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topic 22258

Chromic acid anodizing: introduction and possible problems

A discussion started in 2003 but continuing through 2019


Q. To: All

Presently we are awaiting our customer's approval on our chromic acid anodizing process. As we are waiting, I come to think of possible problems which are usually faced by the processors. What I am referring to are the problems that we could not get away with; kind of a consequence of the process. If that is so, I wanted to be prepared. If the problem is surely expected after a time of processing, then there should be solution already at hand. And what are those expected problems to be dealt with?

I'll appreciate any input.


Nila O. Par
- Winnipeg, Manitoba, Canada


A. Almost all problems with chromic acid anodize are self induced. Parts not clean, inadequate clip (clamp) contact, tank analysis not done often enough, too big a load for the heater to maintain temperature, times not adhered to, bad rinsing and abuse of the parts. If you follow your established procedures, you should have zero problems.

James Watts
- Navarre, Florida


Q. Dear Mr. James Watts, Thank you very much for responding quickly. I am lost when you say bad rinsing and abuse of parts. Will you please expound on these? Thanks again.

Nila O. Par [returning]
- Winnipeg, Manitoba, Canada


A. Bad rinsing- not getting all of a process chemical off the parts and rack before going into the next chemical. This causes as many problems as anything else in anodizing because operators get in a rush and owners rarely have adequate number of rinse tanks, which leads to high waste treatment costs or rinse tanks becoming "foul". It also forms salts on the surface from the chemical reaction of the different chemicals. A single rinse is adequate between the anodizing tank and the seal tank, but not on any of the others!

Abuse- sloppy racking and deracking, banging loaded racks against other racks or objects, throwing parts into a tote bin and . . .

A note of warning that I forgot. When you make up a new anodizing tank, leave about 5% of the old tank or run a couple of dummy loads to get some aluminum into the tank. If you do not, the first load of parts will be a nice bright olive green color instead of the color that you will get on later runs. This is extremely hard to explain to a customer. This strong visual difference will be picked up by the worst sloppy QC incoming inspector.

James Watts
- Navarre, Florida

September 14, 2009

Q. Mr. James Watts, you mentioned that chromic acid anodizing will form salts on the surface from the chemical reaction of the different chemicals.

Can you name these chemicals?

Damian Ho
- Penang, Malaysia

September 14, 2009

A. Hi, Damian. I think what Mr. Watts said was that if you follow the procedures you will have no problems, but inadequate rinsing will form salts on the surface. Those salts would be whatever the parts had been dipped into. In the case of the chromic acid tank itself, it would be chromic acid plus whatever else is in the anodizing solution.


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


Q. Where does the chrome go? Okay, I've been in sales too long. I understand the chemical concepts behind electroplating well enough, but evidently not well enough because switching the problem around, i.e., with anodizing (let's say in a chromic acid solution), I lose it. Besides chromium emissions due to mist (and some due to tank cleaning I'd think, but at low levels with proper precautions), is there an actual oxidation-reduction reaction occurring, and is the Cr+6 reduced to +3 chromium that subsequently bonds to the tank shields and is removed as a shiny solid for clean up? In other words, in a chromic acid batch, where does the chromium element go after the oxygen is provided to the aluminum anode?


Brad Carpenter
Environmental Management Software - Maple Grove, Minnesota, USA


A. By far the biggest loss of chromium is drag out by the racks and parts. Chrome 3 in the solution should be converted to chrome 6 by the high voltage used.

James Watts
- Navarre, Florida

affil. link
"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon

A. Hi Brad. Yes, in a way anodizing is the opposite of plating because the polarity is reversed. But it is not electroplating in the opposite direction (onto the anode). Rather the anodization is a compound of aluminum from the substrate and oxygen from the water in solution. There is no chrome plating involved except incidentally; the chromic acid is simply the electrolyte that the oxidation process takes place in. As Jim says, dragout is the major way chrome leaves the tank.

For more on this topic, you might want to take a look at "The Surface Treatment and Finishing of Aluminum and its Alloys".

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

November 11, 2012

Q. We have just chromic anodised some alloy bodies with lots of blind holes in them. After anodising 40 of them and washing them all off in water, we took each one of the jigs and washed all holes out ... when I noticed 12 out of 40 had a small hole right the way through them. There was a lot of black dust in the blind hole which was not there when we started the job. The solution is checked every month. I have never had a problem with chromic anodising before. I always felt chromic acid would find out if there was a defect in the alloy -- it would show up in some way. Has anyone else had a problem, as now I have to tell my customer something?

