Type II vs Type III Anodizing of 2024 Aluminum

A. The thermal coefficient of expansion of the anodic film has approximately 0.2 times the aluminum metal. 2024 is worse.

1. Bad.
2. Poor. MIL-A-8625 allows hard anodized 2024 to have a Taber abrasive wear loss 7/3 x that of 6061 or 7075, and dyeing + sealing further softens the anodize.

Some effects of alloying are described in the Aluminum Anodizers Council 'Anodizing Reference Guide' http://www.anodizing.org/reference_guide.html

Simplified technical explanation: The Cu in 2000-series alloys forms intermetallics such as CuAl2 which dissolve rather than anodize. About 7 wt% CuAl2 in 2024, so the anodize is ~7% more porous than on a low-Cu alloy. The intermetallics also increase ohmic heating within the alloy, which in combination with thin spots in the anodize, can lead to a nasty, electrochemically-accelerated dissolution phenomenon known as 'burning.'

Many newer alloys have high strength and anodize better than the 1920's vintage 2024. Some with better corrosion resistance in higher strength tempers than the 1943 vintage 7075. The Metallic Materials Properties Development and Standardization (MMPDS) Handbook lists a number of 7000-series alloys: 7010, 7040, 7049/7149, 7050, 7055, 7075, 7150, 7175,7249, 7475; also 6013.

Yield strengths (ksi) for thicknesses up to 2 inches.
2024-T351 42-50
6013-T6 46-49
6013-T651 52
7010-T7451 61-63
7049/7149-T7351 60-64
7050-T73511 56-60
7055-T7751 85-88
7075-T7351 52-59
7150-T7751 74-78
7175-T73511 52-59
7475-T7351 53-61

All values from MMPDS-01 except 6013-T651 (MatWeb). 7040 (heavy plate only, > 3") and 7249 (Hand forgings) were omitted.

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Ed. note Mar. 2016: The anodizing reference guide is no longer available, but a replacement for it, bulletin #4-13, is included in a compendium available from anodizing.org for $75.

A. Type III anodize of 2024 is a risky venture due to the higher concentration of copper within the alloy. Not to say it can't be done, we do it often, but the risk of material degradation is increased.

To answer your other question, 7075 would be more appropriate for a type III coating.

-Bill

A. Hello, Erol. The generic bath is 10% by volume sulfuric acid, operated at 0 °C or slightly below, current density of about 24 ASF, and the time is calculated with "The 720 Rule".

More operational info can be found in letter 11365 ... but it's a long trip from making up a generic bath to successfully hardcoating 2024. It is quite difficult to hard anodize (50 microns) or even anodize to a thickness of 25 microns, and it has been the subject of a dozen letters here already. So if you use the search engine to look for "hard anodize 2024" you'll get some very good insights into the issue.

But the thing is, our readers readily share how to do the moderately easy things, but when they've finally figured out how to do difficult stuff like hardcoating 2024, and they earn their living from their successful techniques, they tend to hold them as proprietary trade secrets.

It's probable that pulse rectification, additions of oxalic acid or glycolic acid, and extraordinarily good agitation will be part of the solution -- I can afford to share what I know both because I don't earn my living that way, and don't actually know how to do it :-)

Luck and regards,

Hi Sanjeev. I hope a reader can help you out, but on-site troubleshooting to establish where the problem is arising is critical, and you haven't been successful with that yet.

It's hard for me to believe that wild guesses will be much help, but I see you've at least included a photo :-)

Surely there are anodizing consultants in the Bangalore area who could quickly get you out of this quagmire, but if not, I think what is missing is a truly orderly troubleshooting plan. Best of luck.

Regards,