scale this down based on what percentage of the hour the tank is empty, and what the actual amperage and voltage are as the voltage ramps up. If you figure the duty cycle is about 60%, then you need about 20 tons of refrigeration capacity.

The part to part spacing should not be critical for hard anodizing, as long as the parts don't touch each other and don't touch the cathodes, and don't shield each other from generous local agitation. Good luck.


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

January 26, 2014

Q. Dear Friends,
I have an issue in hard anodizing, parts get burned. I am using good agitation too; I am using the following parameters:
Concentration of sulfuric acid -- 20% by weight
temperature -- 0 to -5 °C
time -- 45 minutes
current -- 250 amps per square meter
jigs -- Aluminium (by using titanium jigs, parts were getting more burned)

Kindly assist me with what could be the reason of burning the parts after all this arrangement?

In hard anodizing, if concentration of sulfuric acid is less than 10%, then it causes the burning of Aluminium parts which are 7075 T6?

Aijazullah Tajir
- Abu Dhabi, UAE

January 27, 2014

A. Hi Aijazullah. Although the chemistry is very important, good chemistry won't help if the racks lose contact, or can't carry the amperage, or if agitation is insufficient to maintain the temperature of the parts.

Based on Robert Probert's earlier posting about rack capacity, and your observation that titanium racks fare worse, my guess is there is a racking issue. Sometimes an experienced anodizer can tell from a fairly quick look where the problem lies; so if such a person can visit, they may be able to offer expeditious help. Good luck.


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

January 28, 2014

A. At first glance the parameters look OK. A few questions:

1. Are the parts a very thin sheet metal?

2. Where is the burning occurring. At the rack points? In the middle of the parts? On the outer edges?

3. Cathode material, and ratio of the cathodes to the work?

4. Distance from parts to the cathodes?

5. Ramp time?

A picture would help a little bit.

Marc Green
Marc Green
anodizer - Boise, Idaho

January 28, 2014

Q. Dear friends
Does low concentration (say, less than 10%) of sulfuric acid for hard anodizing cause the burning of parts (7075 T6)?I am using 250 amps/m2 for 45-min, temp 0 to -5 ° C.

Aijazullah Tajir [returning]
- Abu Dhabi, UAE

January 30, 2014

A. 10% by weight would be only 2/3 normal strength and, YES, will surely "burn".

10% by volume of 66 degree Be sulfuric acid is about right and should not burn at your current density. Perhaps the air agitation is hitting the part and displacing solution long and often enough to cause burning, or your temperature is too cold for the alloy and current.

Send us some pictures of the racking and whatever you call "burning".

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

January 30, 2014

A. Aijazullah

At 0 °C and 10% acid concentration, your voltage will be higher than at 0 °C and 15% acid concentration. Subsequently the wattage generated will be higher and the need for adequate agitation increased.

Most of the burning I have seen has been the result of inadequate agitation or an excess of dissolved aluminum.

Willie Alexander
- Colorado Springs, Colorado

February 5, 2014

A. Regarding Solution make up and air agitation, I would recommend Robert Probert's book, "Aluminum How To". It really helped me a ton. For the air agitation in our acid solution we changed our air spargers to induce "micro-bubbles" instead of creating a rolling effect with air,and built new liquid manifolds with eductors. The combination of the two has eliminated almost all shadowing and our burning in minimal.
Best money I spent was on this book.

Scott Frazier
- Spokane Valley Washington, USA

Tell me the right hard anodizing formulation with glycolic acid

March 29, 2016

Dear Mr. Robert H Probert

We have an hard anodize process on cookware and various industrial parts. We have an issue of not getting proper black dark colour in hard anodize process. Can you help me to give me proper chemical composition of hard anodize bath what is missing?
We are using sulfuric acid and oxalic acid Anodize bath.

Please sir give me proper chemical composition of hard anodize bath

Can we mix Glycolic/Glycerine in our current bath of sulfuric acid and oxalic acid to protect from burning.

Jigar Panchal
hard anodizing - Gujarat, INDIA

March 2016

thumbs up signHi Jigar. The previously mentioned "Surface Treatment & Finishing of Aluminum ..." text mentions a Reynolds formula of 15-24% sulphuric acid, 1-2% glycolic acid, and 1-2% glycerol. If Mr. Probert or anyone else can suggest a published work that answers your question, we'll be very happy to post the reference to that book or journal article. Or if you retain Mr. Probert as a consultant, he can of course teach you whatever the two of you agree upon as part of his professional services.

But in this field of hard anodizing additives, where various tweaks of oxalic acid, glycolic acid, glycerine, and miscellaneous substances are held as trade secrets going under a dozen or more different proprietary names, we can't publish such formulations. Thanks for your understanding that, considering the anonymity of the internet, we must guard against new employees accidentally blowing trade secrets, or disgruntled ones deliberately doing so :-)


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

(Click graphic for higher resolution)

October 21, 2016

Q. Hi!

We are facing burning for hard anodizing; we used 35V / 8 °C / 20%w/v at 30 minutes; Aluminum concentration 2gm/L., we used titanium jig for hard anodizing and the total area of parts is 24 dm2 with Al 6061 alloys, Can anyone help us to solve this problem?

Anthony Aw

Anthony AwTeow Kheng
- Singapore

October 2016

A. Hi Anthony. The most common hard anodizing temperature in my experience is 0 °C.
8 °C sounds much too warm for conventional practice. Where did that spec come from?

