Anodising thickness not being achieved

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

Also available in Spanish

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A. Since 7075 is very slightly more conductive than the rack 6061 (my page 53), then you can expect the opposite effect, but here, the current had to go somewhere, so it probably went to the rack because the electrical contact was not broad enough, which is a common problem with shops who do Types II and III: production rackers do not notice that 24 ASF takes a broader contact than 12 ASF. Check the rack thickness and come on back.

A. Based upon the limited information (and no mention of burning at contact points), I suspect a high-silicon diecast alloy. The Aluminum Anodizers Council's Anodizing Reference Guide (http://anodizing.org/reference_guide.html) shows 0.0005" maximum hard anodize thickness for alloys 360 and 380. Titanium racks can often be used for hard anodizing very small parts but are generally a disaster for any anodizing of large surface area parts.

A. Due to the high percentage of alloy content in die-castings, it is extremely difficult to achieve thick anodic coatings without burning. The above quoted 0.5 mils is about the maximum for normal anodizing methods. There is, however, at least one proprietary method which can can build up to 2 mils of anodic hardcoat on these high pressure die-castings.

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A. Yes ... it is possible (but unusual) to get 150 microns (.006") depending on the alloy, your use of proper racking techniques, and processing parameters.

A. I think in inches so 0.000 040 X 80 = 0.0032 inches thick. If that is right, then YES, let us now know the alloy and we will come back to you with more data.

? Am I incorrect? I believe approx. 25.4 microns = 1 mil (.001")?

You're both right. 25.4 microns to the mil, so divide the microns by 25.4 to get mils, or multiply the microns by .000 040 to get inches.

? But Ted,

By my calculations he is wanting almost a 6 mil coating, yet by Roberts figures...the coating is 3.2 mil...unless Robert was looking at coating growth on top of the substrate?

A. The coating he is getting is 75-80 microns, equals 3.2 mil. The coating he'd like to get is 150 microns, equals 6 mil.

A. We anodize at 0°C in a 10% H2SO4 electrolyte and can achieve that kind of thickness, temperature is critical because if the part gets too hot due to high current density you will burn the part.

A. Max thickness of hardcoat is limited primarily by the alloy. Since the coating is an electrical insulator, the better the structure (tighter, denser, etc.) the better it insulates and the lower the maximum thickness obtainable will be. On 6xxx alloys, getting anything over 75 microns (roughly 3 mils) is nearly impossible.

On 7xxx alloys on the other hand, getting 150 microns is possible. Ramping is critical (how fast the maximum current density is achieved ). Others may argue that modified power, either superimposed AC over DC or pulse rectification will have a benefit. That would be up to the suppliers of the specific process you employ.

Good luck and let me know how you make out.

For your information, there is an International Hard Anodizing Association (www.ihanodizing.com) that has technical symposiums every 2 years. Attending something like this might establish the network you need to tackle hardcoat concerns.

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 H Probert Robert H Probert Technical Services Garner, North Carolina 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 anodizer - Boise, Idaho 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 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.

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.
Thanks

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.

Regards,

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.

Regards,

A. 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 / MIL-PRF-8625 [on DLA] and ASTM B580 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!

Regards,

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.

Regards,