Martin Jackson
- Hounslow, Middlesex, UK

November 12, 2012

A. Obviously the holes have organic or inorganic soil inside. This could be cutting oil, or aluminum bits from the drilling, and/or work hardening from a dull drill. Next time through, go up/down/up/down/up/down, completely in and out of solution in the cleaner, deox, and all rinses.

robert probert

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

April 19, 2013

Q. Hello,

We are having difficulty with our Chromic Anodizing tank. It was recently made up new, and our operators are having issues with visual appearances particularly with 2024 alloy. Coating weights tests have been done and they are will within the spec limits. Currently, our total chrome is ~56 g/l and the free chrome is 50 g/l. Thoughts?

Trent Elam
- Dayton, Ohio, USA

simultaneous April 19, 2013

A. First, exactly what is the visual problem?
Second, are you using the low or the high voltage method?
Third, what is your rinse and seal cycle.

James Watts
- Navarre, Florida

April 22, 2013

A. Hi Trent,

What in particular is the issue you are having? New anodising solutions tend to have a lighter color than older solutions. As the total chrome rises you will find the color will return to something more akin to what you usually see.

Brian Terry
Aerospace - Yeovil, Somerset, UK

April 24, 2013

Q. The particular visual appearance problem is one that some of the part has a dark gray look across it, while other areas of the part has a lighter gray. Also, it looks like a streak appearance that runs horizontal across test panels, not in a particular area, but across the whole panel. We run at 40 volts, and our sealing times vary from 20-40 minutes. Is it that the total chrome concentration is low enough to cause these problems? It just seems like whenever we make up a new Chromic bath, we have some type of visual issue. Would it behoove of us to change parameters to allow for a higher total & free chrome makeup?

Trent Elam [returning]
- Dayton, Ohio, USA

April 25, 2013

A. What kind of seal? If a hot water seal, then Alcoa said in 1956 and AES repeated in their earlier slide show, that a hot water seal for over 10 minutes does degrade the coating. Granted, the MIL spec calls for 15 minutes and I cannot get them to consider changing it, but I have proven that hot water over 10 minutes degrades the coating. The other important thing is the anode/cathode ratio which must be about 5/1 to 10/1.

robert probert

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

October 29, 2014

Q. I am looking for information on rinse tanks, particularly our anodize rinse tank. We just recently have encountered some 'mysterious' staining on our anodized parts that we have never suffered from before. We have looked at many of the common variables and nothing seems out of the ordinary. We want to examine our rinse tank a little closer than just checking the pH and Dissolved Solids. I did do Cr+6 analysis on a sample of our rinse but have nothing to compare it to. Could someone please give me an idea of what amounts could be expected in our anodize rinse tank due to drag out, etc. Thanks for ANY help at all!

Jillian Penny
- Amherst Nova Scotia, Canada

Chromic Anodizing Electrical Breakdown Problem

October 19, 2015

Q. I am doing a Chromic Anodize process on 2014 T4 Alloy which has to withstand an electrical breakdown current of 50V DC.

The anodize Process:

Every 2 Minute the the voltage is ramped up by 5V up to 40 V. and at 40V it is kept at 35 - 40 mins.

Before anodizing the material is washed in Nitric Acid (50% Nitric Acid and 50% Water) ,
Post Anodizing the material is sealed in Hot water. Sometimes there is no electrical breakdown until 5000 Nos and then the breakdown starts again. I have to then throw the whole bath and make a new one. Is there any other way where I do not have to throw away the water?

Mehul Patodia
Anodizing Shop Owner - Kolkata,India

Acid attack/ pitting

December 10, 2015

Q. we plate a part that is chromic anodised apart from were hard anosing is required. This is where I have the problem. The hard anodising is okay; it's always the side face were it looks like acid attack and pitting.

We wax up as follows: first, we use the side face to jig onto which is a threaded bore with a threaded bar which is hook shaped. We wax first for 30 minutes, then cool, re-wax, cool, then fill any holes, re-wax, cool, remove the jigging bar, clean the area to be hard anodised, then refit straight threaded bar for plating. Before this, seal around the part where you have screwed the new jig in. After plating, strip in hot water tanks. This is when we notice that where the jigging bar has been there is acid attack / pitting.