You don't mention ramp control or current limiting on your voltage, but you certainly can't start at 35 volts.


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

simultaneous October 22, 2016


We have ramping the power supply from 0 volt to 28 volt within 10 mins, every 2 V/min ramping, our power supply was at 500 Amp / 50 Volt, unfortunately when voltage reached 28V, the current gone up to 380 A, when voltage reached up to 32V, current gone up further to 500A and parts burnt finally.

What is your meaning for current limiting on voltage? Our hard anodizing tank was 630 litres only; we do not require very hard coating so we used 8 °C.

Anthony Aw Teow Kheng [returning]
- Singapore

October 26, 2016

A. Anthony

As Ted alluded to, you anodize with current not voltage. Hard anodize is typically processed at 24-36 amps per square foot with a ramp to full current in 3 - 5 minutes.

It appears the burning is located at your contact marks. Aluminum is a better conductor, but if you must use titanium, you likely need to improve the quality of the contact(s) or increase the number of contacts.

Willie Alexander
- Green Mountain Falls. Colorado

August 1, 2017



October 27, 2017

Q. Hi readers,

Need some assistance. We do <25micron anodising and lately having issues burning.
Black color or the silver color anodising is coming out with burns at same spots away from the anode. We have put glycolic acid, and run 20% sulphuric acid.
Checked concentration Al levels, Temperature all in specs.

Current = 1500 A , Load size = 8 sq. m.

Consistent burn marks on the jobs equal distance.
Changed Lead strip cathodes of the burning side.

Please suggest what we might be missing or doing wrong?

Anil Kumar
AEC - Wellington, New Zealand.

November 2017

A. Hi Anil. If I am understanding you, all parameters are in accordance with specifications which formerly were generating good work. If that is the case, it's hard for me to think of anything besides make-&-break contact or bad agitation nozzles which would cause parts to start burning. Good luck.


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

November 5, 2017

Q. Thanks Ted for your time.
I am sharing more details so as to understand the problem a bit better...
We have now welded a polypropylene 5 mm thick sheet to the affected area in the anodiser tank and burns disappear for 2 days and came back again on the same spots but less dense now.

Al conc: 10.9 g/L (12-15 g/L)
Sulphuric acid 183 g/L (180-200 g/L)
Temperature: 21 °C (20-22 °C)

We are having hard time getting work through and most pieces are getting burn marks but not as much used to a week and half ago.

Suggestions from the readers welcome.
Thanks in advance.

Anil Kumar
- Wellington, New Zealand.

November 10, 2017

A. Anil

Your aluminum and sulfuric acid concentration are ok.

Your temperature is ok if you are running regular anodize. 20 °C too high if you are running hard anodize.

Your current density is a little high if you are running regular anodize, and a little low if you are running hard anodize.

If you are running hard anodize you will want to make sure you have adequate agitation. If the burning is occurring in an area where agitation is impaired, you may need to add supplemental agitation.

Willie Alexander
- Green Mountain Falls, Colorado

November 11, 2017

Sounds a lot like chlorides in the anodizing tank.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

November 11, 2017

thumbs up sign Thanks very much for the great response gentlemen. The root cause for burning (which turned out to be color variation in the end) came out to be something we never would have thought of.
While following the load during anodising I spotted some agitation pipe leakage in the Seal tank. That tiny bubble began at the bottom and took the shape of giant triangles by the time it appeared on the surface, thus causing the vulnerable color layer to come off in the same shape. Repaired the valve and now the issue disappeared. Such a great learning curve.

Thanks again to all.


Anil Kumar [returning]
- Wellington, New Zealand.

Hard anodise burning problem

October 13, 2020 -- editor's apologies, not published until Nov. 21

Q. Hiya. I work in a metal treatments shop providing conversion coatings along with chromic and sulphuric anodising. We have recently installed a hard anodise line but we're having a problem with a burning issue which we are trying to get to the bottom of.

Test panels that are jigged on Aluminium jigs anodise fine when they are on their own towards the top. But any panels that are jigged towards the bottom seem to always burn. The jig is masked off par contact areas.

The tank is running at 17.3% Sulphuric acid concentration, Aluminium content is around 5 g/l along with an Oxidite A-42 additive and the temperature is -1 °C with good agitation provided by eductors. We Hard anodise @ 33amps/ft2.

Could this be down to poor agitation at the bottom of the tank? Or maybe provide a more direct connection on both ends of the jigs help solve this issue? I do see a very slow ramp up helps but not 100% perfect

Any advice would be great.

Danny Baker
- Yeovil Somerset

November 2020

A. Hi Danny. Although thin parts can be a problem, if they anodize properly near the top I don't think their thinness or the contact arrangement is the problem.

Slow ramp-up of voltage is of course imperative because the thing that is preventing excessive current is the resistance of the anodized film, so burning will occur if slow ramp-up is not achieved. This is why parts with poor contact burn: they don't slowly build up a film but instead, when the voltage is high enough to 'break through' whatever is deficient in the contacts, highly conductive aluminum is exposed to high voltage and conducts too much current.

In your case I'd be pretty confident that the problem is in fact agitation. Hard anodizing, with its high voltages & high currents produces a lot of heat, and the solution in the immediate vicinity of the part will ruinously heat up very quickly unless there is excellent agitation right on the part. Agitation which is adequate for Type I and Type II anodizing will not be adequate for Type III and its much higher heat generation and cooling requirement. But Chris S's answer above is interesting.

Luck & Regards,

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

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