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Anodising thickness not being achieved

(-----) 2007

Q. Hi all,

We are having trouble achieving a 25µm anodised layer on our components.
The alloy we are anodising is 6026 T6 and our anodising parameters are stated below.

Concentration - 10% sulphuric acid by weight
Temperature - 21°C
Voltage - 13.5 V
Current density - 1 A/dm2
Time - 80 minutes

Is this lack of thickness to do with the current density being too low?
Also, should we be achieving the required 25µm thickness quicker?


John Peacey
production engineer - Gloucester, UK

First of four simultaneous responses -- 2007

A. OK, 25 µm is 0.001 inches. 10%/weight is only 110 gm/L, you need at least 165, so add 55 gm/L (that is 4 fluid ounces per gallon of 66 degree baumé Sulfuric Acid.

1 amps per sq decimeter is too low, raise it to 1.29 (which is 12 amps per square foot.) This should take 16 volts if you are at 165 gm/L.

Temperature is OK.

With the parameters I have suggested you should get thickness in about 55 to 65 minutes.

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

Second of four simultaneous responses -- 2007

A. Yes, your voltage is way too low, and that's why you are not achieving the desired thickness in a reasonable amount of time. Are you limited to only 14 volts with your rectifier? If not, just anodize at the same current density you are, and let the voltage float. At 12-15 amps/sq ft, you should achieve 25 microns in approx. 45 minutes.

Marc Green
Marc Green
anodizer - Boise, Idaho

Third of four simultaneous responses -- 2007

A. First, I doubt if 13.5 volts is giving you 1 amp/DM. I will bet that your rack is pulling part of the amperage.

If you want good anodize in a timely fashion, you will need to be at about 18 volts, depending on alloy.

You would be helped if your tank temp were 1 C lower. The critical part of this, is the tank temp uniform and how close is it maintained? It should not vary by more than 0.5 C from start to finish.

How well is your tank agitated/ It needs to be air or pump or mechanical agitated.

Check for line losses. Do a test of voltage at the rack and set up a run where you can check the amperage at the rack or at least at the bus bar.

The beauty of a lower temp with a higher voltage is you will achieve the required anodize thickness in a lot less time, probably in around 30 min if you have constant temp and good agitation. You will also get a denser/harder anodize.

James Watts
- Navarre, Florida

Fourth of four simultaneous responses -- 2007

A. I'm sure you can get your target thickness with shorter time (around 50 minutes) when you lower the temperature to 18 °C and increase voltage to 17 or 18 volts.

John Hu
- Singapore


A. John

You do not state what thickness you are achieving, but using your parameters, you should be able to achieve 25 µm in about 72 minutes. If you do not have particular coating requirements, you could go to 1.5 asd and achieve the same thickness in about 48 minutes.

Make sure your connections and agitation are good, and that you are not overloading your racks.

Willie Alexander
- Colorado Springs, Colorado

January 2, 2008

A. Consider the formula for anodizing thickness:
Thickness (mil)= ASF x Time (min)/720

It seems to me that your current density is too low and at 80 minute time, you may be dissolving some coating as well.

I suggest increase your current density and make sure the parts enter clean in anodizing bath. 70 F temp should be good.
Concentration of the acid is fine. Some people run anodizing at 15% v/v but we have been running at 10% with no issues.

Hemant Kumar
Hemant Kumar
- Florida, USA

January 6, 2010

Q. Respected Sirs,
I am Prem Easwar from India, I am an under-graduate student and I am involved in an anodizing project for my final year. We are required to optimize the anodizing coating at an "UNIFORM thickness of 25 µm" in a "HARD ANODIZING PROCESS".

Initially we do soap degrease, drag-out rinse, cascade rinse, desmutting (H2SO4) and cascade rinse (demineralised water). The cascade rinse is carried out for 15 seconds at ambient temperature and 250 micro semen/cm.

Then Hard Anodizing is done using H2SO4 for "30 MINS" the temperature being maintained at 0-5 °C and the concentration of H2SO4 is 240-260 g/l and 5-17 g/l aluminium content!

Then cascade rinse is carried out for 15 seconds at ambient temperature and 70 micro semen/cm. Then finally sealing is done with Demineralized water at 95 °C for 10 minutes and pH is 6.5-7.5

We use a Lead-Tin alloy Cathode and current density is 35-45 amp/sq.ft.

Please give me advice as to what to do to optimise the coating thickness at 25 um.

Prem Easwar
- Tamil Nadu ,India

January 6, 2010

A. Hi, Prem. The only thing that sounds funny is the H2SO4 desmutting. What kind of smut will this remove? I don't think it will remove either silicon or copper.

The temperature sounds a bit high, with -1 C to 0 °C being more common. The current density seems high, and you haven't mentioned the voltage, but you will need excellent local agitation to prevent burning. The acid concentration sounds high. Do you have some good anodizing books you are working from?

"Optimize" usually means to improve. How well does the process operate as you have described it? In what way do you see it as less than ideal? Thanks.