We do these parts in loads of 4; not all parts come out like this. Plating is done at 6.00 Amps, temperature 00.00 to 00.05 °C for sixty minutes. We have tried stop-off lacquers, but no good. Thanks.

andrew thomas
plater - uk

December 11, 2015

A. Good day Andrew.

That is quite an interesting problem. You process four pieces at a time, and state not all pieces show the pitting.
Where on the flight bar are the defects? Is it at the ends, as HCD?
How large are the pieces and what is your voltage/ASF?
What is the sulfuric concentration?
What I am getting at is, do you have enough electrical contact area to carry the current?
You state that you remove the jigging, and install a threaded bar and seal before anodize. Are you sealing with the same wax?
I am thinking this sealing is compromised, and my question to you was do you have enough contact area, and how many? Maybe it is a "burn"issue.
I am thinking this is a situation where possibly heat is being generated, allowing anodize solution to "seep" into this area.
Food for thought.
Hope this helps.


Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada

Arcing around plugged holes during chromic acid anodize

December 2, 2016

Q. The anodize operator in our shop is having an issue with arcing / etching around the top edge of threaded holes during the chromic anodize process. The holes were being plugged with green neoprene plugs. We switched to a softer silicone plug and the problem still persists.

22258-1a  22258-1c  22258-1b  22258-1d

My question is, Are the plugs causing this problem at all? Or could something else during the process be the cause?

Jeremy Wright
shop employee (masking) - Salt Lake City, Utah, United States

December 9, 2016

A. I'm not an expert in Chromic acid anodizing, but I've never heard or seen a plug causing this issue in sulfuric acid anodizing. Is it all of the threaded holes on the part? Is it the same hole every time? Are the same plugs being used on other non threaded holes in the part? Something weird is definitely going on here, I do not think it's related to the plug.

Marc Green
Marc Green
anodizer - Boise, Idaho

December 13, 2016

Q. Those are the only holes masked by plug on the part. It's not always the same hole that has issues.

Jeremy Wright [returning]
shop employee (masking) - Salt Lake City, Utah, United States

December 9, 2016

A. Until you give us some more data, here is a very wild guess.I suspect the plugs are leaking, filing up with deoxidizer, then where the deox leaks out, since it is more conductive than the anodizing solution, it attracts high current which burns away the edge of aluminum.

robert probert

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

Brown stains on chromic acid anodising

October 3, 2017

Q. Hi Ted and the community,
Here at our facility we chromic acid anodise parts made of AMS 4050 T7451.
It's a standard ramped process, preceded with aqueous degreasing, alkali etch, and de-smut. The parts are showing brown stains over most of the parts, but especially in recessed areas.


Other parts from other alloys anodise as normal in the same bath, with no signs of stain, it only happens on this one alloy.
One possibility that has been suggested, is that the areas showing the stain may have been stressed during their machining, or via over zealous hand linishing.
If anyone recognises this situation, then I would be glad to hear your remedy to it.
if you need any more information in order to better answer this request, then I can supply it.
Thanks and best regards

Mark Lees
Industrial Chemist - Windy rock in the middle of the Irish Sea

January 2, 2018

A. Hi,
This is called Machine abuse or localized heating during machining, exhibited after anodising.
This can be corrected in machining with Feed, speed and coolant circulations.

All the best

- Belgaum, Karnataka, India

January 5, 2018

thumbs up sign Thanks Shankaranrayanan,
we had our suspicions that some sort of machining damage was at the root of this.
Thanks for your corroboration.
Best regards

Mark Lees [returning]
- A not so warm rock in the irish sea

Increasing aluminum in chromic acid anodizing bath

April 4, 2019

Q. Hi to everybody, we have a new bath of chromic acid anodizing and we need to "age" it before starting the process. Usually for doing it we treat some scrapped aluminum sheets until we reach the minimum Al content of 1.5 g/l. This process is quite long. Is there another solution for ageing the bath?

What happens if we use an anodizing bath with too low Al content in it?

Thanks in advance for your help. Have a nice day.

Fabrizio Galimberti
- Sesto Calende (VA), Italy

October 5, 2019

Q. Hi, it's possible the chromic anodize application could "eat" the material (al alloy) and reduce the part dimensions, is it? Why?. We had this problem.


Eduardo Torres
Landing Gears overhaul - Sunrise, Florida USA

October 11, 2019

A. 1. Some imported chromic acid is high in sulfates.

2. Look for chlorides in your water, especially in Florida.

robert probert

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

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