Burning problem in Hard Anodizing of Aluminum

A. Hi Steve. You did very good. Indeed, pulsing parameters are another tiger to tame, and one that you don't want to have loose until you've tamed the other parameters -- so shutting off the pulsing was the right thing to do.

Although not every anodizer and anodizing consultant is sold on pulse anodizing, some are, and the world expert on the subject is probably Dr. Anne Deacon Juhl who did her doctorate on pulse anodizing and has been conducting an anodizing school for several years now. I think you should write to Anne (anodizingschool.com) and see on what basis you can get her help with this question.

Please remind her where you learned about her ... and that she really should be here as an advertiser and a forum contributor :-)

Luck & Regards,

A. 1.Unless you are doing heat treated 2024 you will get much better hardcoat (smaller pore) with 180 g/L, 225 is too large pore and not Tabor abrasion passing hard coat.

2. The bottom of the rack is the highest current density, so (1) get 8" off the bottom (2) ramp from zero to the target ASF, 24 ASF) for 15 minutes before setting the current . Be sure the air agitation is not hitting the parts and displacing solution/current.

3. With 40 volts you should get up to 0.001 to 0.0015 inch thick, beyond that the resistance calls for higher voltage to maintain the amps, the amps fall off and the pore gets too large. If you want to do 0.002, you need 60 volts.

I have installed and serviced hundreds of hard coat lines, never found anyone who could prove to me any advantage of pulse, so you better stick with Ted Mooney on that subject.

A. I would be amazed if this damage was from static electricity in the packaging. I believe it is either fretting/galling damage (even that is hard to believe), or that the burns were there from the anodizing process.

Are you sure the burns were not there before the parts were stacked?

A. There are many places that claim to do hard coat. If it is a real hard coat not just type 2 with more thickness then I doubt it would be worn off.

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.

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.

Thank you.

Agitation is wonderful thing.

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

A. But is this burning in a HCD area, a spot that might be overheated, or a place where for any reason you would suspect burning to occur?

A. Better agitation should work. Check your sparger design and placement relative to the parts in a good reference book. If it is OK, You may have to increase the air flow. Eductors work for pumped solution. I would buy one and test it in the lab with air.

A. 1. Lower the current density to 24 amps per square foot,
2. be sure the air is not hitting the parts
3. use a hard coat additive, either oxalic or preferably glycolic/glycerine(2%/2%/vol or proprietary)

A. Noppachai

Your post suggests that you have not always had this problem. If your operation has not changed, and the problem is new, have you checked your dissolved aluminum content?

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

Regards,

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.

Regards,

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.

Regards,

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.

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".

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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.

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.

Hi Jigar. The previously mentioned "Surface Treatment & Finishing of Aluminum ..." text mentions a Reynolds formula of 15-24% sulfuric 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 :-)

Regards,

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.

Regards,

A. I WILLING TO BET MOST OF YOUR BURNING IS TAKING PLACE AT THE BOTTOM OF THE RACK. WE HAVE FOUND THAT REGARDLESS OF HOW GOOD THE AGITATION IS AT TIMES, THIS CAN STILL HAPPEN. YOU MAY WANT TO PLACE AN EMPTY RACK AS A ROBBER ON EACH END OF THE WORK BAR AND YOU MAY CHOOSE TO NOT RACK ON THE BOTTOM OF THE RACK. THIS HAS HELPED US GREATLY, ESPECIALLY WHEN RUNNING PARTS THAT ARE VERY THIN.

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.

Regards,

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.

A. Sounds a lot like chlorides in the anodizing tank.

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 on this thread is interesting.

Luck & Regards,

A. Sometimes the best way to answer a question is with a question. So let's start this way: How far do your cathodes go down into the tank? They should go to the bottom of your parts on the rack but really no lower. Also if it is agitation. I would try turning the air off using non-production parts to see if burning gets worse. If it does then increase the air until burning is reduced. It's a start

A. Hi, Danfang. Those values are more or less the same thing since tank size can be expressed in terms of volume. But I think the question is an excellent one because, while air agitation serves several different purposes in plating and anodizing, in hardcoat anodizing its foremost mission is probably heat removal from the parts and the solution touching them because so much power/heat is produced and the acid is more aggressive at higher temperatures.

Wernick, Pinner & Sheasby says the thinner the components, the more agitation is needed, but also quotes a number from Turner of up to 0.45 m3 of air per m2 of tank surface. That sounds good to me.

Luck & Regards,

A. Hi Hassan. There are a number of possibilities, perhaps including Cl- in your bath or other solution imbalance, but if you consistently have a problem only at higher power input, my guess is that you have inadequate agitation and cooling.

Luck & Regards,

A. Obviously the acid is way too low, it is a certain burning recipe, get the acid up to 180 g/L, and call us back.

A. Hassan
If your total acid is 123 g/l and dissolved aluminum 12.6 g/l, your free acid is likely ~ 50 - 60 g/l.

I believe Robert is suggesting you raise your free acid content to ~180 g/l. At your current concentration, the best you can hope for is a thin film.

A. Hello namesake. Pardon my lack of imagination, but I don't understand the pic. Is this a single louvered component or a stack of parts? And what is with all the scratches and gouges, and the sharp but ragged edges -- it looks like the parts haven't been mechanically finished whatsoever.

It doesn't seem to be a case of poor contact because that would probably evidence itself across the entire part, not a section of it.

The temperature of the bath is 32 °F or lower and you have excellent agitation? Are you certain that the part(s) was waterbreak-free clean before the other process steps? Is this a new problem in a long established process or are you just starting up?

Luck & Regards,

A. Until you send us more data, my first guess is that "weathered" oxide was not removed by your deox. It looks like the parts had been stacked for some long time, the exposed edges then covered with natural oxide so thick that the deox could not remove. The plain dark spots would have been the NON-exposed edges during the formation of the weathered oxide.

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A. Hi again. Although brush anodizing is possible it is usually prohibitively difficult and expensive, and insufficiently aesthetic, for this sort of repair. The best repair is usually to strip and re-do the anodizing.

Luck & Regards,