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

January 22, 2010

Q. Respected Sir,

The coating is done on aluminium components and the H2SO4 de-smutting is done to remove the aluminium oxide layer that is formed on the surface of the component prior to anodizing.
Also the Industry says that they do not encounter silicon or copper impurities.

And I am very sorry for having omitted the voltage value in my previous post,the voltage maintained is 22 v.
The industry wants a coating thickness between 23 -27 µm (customer specification) so we thought having it at 25 µm would represent a middle value and also give way for some small variation (though we plan to use some statistical control techniques later to reduce variation)
Sir you have mentioned that the acid concentration, current density sounds high, could you suggest some possible values for acid concentration, current density and voltage?

1) ACID CONC:240 - 260 g/l ,5-17 g/l Al content
2) CURRENT DENSITY: 43 Amp/Sq.ft at one end and 38 Amp/Sq.ft at the other (Where current enters and leaves respectively)
3) VOLTAGE: 22 V

Please advice,eagerly awaiting your response.
Thank You,

Prem Easwar [returning]
- Tamil Nadu, India

January 24, 2010

A. Hi, Prem. It isn't clear to me whether this is an actual anodizing line which is not working properly and you are making adjustments to it, or whether this is a thesis assignment about a hypothetical anodizing line where you are supposed to research this stuff.

You didn't answer yet if you have some good anodizing books to work from. If this is a thesis assignment, you should try to get hold of a copy of "The Surface Treatment and Finishing of Aluminum and its Alloys"; if you need quick practical answers to get a real anodizing line working properly again, I'd suggest Probert's "Aluminum How-to".

This page suggests 165 g/l H2SO4 and if you search the site for "hard anodize" or "hardcoat", you'll see recommendations up to about 210 g/l.

35 to 38 ASF sounds okay for hard anodizing, but there should be no measurable difference from one end of the part to the other; I don't understand that part.

22 volts sounds low for hardcoat, but if you can get 0.001" with that voltage, then it's enough. Please spend some time searching the site as you will find dozens of threads on the topic. Good luck.


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

February 6, 2010

Q. Dear Sir,

I would like to make some clarifications based on your previous post.The problem that I had referred to is in a real anodizing line that is not working properly.

We did go through the various threads in this site and many others over the net as advised by you but we could not find the specific acid concentration that would be required for 25 µm.
Also you have said in your previous post that 22 V sounds low, so could you possibly suggest values for the voltage and acid concentration that would actually be required?

It would be of immense help to us if we had these values with us for further experimentation! Eagerly awaiting your response.

I forgot to mention that we require 25 µm anodizing thickness in HARD ANODIZING!I just thought I would make that clarification. As always I am eager to hear all the responses that I can get.

Thank You,

Prem Easwar [returning]
- Tamil Nadu, India

February 20, 2010

Hi, Prem. Perhaps it wasn't clear, but you don't vary the solution concentration depending on the intended thickness. As previously mentioned, use the 165 g/l suggested by Robert Probert as a minimum, to as much as 210 g/l.

I need to say again that in my opinion to approach the project correctly requires having respected books on the subject on hand. The company should also have specs on hand such as Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil] and ASTM B580 [affil. link to spec at Techstreet] because, regardless of any individual's opinion on a good concentration, voltage, amperage, temperature, or procedure, parts must always be processed according to specifications.

You came here asking our advice because you are new to the technology, and I am trying to help you learn the crucial lesson that although internet forums can be an excellent way to quickly get steered in a proper direction or pick up a clever tip, internet hear-say is not a proper substitute for respected reference materials. If you complete this project without convincing your employer to stock a proper anodizing reference library, then I have failed you in your outreach.

These books and specs will tell you that the required voltage depends upon the alloy that you are processing. But for hard coat of .002" (50 µm) you are probably looking at something like 75 volts. For your .001" (25 µm), maybe about 40 volts. Generally speaking, hardcoat anodizing is considered to be .002" (50 µm thick). Best of luck!


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

April 9, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sir,

I will be very grateful to him if anybody sort out my this problem.

I have job in Aluminium Alloy, 91% Aluminium and 7.2% silicon & rest others.

The required Parameter is Thickness (15-25 Microns), Hardness (HV 400-500), Roughness (Ry-15, Ry-6.3), Area of job is 15.5 sq. inch both sides.

I do this with normal rectifier with sulphuric acid bath, Sp. gravity 1150-1200, temp. 10-15 °C.

Now please advise the required parameters for above standard anodizing like required voltage, required amps, required temperature and required time per cycle.

Please Advise.

Pooran Singh
- Faridabad, India

January 2014

A. Hi Pooran. We appended your inquiry to a thread which should help you determine the ideal operating parameters for your situation.

However, you have also introduced the issue of surface roughness; and for that topic you might want to go on to letter 51222, "Maintaining a Surface Finish after Anodizing" and letter 22123, "Hardcoat anodizing surface roughness, Ra/RMS issues, Honing". Good luck.


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

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