Hard Anodize Problem. 2XXX Aluminum Burning

A. Hi David.

When you've done decades of successful processing, and then things stop working, it's much easier to figure out what has changed than to start over, so I would urge you to not change too much  🙂

And because you have so much process-related knowledge, it might be best to consider things that are not a direct part of the electrochemistry ...

Are you sure that the air agitation ⇦ huh? (or sparger agitation) has not changed significantly? A great deal of localized cooling is required for hard anodizing. Are you sure that downtime related to rectifier changeover didn't introduce poor rack, workbar, or saddle contacts such that you're suffering voltage breakthrough after the ramp up?

If nothing that has been recorded changed, are you certain that the previous thickness you were obtaining matches what you are now shooting for?

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It is rather a mystery. Can you check the voltage with an oscilloscope to make certain that the rectifier isn't single-phasing or suffering some other defect? These days high end digital electrical V-O-M multimeters can display oscilloscope traces.

Luck & Regards,


Ted Mooney, P.E. RET
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A. Hi David,

I know your company very well; if Arron can't sort it out, then it must be a conundrum!

Good luck, an that.

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A. Hi David. You can buy a separate Ampere-Hour meter -- that problem is easily solved.

But if you were able to successfully hardcoat these difficult to anodize alloys only by using a rectifier that had a pulsing profile you can no longer generate, that's quite a problem. You might try to contact Ann Deacon Juhl for advice if you haven't already -- she has specializing in pulse anodizing for about 38 years  🙂

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.

A. Hello David
It seems to me that for hard anodizing high copper alloy, your sulfuric concentration is rather low. My recollection is that it should be somewhere near the 18- 20% range. I am thinking that 14% is much closer to the norm for type II anodizing. I am not able to address pulse anodizing as this is not within my scope of experience.

A. I would drag the bottom of your tanks to be sure whether an older part or material that is in the tank is causing the burning

Ed. note: Excellent idea!

A. Hello David.
Opportunities such as you have are the thing that kept me coming back to the job every day aside from enjoying my job as much as i did. (35 years of doing battle). What worked yesterday does not work today. I would make this suggestion to you. Look backwards and not forward or to the present. I responded back in July that 2000 series aluminum needs a sulfuric concentration of near 20 per cent to prevent burning.

Your old rectifier gave out on you so you purchased a new one. Maybe that was the problem. You were not attaining proper current to your parts (anodizing more slowly) so the acid concentration was not an issue. Now with this new baby, things are happening how they are supposed to and this is foreign to you. It is kind of like your old rectifier was training you incorrectly and nobody knew any different so you all just kind of went with it and the experience you were gaining caused you to build on a faulty foundation.

I can tell you from experience, a less than 20% concentration for 2000 series aluminum is going to lend itself to burning. The proof is in plain sight. If you see that 3000, 5000 and 6000 series aluminum are processing fine with no burning and 2000 series is burning, it tells you something is wrong with your process, not the rectifier.

You can process other alloys in a tank with higher concentration, but you should not process 2000 series in a tank with lower acid concentration. 14% sulfuric is a great solution strength for type ll anodizing including 2000 series but it does not work so well for hard coat. If you have the space or multiple processing tanks, it would be beneficial to convert one of your tanks to the higher sulfuric concentration and process your 2000 and possibly your 7000 series in said tank.
Keep fighting the good fight.

A. Sir,

2024 is a high copper alloy & cannot be hardcoated in conventional hardcoat systems. Here in our facility we use 2 different hardcoat systems, Martin HC & Imperv-X HC. Martin will process most alloys BUT NOT high copper ones. The Imperv-X system is specifically designed for high copper alloys and we hardcoat them daily with no problems.

A. Richard,

Your information is not complete which makes an answer difficult or at least requires assumptions to be made which is not good.

Al 1.0 oz/gal or g/l or ? Sulfuric Acid ounce by weight or fluid ounce?

Normal hard anodize is 32 °F. Are you using a proprietary or an additive? Straight DC or superimposed AC or pulse?

The high copper content in your alloy makes the anodizing more difficult, but you should be able to get 1 mil.

If your final voltage is 16, you are hard pressed to call it type III anodize, and 1 mil will be very hard to get.

Include all of the process variables and it will be easier for people that are more knowledgeable than me to help you.

Jim

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A. Your processing parameters are more appropriate for an anodize than a hardcoat. I doubt that this coating would meet the minimum wear requirements for a Type III hardcoating per MIL-A-8625. This could be contributing to your difficulties in achieving the desired thickness. Typical hardcoat anodizes utilize higher voltages and current densities with lower solution temperatures. The trade-off for these harder, thicker coatings is an increased tendency for "burning", particularly in the 2XXX series alloys. Special procedures and process controls can help minimize this phenomenon. Also, proper design and placement of auxiliary cathodes will help with coating uniformity.

A. Hi Mehdi.

Ambient temperature hard anodizing is not true hard anodizing, so some people call it Type 2-1/2 anodizing. It doesn't operate at freezing temperatures, but I don't think it's truly ambient, maybe more like 50-60 °F (10-15 °C). These processes are difficult and almost surely require proprietary additives. If you don't have access to such, it's probably better to lower the temperature to freezing, although Robert Probert can probably consult on non-proprietary additives.

By far the most complete reference to every type of anodizing is The Surface Treatment & Finishing of Aluminium and Its Alloys ⇨
It's a large, expensive, 2-volume book ... but hopefully there's a copy in a public or university library. It covers everything you can imagine .

Regards,

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

A. Hi

2014 is copper Aluminum Alloy, which may give burning problem, may not support much for hard anodizing (Sulfuric Acid electrolyte), where you need higher current density.

A. 2024 and especially heat treated 2024 requires higher free acid, try 220 gm/L, then maybe 230 gm/L, let us know.
Always ramp 15 minutes up to a pre-determined 24 amps per square foot amperage then switch over to current control and let the voltage climb as the resistant thickness builds.

A. Hi, Ali.

In my rather limited experience in that area, this is usually caused by poor contact. What happens with poor contact is that the item initially gets little or even no anodizing thickness, then as the voltage goes higher, it breaks through whatever resistance was on the contact surfaces; the thinly anodized or non-anodized parts subjected to 45-50 V causes large currents, which immediately causes burning.

Regards,

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

A. How are you measuring hardness? When Reynolds published their first work on the glycolic/glycerin additive they got a average of 320 and a max of 365 Knoop on 2024-T3 at 1.8 mils and avg. 347 and max of 376 Knoop at 2.3 mils. So 300 is attainable. Be reminded that the Knoop diamond comes in on the cross section -- if close to the bottom of the thickness where the pore is small it gets a higher thickness, but when it comes in near the top of the inverse taper pore, it crunches and shows a lower thickness.
Now for ways to get a smaller pore (read harder): lower temperature, lower free acid, higher current density, and 4%/vol of a 50/50 glycolic/glycerin additive. Note also that all other allows get harder than the 2000's.