Type III hard anodizing and Taber Abrasion testing

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Hi Nader
Robert Probert lists below some conditions that lead to better Taber abrasion, but you should also review topic topic 11872 as it notes that test conditions like humidity can affect abrasion test results.

"A lot of aluminum parts on the bottom of the bath" can contribute to too much aluminum concentration, of course, but also indicates very poor attention to detail. Times have been changing and parts falling off of racks unremarked is no longer acceptable.
Luck & Regards,


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

A. Dear Wayne Hale,

Probably you need to cool down your solution and have a much more vigorous agitation of it.

Another possibility is to use pulse current, this could be pulsing between two levels of the current or using DC/AC as in e.g. Sanford process.

To reach more specific improvements, you have to give more information about how you are processing now.

A. Recommend sending HA 7075 test panels to independent laboratory that performs Taber Resistance Testing. After 2 months of failures, I noticed that our internal lab was performing test with improper weight load and uneven grind wheels!

Increase surface area in tank. Plate panels to 2.3 mils or highest allowed thickness per spec. Agitation?

? Without knowing your anodizing process, hardly give suggestion.

A. Hi, Rajanish. It's hard to say what went wrong until you describe what was changed, but on this page Robert Probert tries to describe what processing conditions lead to better Taber abrasion results, and you might find topic 11872 interesting as it speaks about test conditions like humidity which can affect the results.
Good luck.

Regards,

A. You specify what you want in the purchase order along with how frequently it will be done, how it will be done and by who.

A. MIL-A-8625F Type III only specifies a minimum wear standard for a dry Taber abraser test on a relatively easy to hardcoat 6061 alloy. Most hardcoat anodizers have their own proprietary processes. They are not all equivalent. Unfortunately, other than actual testing under your operating conditions, there is no good way to compare coatings.

The International Hard Anodizing Association (www.ihanodizing.com) has been trying, unsuccessfully, to develop a more discriminating test to identify "world class" hardcoaters. I suggest you stick with the vender(s) whose coating performs the best.

A. You may want to ask if one of your vendors is using an additive in the hard coat process. If they are, you are not getting a true hardcoat.

A. Interesting dialogue on hardcoat anodize and what leads to a "quality", wear-resistant anodic layer. Building on what the previous responders have suggested, my guess is that some of your anodize providers are going through the motions to provide anodize per MIL-A-8625 but don't have the quality system or periodic testing necessary to ensure that it's compliant each and every time.

There are subtle differences between conventional (Type II) sulfuric anodizing and "genuine" Type III hardcoat. Some chemical suppliers sell an additive that added to a conventional bath that will yield borderline compliant hardcoat values. The fact that a house uses an "additive" doesn't necessarily make them a "hardcoater." In most cases it takes (1) an additive, (2) reduced electrolyte temperature, and/or (3) increased power (typically expressed as amps per square foot/decimeter). In addition to these seemingly "subtle" differences in operation, to be fully compliant one would also have to monitor (at very least monthly I'd say) the concentration of all baths, calibrate temperature, time and power controls, monitor the thickness of each load, and perform Taber abrasion tests monthly.

If I were in your shoes, I'd audit each of the outside processing houses checking for operating conditions, records of all process control and periodic inspection checks, and evidence of work instructions to ensure the process can be repeated. Good luck.

A. Hardness (abrasion resistance) is favored by lower temp, lower concentration, higher current density, and the glycolic/glycerin additive. AND ramp up to the desired current density over a period of 15 minutes before setting the current to remain constant while the voltage climbs.

Also, understand, that because of the inverse conical shape of the pore, the pore is smaller (and more abrasion resistant) at the lowest thickness.

A. Franz, Do you have the option to follow the anodizing step with a dye step? We immerse hard anodized 6061 parts in a black dye, and get a very dark finish. Michael Michael Costello - Grand Junction, Colorado, USA A. Darkness is favored by lower concentration, higher current density, and lower temperature all of which also favor burning. If that is not dark enough then do the reverse of the above (to get a larger pore) and dye black Robert H Probert Robert H Probert Technical Services Garner, North Carolina

A. Just remember, if you choose to dye the part you must seal it afterward to prevent rapid fading. This will slightly lower the hardness, but dramatically increase the corrosion resistance..
Good luck!

A. Experts, please correct me if I am wrong.
Am also given to understand that addition of Oxalic Acid (in the range of 10-15g/l)in the sulfuric acid bath also assists in faster and harder coating growth as well as darker color.

A. Ravi,

We hard anodize (Type III) at 10 ± 2 °C, then dye black and seal. I can't give you a hardness number. The customer spec is based on passing a 10,000 cycle Taber abrasion test, and our parts always pass. I hope this is useful to you.

A. Replace the deox, then tell us the free acid, temp, and current density, thickness of coating...

A. Hi zander,

Is Taber the only destructive test you send out? You've already mentioned deox issues, are you seeing worse results / trending in your other tests (if you have any)?

You gotta seriously maintain a deox tank, in my humble opinion, more carefully than any other tank on the line. A bad deox will BREAK you.

And the operator should be doing a smudge test coming out of the deox bath. Gloved finger or white cloth (or if you're my lunatic colleague, just poke it barehanded) if there's evidence of unflushed smut, and the line technician isn't literally yelling at the lab technician [disclosure: I'm a lab technician and would HOPE to be yelled at in a pinch]... you have a ground-up retraining situation on your hands.

Have your lab tech run a test of the deox in front of you. If it's good, move on to the next step.

Ate you running am anti-burn additive (proprietary organic acids, originally oxalic, but...ooh I've been through this). Are you running it at full manufacturer strength. And are you fully dehydrating the specimens prior to shipment to the lab.

I've found MINIMAL anti burn additives plus great care in full dehydration of parts is the best abrasion resistance.

There is a fascinating academic study on the subject of dehydration of hardcoat anodize.

But if you aren't flushing the smut, you're not going to start your build properly. That's where you need to begin.